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Speaker 1: Hey, Daniel. When you look at the night sky on

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Speaker 1: a good night, how many stars can you see?

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Speaker 2: Well, if it's a really good dark night without any

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Speaker 2: light pollution, you might see several thousand.

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Speaker 1: Stars, thousands in Los Angeles.

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Speaker 2: Maybe forgotten Joshua Tree.

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Speaker 1: But that's it. Only a thousand stars and the night

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Speaker 1: sky aren't there like trillions of stars out there?

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Speaker 2: There are oodles of stars out there, but most of

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Speaker 2: them are too dim to see even though they're out.

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Speaker 1: There, like their photons are not reaching us, or they're

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Speaker 1: too weak.

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Speaker 2: Photons can travel in infinite distance without ever getting tired,

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Speaker 2: but their photons are just so rare that you need

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Speaker 2: like a really big eyeball, maybe Hubble or James Webb

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Speaker 2: to capture one of them.

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Speaker 1: Or just like a long exposure too.

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Speaker 2: Right, Yeah, if you set up your camera for months

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Speaker 2: and months, you'll probably see one of those dim stars you.

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Speaker 1: Need, like a slow eyeball.

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Speaker 2: A big or slow eyeball or both.

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Speaker 1: So I guess what else is out there? Like, what

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Speaker 1: else would you see? Would you see stars and anything else?

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Speaker 2: We don't know. What's out there could just be stars

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Speaker 2: and galaxies, or there could be like chocolate bars and

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Speaker 2: frozen bananas waiting for us to discover them.

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Speaker 1: WHOA, sounds delicious? But will we see those things? Wouldn't

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Speaker 1: they have to glow somehow? Would these be glowing frozen bananas?

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Speaker 2: Glowing or reflective?

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Speaker 1: Which case? Do you want to eat them?

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Speaker 2: I'm glowing with excitement to try them?

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Speaker 1: I do. I know it sounds a little slippery. Hi.

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Speaker 1: I'm Jorge make, cartoonist and the author of Oliver's Greek

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Speaker 1: Big Universe. Hi.

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Speaker 2: I'm Daniel. I'm a particle physicist and a professor at

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Speaker 2: UC Irvine, and I'm so excited to see what's hiding

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Speaker 2: out there in the universe waiting for us to discover it.

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Speaker 1: Yeah, there seems to be a lot out there. But

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Speaker 1: do you think it's hiding or we're just not good

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Speaker 1: at seeing I.

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Speaker 2: Don't think it knows or cares about whether we are

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Speaker 2: seeing it. But it has not yet been revealed, so

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Speaker 2: in that sense, it is concealed.

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Speaker 1: Oh yeah, yeah, But is it really the case that

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Speaker 1: it could anything could be out there? You think then

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Speaker 1: we have a pretty good sense from what we can

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Speaker 1: see around us.

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Speaker 2: There's still a lot of big questions about what's out there,

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Speaker 2: the stuff nearby, the stuff far away the stuff in between,

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Speaker 2: and we should never make the assumption that the stuff

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Speaker 2: that's close to us is typical and that it could

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Speaker 2: be used to explain the whole universe.

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Speaker 1: Well, I guess there could be things hiding within our

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Speaker 1: own galaxy that we can't see it, right, Like we

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Speaker 1: haven't seen all of the Milky Way galaxy.

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Speaker 2: Oh absolutely, And the center of the galaxy, the most

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Speaker 2: interesting place, is the hardest to see. But the Milky

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Speaker 2: Way itself is so bright and so big, and that

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Speaker 2: it makes it really hard to see beyond it to

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Speaker 2: other galaxies.

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Speaker 1: And these other galaxies could be totally different from.

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Speaker 2: Ours, right, They could be very different. They could have

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Speaker 2: a very different history. There could be all sorts of

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Speaker 2: stuff going on out there deep in the universe that

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Speaker 2: we haven't yet figured out. Most of the photons that

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Speaker 2: come to Earth we don't gather. We mostly ignore them.

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Speaker 1: You mean, like we're getting all this information from all

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Speaker 1: the cross the universe, but we're not doing anything with it.

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Speaker 1: We're not paying attention.

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Speaker 2: Yeah, the universe is screaming at us in photons. Everything

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Speaker 2: that's out there in the universe is telling us all

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Speaker 2: about itself. So we have like the whole history of

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Speaker 2: the universe is out there being literally beamed at us,

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Speaker 2: but mostly we're not paying attention. Mostly those photons just

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Speaker 2: like splash on the sidewalk.

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Speaker 1: I wonder if that could be a good thing. Like

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Speaker 1: I wonder if at some point it's like TMI Universe.

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Speaker 1: There's some things I don't want to never TMI.

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Speaker 2: With science, you always want more data so you can

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Speaker 2: know more about the universe. I want to know the

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Speaker 2: Universe's deepest, darkest, most embarrassing secrets.

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Speaker 1: I see, you're more of an any person.

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Speaker 2: Not enough information, Absolutely no information is too embarrassing.

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Speaker 1: Well I hope that's true. But anyways, welcome to our podcast,

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Speaker 1: Daniel and Jorge Explain the Universe, a production of Our

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Speaker 1: Heart Radio.

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Speaker 2: In which no question is too weird, too gross, too

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Speaker 2: icky for us to explore it. We want to know

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Speaker 2: all of the embarrassing details about how the universe was born,

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Speaker 2: how it grew up, and all the messes it made

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Speaker 2: along the way. We want to unravel the deep history

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Speaker 2: of time and understand how the universe got to be

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Speaker 2: the way that it is and why it operates in

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Speaker 2: such an incredible, amazing and beautiful fashion.

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Speaker 1: Man, you make it sound like crazy fans of the

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Speaker 1: universe we are, or as the kids say these days, stand.

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Speaker 2: I totally stand our universe absolutely. I will defend it

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Speaker 2: online against haters.

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Speaker 1: There you go until you turn on it.

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Speaker 2: If in season seven it does something really weird, then yes,

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Speaker 2: I will turn on it.

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Speaker 1: But so far, if it jumps the shark, if it

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Speaker 1: jumps the galactic shark, you're like, I like the earlier

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Speaker 1: seasons better.

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Speaker 2: So far, it's been pretty awesome, and everything we've learned

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Speaker 2: has blown our minds and revealed incredible things about the

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Speaker 2: way the universe works. Not only are the law that

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Speaker 2: it follows really fascinating and incredible and have weird philosophical

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Speaker 2: consequences for what the nature of reality is, but the

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Speaker 2: stuff that bubbles up from those tiny laws, the huge,

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Speaker 2: the big stuff, the black holes, the galaxies, the quasars,

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Speaker 2: the blazars, all that stuff is just so mind blowingly awesome.

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Speaker 1: Yeah, it's pretty amazing. As we said before, how much

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Speaker 1: we've been able to figure out about the larger universe,

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Speaker 1: even just about our galaxy and beyond, just from sitting

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Speaker 1: on this little tiny rock in one corner of the galaxy.

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Speaker 1: It's pretty incredible. If you think about the scale things

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Speaker 1: and how much we know about what's out there.

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Speaker 2: But we've only really just begun to observe our universe.

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Speaker 2: We have a few eyeballs capable of picking up really

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Speaker 2: distant objects, but most of the light that's out there,

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Speaker 2: billions and billions and trillions and trillions of photons that

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Speaker 2: contain super fascinating important information about the history of our universe,

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Speaker 2: we're not capturing them. They're mostly drowned out by bigger,

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Speaker 2: brighter stuff like the lights of Los Angeles.

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Speaker 1: We're sort of washed with information from the universe and

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Speaker 1: we're not really I guess we are capturing it, we're

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Speaker 1: just not recording it, is maybe what you mean. We're like,

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Speaker 1: we're getting night light, starlight, you know when I step

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Speaker 1: outside tonight, but I'm not going to be thinking about

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Speaker 1: it or recording it or trying to figure out what

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Speaker 1: it says.

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Speaker 2: Yeah, most of it just hits the earth, and unless

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Speaker 2: some like Gecko is looking up at the night sky,

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Speaker 2: there's no being that's gathering that information. It just like

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Speaker 2: gently warms some rock or some like gum wrapper that's

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Speaker 2: lying on the ground.

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Speaker 1: But do you think we're missing stuff? Like if I

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Speaker 1: just point a telescope there every once in a while.

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Speaker 1: Am I really going to miss anything?

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Speaker 2: I think there's an incredible amount of deep history out

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Speaker 2: there in the night sky. And if we build zillions

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Speaker 2: of telescopes and point to them in all those directions

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Speaker 2: and just let them accumulate information, we would learn so

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Speaker 2: much about the history of the universe from these really

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Speaker 2: faint sources.

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Speaker 1: But ironically, if you cover the night sky with telescopes

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Speaker 1: and you wouldn't be able to see the night sky.

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Speaker 2: See it in a different way.

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Speaker 1: Well, I guess that's true. We can see it on

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Speaker 1: our phones. It's a little less poetic, and we can

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Speaker 1: see it scientifically. But anyways, it's kind of interesting what

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Speaker 1: is out there and what kind of information we are getting,

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Speaker 1: including even the stuff we might consider being in the background.

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Speaker 2: Yeah, that's right. The night sky is chock full of

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Speaker 2: stars from our galaxy, but there's a lot of really

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Speaker 2: useful information in the background, information we're mostly missing.

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Speaker 1: So today on the program, we'll be tackling the question

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Speaker 1: what is extra galactic background light. It's a lot of

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Speaker 1: syllables there for one term.

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Speaker 2: Yeah, astronomers, you know this one. I think they actually

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Speaker 2: name pretty well.

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Speaker 1: We'll see, we'll see. You've always promised that, and most

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Speaker 1: of the time it disappoints.

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Speaker 2: You have a very unrealistic standard.

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Speaker 1: If I have to say so, I'm just saying, you know,

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Speaker 1: take a minute to think about it, all.

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Speaker 2: Right, all right, I'll suspend my judgment.

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Speaker 1: But yeah, it's an interesting question. A lot of words here,

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Speaker 1: extra galactic background light, which should have sounds self apparent,

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Speaker 1: but maybe the extra it throws me off a little bit,

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Speaker 1: like it's extra, like we don't need it. Or is

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Speaker 1: it extra like like you get a bonus, Like you know,

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Speaker 1: I pay for a certain amount of galactic background light

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Speaker 1: and I'm getting some extra serving of it.

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Speaker 2: Or maybe it's just like a bit much universe Like

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Speaker 2: why are you so extra? Yeah, that's how my teenage

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Speaker 2: daughter would interpret it.

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Speaker 1: Mmmm, you're being too extra?

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Speaker 2: Yeah, Dad, you're so extra.

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Speaker 1: Oh my god, that's better than me. Mid, that's like

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Speaker 1: the that's not the worst insult from a teen right now,

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Speaker 1: that's kind of Mid. Oh boy university, Mid, I've seen

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Speaker 1: better life existence, Mid. But yeah, I guess we'll dig

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Speaker 1: into what all these turns mean and why it's interesting

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Speaker 1: to think about the extra galactic background light us you'll

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Speaker 1: be for wondering how many people out there know about

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Speaker 1: this or have any thoughts about what it might be.

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Speaker 2: Thanks very much to our panel of volunteers who comment

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Speaker 2: on these well named astronomical phenomena and offer their opinions

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Speaker 2: without the chance to google about it. If you would

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Speaker 2: like to play for a future episode, please don't be shy.

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Speaker 2: Write to me two questions at Danielandjorge dot com.

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Speaker 1: So think about it for a second. What do you

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Speaker 1: think is the extra galactic background light? Here's what people

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Speaker 1: have to say. It must be all the light coming

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Speaker 1: from outside our galaxy, either that or one of those

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Speaker 1: LED sets that you can use to make your bedroom

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Speaker 1: lighting extra galactic.

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Speaker 3: Extra galactic background light is the light produced by Club

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Speaker 3: Andromeda when the Aliens are having a rave. I don't

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Speaker 3: know something to do with our galaxy and there's too

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Speaker 3: much light for what there should be by some theory

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Speaker 3: that's calculating, Uh, you know how it fits within the

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Speaker 3: light it should get from other galaxies or our sun,

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Speaker 3: and somehow it's being amplified.

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Speaker 4: I've heard of it, but I'm thinking that our other

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Speaker 4: galaxies out of our galaxy and there it's a large background.

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Speaker 4: Most of the stars we see is the background galaxies,

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Speaker 4: and that's the range of galaxies we see in their lives.

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Speaker 1: Yes, all right, a lot of creative answers here. Some

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Speaker 1: people think extra means party.

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Speaker 2: I was wondering if anybody was going to say, it's

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Speaker 2: like breaking news, like extra extra, read all about it.

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Speaker 1: Oh right, that's another use of the word extra. Yeah,

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Speaker 1: if you're from the nineteen twenties, I.

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Speaker 2: Got a jaunty cap on. I'm standing on a so

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Speaker 2: far I'm selling newspaper.

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Speaker 1: You get a vest on. Yeah, yeah, yeah, you're on

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Speaker 1: the street. I got on my cheeks hanging out scientific papers.

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Speaker 2: That's how we distribute science these days. We passed it

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Speaker 2: out to OHI g mister, have you read white since

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Speaker 2: latest paper? It's a hoot.

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Speaker 1: But yeah, that's one way to think about it. But

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Speaker 1: a lot of creative answers here. I guess it's sort

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Speaker 1: of stelf a. But like I said, there's some ambiguity

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Speaker 1: in the terms.

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Speaker 2: Yes, as always.

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Speaker 1: All right, well let's dig into it, Daniel. What is

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Speaker 1: the extragalactic background light?

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Speaker 2: Basically, the extragalactic background light, or EBL, as astronomers like

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Speaker 2: to call it, is all the light emitted by everything

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Speaker 2: else in the universe except for the Milky Way, during

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Speaker 2: the entire history of the universe. So it's like all

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Speaker 2: of the photons in the universe not emitted by our galaxy.

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Speaker 1: Wait what but also not emitted by the background microwave

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Speaker 1: background radiation.

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Speaker 2: Now the CMB, the cosmic microwave background is part of

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Speaker 2: the EBL. The EBL is like the super general version

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Speaker 2: of the CMB. CMB is only at one wavelength, that's

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Speaker 2: the cosmic microwave background. The EBL is like, well, what's

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Speaker 2: the background in all of the wavelengths?

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Speaker 1: Oh wait, wait, so the EBL is part of this CMB.

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Speaker 2: No, the CMB is part of the EBL.

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Speaker 1: Oh okay, but it doesn't come from galaxies, does it.

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Speaker 2: It doesn't have to come from galaxy. This is the

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Speaker 2: light emitted by everything else in the universe other than

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Speaker 2: our galaxy. So extragalactic means not the Milky Way. So

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Speaker 2: any other star, any other object, any frozen banana floating

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Speaker 2: out there in space, any primordial soup of gas and plasma,

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Speaker 2: everything in the whole history of the universe except for

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Speaker 2: the Milky Way.

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Speaker 1: Oh I see, this is not galactic light. It's like

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Speaker 1: extra galactic in the sense of being outside of our gutsy.

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Speaker 2: Yes, take all the photons in the universe and subtract

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Speaker 2: the ones emitted by our galaxy the ones left over,

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Speaker 2: that's the extragalactic background light. So like most of the

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Speaker 2: photons in the universe are the EBL photons I see.

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Speaker 1: So any photon that we see out there in space

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Speaker 1: that doesn't come from a star within or any other

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Speaker 1: source within our Milky Way galaxy exactly.

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Speaker 2: And the estimate is that there's like ten to the

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Speaker 2: eighty four extragalactic background photons in the universe. So there's

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Speaker 2: a whole lot of them.

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Speaker 1: Ten to the eighty four.

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Speaker 2: Or ten with eighty four zeros in front of it.

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Speaker 2: It's a lot.

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Speaker 1: It sounds like a lot, But I don't know how

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Speaker 1: many come from our galaxy.

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Speaker 2: I mean, the whole history of the universe, the tiniest

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Speaker 2: fraction come from our galaxy. Almost every single photon in

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Speaker 2: the universe doesn't come from our galaxy. So almost every

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Speaker 2: single photon in the universe is an EBL photon.

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Speaker 1: Interesting, and this comes from other galaxies, or as we

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Speaker 1: talked about, maybe directly from the Big Bang, right.

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Speaker 2: It comes from the whole history of the universe, and

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Speaker 2: that's what's super fascinating about it. Some of these photons

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Speaker 2: were made before there were galaxies. Some of them were

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Speaker 2: made before there were stars. Some of them were made

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Speaker 2: during reionization, when these big clouds of neutral gas first

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Speaker 2: started to clump together to form stars. The whole history

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Speaker 2: of the universe is written in these photons. They're out there,

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Speaker 2: they're floating around. They contain all of this information, but

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Speaker 2: they're very, very tricky to see.

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Speaker 1: Right, So, like we're getting the lighte from Andromeda, which

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Speaker 1: is close to us, but we're also getting light from

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Speaker 1: super far away, which also happens to be really a

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Speaker 1: long time ago. That's all mixed in with this in

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Speaker 1: this micro background.

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Speaker 2: Like that's right, all that counts as EBL because it's

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Speaker 2: not coming from the Milky Way.

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Speaker 1: Maybe should be like extra Milky Way background light, right,

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Speaker 1: because it's not just like extra galactic like in the

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Speaker 1: general sense, it's just like our anything outside of our galaxy.

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Speaker 2: Yeah, extra our galactic background light.

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Speaker 1: Yeah, there you go. Yeah, because do so many in Andromeda,

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Speaker 1: the EBL would be different.

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Speaker 2: Right, Yeah, that's true. Those astronomers would argue with our

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Speaker 2: astronomers about how to name this thing. That would be fun.

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Speaker 1: Yeah, there a cartoonists would call my cartoonists. We would

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Speaker 1: duke it out, and then I, you know, I would

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Speaker 1: take them out and then I get to name it

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Speaker 1: for the whole universe.

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Speaker 2: You know, you wouldn't be the first cartoonist to actually

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Speaker 2: name something scientific. You know, Gary Larson actually had an

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Speaker 2: impact on science.

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Speaker 1: Oh yeah, what did he name?

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Speaker 2: Yes, this hilarious cartoon of a caveman giving a name

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Speaker 2: to those four pokey spikes on the back of a

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Speaker 2: status He calls it the phagomizer, after thag who was

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Speaker 2: killed by a stegosaurus. And it turns out that nobody

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Speaker 2: had actually named that before. So then scientists actually started

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Speaker 2: using phagomizer in their science papers. And now it's the

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Speaker 2: official name for the stegosaurus table.

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Speaker 1: Whoa. See, Now that's a well named thing in science.

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Speaker 1: You should put cartoonis in charge of everything.

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Speaker 2: Right, Yes, we should name everything after the caveman that

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Speaker 2: was killed by it.

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Speaker 1: Yeah. No, I'm just saying trust cartoonists, you know, with

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Speaker 1: anything science.

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Speaker 2: That's the lesson I learned from that. Yeah for sure.

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Speaker 1: All right, So this seems like a really broad concept

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Speaker 1: all the light. Basically, it's all the light and universe

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Speaker 1: we're getting from outside our galaxy. It seems like a lot.

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Speaker 1: Can we make any sense of it? Or is it

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Speaker 1: all just like a wash?

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Speaker 2: It's a lot, and it contains an incredible amount of information,

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Speaker 2: and it's really varied. It varies across the spectrum from

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Speaker 2: like super high energy gamma rays produced by really distant

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Speaker 2: active galactic nuclei like blazers, all the way down to

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Speaker 2: like really long wavelength radio that might be produced by

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Speaker 2: like dark matter decaying. There's an incredible amount of information

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Speaker 2: across the spectrum, but we want to see across the spectrum.

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Speaker 2: But we want to see it all the way across

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Speaker 2: the spectrum, and we also want to see where it's

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Speaker 2: coming from in the universe. So it's not just like

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Speaker 2: let's just see it. Let's see where it's coming from

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Speaker 2: and what the energy distributions are. Let's use that to

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Speaker 2: learn about the history of the universe.

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Speaker 1: Because I guess I wonder, like, how do you tell

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Speaker 1: the difference if it's how do you know it's coming

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Speaker 1: if it's coming from our galaxy or from outside of

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Speaker 1: our galaxy.

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Speaker 2: Yeah, that's really tricky because our galaxy turns out to

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Speaker 2: be really bright. You might think, well, can't you just

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Speaker 2: point your telescope in the night sky and gather some

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Speaker 2: EBL photons. Yeah, but it's sort of like trying to

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Speaker 2: see the Milky Way. If you're in times square. Times

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Speaker 2: Square is so bright you can't even see any stars

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Speaker 2: in the sky. So our Milky Way is so bright

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Speaker 2: that we can't see the distant dim things that are

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Speaker 2: hiding behind it. So there's a lot of competition from

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Speaker 2: the Milky Way, and the Milky Way is not something

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Speaker 2: we understand super well, so it's difficult to disentangle which

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Speaker 2: photons come from the extragalactic background light and which photons

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Speaker 2: come from our own Milky Way. There's an interplay there

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Speaker 2: where if we knew really, really well what the extragalactic

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Speaker 2: background light was, it would help us understand the Milky Way,

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Speaker 2: Or if we understood the Milky Way better, we could

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Speaker 2: subtract it from what we see in the night sky

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Speaker 2: and understand better what the extragalactic light is. We would

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Speaker 2: learn so much either way, But right now, the Milky

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Speaker 2: Way outshines everything, and the whole thing is kind of entangled.

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Speaker 2: It's a big mess.

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Speaker 1: So when you look at the stars of the nice Guy,

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Speaker 1: every star he sees is in the Milky Way galaxy, right,

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Speaker 1: you can really see stars that are outside of the

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Speaker 1: Milky Way. So any pinpoint you see out there is

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Speaker 1: in the Milky Way galaxy. So if I wanted to

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Speaker 1: see something outside of our galaxy, I would maybe point

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Speaker 1: my discope at a spot between the stars. Absolutely, do

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Speaker 1: you get raw light from outside of our galaxy or

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Speaker 1: is there still like dust from our galaxy there that

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Speaker 1: is maybe polluting that light?

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Speaker 2: So the answer depends on the frequency of light that

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Speaker 2: you're looking for. You're always looking through the Milky Way,

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Speaker 2: and there's always going to be dust that interferes, But

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Speaker 2: it depends on the wavelength. Some wavelengths of light can

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Speaker 2: penetrate that dust, some wavelengths of light can't. So it's

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Speaker 2: really a different puzzle at different wavelengths, from GAMER rays

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Speaker 2: to X rays to ultraviolet light. There's different sources of

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Speaker 2: photons that get confused between the Milky Way and the

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Speaker 2: extragalactic background light. A big factor is the zodiacal light

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Speaker 2: scattering of dust from within our solar system, which makes

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Speaker 2: everything very tricky, all right.

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Speaker 1: It sounds like we need to parse it by frequency,

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Speaker 1: and so let's break down the spectrum of light from

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Speaker 1: outside of our galaxy and see what it tells us

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Speaker 1: about what's out there. But first let's take a quick break.

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Speaker 1: All right, we're talking about the extra Milky Way background light.

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Speaker 1: I just renamed it. I called the extra tag tag

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Speaker 1: is a sizer background light.

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Speaker 2: I'm not even gonnam. It's all up to you at

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Speaker 2: this point.

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Speaker 1: I'll just called the extra larsen or here here we go,

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Speaker 1: we'll call it the extra far Side background line because

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Speaker 1: and that is both true and a homage to Gary Larson. Yeah,

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Speaker 1: absolutely right, because anything outside of our galaxy is technically

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Speaker 1: on the far side of the universe.

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Speaker 2: That's probably what he meant when he was talking about

401
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Speaker 2: the far side. He meant the other galaxies.

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Speaker 1: Yes, well, no, I don't think so. I'm saying in

403
00:19:26,600 --> 00:19:29,360
Speaker 1: this context it seems like an appropriateate. But anyways, let's

404
00:19:29,359 --> 00:19:31,920
Speaker 1: break it down by frequency, you said, because we're getting

405
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Speaker 1: a wash by all this life from outside of our galaxy.

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Speaker 1: We don't even know if it's coming from outside of

407
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Speaker 1: our galaxy. But if you break it down by frequency,

408
00:19:38,920 --> 00:19:40,720
Speaker 1: you can get a better handle on what's going on.

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Speaker 2: And the main challenges are that most of this stuff

410
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Speaker 2: is really dim because the sources are really distant. Everything

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Speaker 2: we're talking about is coming from outside our galaxy, which

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Speaker 2: means it's probably millions or billions of light years away,

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Speaker 2: So it's photons have been spread out. They don't get tired,

414
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Speaker 2: but they do get spread out. So everything out there

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Speaker 2: is really really dim, or there are sources in the

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Speaker 2: milky Way that are brighter than it, or there's like

417
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Speaker 2: dust and stuff interfering. The challenge is change as you

418
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Speaker 2: go through the frequency, just the same way that the

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Speaker 2: night sky changes in frequency. If you look at the

420
00:20:12,280 --> 00:20:14,919
Speaker 2: night sky in the optical or in the infrared or

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Speaker 2: in the UV, you see a very different picture.

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Speaker 1: All right, let's break it down, and let's start maybe

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Speaker 1: with the higher frequencies. What do we see at the

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Speaker 1: high frequencies.

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Speaker 2: So at the very highest frequencies, the highest energy photons,

426
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Speaker 2: we call these things gamma rays for silly historical reasons.

427
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Speaker 1: Related to the Hulk, right, That's where it started.

428
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Speaker 2: It started because in the early part of the century

429
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Speaker 2: we didn't really understand quantum mechanics or radiation, so we

430
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Speaker 2: just started naming things like alpha rays, beta rays, gamma rays.

431
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Speaker 2: We had no idea what gamma rays were. Then later

432
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Speaker 2: we understood, oh, they're just high energry photons, but they

433
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Speaker 2: already had this name. So you can't just say, oh,

434
00:20:50,280 --> 00:20:52,520
Speaker 2: gamma rays and X rays are really the same thing.

435
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Speaker 2: There's artificial distinction between them. We just sort of.

436
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Speaker 1: Stuck with it, and so they had the Hulk back

437
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Speaker 1: at the Court of the century.

438
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Speaker 2: Hulk was actually the one doing these science experiments. Remember,

439
00:21:02,359 --> 00:21:04,600
Speaker 2: Bruce Banner is a scientist man. You should read his

440
00:21:04,640 --> 00:21:06,159
Speaker 2: paper right now. They're great stuff.

441
00:21:07,040 --> 00:21:10,160
Speaker 1: It's the famous correspondence between Bruce Banner and Albert Einstein

442
00:21:10,240 --> 00:21:13,959
Speaker 1: right where they're like, pal come them.

443
00:21:14,080 --> 00:21:15,840
Speaker 2: It was in German, though, right, wasn't it in German?

444
00:21:17,240 --> 00:21:18,800
Speaker 1: I'm just messing with you, all right? Sorry, I keep going,

445
00:21:18,880 --> 00:21:20,840
Speaker 1: keep going. What do we see in the gamma rays?

446
00:21:20,880 --> 00:21:23,239
Speaker 2: So the gamma ray night sky is mostly surveyed by

447
00:21:23,280 --> 00:21:26,119
Speaker 2: a space probe called Fermei LAT, which is basically a

448
00:21:26,119 --> 00:21:29,880
Speaker 2: particle physics detector in space. Photons hid it and they

449
00:21:29,920 --> 00:21:32,639
Speaker 2: convert into a pair of electrons and positrons and we

450
00:21:32,760 --> 00:21:35,320
Speaker 2: track those. We can use that to measure the energy.

451
00:21:35,800 --> 00:21:38,080
Speaker 2: This is our best way to see the night sky.

452
00:21:38,280 --> 00:21:40,680
Speaker 2: In gamma rays. And if you look out in the

453
00:21:40,760 --> 00:21:42,879
Speaker 2: night sky you do see a bunch of gamma rays.

454
00:21:42,960 --> 00:21:45,320
Speaker 2: You see a huge source coming from the center of

455
00:21:45,359 --> 00:21:47,399
Speaker 2: our galaxy. Like the center of the galaxy emits a

456
00:21:47,520 --> 00:21:48,800
Speaker 2: very large number.

457
00:21:48,520 --> 00:21:50,520
Speaker 1: And that's from the black hole there, right.

458
00:21:50,480 --> 00:21:53,040
Speaker 2: That's from all sorts of crazy processes happening in the

459
00:21:53,040 --> 00:21:55,600
Speaker 2: center of the Milky Way. There's pulsars that emit gamma rays.

460
00:21:55,640 --> 00:21:57,840
Speaker 2: These are point sources. There's also just a lot of

461
00:21:57,880 --> 00:22:02,080
Speaker 2: diffuse emission of gamma rays from like really high energy processes,

462
00:22:02,119 --> 00:22:06,119
Speaker 2: like electrons getting accelerated really really hard and emitting gamma

463
00:22:06,200 --> 00:22:09,560
Speaker 2: ray photons. The problem is that we don't really understand

464
00:22:09,600 --> 00:22:11,480
Speaker 2: the center of the galaxy. So if you just look

465
00:22:11,480 --> 00:22:13,359
Speaker 2: at like the distribution of gamma rays in the center

466
00:22:13,440 --> 00:22:16,439
Speaker 2: of our galaxy, we don't understand it. We can't explain it.

467
00:22:16,520 --> 00:22:19,720
Speaker 2: There's lots of mysteries there, and so that's a real problem.

468
00:22:19,800 --> 00:22:22,480
Speaker 2: If you want to subtract out the Milky Ways contribution

469
00:22:22,560 --> 00:22:24,680
Speaker 2: and look at the rest of the universe, you don't

470
00:22:24,680 --> 00:22:26,720
Speaker 2: really understand what to subtract out.

471
00:22:27,800 --> 00:22:29,600
Speaker 1: You just like point your telescope at the center of

472
00:22:29,640 --> 00:22:31,520
Speaker 1: the galaxy and then point it away from the center

473
00:22:31,520 --> 00:22:33,560
Speaker 1: of the galaxy to kind of get a sense of

474
00:22:33,600 --> 00:22:35,439
Speaker 1: what's coming from the center and what is not.

475
00:22:35,920 --> 00:22:38,560
Speaker 2: Yeah, you can measure what's happening at the center, but

476
00:22:38,600 --> 00:22:41,040
Speaker 2: then you also want to understand how that varies across

477
00:22:41,040 --> 00:22:42,960
Speaker 2: the night sky. To do that, you really need some

478
00:22:43,040 --> 00:22:45,800
Speaker 2: sort of model so you can interpolate, so then when

479
00:22:45,840 --> 00:22:47,919
Speaker 2: you're looking at some other place, you can say how

480
00:22:48,000 --> 00:22:49,720
Speaker 2: much of this is from the center of the galaxy

481
00:22:49,960 --> 00:22:52,159
Speaker 2: and how much isn't. You can't just like turn off

482
00:22:52,160 --> 00:22:53,720
Speaker 2: the center of the galaxy in nature and see the

483
00:22:53,760 --> 00:22:55,960
Speaker 2: rest of it, or turn it back on or vary it.

484
00:22:56,000 --> 00:22:57,840
Speaker 2: So we need some sort of like understanding of the

485
00:22:57,840 --> 00:23:00,640
Speaker 2: center of the galaxy in or to extrapolate away from

486
00:23:00,640 --> 00:23:04,800
Speaker 2: it and like subtract it out from underneath the extragalactic background.

487
00:23:04,480 --> 00:23:07,760
Speaker 1: Light, so we can study the extragalactic background, or so

488
00:23:07,800 --> 00:23:10,400
Speaker 1: we can study the center of the galaxy or both both.

489
00:23:10,480 --> 00:23:13,840
Speaker 2: Because mostly the extragalactic background light in the gamma rays

490
00:23:13,880 --> 00:23:16,960
Speaker 2: we think is coming from the centers of other galaxies.

491
00:23:17,359 --> 00:23:20,560
Speaker 2: Right other galaxies we think are also emitting gamma rays

492
00:23:20,680 --> 00:23:23,520
Speaker 2: at high rates, and the extragalactic background light in the

493
00:23:23,520 --> 00:23:26,320
Speaker 2: gamma rays is then mostly from those other galactic nuclei,

494
00:23:26,480 --> 00:23:28,680
Speaker 2: and so if we could see those, we could understand

495
00:23:28,720 --> 00:23:30,880
Speaker 2: our own galaxy better, or if we understood our own

496
00:23:30,880 --> 00:23:33,440
Speaker 2: galaxy better, we could subtract it out and then understand

497
00:23:33,480 --> 00:23:36,280
Speaker 2: those other galaxies. So it's sort of a chicken and

498
00:23:36,320 --> 00:23:38,320
Speaker 2: egg problem. We don't really know how to pull this.

499
00:23:38,280 --> 00:23:40,439
Speaker 1: Apart, all right, So then what can we see in

500
00:23:40,480 --> 00:23:42,120
Speaker 1: the gamma rays when we look around us.

501
00:23:42,160 --> 00:23:44,280
Speaker 2: Well, there's this long standing mystery, but the center of

502
00:23:44,280 --> 00:23:47,720
Speaker 2: our galaxy, whether it's sending us signals of dark matter

503
00:23:47,880 --> 00:23:49,560
Speaker 2: like dark matter, we know this a lot of it

504
00:23:49,600 --> 00:23:51,960
Speaker 2: in the center of the galaxy, and we wonder if

505
00:23:51,960 --> 00:23:54,800
Speaker 2: sometimes when two dark matter particles bounce off each other

506
00:23:55,000 --> 00:23:57,639
Speaker 2: they actually annihilate, Like there might be dark matter and

507
00:23:57,760 --> 00:23:59,920
Speaker 2: anti dark matter, and it might be possible for it

508
00:24:00,080 --> 00:24:03,640
Speaker 2: to annihilate and actually produce photons. This is counterintuitive because

509
00:24:03,640 --> 00:24:06,080
Speaker 2: you think of dark matter as dark, not shining in

510
00:24:06,119 --> 00:24:09,560
Speaker 2: any electromagnetic spectrum. But there are theories where dark matter

511
00:24:09,600 --> 00:24:13,119
Speaker 2: will annihilate itself and make very high energy photons. And

512
00:24:13,160 --> 00:24:14,879
Speaker 2: in fact, there's a signal from the center of the

513
00:24:14,880 --> 00:24:17,240
Speaker 2: galaxy that we don't understand that a bunch of people

514
00:24:17,240 --> 00:24:19,960
Speaker 2: think is from dark matter. So we don't understand that

515
00:24:20,080 --> 00:24:22,000
Speaker 2: very well, and we'd love to look for that signal

516
00:24:22,119 --> 00:24:24,920
Speaker 2: in the centers of other galaxies, basically see if we

517
00:24:24,960 --> 00:24:27,800
Speaker 2: can reproduce this in the extra galactic background light. But

518
00:24:27,880 --> 00:24:29,480
Speaker 2: so far we have themen able to pull those things

519
00:24:29,520 --> 00:24:33,560
Speaker 2: apart and understand which photons come from other galaxies.

520
00:24:34,040 --> 00:24:36,320
Speaker 1: Wait, wait, wait, First of all, are you saying dark

521
00:24:36,359 --> 00:24:37,960
Speaker 1: matter is not maybe really dark?

522
00:24:38,080 --> 00:24:40,560
Speaker 2: Yeah, dark matter might be shining brightly from the center

523
00:24:40,600 --> 00:24:41,360
Speaker 2: of our galaxy.

524
00:24:41,480 --> 00:24:44,240
Speaker 1: Oh boy, And then can't we just point our telescope

525
00:24:44,280 --> 00:24:46,720
Speaker 1: at another galaxy to see what kind of signal we

526
00:24:46,840 --> 00:24:47,120
Speaker 1: get from.

527
00:24:47,119 --> 00:24:48,800
Speaker 2: Then, yeah, we can do that, and we can see

528
00:24:48,840 --> 00:24:51,800
Speaker 2: some other galaxies that are very clear, like galaxies with

529
00:24:51,920 --> 00:24:55,119
Speaker 2: quasars in them or blazars, you know, quasars that are

530
00:24:55,119 --> 00:24:57,560
Speaker 2: pointed right at us. Those are shooting really high energy

531
00:24:57,560 --> 00:25:00,200
Speaker 2: photons right at us, and that we can tell like, Okay,

532
00:25:00,280 --> 00:25:03,160
Speaker 2: it's definitely there. It's definitely there. So that's a part

533
00:25:03,240 --> 00:25:06,000
Speaker 2: of the extragalactic background light that we can tell. But

534
00:25:06,040 --> 00:25:09,160
Speaker 2: not every galaxy has an active nucleus, and we're interested

535
00:25:09,160 --> 00:25:11,760
Speaker 2: in studying those that aren't active because those are the

536
00:25:11,760 --> 00:25:14,280
Speaker 2: best ones for studying dark matter, but it's not always

537
00:25:14,359 --> 00:25:17,040
Speaker 2: clear which photons are coming from our galaxy in which

538
00:25:17,040 --> 00:25:20,600
Speaker 2: photons are coming from other galaxies, because remember we're inside

539
00:25:20,600 --> 00:25:22,960
Speaker 2: the Milky Way, and not all of the gamma rays

540
00:25:23,040 --> 00:25:25,400
Speaker 2: come directly from the center. Some of them are emitted

541
00:25:25,440 --> 00:25:28,359
Speaker 2: along the galactic plane, and those are still brighter than

542
00:25:28,359 --> 00:25:29,480
Speaker 2: the emissions from other.

543
00:25:29,359 --> 00:25:34,720
Speaker 1: Galaxies emitted by what we are we this would be

544
00:25:34,800 --> 00:25:37,760
Speaker 1: emitted by dark matter in the rim of the galaxy

545
00:25:37,800 --> 00:25:38,040
Speaker 1: or what.

546
00:25:38,680 --> 00:25:40,840
Speaker 2: Maybe dark matter in the room the galaxy. But anytime

547
00:25:40,880 --> 00:25:43,440
Speaker 2: an electron is accelerated, it's going to emit a photon,

548
00:25:43,920 --> 00:25:46,720
Speaker 2: and so there's some emission of photons from electrons in

549
00:25:46,760 --> 00:25:49,720
Speaker 2: the galactic plane that cloud our observations.

550
00:25:50,080 --> 00:25:54,280
Speaker 1: MM interesting. So looking at these gamma rays might reveal

551
00:25:54,400 --> 00:25:56,160
Speaker 1: the what's going on with dark matter.

552
00:25:56,240 --> 00:25:57,919
Speaker 2: Yeah, it would be super cool if we could make

553
00:25:58,040 --> 00:26:00,919
Speaker 2: like a map of these gamma rays from other galaxies

554
00:26:01,040 --> 00:26:03,760
Speaker 2: and then cross correlated with our understanding of like where

555
00:26:03,800 --> 00:26:06,439
Speaker 2: the density of matter is, Like we have a pretty

556
00:26:06,440 --> 00:26:08,960
Speaker 2: good understanding of where the galaxies are and the whole

557
00:26:08,960 --> 00:26:12,639
Speaker 2: cosmic web. If we could cross correlate these gamma rays

558
00:26:12,640 --> 00:26:15,600
Speaker 2: from like clumps of dark matter, then to be really

559
00:26:15,600 --> 00:26:18,679
Speaker 2: powerful evidence that maybe these gamma rays really are coming

560
00:26:19,080 --> 00:26:21,800
Speaker 2: from dark matter and not just from other sources of

561
00:26:21,880 --> 00:26:22,520
Speaker 2: gamma rays.

562
00:26:22,960 --> 00:26:25,639
Speaker 1: WHOA, would they have like a special signature if they

563
00:26:25,640 --> 00:26:26,520
Speaker 1: came from dark matter?

564
00:26:26,640 --> 00:26:29,399
Speaker 2: Unfortunately not, they're like energy distribution of these things is

565
00:26:29,400 --> 00:26:32,480
Speaker 2: not that different from the energy distribution we see from

566
00:26:32,520 --> 00:26:34,919
Speaker 2: other sources, which is what makes it so challenging and

567
00:26:34,960 --> 00:26:38,440
Speaker 2: why it's so important to understand all the other sources

568
00:26:38,480 --> 00:26:41,080
Speaker 2: of gamma rays so we can figure out which ones

569
00:26:41,200 --> 00:26:43,440
Speaker 2: might be coming from dark matter. It's like you're trying

570
00:26:43,440 --> 00:26:45,679
Speaker 2: to explain the spectrum with a few different blobs, but

571
00:26:45,720 --> 00:26:48,000
Speaker 2: the blobs aren't that different, so it's hard to tell

572
00:26:48,040 --> 00:26:50,199
Speaker 2: how much of each blob you need to use to

573
00:26:50,240 --> 00:26:51,760
Speaker 2: explain the spectrum that you're seeing.

574
00:26:52,080 --> 00:26:54,960
Speaker 1: WHOA. So, then if it turns out dark matter is shiny,

575
00:26:54,960 --> 00:26:56,320
Speaker 1: would you need to change the name of it.

576
00:26:57,160 --> 00:26:58,520
Speaker 2: Yes, and we'd come to you first.

577
00:27:00,600 --> 00:27:04,080
Speaker 1: I'll call it dark light as ray star war Z.

578
00:27:04,480 --> 00:27:06,679
Speaker 1: All right, Well, that's gamma rays, it might tell us

579
00:27:06,680 --> 00:27:10,480
Speaker 1: about dark matter. What about the next range of frequencies

580
00:27:10,520 --> 00:27:12,359
Speaker 1: in the spectrum of this background light?

581
00:27:12,560 --> 00:27:14,760
Speaker 2: So taking a step down and energy, you get to

582
00:27:15,119 --> 00:27:18,440
Speaker 2: X rays, and again there's just an arbitrary distinction between

583
00:27:18,440 --> 00:27:21,159
Speaker 2: gamma rays and X rays, but X rays are lower energy,

584
00:27:21,240 --> 00:27:23,280
Speaker 2: and here the technology is sort of like a bridge

585
00:27:23,320 --> 00:27:26,840
Speaker 2: between particle physics detectors that see gamma rays and more

586
00:27:26,880 --> 00:27:30,679
Speaker 2: traditional telescopes that see lower energy light. Here we have

587
00:27:31,000 --> 00:27:34,080
Speaker 2: X ray telescopes, and these use like weird X ray

588
00:27:34,119 --> 00:27:36,960
Speaker 2: optics because X rays are really energetic and really hard

589
00:27:36,960 --> 00:27:40,119
Speaker 2: to bend using optics, So there's all sorts of weird

590
00:27:40,160 --> 00:27:42,879
Speaker 2: tricks they use to try to gather and focus X rays.

591
00:27:43,040 --> 00:27:44,920
Speaker 2: But we have a couple of cool space telescopes and

592
00:27:44,960 --> 00:27:48,320
Speaker 2: New Star and Chandra up there observing the sky in

593
00:27:48,400 --> 00:27:49,600
Speaker 2: the X ray What do.

594
00:27:49,600 --> 00:27:51,040
Speaker 1: You mean they are hard to bend like that, You

595
00:27:51,119 --> 00:27:53,679
Speaker 1: can't focus them. You can focus them with a lens.

596
00:27:53,560 --> 00:27:55,800
Speaker 2: Yeah, exactly, because of the super high energy, they just

597
00:27:55,840 --> 00:27:58,199
Speaker 2: don't bend very much through a lens, and so you

598
00:27:58,240 --> 00:28:01,280
Speaker 2: need special techniques to shape these things, basically like wave

599
00:28:01,320 --> 00:28:04,200
Speaker 2: guides and weird constructions. We had a whole episode about

600
00:28:04,280 --> 00:28:06,840
Speaker 2: X ray telescopes and Chandra. Check it out for more

601
00:28:06,880 --> 00:28:09,520
Speaker 2: details on how to build your own X ray telescope.

602
00:28:09,840 --> 00:28:11,639
Speaker 1: Right, you can make them out of bones, right this,

603
00:28:11,960 --> 00:28:13,159
Speaker 1: X rays don't go through bones.

604
00:28:16,840 --> 00:28:20,120
Speaker 2: Yes, and we're calling the next one the Fred Flintstone Telescope. Exactly.

605
00:28:20,480 --> 00:28:21,240
Speaker 1: Yeah, there is.

606
00:28:21,960 --> 00:28:23,560
Speaker 2: We're gonna have a Stegosaurus.

607
00:28:23,040 --> 00:28:24,359
Speaker 1: Operator after Hanna Barbara.

608
00:28:24,520 --> 00:28:27,680
Speaker 2: Of course, if Hannah Barbera wanted a fund one, we

609
00:28:27,720 --> 00:28:29,919
Speaker 2: would name it after them, for sure. There you go.

610
00:28:30,160 --> 00:28:31,720
Speaker 1: I'm not sure there's still a.

611
00:28:31,800 --> 00:28:35,920
Speaker 2: Round somebody owns that iph.

612
00:28:35,080 --> 00:28:36,560
Speaker 1: Well you should follow the application.

613
00:28:37,480 --> 00:28:39,800
Speaker 2: But the night sky in the X ray is really

614
00:28:39,840 --> 00:28:43,800
Speaker 2: fascinating because this mostly comes from electrons. We were talking

615
00:28:43,840 --> 00:28:47,880
Speaker 2: earlier about electrons emitting super high energy photons. They also

616
00:28:47,920 --> 00:28:50,360
Speaker 2: emit X rays and this is a really cool German

617
00:28:50,480 --> 00:28:54,240
Speaker 2: name for it. It's called bremstra Lung, which means breaking light.

618
00:28:54,600 --> 00:28:56,880
Speaker 2: Since you have an electron it changes direction because it

619
00:28:56,960 --> 00:28:59,160
Speaker 2: HiT's like a magnetic field or something, it has to

620
00:28:59,160 --> 00:29:00,920
Speaker 2: give off a photon in order to do that, and

621
00:29:00,920 --> 00:29:03,680
Speaker 2: based on the energy of those electrons the amount of curvature,

622
00:29:03,800 --> 00:29:05,480
Speaker 2: how from, we give off X rays. So a lot

623
00:29:05,520 --> 00:29:08,400
Speaker 2: of the night sky in X ray comes from these

624
00:29:08,440 --> 00:29:10,320
Speaker 2: electrons giving off bremstralag.

625
00:29:10,680 --> 00:29:12,760
Speaker 1: I mean, these are electrons that are just floating out

626
00:29:12,800 --> 00:29:16,920
Speaker 1: there in space and if somehow they change direction, they

627
00:29:16,960 --> 00:29:18,400
Speaker 1: emit an X ray.

628
00:29:18,480 --> 00:29:20,680
Speaker 2: Yeah, exactly, electrons can change direction when they hit a

629
00:29:20,680 --> 00:29:23,600
Speaker 2: magnetic field or if they like zoom around the black

630
00:29:23,640 --> 00:29:26,520
Speaker 2: hole or something. Any sort of change of direction or

631
00:29:26,600 --> 00:29:29,480
Speaker 2: change of velocity, an electron will emit a photon.

632
00:29:29,480 --> 00:29:31,840
Speaker 1: And so the universe is just full of these electrons

633
00:29:31,880 --> 00:29:34,240
Speaker 1: or what They're just floating out there like dust or

634
00:29:34,280 --> 00:29:36,320
Speaker 1: are they like in galaxies or is this the stuff

635
00:29:36,320 --> 00:29:37,240
Speaker 1: between galaxies?

636
00:29:37,360 --> 00:29:40,680
Speaker 2: Electrons are everywhere, man, just thew way protons are. You know,

637
00:29:40,760 --> 00:29:43,360
Speaker 2: most of the universe is hydrogen, but by that we

638
00:29:43,480 --> 00:29:47,080
Speaker 2: mean protons and electrons, and often it's in plasma form.

639
00:29:47,160 --> 00:29:50,120
Speaker 2: It's not neutral hydrogen, so there are also clouds of

640
00:29:50,200 --> 00:29:52,720
Speaker 2: neutral hydrogen. But there's also just a lot of protons

641
00:29:52,720 --> 00:29:57,040
Speaker 2: and electrons flying out there, both in galaxies and between galaxies.

642
00:29:57,520 --> 00:30:00,880
Speaker 2: Remember that between galaxies is not as bright. There aren't stars,

643
00:30:01,120 --> 00:30:04,000
Speaker 2: but a huge fraction of baryonic matter in the universe,

644
00:30:04,040 --> 00:30:07,920
Speaker 2: meaning protons and electrons, is actually between the galaxies, not

645
00:30:08,080 --> 00:30:11,080
Speaker 2: in the galaxies. So yeah, electrons are everywhere, and they're

646
00:30:11,160 --> 00:30:13,400
Speaker 2: emitting X rays whenever they change direction.

647
00:30:13,600 --> 00:30:16,560
Speaker 1: Do you consider this noise or is this part of

648
00:30:16,560 --> 00:30:18,240
Speaker 1: what you want to see or is this getting in

649
00:30:18,240 --> 00:30:20,680
Speaker 1: the way of the interesting things you want to see.

650
00:30:20,760 --> 00:30:22,600
Speaker 2: This is definitely something you want to see because you

651
00:30:22,640 --> 00:30:24,960
Speaker 2: want to understand all the sources of it. But it's

652
00:30:25,000 --> 00:30:27,600
Speaker 2: really hard to pin down these sources. Some of it

653
00:30:27,600 --> 00:30:30,280
Speaker 2: we can associate with the centers of galaxies, so like

654
00:30:30,360 --> 00:30:34,360
Speaker 2: active galactic nuclei are pumping out these high energy X rays,

655
00:30:34,600 --> 00:30:36,520
Speaker 2: but a lot of it we can't. I read one

656
00:30:36,560 --> 00:30:39,520
Speaker 2: study that said that one percent of X rays can

657
00:30:39,560 --> 00:30:41,920
Speaker 2: be associated with known objects. The rest of it, we're

658
00:30:42,000 --> 00:30:44,640
Speaker 2: just like, we don't know what made this. So something

659
00:30:44,680 --> 00:30:47,080
Speaker 2: out there in the universe is like shooting out X

660
00:30:47,160 --> 00:30:49,280
Speaker 2: rays and we don't know what it is. Maybe it's

661
00:30:49,320 --> 00:30:51,720
Speaker 2: just a bunch of more active galactic nuclei that have

662
00:30:51,800 --> 00:30:54,400
Speaker 2: been like red shifted and are faint, but it could

663
00:30:54,440 --> 00:30:58,760
Speaker 2: also be other weird stuff like early universe black holes,

664
00:30:58,920 --> 00:31:02,800
Speaker 2: direct collapse black holes that formed during the early universe

665
00:31:02,920 --> 00:31:04,000
Speaker 2: and emitted X rays.

666
00:31:04,120 --> 00:31:07,480
Speaker 1: What do you mean only one percent, like one percent

667
00:31:07,600 --> 00:31:11,320
Speaker 1: is coming from these electrons floating around, or we're just

668
00:31:11,360 --> 00:31:13,760
Speaker 1: getting them from things that might have existed in the

669
00:31:13,840 --> 00:31:16,120
Speaker 1: universe a long time ago that we can't sort of

670
00:31:16,120 --> 00:31:17,560
Speaker 1: see it with their naked eye.

671
00:31:17,640 --> 00:31:20,520
Speaker 2: Yeah, we can't associate them with anything we've seen, so

672
00:31:20,560 --> 00:31:22,200
Speaker 2: We don't know what's making them. They could just be

673
00:31:22,240 --> 00:31:24,280
Speaker 2: diffuse electrons. It could be a big chunk of it.

674
00:31:24,360 --> 00:31:26,920
Speaker 2: There could also be a bunch of new astrophysical objects

675
00:31:26,960 --> 00:31:29,800
Speaker 2: out there emitting X rays that we've just never seen

676
00:31:29,880 --> 00:31:32,320
Speaker 2: before because we haven't been able to measure the X

677
00:31:32,400 --> 00:31:35,720
Speaker 2: ray spectrum outside of our galaxy. So there could be

678
00:31:35,760 --> 00:31:39,520
Speaker 2: these like direct collapse black holes that formed. Like remember

679
00:31:39,560 --> 00:31:41,680
Speaker 2: we were talking about having the early universe with this

680
00:31:41,800 --> 00:31:45,400
Speaker 2: famous moment when the universe became neutral. Protons and electrons

681
00:31:45,520 --> 00:31:48,240
Speaker 2: came together to make hydrogen, and that was the Dark Ages.

682
00:31:48,320 --> 00:31:50,719
Speaker 2: You have all this neutral hydrogen floating around, but there

683
00:31:50,720 --> 00:31:52,760
Speaker 2: were no stars yet. At the end of that, there's

684
00:31:52,800 --> 00:31:55,480
Speaker 2: a moment we call reionization, when the universe is then

685
00:31:55,560 --> 00:31:58,719
Speaker 2: pulling those atoms apart again. That's when we think stars

686
00:31:58,720 --> 00:32:01,959
Speaker 2: started to form. All possible that black holes formed at

687
00:32:02,000 --> 00:32:05,080
Speaker 2: the same moment. They didn't just collapse into massive stars.

688
00:32:05,360 --> 00:32:08,760
Speaker 2: Some clouds might have collapsed directly into black holes, and

689
00:32:08,840 --> 00:32:11,600
Speaker 2: those formations we think left their imprint in the X

690
00:32:11,720 --> 00:32:14,520
Speaker 2: ray spectrum. So if we could measure really really well,

691
00:32:14,560 --> 00:32:17,440
Speaker 2: we might see hints of direct collapse black holes from

692
00:32:17,480 --> 00:32:18,280
Speaker 2: the early universe.

693
00:32:18,800 --> 00:32:20,840
Speaker 1: Oh I see like, this is stuff happened that happened

694
00:32:20,840 --> 00:32:23,680
Speaker 1: a long time ago, but it happened so far away.

695
00:32:23,760 --> 00:32:26,840
Speaker 1: We're only just now getting the evidence of these things

696
00:32:26,880 --> 00:32:28,360
Speaker 1: that happened a long time ago, exactly.

697
00:32:28,400 --> 00:32:31,160
Speaker 2: And it's very dim, much dimmer than X ray sources

698
00:32:31,160 --> 00:32:34,120
Speaker 2: in our galaxy, and it's very hard to pull apart.

699
00:32:34,360 --> 00:32:37,520
Speaker 2: So if we understood the X ray spectrum super duper well,

700
00:32:37,680 --> 00:32:40,360
Speaker 2: we could ask questions like is there evidence in there

701
00:32:40,360 --> 00:32:43,200
Speaker 2: for direct collapse black holes or not? But right now

702
00:32:43,240 --> 00:32:45,360
Speaker 2: it's a big question mark. We don't know which photons

703
00:32:45,360 --> 00:32:48,120
Speaker 2: come from our galaxy, which photons come from outside the galaxy.

704
00:32:48,800 --> 00:32:51,320
Speaker 2: So we're like looking for a really tiny signal and

705
00:32:51,360 --> 00:32:52,480
Speaker 2: we have big question.

706
00:32:52,280 --> 00:32:55,600
Speaker 1: Marks, right, big ones. It ninety nine percent. We don't

707
00:32:55,640 --> 00:32:58,200
Speaker 1: know what it is, question exactly. All right, let's go

708
00:32:58,280 --> 00:33:01,800
Speaker 1: down to the next frequency. This is ultraviolet.

709
00:33:02,080 --> 00:33:06,040
Speaker 2: Yeah, so ultraviolet photons are super interesting, and our best

710
00:33:06,080 --> 00:33:10,000
Speaker 2: measurements of the ultraviolet extragalactic background light actually come from

711
00:33:10,040 --> 00:33:13,320
Speaker 2: the voyager probes. Remember those probes we sent out into

712
00:33:13,360 --> 00:33:15,280
Speaker 2: the Solar System to take pictures of the planets and

713
00:33:15,320 --> 00:33:19,040
Speaker 2: then just continue on out into space. They had instruments

714
00:33:19,080 --> 00:33:22,840
Speaker 2: on board for measuring ultraviolet photons because they were interested

715
00:33:22,960 --> 00:33:26,479
Speaker 2: in the atmospheres of those planets and seeing ultraviolet light

716
00:33:26,560 --> 00:33:29,760
Speaker 2: emitted from those planets to like study atmospheres and all

717
00:33:29,800 --> 00:33:31,200
Speaker 2: sorts of planetary physics.

718
00:33:31,520 --> 00:33:35,200
Speaker 1: WHOA. So even today we're still getting data from that spacecraft.

719
00:33:35,320 --> 00:33:37,840
Speaker 2: I think actually Voyager just shut down. It ran out

720
00:33:37,880 --> 00:33:40,160
Speaker 2: of power and we're no longer hearing from it. But

721
00:33:40,280 --> 00:33:44,000
Speaker 2: until very recently we were getting measurements from Voyager. It's

722
00:33:44,000 --> 00:33:47,320
Speaker 2: a really long lasting probe and it's one of our

723
00:33:47,360 --> 00:33:49,920
Speaker 2: best ways to understand the UV night sky.

724
00:33:50,480 --> 00:33:52,280
Speaker 1: Now what else can we see in this UV light?

725
00:33:52,400 --> 00:33:55,000
Speaker 2: So we can't see very much unfortunately, because the galaxy

726
00:33:55,080 --> 00:33:58,640
Speaker 2: is pretty bright in this UV light. Like planets emit

727
00:33:58,680 --> 00:34:02,800
Speaker 2: in the UV. Neutral hydrogen in our galaxy absorbs this

728
00:34:02,800 --> 00:34:05,600
Speaker 2: stuff and emits in the UV. But it's important for

729
00:34:05,800 --> 00:34:09,000
Speaker 2: understanding the distribution of matter, Like we'd love to know

730
00:34:09,160 --> 00:34:13,319
Speaker 2: more about the baryonic matter, the hydrogen that's between the galaxies.

731
00:34:13,719 --> 00:34:16,319
Speaker 2: That's where most of the hydrogen in the universe is.

732
00:34:16,600 --> 00:34:19,000
Speaker 2: If we could separate the UV light that's coming from

733
00:34:19,040 --> 00:34:21,120
Speaker 2: within our galaxy from the UV light that's coming from

734
00:34:21,160 --> 00:34:24,200
Speaker 2: outside the galaxy, then we could understand this better. But

735
00:34:24,239 --> 00:34:26,719
Speaker 2: we don't really have great measurements here, Like Voyager was

736
00:34:26,760 --> 00:34:29,640
Speaker 2: not set up to measure the extragalactic background light in

737
00:34:29,640 --> 00:34:32,000
Speaker 2: the UV spectrum. It's just like the only thing we have.

738
00:34:32,680 --> 00:34:34,600
Speaker 2: So it's if you look at the whole spectrum of

739
00:34:34,600 --> 00:34:37,480
Speaker 2: extragalactic background light, there's like a big gap there in

740
00:34:37,520 --> 00:34:40,880
Speaker 2: the UV because we really have almost no published studies

741
00:34:40,920 --> 00:34:42,760
Speaker 2: at all. It's just like a big blank.

742
00:34:43,600 --> 00:34:47,560
Speaker 1: But I guess what's making these you raise within our galaxy?

743
00:34:47,680 --> 00:34:51,120
Speaker 2: So mostly it's clouds of neutral hydrogen. Hydrogen, Remember, is

744
00:34:51,160 --> 00:34:54,400
Speaker 2: an atom, and electrons in the atom have certain energy levels,

745
00:34:54,480 --> 00:34:57,680
Speaker 2: and one of those energy level transitions corresponds with the

746
00:34:57,760 --> 00:35:01,719
Speaker 2: ultraviolet spectrum, and so neutral hydrogen tends to glow in

747
00:35:01,760 --> 00:35:04,000
Speaker 2: many different spectrum but one of them is in the UV.

748
00:35:04,400 --> 00:35:06,560
Speaker 1: UH just closed from being hot.

749
00:35:06,360 --> 00:35:08,680
Speaker 2: Yeah, exactly. You think of space as cold, but a

750
00:35:08,680 --> 00:35:12,040
Speaker 2: lot of this interstellar gas and intergalactic gas is actually

751
00:35:12,160 --> 00:35:14,880
Speaker 2: quite hot in the sense that it has high velocity

752
00:35:14,920 --> 00:35:17,200
Speaker 2: and each atom can have a significant amount of energy.

753
00:35:17,280 --> 00:35:18,799
Speaker 2: Even if we know that if you went out there

754
00:35:18,840 --> 00:35:21,560
Speaker 2: you'd freeze to death, you'd be surrounded by very sparse

755
00:35:21,680 --> 00:35:24,160
Speaker 2: hot gas. And when we study this stuff, there's a

756
00:35:24,200 --> 00:35:26,560
Speaker 2: lot that we don't understand, Like we can try to

757
00:35:26,600 --> 00:35:30,319
Speaker 2: subtract the Milky Way contribution in the UV, and then

758
00:35:30,360 --> 00:35:33,840
Speaker 2: there's all sorts of hints that other galaxies are emitting

759
00:35:33,920 --> 00:35:36,520
Speaker 2: in the UV in ways that we don't understand, Like

760
00:35:36,560 --> 00:35:39,280
Speaker 2: the Coma cluster is a famous puzzle. We don't understand

761
00:35:39,280 --> 00:35:42,080
Speaker 2: the UV spectrum from the Coma cluster. What's going on there?

762
00:35:42,160 --> 00:35:45,359
Speaker 2: Are those galaxies different from ours? Is there something else

763
00:35:45,400 --> 00:35:47,920
Speaker 2: between us and that galaxy? Are we just not understanding

764
00:35:48,120 --> 00:35:51,560
Speaker 2: the Milky Way contributions? It's a really open field to study.

765
00:35:52,360 --> 00:35:54,920
Speaker 1: All right, pretty cool. I guess I wonder if it

766
00:35:54,960 --> 00:35:58,120
Speaker 1: was healthy we stopped putting sunblock on all of our telscipes.

767
00:36:00,200 --> 00:36:01,640
Speaker 2: Or directly on our eyeballs.

768
00:36:02,600 --> 00:36:03,600
Speaker 1: Yeah, that never helps.

769
00:36:03,840 --> 00:36:05,920
Speaker 2: I think that's a bad idea. Don't do that people.

770
00:36:06,160 --> 00:36:09,239
Speaker 1: That's right, good health advice here on the Physics podcast.

771
00:36:10,120 --> 00:36:14,120
Speaker 1: All right, let's get into our maybe more interesting frequency spectrum,

772
00:36:14,160 --> 00:36:18,200
Speaker 1: the optical or visible light spectrum and infrared to see

773
00:36:18,280 --> 00:36:22,080
Speaker 1: what is out there beyond the bounds of our galaxy.

774
00:36:22,680 --> 00:36:38,080
Speaker 1: But first, let's take another quick break. Or we're talking

775
00:36:38,120 --> 00:36:41,360
Speaker 1: about light that comes from outside of our galaxy, which

776
00:36:41,719 --> 00:36:44,799
Speaker 1: turns out is like most of the light that we

777
00:36:44,840 --> 00:36:45,640
Speaker 1: can see.

778
00:36:45,480 --> 00:36:48,000
Speaker 2: Yeah, most of the photons in the universe didn't come

779
00:36:48,040 --> 00:36:50,960
Speaker 2: from our galaxy. And yet those are the photons that

780
00:36:50,960 --> 00:36:54,560
Speaker 2: are hardest to see because they're overwhelmed by the light

781
00:36:54,600 --> 00:36:55,920
Speaker 2: from our galaxy.

782
00:36:55,800 --> 00:36:58,680
Speaker 1: Right, because we're so close to our galaxy, we're in it.

783
00:36:59,120 --> 00:37:01,200
Speaker 1: But also I feel like it's hard because like it's

784
00:37:01,200 --> 00:37:03,400
Speaker 1: a big universe out there that's been around for a

785
00:37:03,400 --> 00:37:05,640
Speaker 1: long time. So we're getting stuff at the same time,

786
00:37:05,880 --> 00:37:08,560
Speaker 1: stuff that's closed, stuff that's far, stuff that's happening now,

787
00:37:08,600 --> 00:37:11,879
Speaker 1: stuff that's happened, you know, fourteen billion years ago. So

788
00:37:11,920 --> 00:37:14,000
Speaker 1: it's sort of like this big wash of light that

789
00:37:14,000 --> 00:37:16,279
Speaker 1: we're getting that you're trying to sift through.

790
00:37:16,440 --> 00:37:18,480
Speaker 2: Yeah, it's sort of amazing that you can understand any

791
00:37:18,520 --> 00:37:20,799
Speaker 2: of it, you know, But the tricks we use are

792
00:37:20,840 --> 00:37:24,040
Speaker 2: pretty basic. Look at the direction that came from, study

793
00:37:24,080 --> 00:37:26,520
Speaker 2: it over time, study it as a function of energy.

794
00:37:27,320 --> 00:37:29,279
Speaker 2: Using all those ideas, you can try to pull this

795
00:37:29,440 --> 00:37:31,879
Speaker 2: apart to make a consistent picture of what's out there

796
00:37:31,880 --> 00:37:34,680
Speaker 2: in the universe generating all these photons, and in the end,

797
00:37:34,800 --> 00:37:36,920
Speaker 2: that's the goal. Come up with the whole history of

798
00:37:36,920 --> 00:37:39,200
Speaker 2: the universe that can explain all the photons that we

799
00:37:39,239 --> 00:37:42,520
Speaker 2: can see. But first you got to see those photons.

800
00:37:42,600 --> 00:37:44,520
Speaker 1: Right, or I guess you can see them, you just

801
00:37:44,520 --> 00:37:47,000
Speaker 1: don't know which what it is that you're looking at.

802
00:37:47,600 --> 00:37:49,360
Speaker 2: Yeah, we see a bunch of photons. We'd love to

803
00:37:49,400 --> 00:37:51,640
Speaker 2: explain them all, and we're hoping that some of those

804
00:37:51,680 --> 00:37:54,919
Speaker 2: photons give us hints about what's outside our galaxy, not

805
00:37:55,000 --> 00:37:56,520
Speaker 2: just what's inside our galaxy.

806
00:37:56,760 --> 00:37:59,960
Speaker 1: Right. Well, so we've been going through different frequency rate

807
00:38:00,000 --> 00:38:01,840
Speaker 1: and just in the light that we get from outside

808
00:38:01,880 --> 00:38:04,040
Speaker 1: of our galaxy and how they can reveal different things,

809
00:38:04,480 --> 00:38:06,920
Speaker 1: and we're down to the visible light spectrum, like what

810
00:38:06,960 --> 00:38:08,720
Speaker 1: you could see with the naked eye.

811
00:38:08,680 --> 00:38:12,359
Speaker 2: Yeah, exactly. And so in the optical of course, we're

812
00:38:12,440 --> 00:38:15,600
Speaker 2: very curious about what's out there in the universe, and

813
00:38:15,719 --> 00:38:18,000
Speaker 2: a lot of the light that's in the optical spectrum

814
00:38:18,000 --> 00:38:21,480
Speaker 2: comes from stars. Right. The optical spectrum exists because we

815
00:38:21,560 --> 00:38:23,520
Speaker 2: evolve to be able to see light from our sun,

816
00:38:23,600 --> 00:38:26,560
Speaker 2: which makes us also able to see with our eyeballs

817
00:38:26,600 --> 00:38:29,319
Speaker 2: directly light from other stars, and of course there are

818
00:38:29,360 --> 00:38:32,040
Speaker 2: stars outside the Milky Way, and so a lot of

819
00:38:32,080 --> 00:38:35,040
Speaker 2: the optical light that's in the extra galactic background light

820
00:38:35,239 --> 00:38:38,200
Speaker 2: is emitted by stars in other galaxies.

821
00:38:37,840 --> 00:38:40,520
Speaker 1: Right, because our sun is a pretty typical star in

822
00:38:40,560 --> 00:38:42,920
Speaker 1: the universe. Like, what the kind of light that our

823
00:38:42,960 --> 00:38:47,200
Speaker 1: sun puts out is pretty tychopical all stars in the universe.

824
00:38:46,880 --> 00:38:48,920
Speaker 2: It's not that unusual. Our star is actually on the

825
00:38:49,040 --> 00:38:52,160
Speaker 2: larger side compared to your typical star. The most common

826
00:38:52,239 --> 00:38:54,440
Speaker 2: kind of star in the universe is a red dwarf,

827
00:38:54,800 --> 00:38:57,359
Speaker 2: which is a little redder and dimmer than our star,

828
00:38:57,600 --> 00:38:59,400
Speaker 2: but it's not a big deal. Red dwarfs are still

829
00:38:59,400 --> 00:38:59,960
Speaker 2: mostly in.

830
00:39:00,120 --> 00:39:03,160
Speaker 1: Optical Like, if we had been born under a red sun,

831
00:39:03,200 --> 00:39:06,799
Speaker 1: we would maybe you have different eyeball right.

832
00:39:07,400 --> 00:39:09,800
Speaker 2: Yeah, exactly, life could be very different if we evolved

833
00:39:09,840 --> 00:39:12,280
Speaker 2: around a red dwarf. We have a whole episode about

834
00:39:12,280 --> 00:39:15,040
Speaker 2: like how unlikely it is for life to have evolved

835
00:39:15,080 --> 00:39:16,759
Speaker 2: around a weird star like the Sun.

836
00:39:16,920 --> 00:39:19,399
Speaker 1: All right, So you're saying most of the visible light

837
00:39:19,480 --> 00:39:22,080
Speaker 1: that we get from outside the galaxy comes from stars

838
00:39:22,160 --> 00:39:24,799
Speaker 1: that are outside the galaxy, and these are mostly in

839
00:39:24,920 --> 00:39:27,160
Speaker 1: other galaxies, right, Like, there aren't a lot of stars

840
00:39:27,320 --> 00:39:29,320
Speaker 1: floating around, not in galaxies.

841
00:39:29,440 --> 00:39:31,440
Speaker 2: Yeah, there aren't a lot of stars out there, but

842
00:39:31,440 --> 00:39:34,239
Speaker 2: there are some, you know, there are stars in like

843
00:39:34,400 --> 00:39:37,959
Speaker 2: extended halos of galaxies or that were stripped out from

844
00:39:37,960 --> 00:39:40,799
Speaker 2: the galaxies during a merger or something and are now

845
00:39:40,840 --> 00:39:43,879
Speaker 2: just like floating out there in space. And that's something

846
00:39:43,920 --> 00:39:46,160
Speaker 2: we'd really like to understand. How much light is coming

847
00:39:46,160 --> 00:39:49,680
Speaker 2: from outside our galaxy, but also outside the galaxies we

848
00:39:49,719 --> 00:39:53,040
Speaker 2: can identify, you know, from between galaxies.

849
00:39:53,480 --> 00:39:55,839
Speaker 1: WHOA wait, you mean there could be a sun out

850
00:39:55,840 --> 00:39:59,360
Speaker 1: there in between galaxies all by itself with maybe a

851
00:39:59,400 --> 00:40:03,040
Speaker 1: planet orbiting around it with life. And what would they

852
00:40:03,080 --> 00:40:03,960
Speaker 1: see in the night sky?

853
00:40:04,120 --> 00:40:06,319
Speaker 2: They would only see galaxies, right, so their night sky

854
00:40:06,320 --> 00:40:09,360
Speaker 2: would be much much darker. They would only see smudges,

855
00:40:09,400 --> 00:40:12,360
Speaker 2: no pinpoints of light. Oh, they wouldn't see stars. They

856
00:40:12,360 --> 00:40:16,960
Speaker 2: wouldn't know that their sun is maybe just another star. Necessarily,

857
00:40:17,120 --> 00:40:21,040
Speaker 2: it's maybe unlikely because a star that experiences those kind

858
00:40:21,040 --> 00:40:24,040
Speaker 2: of forces would probably also lose its planets and then

859
00:40:24,080 --> 00:40:26,600
Speaker 2: to the extreme gravity being like tossed out of a galaxy.

860
00:40:26,719 --> 00:40:29,040
Speaker 2: But it's possible that the planets come along for the ride.

861
00:40:29,080 --> 00:40:31,520
Speaker 1: Yeah, it makes me a little sad because they wouldn't

862
00:40:31,520 --> 00:40:32,600
Speaker 1: be able to wish upon a star.

863
00:40:34,600 --> 00:40:36,480
Speaker 2: So you might think that this is like the easiest

864
00:40:36,520 --> 00:40:39,080
Speaker 2: extragalactic background like to see because you just like point

865
00:40:39,120 --> 00:40:42,279
Speaker 2: the hubble or point your eyeball between galaxies and see

866
00:40:42,280 --> 00:40:45,640
Speaker 2: what's there. But our galaxies actually really bright in this

867
00:40:45,719 --> 00:40:48,399
Speaker 2: kind of light, not just from the stars, also from

868
00:40:48,440 --> 00:40:52,319
Speaker 2: the scattering of dust. We talked about zodiacal light that

869
00:40:52,360 --> 00:40:55,080
Speaker 2: you can see from Earth. It's like at sunset you

870
00:40:55,080 --> 00:40:57,160
Speaker 2: can see this like a cone of light, like a

871
00:40:57,160 --> 00:41:00,400
Speaker 2: hazy pyramid just above the sunrise or sunset. But this

872
00:41:00,480 --> 00:41:03,520
Speaker 2: is the scattering of light off of dust in our

873
00:41:03,560 --> 00:41:06,600
Speaker 2: Solar system, and it's exactly at the frequency we want

874
00:41:06,640 --> 00:41:09,920
Speaker 2: to use to observe the extragalactic background light. So it's

875
00:41:09,960 --> 00:41:10,720
Speaker 2: a big problem.

876
00:41:11,120 --> 00:41:13,240
Speaker 1: You mean, like the dust that's out there in space

877
00:41:13,400 --> 00:41:17,200
Speaker 1: reflects visible light exactly, but it must reflect other kinds

878
00:41:17,200 --> 00:41:20,319
Speaker 1: of lights as well, doesn't it, Or only does it

879
00:41:20,400 --> 00:41:22,520
Speaker 1: reflect visible light especially well.

880
00:41:22,360 --> 00:41:24,480
Speaker 2: It reflects visible light especially well. It has to do

881
00:41:24,520 --> 00:41:27,080
Speaker 2: with like the size of these dust grains that are

882
00:41:27,120 --> 00:41:30,959
Speaker 2: like usually one to a few hundred microns, and these

883
00:41:31,000 --> 00:41:34,080
Speaker 2: are a huge cloud of dust all through the center

884
00:41:34,200 --> 00:41:36,480
Speaker 2: of the Solar system. It extends out we think, like

885
00:41:36,680 --> 00:41:39,600
Speaker 2: just past Mars, and it might all be Mars's fault. Actually,

886
00:41:39,800 --> 00:41:42,120
Speaker 2: dust storms on Mars might be kicking up a lot

887
00:41:42,120 --> 00:41:44,360
Speaker 2: of this stuff, but it reflects light typically in the

888
00:41:44,360 --> 00:41:47,759
Speaker 2: optical and also in the infrared, and it makes it

889
00:41:47,800 --> 00:41:50,319
Speaker 2: really really hard to see the extragalactic background light.

890
00:41:51,040 --> 00:41:53,680
Speaker 1: Well, this is it, I mean it. This makes it

891
00:41:53,719 --> 00:41:55,920
Speaker 1: hard to see even the galactic light, right because all

892
00:41:55,960 --> 00:41:59,240
Speaker 1: this dust pollution is within our solar system.

893
00:41:59,440 --> 00:42:02,239
Speaker 2: Yeah, exactly, it's a big problem for seeing outside of

894
00:42:02,239 --> 00:42:05,440
Speaker 2: our solar system. You're right, Even understanding our galaxy, the

895
00:42:05,520 --> 00:42:06,880
Speaker 2: zodiacal light is a big.

896
00:42:06,719 --> 00:42:09,960
Speaker 1: Issue, all right, Well, what can we tell from the

897
00:42:10,040 --> 00:42:12,880
Speaker 1: light that is coming from outside of our galaxy in

898
00:42:12,880 --> 00:42:13,800
Speaker 1: the visible spectrum?

899
00:42:13,920 --> 00:42:15,800
Speaker 2: Well, if you look at all the light that's coming

900
00:42:15,800 --> 00:42:18,640
Speaker 2: from outside of our galaxy, we can't explain it. Like

901
00:42:18,680 --> 00:42:20,440
Speaker 2: there's a bunch of photons that are out there that

902
00:42:20,560 --> 00:42:23,440
Speaker 2: just don't match our predictions. Like if you try to say, here,

903
00:42:23,480 --> 00:42:26,120
Speaker 2: I understand what's in the universe. Let me predict all

904
00:42:26,160 --> 00:42:28,799
Speaker 2: the photons we'll see in the optical spectrum, and then

905
00:42:28,840 --> 00:42:30,960
Speaker 2: you compare that to what we do see, there's a

906
00:42:30,960 --> 00:42:33,880
Speaker 2: big gap. Like there must be something wrong with our

907
00:42:33,960 --> 00:42:36,279
Speaker 2: modeling of what's out there in the universe or how

908
00:42:36,320 --> 00:42:39,720
Speaker 2: it emits. Like the data and our predictions do not agree.

909
00:42:40,000 --> 00:42:42,399
Speaker 1: But what do you mean the gap like we're missing light.

910
00:42:42,600 --> 00:42:46,520
Speaker 2: Yeah, there are more optical photons in the extragalactic background

911
00:42:46,760 --> 00:42:49,840
Speaker 2: then we can explain. So that means either there's something

912
00:42:49,880 --> 00:42:52,279
Speaker 2: else out there emitting photons we haven't accounted for it,

913
00:42:52,680 --> 00:42:55,480
Speaker 2: or maybe we've underestimated the amount of scattering from this

914
00:42:55,600 --> 00:42:58,839
Speaker 2: dust the zodiacal background light. But there are photons out

915
00:42:58,840 --> 00:43:00,360
Speaker 2: there that we can't explain.

916
00:43:00,560 --> 00:43:03,920
Speaker 1: Well, meaning like we look out there and we were

917
00:43:03,960 --> 00:43:06,399
Speaker 1: getting light, but there's no object there to see.

918
00:43:06,600 --> 00:43:08,279
Speaker 2: Yeah, a lot of this stuff is diffuse, right, We

919
00:43:08,280 --> 00:43:10,759
Speaker 2: don't know what's out there emitting it. We can't associate

920
00:43:10,760 --> 00:43:13,560
Speaker 2: it with any points source, and so we don't know

921
00:43:13,600 --> 00:43:16,359
Speaker 2: what diffuse sources of this light there are out there

922
00:43:16,360 --> 00:43:18,480
Speaker 2: in the galaxy. Maybe there are more of these like

923
00:43:18,640 --> 00:43:20,760
Speaker 2: rogue stars out there in the middle of the galaxy

924
00:43:20,760 --> 00:43:22,919
Speaker 2: and all their light like adds up to this big

925
00:43:22,960 --> 00:43:25,959
Speaker 2: diffuse component to explain it. Or maybe it's something simpl

926
00:43:26,000 --> 00:43:27,600
Speaker 2: or like misunderstanding the Milky Way.

927
00:43:27,960 --> 00:43:32,280
Speaker 1: Whoa wait, So maybe this mysterious light is just Milky

928
00:43:32,280 --> 00:43:34,560
Speaker 1: Way light. It's not necessarily extra galacti.

929
00:43:34,680 --> 00:43:37,200
Speaker 2: Yeah, exactly. We can't quite pull it apart. But there's

930
00:43:37,239 --> 00:43:40,160
Speaker 2: a really cool recent technique they're using to try to

931
00:43:40,200 --> 00:43:42,640
Speaker 2: get a sense for what's outside of our galaxy. So

932
00:43:42,680 --> 00:43:44,440
Speaker 2: they're can to help pull this thing apart, and that's

933
00:43:44,480 --> 00:43:48,080
Speaker 2: by using even higher energy photons that come from quasars.

934
00:43:48,560 --> 00:43:50,640
Speaker 2: So like the super high energy gamma rays we were

935
00:43:50,640 --> 00:43:52,560
Speaker 2: talking about earlier that are emitted from the centers of

936
00:43:52,680 --> 00:43:55,759
Speaker 2: very distant galaxies. We think we understand the spectrum that

937
00:43:55,800 --> 00:43:58,200
Speaker 2: should be coming from them, but as they travel through

938
00:43:58,200 --> 00:44:02,560
Speaker 2: the universe, sometimes they inter with lower energy photons they

939
00:44:02,560 --> 00:44:05,960
Speaker 2: can get like scattered or reabsorbed or change to another energy.

940
00:44:06,080 --> 00:44:08,120
Speaker 2: So if you look at the spectrum that comes from

941
00:44:08,120 --> 00:44:11,480
Speaker 2: those distant blazers and you see how it's modified from

942
00:44:11,520 --> 00:44:13,600
Speaker 2: what we expect, you can use that to try to

943
00:44:13,640 --> 00:44:17,799
Speaker 2: like map how many extragalactic background photons they ran into

944
00:44:18,120 --> 00:44:18,960
Speaker 2: along the way.

945
00:44:19,280 --> 00:44:22,040
Speaker 1: Wait wait, wait, light can run into light. I thought

946
00:44:22,160 --> 00:44:24,840
Speaker 1: light couldn't collide with other light particles.

947
00:44:24,920 --> 00:44:27,640
Speaker 2: No, you're exactly right. In general, light does not interact

948
00:44:27,640 --> 00:44:30,400
Speaker 2: with light because photons do not have electric charges, so

949
00:44:30,440 --> 00:44:34,360
Speaker 2: they don't interact directly, but they can interact indirectly, like

950
00:44:34,400 --> 00:44:37,120
Speaker 2: a photon can pair produce turn into an electron and

951
00:44:37,160 --> 00:44:38,279
Speaker 2: a positron.

952
00:44:37,880 --> 00:44:39,400
Speaker 1: Momentarily like randomly.

953
00:44:39,600 --> 00:44:41,680
Speaker 2: Yeah, every photon has a probability to pair produce at

954
00:44:41,719 --> 00:44:44,520
Speaker 2: any moment, and so there's a probability for two photons

955
00:44:44,520 --> 00:44:47,920
Speaker 2: to interact. We did whole episode about light beams crossing

956
00:44:47,960 --> 00:44:50,680
Speaker 2: and how you can actually study this. It's a rare process,

957
00:44:50,920 --> 00:44:53,640
Speaker 2: but it does happen. It requires this indirect process through

958
00:44:53,680 --> 00:44:54,640
Speaker 2: the electron field.

959
00:44:54,920 --> 00:44:58,520
Speaker 1: Okay, So then by looking at a source like equasor

960
00:44:58,560 --> 00:45:00,799
Speaker 1: that we know kind of pretty well and see how

961
00:45:00,800 --> 00:45:02,839
Speaker 1: the light is modified when it gets to us. By

962
00:45:02,880 --> 00:45:04,360
Speaker 1: the time it gets to us, you can sort of

963
00:45:04,360 --> 00:45:06,360
Speaker 1: get a sense of what's out there in between.

964
00:45:06,640 --> 00:45:09,239
Speaker 2: Yeah, you can try. It's tricky, like, first of all,

965
00:45:09,239 --> 00:45:11,440
Speaker 2: you have to be very confident that you understand the

966
00:45:11,640 --> 00:45:14,560
Speaker 2: unattenuated light, the light that was emitted by the quays are,

967
00:45:14,719 --> 00:45:16,759
Speaker 2: and then compare it to what we see. Then you

968
00:45:16,800 --> 00:45:19,279
Speaker 2: also have to convince yourself you understand everything else that

969
00:45:19,280 --> 00:45:22,000
Speaker 2: could happen to those photons be absorbed or influenced by

970
00:45:22,000 --> 00:45:24,480
Speaker 2: other things in the universe. That's why they like to

971
00:45:24,560 --> 00:45:27,000
Speaker 2: use these very high energy sources because they tend to

972
00:45:27,040 --> 00:45:30,080
Speaker 2: interact less than other photons, so they're more pristine.

973
00:45:30,320 --> 00:45:33,680
Speaker 1: Oh. Interesting, And again it's sort of weird that we're

974
00:45:33,719 --> 00:45:35,520
Speaker 1: getting all this light and we don't know where it's

975
00:45:35,560 --> 00:45:36,040
Speaker 1: coming from.

976
00:45:36,160 --> 00:45:38,400
Speaker 2: Yeah, exactly, But it's cool to be using these like

977
00:45:38,520 --> 00:45:41,799
Speaker 2: pencil beams of super high energy photons to get a

978
00:45:41,840 --> 00:45:45,080
Speaker 2: measure of like the other low energy photons along the way.

979
00:45:45,200 --> 00:45:47,640
Speaker 2: It's like we're getting information about photons that would never

980
00:45:47,680 --> 00:45:48,520
Speaker 2: have reached Earth.

981
00:45:48,760 --> 00:45:49,719
Speaker 1: Pretty cool, all right.

982
00:45:49,760 --> 00:45:49,960
Speaker 3: Now.

983
00:45:50,080 --> 00:45:53,160
Speaker 1: The last frequency range is the low frequency range of light,

984
00:45:53,320 --> 00:45:54,600
Speaker 1: and that's the infrared.

985
00:45:54,800 --> 00:45:57,400
Speaker 2: Yeah. The lowest frequency range is interesting because you know,

986
00:45:57,400 --> 00:46:00,239
Speaker 2: it's dominated by the microwaves, which we've studied very very

987
00:46:00,280 --> 00:46:02,239
Speaker 2: well and talked about and it's sort of like a

988
00:46:02,239 --> 00:46:04,799
Speaker 2: good example of what you can learn just by looking

989
00:46:04,840 --> 00:46:07,279
Speaker 2: at the night sky in the microwave. We've learned so

990
00:46:07,520 --> 00:46:10,920
Speaker 2: much about the early universe because there were microwaves emitted

991
00:46:10,960 --> 00:46:14,000
Speaker 2: in the very early universe, this moment when protons and

992
00:46:14,040 --> 00:46:17,879
Speaker 2: electrons came together in the universe became transparent again. Those

993
00:46:17,880 --> 00:46:20,840
Speaker 2: photons are still around and we've captured them and measured

994
00:46:20,840 --> 00:46:24,480
Speaker 2: things about the early universe. Really revolutionized all of cosmology.

995
00:46:24,800 --> 00:46:26,839
Speaker 2: That's just like a taste of what we could learn

996
00:46:26,960 --> 00:46:29,760
Speaker 2: from the other spectra. The microwaves, though, were like thirty

997
00:46:29,840 --> 00:46:33,360
Speaker 2: or forty times brighter than every other wavelength. For the

998
00:46:33,400 --> 00:46:36,920
Speaker 2: extragalactic background light, it's like the brightest part of the spectrum,

999
00:46:36,960 --> 00:46:39,040
Speaker 2: which is why it's sort of like the easy thing.

1000
00:46:39,520 --> 00:46:42,719
Speaker 2: The microwaves are like the first by the d apple, But.

1001
00:46:42,960 --> 00:46:45,279
Speaker 1: We skipped over i infrared though, didn't we? In terms

1002
00:46:45,320 --> 00:46:48,719
Speaker 1: of frequency, infrared is higher frequency than microwaves.

1003
00:46:49,000 --> 00:46:52,200
Speaker 2: Yeah, absolutely, Infrared is higher energy than microwaves. And the

1004
00:46:52,239 --> 00:46:56,759
Speaker 2: cosmic infrared background is also super fascinating and we'd love

1005
00:46:56,840 --> 00:46:59,319
Speaker 2: to study it in more detail because it might have

1006
00:46:59,600 --> 00:47:02,640
Speaker 2: might be rich with information. Infrared light has lots of

1007
00:47:02,680 --> 00:47:06,400
Speaker 2: really interesting point sources, like star forming regions and galaxies

1008
00:47:06,640 --> 00:47:09,319
Speaker 2: at very high redshift that have been red shifted into

1009
00:47:09,320 --> 00:47:12,880
Speaker 2: the infrared. Super interesting to study. And this was actually

1010
00:47:12,920 --> 00:47:16,000
Speaker 2: studied by the same satellite that measured the cosmic microwave

1011
00:47:16,000 --> 00:47:18,319
Speaker 2: background light. There was an instrument on board that was

1012
00:47:18,320 --> 00:47:21,280
Speaker 2: capable of picking up infrared light, but it's much dimmer

1013
00:47:21,320 --> 00:47:23,720
Speaker 2: than the microwave light, and so it's much more difficult

1014
00:47:23,960 --> 00:47:24,480
Speaker 2: to study.

1015
00:47:24,680 --> 00:47:26,319
Speaker 1: Well, I wonder if all these things just kind of

1016
00:47:26,320 --> 00:47:29,279
Speaker 1: get smooched together, because you know, I know we've talked

1017
00:47:29,280 --> 00:47:32,480
Speaker 1: about that things that are really far away. They might

1018
00:47:32,520 --> 00:47:34,200
Speaker 1: emit light, but by the time that light gets to it,

1019
00:47:34,200 --> 00:47:37,440
Speaker 1: because of the expanding universe, that light gets red shifted,

1020
00:47:37,480 --> 00:47:39,839
Speaker 1: it becomes more red er. So like if you get

1021
00:47:39,880 --> 00:47:43,560
Speaker 1: red light, it could come from basically anything, right.

1022
00:47:43,480 --> 00:47:47,440
Speaker 2: Yeah, absolutely, you could have super high energy galactic nuclei

1023
00:47:47,640 --> 00:47:50,319
Speaker 2: emitting very high energy photons and by the time they

1024
00:47:50,360 --> 00:47:54,120
Speaker 2: get to us there infrared like even the cosmic microwave

1025
00:47:54,120 --> 00:47:57,440
Speaker 2: background light. Those photons are super long wavelength, but when

1026
00:47:57,440 --> 00:47:59,799
Speaker 2: they were emitted, they weren't. The plasma that made them

1027
00:47:59,840 --> 00:48:02,600
Speaker 2: was super high energy. They were admitted a very very

1028
00:48:02,680 --> 00:48:05,200
Speaker 2: high frequency. It's only the expansion of the universe that

1029
00:48:05,360 --> 00:48:07,799
Speaker 2: stretched them out all the way down to the microwave.

1030
00:48:08,120 --> 00:48:10,040
Speaker 2: So you're right, everything is piled on top of each

1031
00:48:10,040 --> 00:48:11,440
Speaker 2: other a minute at one way.

1032
00:48:11,719 --> 00:48:13,640
Speaker 1: Is this like the messiest kind of light we're getting?

1033
00:48:14,239 --> 00:48:15,800
Speaker 1: Do you know what I mean? Like, because everything piles

1034
00:48:15,800 --> 00:48:19,879
Speaker 1: onto that frequency spectrum as opposed to like the higher frequencies. Thinks,

1035
00:48:19,920 --> 00:48:20,600
Speaker 1: don't get bluer.

1036
00:48:20,719 --> 00:48:22,680
Speaker 2: Yeah, it's a beautiful mess down there at the long

1037
00:48:22,719 --> 00:48:25,480
Speaker 2: wave lengths because everything's red shifted down there into the

1038
00:48:25,560 --> 00:48:26,720
Speaker 2: dustbin of the universe.

1039
00:48:26,960 --> 00:48:30,360
Speaker 1: Now, this gets us into the microwave range, which we

1040
00:48:30,719 --> 00:48:34,919
Speaker 1: talked a lot about before. The cosmic microwave background, which

1041
00:48:34,920 --> 00:48:38,800
Speaker 1: you said is included in this idea of the extragalactic background.

1042
00:48:38,440 --> 00:48:42,360
Speaker 2: Exactly because it's generated by plasma that's outside the Milky Way,

1043
00:48:42,840 --> 00:48:45,960
Speaker 2: and so it's definitely extra galactic. It's also the brightest

1044
00:48:46,000 --> 00:48:48,600
Speaker 2: part of the spectrum, and so it's easiest to tackle,

1045
00:48:48,640 --> 00:48:50,960
Speaker 2: and it tells us a lot about the very early universe.

1046
00:48:51,360 --> 00:48:53,080
Speaker 2: So that's why it's sort of the best well known

1047
00:48:53,120 --> 00:48:54,200
Speaker 2: and the best well studied.

1048
00:48:54,480 --> 00:48:56,279
Speaker 1: So then the stuff we get that we call this

1049
00:48:56,400 --> 00:48:59,239
Speaker 1: CMB the cosmic microwave background, we know for sure what

1050
00:48:59,360 --> 00:49:02,000
Speaker 1: it is. We always say it's light from the beginning

1051
00:49:02,040 --> 00:49:04,719
Speaker 1: of the universe. How well do we actually know that?

1052
00:49:04,920 --> 00:49:08,319
Speaker 1: Wouldn't it be kind of confounded or mixed together with

1053
00:49:08,920 --> 00:49:11,120
Speaker 1: you know, distance, stars exploding and things like that.

1054
00:49:11,280 --> 00:49:14,720
Speaker 2: Absolutely, there are other sources in the microwave, including sources

1055
00:49:14,760 --> 00:49:18,279
Speaker 2: from the Milky Way, but everything is easier when you're

1056
00:49:18,280 --> 00:49:21,440
Speaker 2: looking for a bigger signal, right, it's easier to establish,

1057
00:49:21,560 --> 00:49:23,959
Speaker 2: it's easier to subtract a way. Uncertainties in the Milky

1058
00:49:24,000 --> 00:49:26,880
Speaker 2: Way can be larger without affecting your measurements because you

1059
00:49:26,880 --> 00:49:28,359
Speaker 2: have a larger signal you're looking for.

1060
00:49:28,719 --> 00:49:30,439
Speaker 1: What do you mean it's a bigger signal, Like it's

1061
00:49:30,480 --> 00:49:32,880
Speaker 1: more powerful, or just a wavelength is bigger.

1062
00:49:32,920 --> 00:49:35,600
Speaker 2: I mean it's more powerful. There are more photons, like

1063
00:49:35,840 --> 00:49:38,600
Speaker 2: there are forty times as many photons in the microwave

1064
00:49:38,800 --> 00:49:41,080
Speaker 2: than there are in the radio or in the UV

1065
00:49:41,640 --> 00:49:44,239
Speaker 2: or in the optical. The universe is brighter in the

1066
00:49:44,280 --> 00:49:47,080
Speaker 2: microwaves than they are any other spectrum.

1067
00:49:47,160 --> 00:49:49,919
Speaker 1: Yes, because of the Big Bang, because of the Big

1068
00:49:49,960 --> 00:49:51,120
Speaker 1: Bang was a huge source of it?

1069
00:49:51,280 --> 00:49:54,160
Speaker 2: Or is it because of this early universe plasma? Yeah,

1070
00:49:54,320 --> 00:49:56,880
Speaker 2: not technically the Big Bang, but there's a lot of

1071
00:49:56,920 --> 00:49:59,960
Speaker 2: emission in the very early universe, and that and then

1072
00:49:59,800 --> 00:50:01,960
Speaker 2: an and that got slid all the way down by

1073
00:50:01,960 --> 00:50:04,480
Speaker 2: the expansion of the universe to the microwave. And so

1074
00:50:04,520 --> 00:50:06,360
Speaker 2: it's sitting there as a very bright.

1075
00:50:06,160 --> 00:50:08,480
Speaker 1: Signal and we sort of know what it is. But

1076
00:50:08,520 --> 00:50:12,120
Speaker 1: then that means it also mean that one big source

1077
00:50:12,200 --> 00:50:14,520
Speaker 1: the beginning of the universe is drowning out anything else

1078
00:50:14,560 --> 00:50:16,000
Speaker 1: we might want to see in the microwave.

1079
00:50:16,080 --> 00:50:18,720
Speaker 2: Absolutely, and that's why we've been studying into great detail

1080
00:50:18,760 --> 00:50:20,680
Speaker 2: and trying to understand all the ripples in it and

1081
00:50:20,680 --> 00:50:22,719
Speaker 2: the other sources of it. Again, when you have a

1082
00:50:22,760 --> 00:50:26,319
Speaker 2: brighter signal, just everything is easier scientifically. You have more

1083
00:50:26,400 --> 00:50:28,520
Speaker 2: data to play with, so you can do more tricks

1084
00:50:28,560 --> 00:50:31,440
Speaker 2: like extrapolating from one region to the other. You have

1085
00:50:31,480 --> 00:50:34,160
Speaker 2: better ways to validate all of your models. It's much

1086
00:50:34,239 --> 00:50:36,719
Speaker 2: much more difficult when you're dealing with faint sources that

1087
00:50:36,760 --> 00:50:39,120
Speaker 2: you're not even sure you're seeing. So seeing that in

1088
00:50:39,160 --> 00:50:41,719
Speaker 2: the microwave and seeing it exactly the temperature we expected

1089
00:50:42,040 --> 00:50:45,160
Speaker 2: was a great confirmation of our understanding of that whole process.

1090
00:50:45,520 --> 00:50:50,439
Speaker 1: Mm interesting and it also heats up barburritos, all right.

1091
00:50:50,480 --> 00:50:53,440
Speaker 1: So then the last frequency range is the radio waves,

1092
00:50:53,960 --> 00:50:58,879
Speaker 1: which is like the lowest frigency, longest wavelength light that's

1093
00:50:58,920 --> 00:50:59,920
Speaker 1: out there exactly.

1094
00:51:00,080 --> 00:51:04,040
Speaker 2: And these are really cool experiments to see radio emissions

1095
00:51:04,120 --> 00:51:08,080
Speaker 2: from the deep universe. We have these balloon experiments. Well,

1096
00:51:08,120 --> 00:51:10,440
Speaker 2: you have like a radio antenna, but you cool it

1097
00:51:10,520 --> 00:51:14,120
Speaker 2: down using liquid helium, and then you'll launch it up

1098
00:51:14,239 --> 00:51:17,880
Speaker 2: like thirty seven kilometers above Texas or sometimes above the

1099
00:51:17,920 --> 00:51:20,760
Speaker 2: South Pole, so we can gather radio signals from space.

1100
00:51:21,080 --> 00:51:23,360
Speaker 2: You shield it so that it's not just like getting

1101
00:51:23,400 --> 00:51:26,400
Speaker 2: your local NPR station, and you cool it down so

1102
00:51:26,440 --> 00:51:28,640
Speaker 2: it can pick up the most faint signals, and then

1103
00:51:28,680 --> 00:51:31,319
Speaker 2: you try to gather radio signals from deep space. We

1104
00:51:31,360 --> 00:51:33,560
Speaker 2: measure a lot of these things, but we don't understand

1105
00:51:33,600 --> 00:51:36,279
Speaker 2: all the radio waves that we see. Maybe some of

1106
00:51:36,320 --> 00:51:39,279
Speaker 2: them are from dark matter colliding and emitting in the

1107
00:51:39,400 --> 00:51:43,799
Speaker 2: radio Maybe there's some other diffuse emission of radio. Maybe

1108
00:51:43,880 --> 00:51:46,480
Speaker 2: it's just something in the galactic foreground, something else in

1109
00:51:46,520 --> 00:51:48,799
Speaker 2: the galaxy that's emitting in the radio waves we don't

1110
00:51:48,880 --> 00:51:52,560
Speaker 2: yet understand. So we don't actually know if these photons

1111
00:51:52,600 --> 00:51:56,640
Speaker 2: that we're seeing are EBL radio photons or just vanilla

1112
00:51:56,760 --> 00:51:58,919
Speaker 2: milky way photons.

1113
00:52:00,080 --> 00:52:03,400
Speaker 1: If they have an actual like source generating them, or

1114
00:52:03,400 --> 00:52:05,520
Speaker 1: they're just kind of like noise. Is that what you mean.

1115
00:52:05,880 --> 00:52:08,759
Speaker 2: Yeah, if there's a point source generating all these radio

1116
00:52:08,960 --> 00:52:12,160
Speaker 2: frequency photons, we probably would have figured that out, could

1117
00:52:12,200 --> 00:52:14,120
Speaker 2: identify it with like the center of the galaxy or

1118
00:52:14,120 --> 00:52:17,520
Speaker 2: some black hole or some pulsar or something. So that

1119
00:52:17,560 --> 00:52:19,160
Speaker 2: would have been easy, and we haven't been able to

1120
00:52:19,160 --> 00:52:22,000
Speaker 2: do that, which means probably it's something diffuse, like maybe

1121
00:52:22,160 --> 00:52:25,000
Speaker 2: dark matter and the whole halo colliding with itself or

1122
00:52:25,280 --> 00:52:28,759
Speaker 2: something else. We don't really understand the sources here. WHOA.

1123
00:52:29,640 --> 00:52:32,799
Speaker 1: All right, So to recap we're getting a lot of

1124
00:52:32,840 --> 00:52:35,880
Speaker 1: lights from the universe. Only about one percent of that

1125
00:52:36,000 --> 00:52:38,360
Speaker 1: light comes from our galaxy. Ninety nine percent of the

1126
00:52:38,440 --> 00:52:40,600
Speaker 1: light that we see that when you look at the

1127
00:52:40,680 --> 00:52:44,440
Speaker 1: nice sky comes from outside of the galaxy. And it

1128
00:52:44,480 --> 00:52:48,160
Speaker 1: seems like ninety eight percent of that is all mystery light,

1129
00:52:48,239 --> 00:52:50,360
Speaker 1: Like we don't know what is making that light, where's

1130
00:52:50,360 --> 00:52:52,200
Speaker 1: it coming from? Right, That's kind of what it seems like.

1131
00:52:52,560 --> 00:52:55,160
Speaker 2: Well, most of the light in the universe is generated

1132
00:52:55,160 --> 00:52:57,160
Speaker 2: outside our galaxy, but most of the light that we

1133
00:52:57,239 --> 00:53:01,120
Speaker 2: see is generated inside our galaxy because we're inside our galaxy.

1134
00:53:01,440 --> 00:53:02,800
Speaker 2: So most of the light that we see in the

1135
00:53:02,880 --> 00:53:05,319
Speaker 2: night sky is coming from the Milky Way. But that's

1136
00:53:05,360 --> 00:53:08,200
Speaker 2: a tiny fraction of all the photons in the universe,

1137
00:53:08,760 --> 00:53:10,719
Speaker 2: and so the rest of the universe is quite dim

1138
00:53:10,760 --> 00:53:12,800
Speaker 2: in comparison to the Milky Way. But that's most of

1139
00:53:12,840 --> 00:53:16,440
Speaker 2: the interesting stuff, and these photons contain the whole history

1140
00:53:16,480 --> 00:53:19,600
Speaker 2: of the universe, but they're mostly outshined by the brightness

1141
00:53:19,680 --> 00:53:21,920
Speaker 2: of the Milky Way, which just you know, happens to

1142
00:53:21,920 --> 00:53:22,560
Speaker 2: be nearby.

1143
00:53:23,080 --> 00:53:25,719
Speaker 1: Oh, I see, there's a lot of light out there,

1144
00:53:25,760 --> 00:53:27,440
Speaker 1: but we don't get all of it is what you're saying.

1145
00:53:28,040 --> 00:53:30,360
Speaker 1: Because we're so close the Milky Way galaxy, most of

1146
00:53:30,360 --> 00:53:32,279
Speaker 1: the light that we get goes from the Milky Way.

1147
00:53:32,360 --> 00:53:34,960
Speaker 2: Yeah, it's like you're standing right next to a lighthouse,

1148
00:53:35,000 --> 00:53:37,319
Speaker 2: and so you can't really see anything that's far away.

1149
00:53:37,360 --> 00:53:39,840
Speaker 2: Even if those photons are coming to you, they're mostly

1150
00:53:39,880 --> 00:53:41,600
Speaker 2: outshined by the local sources.

1151
00:53:41,800 --> 00:53:44,520
Speaker 1: But well, we can't see the cosmic background outside of

1152
00:53:44,520 --> 00:53:47,720
Speaker 1: our galaxy. It's all sort of shrouded in mystery, it seems.

1153
00:53:47,840 --> 00:53:50,719
Speaker 2: Yeah, it's tricky to disentangle the Milky Way from the

1154
00:53:50,760 --> 00:53:53,239
Speaker 2: other sources. There's a lot of photons we don't understand,

1155
00:53:53,400 --> 00:53:55,879
Speaker 2: a lot of question marks. Over the next few years,

1156
00:53:55,880 --> 00:53:58,600
Speaker 2: we're hoping to turn on more sensitive instruments that can

1157
00:53:58,640 --> 00:54:02,600
Speaker 2: disentangle these things make at our measurements. Maybe this extragalactic

1158
00:54:02,600 --> 00:54:05,400
Speaker 2: background light's gonna come into sharper relief, and it might

1159
00:54:05,440 --> 00:54:07,719
Speaker 2: teach us things about the universe. It could reveal all

1160
00:54:07,800 --> 00:54:10,680
Speaker 2: sorts of weird stuff going on in the deep reaches

1161
00:54:10,719 --> 00:54:12,919
Speaker 2: of space and in the deep history of time.

1162
00:54:13,200 --> 00:54:16,080
Speaker 1: Yeah, and about the makeup of the universe as well.

1163
00:54:16,200 --> 00:54:19,720
Speaker 1: If it tells us about dark matter and maybe dark energy.

1164
00:54:20,000 --> 00:54:21,800
Speaker 1: I think Daniel. Maybe the only solution here is that

1165
00:54:21,840 --> 00:54:25,400
Speaker 1: you're gonna have to leave the galaxy to get a

1166
00:54:25,440 --> 00:54:27,320
Speaker 1: good look at this light. I'll pack a lot of children.

1167
00:54:27,360 --> 00:54:30,640
Speaker 1: Do we have an ex make an extra Daniel, extra

1168
00:54:30,719 --> 00:54:32,719
Speaker 1: galactic Daniel adventure here.

1169
00:54:33,480 --> 00:54:35,399
Speaker 2: That's going to take a very very long time.

1170
00:54:37,480 --> 00:54:39,560
Speaker 1: I'll report we got time, We got time, right.

1171
00:54:39,600 --> 00:54:41,600
Speaker 2: I gotta make it back before next week's episode. Man,

1172
00:54:41,760 --> 00:54:43,160
Speaker 2: I better build a fast ship.

1173
00:54:43,600 --> 00:54:45,520
Speaker 1: Yeah, there you go. Well, first of all, invent the

1174
00:54:45,560 --> 00:54:48,439
Speaker 1: faster than light space ship, and then then we're talking.

1175
00:54:48,680 --> 00:54:51,279
Speaker 1: Sounds good, all right? Well, another reminder of how much

1176
00:54:51,320 --> 00:54:54,160
Speaker 1: of the mystery we still have to observe and discover

1177
00:54:54,360 --> 00:54:56,960
Speaker 1: and figure out. There are still lots of mysteries out there,

1178
00:54:57,160 --> 00:55:00,600
Speaker 1: even in the light that bathes us every night, every day,

1179
00:55:00,960 --> 00:55:01,799
Speaker 1: all around the Earth.

1180
00:55:02,239 --> 00:55:04,239
Speaker 2: That's right, There is so much of the universe we

1181
00:55:04,360 --> 00:55:07,120
Speaker 2: have not yet observed or understood, so much left to

1182
00:55:07,200 --> 00:55:10,560
Speaker 2: discover for you young scientists out there, the next generation

1183
00:55:10,880 --> 00:55:12,640
Speaker 2: of curious explorers.

1184
00:55:12,640 --> 00:55:13,759
Speaker 1: Or the old ones, do right.

1185
00:55:15,239 --> 00:55:17,040
Speaker 2: I'm just going to retire and let everybody else figure

1186
00:55:17,040 --> 00:55:18,600
Speaker 2: it out and explain it to me at this point.

1187
00:55:20,680 --> 00:55:22,480
Speaker 1: Well, hopefully they do it in time, because come on,

1188
00:55:22,880 --> 00:55:26,000
Speaker 1: we're not getting any younger. Daniel, all right, well, we

1189
00:55:26,040 --> 00:55:29,120
Speaker 1: hope you enjoyed that. Thanks for joining us, See you

1190
00:55:29,200 --> 00:55:29,600
Speaker 1: next time.

1191
00:55:34,400 --> 00:55:37,600
Speaker 2: For more science and curiosity, come find us on social media,

1192
00:55:37,719 --> 00:55:41,839
Speaker 2: where we answer questions and post videos. We're on Twitter, This, Org,

1193
00:55:41,920 --> 00:55:45,520
Speaker 2: Instant and now TikTok. Thanks for listening and remember that

1194
00:55:45,719 --> 00:55:49,520
Speaker 2: Daniel and Jorge Explain the Universe is a production of iHeartRadio.

1195
00:55:49,800 --> 00:55:54,880
Speaker 2: For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts,

1196
00:55:55,080 --> 00:55:57,400
Speaker 2: or wherever you listen to your favorite shows.