Speaker 1: Hey, Daniel. When you look at the night sky on
Speaker 1: a good night, how many stars can you see?
Speaker 2: Well, if it's a really good dark night without any
Speaker 2: light pollution, you might see several thousand.
Speaker 1: Stars, thousands in Los Angeles.
Speaker 2: Maybe forgotten Joshua Tree.
Speaker 1: But that's it. Only a thousand stars and the night
Speaker 1: sky aren't there like trillions of stars out there?
Speaker 2: There are oodles of stars out there, but most of
Speaker 2: them are too dim to see even though they're out.
Speaker 1: There, like their photons are not reaching us, or they're
Speaker 1: too weak.
Speaker 2: Photons can travel in infinite distance without ever getting tired,
Speaker 2: but their photons are just so rare that you need
Speaker 2: like a really big eyeball, maybe Hubble or James Webb
Speaker 2: to capture one of them.
Speaker 1: Or just like a long exposure too.
Speaker 2: Right, Yeah, if you set up your camera for months
Speaker 2: and months, you'll probably see one of those dim stars you.
Speaker 1: Need, like a slow eyeball.
Speaker 2: A big or slow eyeball or both.
Speaker 1: So I guess what else is out there? Like, what
Speaker 1: else would you see? Would you see stars and anything else?
Speaker 2: We don't know. What's out there could just be stars
Speaker 2: and galaxies, or there could be like chocolate bars and
Speaker 2: frozen bananas waiting for us to discover them.
Speaker 1: WHOA, sounds delicious? But will we see those things? Wouldn't
Speaker 1: they have to glow somehow? Would these be glowing frozen bananas?
Speaker 2: Glowing or reflective?
Speaker 1: Which case? Do you want to eat them?
Speaker 2: I'm glowing with excitement to try them?
Speaker 1: I do. I know it sounds a little slippery. Hi.
Speaker 1: I'm Jorge make, cartoonist and the author of Oliver's Greek
Speaker 1: Big Universe. Hi.
Speaker 2: I'm Daniel. I'm a particle physicist and a professor at
Speaker 2: UC Irvine, and I'm so excited to see what's hiding
Speaker 2: out there in the universe waiting for us to discover it.
Speaker 1: Yeah, there seems to be a lot out there. But
Speaker 1: do you think it's hiding or we're just not good
Speaker 1: at seeing I.
Speaker 2: Don't think it knows or cares about whether we are
Speaker 2: seeing it. But it has not yet been revealed, so
Speaker 2: in that sense, it is concealed.
Speaker 1: Oh yeah, yeah, But is it really the case that
Speaker 1: it could anything could be out there? You think then
Speaker 1: we have a pretty good sense from what we can
Speaker 1: see around us.
Speaker 2: There's still a lot of big questions about what's out there,
Speaker 2: the stuff nearby, the stuff far away the stuff in between,
Speaker 2: and we should never make the assumption that the stuff
Speaker 2: that's close to us is typical and that it could
Speaker 2: be used to explain the whole universe.
Speaker 1: Well, I guess there could be things hiding within our
Speaker 1: own galaxy that we can't see it, right, Like we
Speaker 1: haven't seen all of the Milky Way galaxy.
Speaker 2: Oh absolutely, And the center of the galaxy, the most
Speaker 2: interesting place, is the hardest to see. But the Milky
Speaker 2: Way itself is so bright and so big, and that
Speaker 2: it makes it really hard to see beyond it to
Speaker 2: other galaxies.
Speaker 1: And these other galaxies could be totally different from.
Speaker 2: Ours, right, They could be very different. They could have
Speaker 2: a very different history. There could be all sorts of
Speaker 2: stuff going on out there deep in the universe that
Speaker 2: we haven't yet figured out. Most of the photons that
Speaker 2: come to Earth we don't gather. We mostly ignore them.
Speaker 1: You mean, like we're getting all this information from all
Speaker 1: the cross the universe, but we're not doing anything with it.
Speaker 1: We're not paying attention.
Speaker 2: Yeah, the universe is screaming at us in photons. Everything
Speaker 2: that's out there in the universe is telling us all
Speaker 2: about itself. So we have like the whole history of
Speaker 2: the universe is out there being literally beamed at us,
Speaker 2: but mostly we're not paying attention. Mostly those photons just
Speaker 2: like splash on the sidewalk.
Speaker 1: I wonder if that could be a good thing. Like
Speaker 1: I wonder if at some point it's like TMI Universe.
Speaker 1: There's some things I don't want to never TMI.
Speaker 2: With science, you always want more data so you can
Speaker 2: know more about the universe. I want to know the
Speaker 2: Universe's deepest, darkest, most embarrassing secrets.
Speaker 1: I see, you're more of an any person.
Speaker 2: Not enough information, Absolutely no information is too embarrassing.
Speaker 1: Well I hope that's true. But anyways, welcome to our podcast,
Speaker 1: Daniel and Jorge Explain the Universe, a production of Our
Speaker 1: Heart Radio.
Speaker 2: In which no question is too weird, too gross, too
Speaker 2: icky for us to explore it. We want to know
Speaker 2: all of the embarrassing details about how the universe was born,
Speaker 2: how it grew up, and all the messes it made
Speaker 2: along the way. We want to unravel the deep history
Speaker 2: of time and understand how the universe got to be
Speaker 2: the way that it is and why it operates in
Speaker 2: such an incredible, amazing and beautiful fashion.
Speaker 1: Man, you make it sound like crazy fans of the
Speaker 1: universe we are, or as the kids say these days, stand.
Speaker 2: I totally stand our universe absolutely. I will defend it
Speaker 2: online against haters.
Speaker 1: There you go until you turn on it.
Speaker 2: If in season seven it does something really weird, then yes,
Speaker 2: I will turn on it.
Speaker 1: But so far, if it jumps the shark, if it
Speaker 1: jumps the galactic shark, you're like, I like the earlier
Speaker 1: seasons better.
Speaker 2: So far, it's been pretty awesome, and everything we've learned
Speaker 2: has blown our minds and revealed incredible things about the
Speaker 2: way the universe works. Not only are the law that
Speaker 2: it follows really fascinating and incredible and have weird philosophical
Speaker 2: consequences for what the nature of reality is, but the
Speaker 2: stuff that bubbles up from those tiny laws, the huge,
Speaker 2: the big stuff, the black holes, the galaxies, the quasars,
Speaker 2: the blazars, all that stuff is just so mind blowingly awesome.
Speaker 1: Yeah, it's pretty amazing. As we said before, how much
Speaker 1: we've been able to figure out about the larger universe,
Speaker 1: even just about our galaxy and beyond, just from sitting
Speaker 1: on this little tiny rock in one corner of the galaxy.
Speaker 1: It's pretty incredible. If you think about the scale things
Speaker 1: and how much we know about what's out there.
Speaker 2: But we've only really just begun to observe our universe.
Speaker 2: We have a few eyeballs capable of picking up really
Speaker 2: distant objects, but most of the light that's out there,
Speaker 2: billions and billions and trillions and trillions of photons that
Speaker 2: contain super fascinating important information about the history of our universe,
Speaker 2: we're not capturing them. They're mostly drowned out by bigger,
Speaker 2: brighter stuff like the lights of Los Angeles.
Speaker 1: We're sort of washed with information from the universe and
Speaker 1: we're not really I guess we are capturing it, we're
Speaker 1: just not recording it, is maybe what you mean. We're like,
Speaker 1: we're getting night light, starlight, you know when I step
Speaker 1: outside tonight, but I'm not going to be thinking about
Speaker 1: it or recording it or trying to figure out what
Speaker 1: it says.
Speaker 2: Yeah, most of it just hits the earth, and unless
Speaker 2: some like Gecko is looking up at the night sky,
Speaker 2: there's no being that's gathering that information. It just like
Speaker 2: gently warms some rock or some like gum wrapper that's
Speaker 2: lying on the ground.
Speaker 1: But do you think we're missing stuff? Like if I
Speaker 1: just point a telescope there every once in a while.
Speaker 1: Am I really going to miss anything?
Speaker 2: I think there's an incredible amount of deep history out
Speaker 2: there in the night sky. And if we build zillions
Speaker 2: of telescopes and point to them in all those directions
Speaker 2: and just let them accumulate information, we would learn so
Speaker 2: much about the history of the universe from these really
Speaker 2: faint sources.
Speaker 1: But ironically, if you cover the night sky with telescopes
Speaker 1: and you wouldn't be able to see the night sky.
Speaker 2: See it in a different way.
Speaker 1: Well, I guess that's true. We can see it on
Speaker 1: our phones. It's a little less poetic, and we can
Speaker 1: see it scientifically. But anyways, it's kind of interesting what
Speaker 1: is out there and what kind of information we are getting,
Speaker 1: including even the stuff we might consider being in the background.
Speaker 2: Yeah, that's right. The night sky is chock full of
Speaker 2: stars from our galaxy, but there's a lot of really
Speaker 2: useful information in the background, information we're mostly missing.
Speaker 1: So today on the program, we'll be tackling the question
Speaker 1: what is extra galactic background light. It's a lot of
Speaker 1: syllables there for one term.
Speaker 2: Yeah, astronomers, you know this one. I think they actually
Speaker 2: name pretty well.
Speaker 1: We'll see, we'll see. You've always promised that, and most
Speaker 1: of the time it disappoints.
Speaker 2: You have a very unrealistic standard.
Speaker 1: If I have to say so, I'm just saying, you know,
Speaker 1: take a minute to think about it, all.
Speaker 2: Right, all right, I'll suspend my judgment.
Speaker 1: But yeah, it's an interesting question. A lot of words here,
Speaker 1: extra galactic background light, which should have sounds self apparent,
Speaker 1: but maybe the extra it throws me off a little bit,
Speaker 1: like it's extra, like we don't need it. Or is
Speaker 1: it extra like like you get a bonus, Like you know,
Speaker 1: I pay for a certain amount of galactic background light
Speaker 1: and I'm getting some extra serving of it.
Speaker 2: Or maybe it's just like a bit much universe Like
Speaker 2: why are you so extra? Yeah, that's how my teenage
Speaker 2: daughter would interpret it.
Speaker 1: Mmmm, you're being too extra?
Speaker 2: Yeah, Dad, you're so extra.
Speaker 1: Oh my god, that's better than me. Mid, that's like
Speaker 1: the that's not the worst insult from a teen right now,
Speaker 1: that's kind of Mid. Oh boy university, Mid, I've seen
Speaker 1: better life existence, Mid. But yeah, I guess we'll dig
Speaker 1: into what all these turns mean and why it's interesting
Speaker 1: to think about the extra galactic background light us you'll
Speaker 1: be for wondering how many people out there know about
Speaker 1: this or have any thoughts about what it might be.
Speaker 2: Thanks very much to our panel of volunteers who comment
Speaker 2: on these well named astronomical phenomena and offer their opinions
Speaker 2: without the chance to google about it. If you would
Speaker 2: like to play for a future episode, please don't be shy.
Speaker 2: Write to me two questions at Danielandjorge dot com.
Speaker 1: So think about it for a second. What do you
Speaker 1: think is the extra galactic background light? Here's what people
Speaker 1: have to say. It must be all the light coming
Speaker 1: from outside our galaxy, either that or one of those
Speaker 1: LED sets that you can use to make your bedroom
Speaker 1: lighting extra galactic.
Speaker 3: Extra galactic background light is the light produced by Club
Speaker 3: Andromeda when the Aliens are having a rave. I don't
Speaker 3: know something to do with our galaxy and there's too
Speaker 3: much light for what there should be by some theory
Speaker 3: that's calculating, Uh, you know how it fits within the
Speaker 3: light it should get from other galaxies or our sun,
Speaker 3: and somehow it's being amplified.
Speaker 4: I've heard of it, but I'm thinking that our other
Speaker 4: galaxies out of our galaxy and there it's a large background.
Speaker 4: Most of the stars we see is the background galaxies,
Speaker 4: and that's the range of galaxies we see in their lives.
Speaker 1: Yes, all right, a lot of creative answers here. Some
Speaker 1: people think extra means party.
Speaker 2: I was wondering if anybody was going to say, it's
Speaker 2: like breaking news, like extra extra, read all about it.
Speaker 1: Oh right, that's another use of the word extra. Yeah,
Speaker 1: if you're from the nineteen twenties, I.
Speaker 2: Got a jaunty cap on. I'm standing on a so
Speaker 2: far I'm selling newspaper.
Speaker 1: You get a vest on. Yeah, yeah, yeah, you're on
Speaker 1: the street. I got on my cheeks hanging out scientific papers.
Speaker 2: That's how we distribute science these days. We passed it
Speaker 2: out to OHI g mister, have you read white since
Speaker 2: latest paper? It's a hoot.
Speaker 1: But yeah, that's one way to think about it. But
Speaker 1: a lot of creative answers here. I guess it's sort
Speaker 1: of stelf a. But like I said, there's some ambiguity
Speaker 1: in the terms.
Speaker 2: Yes, as always.
Speaker 1: All right, well let's dig into it, Daniel. What is
Speaker 1: the extragalactic background light?
Speaker 2: Basically, the extragalactic background light, or EBL, as astronomers like
Speaker 2: to call it, is all the light emitted by everything
Speaker 2: else in the universe except for the Milky Way, during
Speaker 2: the entire history of the universe. So it's like all
Speaker 2: of the photons in the universe not emitted by our galaxy.
Speaker 1: Wait what but also not emitted by the background microwave
Speaker 1: background radiation.
Speaker 2: Now the CMB, the cosmic microwave background is part of
Speaker 2: the EBL. The EBL is like the super general version
Speaker 2: of the CMB. CMB is only at one wavelength, that's
Speaker 2: the cosmic microwave background. The EBL is like, well, what's
Speaker 2: the background in all of the wavelengths?
Speaker 1: Oh wait, wait, so the EBL is part of this CMB.
Speaker 2: No, the CMB is part of the EBL.
Speaker 1: Oh okay, but it doesn't come from galaxies, does it.
Speaker 2: It doesn't have to come from galaxy. This is the
Speaker 2: light emitted by everything else in the universe other than
Speaker 2: our galaxy. So extragalactic means not the Milky Way. So
Speaker 2: any other star, any other object, any frozen banana floating
Speaker 2: out there in space, any primordial soup of gas and plasma,
Speaker 2: everything in the whole history of the universe except for
Speaker 2: the Milky Way.
Speaker 1: Oh I see, this is not galactic light. It's like
Speaker 1: extra galactic in the sense of being outside of our gutsy.
Speaker 2: Yes, take all the photons in the universe and subtract
Speaker 2: the ones emitted by our galaxy the ones left over,
Speaker 2: that's the extragalactic background light. So like most of the
Speaker 2: photons in the universe are the EBL photons I see.
Speaker 1: So any photon that we see out there in space
Speaker 1: that doesn't come from a star within or any other
Speaker 1: source within our Milky Way galaxy exactly.
Speaker 2: And the estimate is that there's like ten to the
Speaker 2: eighty four extragalactic background photons in the universe. So there's
Speaker 2: a whole lot of them.
Speaker 1: Ten to the eighty four.
Speaker 2: Or ten with eighty four zeros in front of it.
Speaker 2: It's a lot.
Speaker 1: It sounds like a lot, But I don't know how
Speaker 1: many come from our galaxy.
Speaker 2: I mean, the whole history of the universe, the tiniest
Speaker 2: fraction come from our galaxy. Almost every single photon in
Speaker 2: the universe doesn't come from our galaxy. So almost every
Speaker 2: single photon in the universe is an EBL photon.
Speaker 1: Interesting, and this comes from other galaxies, or as we
Speaker 1: talked about, maybe directly from the Big Bang, right.
Speaker 2: It comes from the whole history of the universe, and
Speaker 2: that's what's super fascinating about it. Some of these photons
Speaker 2: were made before there were galaxies. Some of them were
Speaker 2: made before there were stars. Some of them were made
Speaker 2: during reionization, when these big clouds of neutral gas first
Speaker 2: started to clump together to form stars. The whole history
Speaker 2: of the universe is written in these photons. They're out there,
Speaker 2: they're floating around. They contain all of this information, but
Speaker 2: they're very, very tricky to see.
Speaker 1: Right, So, like we're getting the lighte from Andromeda, which
Speaker 1: is close to us, but we're also getting light from
Speaker 1: super far away, which also happens to be really a
Speaker 1: long time ago. That's all mixed in with this in
Speaker 1: this micro background.
Speaker 2: Like that's right, all that counts as EBL because it's
Speaker 2: not coming from the Milky Way.
Speaker 1: Maybe should be like extra Milky Way background light, right,
Speaker 1: because it's not just like extra galactic like in the
Speaker 1: general sense, it's just like our anything outside of our galaxy.
Speaker 2: Yeah, extra our galactic background light.
Speaker 1: Yeah, there you go. Yeah, because do so many in Andromeda,
Speaker 1: the EBL would be different.
Speaker 2: Right, Yeah, that's true. Those astronomers would argue with our
Speaker 2: astronomers about how to name this thing. That would be fun.
Speaker 1: Yeah, there a cartoonists would call my cartoonists. We would
Speaker 1: duke it out, and then I, you know, I would
Speaker 1: take them out and then I get to name it
Speaker 1: for the whole universe.
Speaker 2: You know, you wouldn't be the first cartoonist to actually
Speaker 2: name something scientific. You know, Gary Larson actually had an
Speaker 2: impact on science.
Speaker 1: Oh yeah, what did he name?
Speaker 2: Yes, this hilarious cartoon of a caveman giving a name
Speaker 2: to those four pokey spikes on the back of a
Speaker 2: status He calls it the phagomizer, after thag who was
Speaker 2: killed by a stegosaurus. And it turns out that nobody
Speaker 2: had actually named that before. So then scientists actually started
Speaker 2: using phagomizer in their science papers. And now it's the
Speaker 2: official name for the stegosaurus table.
Speaker 1: Whoa. See, Now that's a well named thing in science.
Speaker 1: You should put cartoonis in charge of everything.
Speaker 2: Right, Yes, we should name everything after the caveman that
Speaker 2: was killed by it.
Speaker 1: Yeah. No, I'm just saying trust cartoonists, you know, with
Speaker 1: anything science.
Speaker 2: That's the lesson I learned from that. Yeah for sure.
Speaker 1: All right, So this seems like a really broad concept
Speaker 1: all the light. Basically, it's all the light and universe
Speaker 1: we're getting from outside our galaxy. It seems like a lot.
Speaker 1: Can we make any sense of it? Or is it
Speaker 1: all just like a wash?
Speaker 2: It's a lot, and it contains an incredible amount of information,
Speaker 2: and it's really varied. It varies across the spectrum from
Speaker 2: like super high energy gamma rays produced by really distant
Speaker 2: active galactic nuclei like blazers, all the way down to
Speaker 2: like really long wavelength radio that might be produced by
Speaker 2: like dark matter decaying. There's an incredible amount of information
Speaker 2: across the spectrum, but we want to see across the spectrum.
Speaker 2: But we want to see it all the way across
Speaker 2: the spectrum, and we also want to see where it's
Speaker 2: coming from in the universe. So it's not just like
Speaker 2: let's just see it. Let's see where it's coming from
Speaker 2: and what the energy distributions are. Let's use that to
Speaker 2: learn about the history of the universe.
Speaker 1: Because I guess I wonder, like, how do you tell
Speaker 1: the difference if it's how do you know it's coming
Speaker 1: if it's coming from our galaxy or from outside of
Speaker 1: our galaxy.
Speaker 2: Yeah, that's really tricky because our galaxy turns out to
Speaker 2: be really bright. You might think, well, can't you just
Speaker 2: point your telescope in the night sky and gather some
Speaker 2: EBL photons. Yeah, but it's sort of like trying to
Speaker 2: see the Milky Way. If you're in times square. Times
Speaker 2: Square is so bright you can't even see any stars
Speaker 2: in the sky. So our Milky Way is so bright
Speaker 2: that we can't see the distant dim things that are
Speaker 2: hiding behind it. So there's a lot of competition from
Speaker 2: the Milky Way, and the Milky Way is not something
Speaker 2: we understand super well, so it's difficult to disentangle which
Speaker 2: photons come from the extragalactic background light and which photons
Speaker 2: come from our own Milky Way. There's an interplay there
Speaker 2: where if we knew really, really well what the extragalactic
Speaker 2: background light was, it would help us understand the Milky Way,
Speaker 2: Or if we understood the Milky Way better, we could
Speaker 2: subtract it from what we see in the night sky
Speaker 2: and understand better what the extragalactic light is. We would
Speaker 2: learn so much either way, But right now, the Milky
Speaker 2: Way outshines everything, and the whole thing is kind of entangled.
Speaker 2: It's a big mess.
Speaker 1: So when you look at the stars of the nice Guy,
Speaker 1: every star he sees is in the Milky Way galaxy, right,
Speaker 1: you can really see stars that are outside of the
Speaker 1: Milky Way. So any pinpoint you see out there is
Speaker 1: in the Milky Way galaxy. So if I wanted to
Speaker 1: see something outside of our galaxy, I would maybe point
Speaker 1: my discope at a spot between the stars. Absolutely, do
Speaker 1: you get raw light from outside of our galaxy or
Speaker 1: is there still like dust from our galaxy there that
Speaker 1: is maybe polluting that light?
Speaker 2: So the answer depends on the frequency of light that
Speaker 2: you're looking for. You're always looking through the Milky Way,
Speaker 2: and there's always going to be dust that interferes, But
Speaker 2: it depends on the wavelength. Some wavelengths of light can
Speaker 2: penetrate that dust, some wavelengths of light can't. So it's
Speaker 2: really a different puzzle at different wavelengths, from GAMER rays
Speaker 2: to X rays to ultraviolet light. There's different sources of
Speaker 2: photons that get confused between the Milky Way and the
Speaker 2: extragalactic background light. A big factor is the zodiacal light
Speaker 2: scattering of dust from within our solar system, which makes
Speaker 2: everything very tricky, all right.
Speaker 1: It sounds like we need to parse it by frequency,
Speaker 1: and so let's break down the spectrum of light from
Speaker 1: outside of our galaxy and see what it tells us
Speaker 1: about what's out there. But first let's take a quick break.
Speaker 1: All right, we're talking about the extra Milky Way background light.
Speaker 1: I just renamed it. I called the extra tag tag
Speaker 1: is a sizer background light.
Speaker 2: I'm not even gonnam. It's all up to you at
Speaker 2: this point.
Speaker 1: I'll just called the extra larsen or here here we go,
Speaker 1: we'll call it the extra far Side background line because
Speaker 1: and that is both true and a homage to Gary Larson. Yeah,
Speaker 1: absolutely right, because anything outside of our galaxy is technically
Speaker 1: on the far side of the universe.
Speaker 2: That's probably what he meant when he was talking about
Speaker 2: the far side. He meant the other galaxies.
Speaker 1: Yes, well, no, I don't think so. I'm saying in
Speaker 1: this context it seems like an appropriateate. But anyways, let's
Speaker 1: break it down by frequency, you said, because we're getting
Speaker 1: a wash by all this life from outside of our galaxy.
Speaker 1: We don't even know if it's coming from outside of
Speaker 1: our galaxy. But if you break it down by frequency,
Speaker 1: you can get a better handle on what's going on.
Speaker 2: And the main challenges are that most of this stuff
Speaker 2: is really dim because the sources are really distant. Everything
Speaker 2: we're talking about is coming from outside our galaxy, which
Speaker 2: means it's probably millions or billions of light years away,
Speaker 2: So it's photons have been spread out. They don't get tired,
Speaker 2: but they do get spread out. So everything out there
Speaker 2: is really really dim, or there are sources in the
Speaker 2: milky Way that are brighter than it, or there's like
Speaker 2: dust and stuff interfering. The challenge is change as you
Speaker 2: go through the frequency, just the same way that the
Speaker 2: night sky changes in frequency. If you look at the
Speaker 2: night sky in the optical or in the infrared or
Speaker 2: in the UV, you see a very different picture.
Speaker 1: All right, let's break it down, and let's start maybe
Speaker 1: with the higher frequencies. What do we see at the
Speaker 1: high frequencies.
Speaker 2: So at the very highest frequencies, the highest energy photons,
Speaker 2: we call these things gamma rays for silly historical reasons.
Speaker 1: Related to the Hulk, right, That's where it started.
Speaker 2: It started because in the early part of the century
Speaker 2: we didn't really understand quantum mechanics or radiation, so we
Speaker 2: just started naming things like alpha rays, beta rays, gamma rays.
Speaker 2: We had no idea what gamma rays were. Then later
Speaker 2: we understood, oh, they're just high energry photons, but they
Speaker 2: already had this name. So you can't just say, oh,
Speaker 2: gamma rays and X rays are really the same thing.
Speaker 2: There's artificial distinction between them. We just sort of.
Speaker 1: Stuck with it, and so they had the Hulk back
Speaker 1: at the Court of the century.
Speaker 2: Hulk was actually the one doing these science experiments. Remember,
Speaker 2: Bruce Banner is a scientist man. You should read his
Speaker 2: paper right now. They're great stuff.
Speaker 1: It's the famous correspondence between Bruce Banner and Albert Einstein
Speaker 1: right where they're like, pal come them.
Speaker 2: It was in German, though, right, wasn't it in German?
Speaker 1: I'm just messing with you, all right? Sorry, I keep going,
Speaker 1: keep going. What do we see in the gamma rays?
Speaker 2: So the gamma ray night sky is mostly surveyed by
Speaker 2: a space probe called Fermei LAT, which is basically a
Speaker 2: particle physics detector in space. Photons hid it and they
Speaker 2: convert into a pair of electrons and positrons and we
Speaker 2: track those. We can use that to measure the energy.
Speaker 2: This is our best way to see the night sky.
Speaker 2: In gamma rays. And if you look out in the
Speaker 2: night sky you do see a bunch of gamma rays.
Speaker 2: You see a huge source coming from the center of
Speaker 2: our galaxy. Like the center of the galaxy emits a
Speaker 2: very large number.
Speaker 1: And that's from the black hole there, right.
Speaker 2: That's from all sorts of crazy processes happening in the
Speaker 2: center of the Milky Way. There's pulsars that emit gamma rays.
Speaker 2: These are point sources. There's also just a lot of
Speaker 2: diffuse emission of gamma rays from like really high energy processes,
Speaker 2: like electrons getting accelerated really really hard and emitting gamma
Speaker 2: ray photons. The problem is that we don't really understand
Speaker 2: the center of the galaxy. So if you just look
Speaker 2: at like the distribution of gamma rays in the center
Speaker 2: of our galaxy, we don't understand it. We can't explain it.
Speaker 2: There's lots of mysteries there, and so that's a real problem.
Speaker 2: If you want to subtract out the Milky Ways contribution
Speaker 2: and look at the rest of the universe, you don't
Speaker 2: really understand what to subtract out.
Speaker 1: You just like point your telescope at the center of
Speaker 1: the galaxy and then point it away from the center
Speaker 1: of the galaxy to kind of get a sense of
Speaker 1: what's coming from the center and what is not.
Speaker 2: Yeah, you can measure what's happening at the center, but
Speaker 2: then you also want to understand how that varies across
Speaker 2: the night sky. To do that, you really need some
Speaker 2: sort of model so you can interpolate, so then when
Speaker 2: you're looking at some other place, you can say how
Speaker 2: much of this is from the center of the galaxy
Speaker 2: and how much isn't. You can't just like turn off
Speaker 2: the center of the galaxy in nature and see the
Speaker 2: rest of it, or turn it back on or vary it.
Speaker 2: So we need some sort of like understanding of the
Speaker 2: center of the galaxy in or to extrapolate away from
Speaker 2: it and like subtract it out from underneath the extragalactic background.
Speaker 1: Light, so we can study the extragalactic background, or so
Speaker 1: we can study the center of the galaxy or both both.
Speaker 2: Because mostly the extragalactic background light in the gamma rays
Speaker 2: we think is coming from the centers of other galaxies.
Speaker 2: Right other galaxies we think are also emitting gamma rays
Speaker 2: at high rates, and the extragalactic background light in the
Speaker 2: gamma rays is then mostly from those other galactic nuclei,
Speaker 2: and so if we could see those, we could understand
Speaker 2: our own galaxy better, or if we understood our own
Speaker 2: galaxy better, we could subtract it out and then understand
Speaker 2: those other galaxies. So it's sort of a chicken and
Speaker 2: egg problem. We don't really know how to pull this.
Speaker 1: Apart, all right, So then what can we see in
Speaker 1: the gamma rays when we look around us.
Speaker 2: Well, there's this long standing mystery, but the center of
Speaker 2: our galaxy, whether it's sending us signals of dark matter
Speaker 2: like dark matter, we know this a lot of it
Speaker 2: in the center of the galaxy, and we wonder if
Speaker 2: sometimes when two dark matter particles bounce off each other
Speaker 2: they actually annihilate, Like there might be dark matter and
Speaker 2: anti dark matter, and it might be possible for it
Speaker 2: to annihilate and actually produce photons. This is counterintuitive because
Speaker 2: you think of dark matter as dark, not shining in
Speaker 2: any electromagnetic spectrum. But there are theories where dark matter
Speaker 2: will annihilate itself and make very high energy photons. And
Speaker 2: in fact, there's a signal from the center of the
Speaker 2: galaxy that we don't understand that a bunch of people
Speaker 2: think is from dark matter. So we don't understand that
Speaker 2: very well, and we'd love to look for that signal
Speaker 2: in the centers of other galaxies, basically see if we
Speaker 2: can reproduce this in the extra galactic background light. But
Speaker 2: so far we have themen able to pull those things
Speaker 2: apart and understand which photons come from other galaxies.
Speaker 1: Wait, wait, wait, First of all, are you saying dark
Speaker 1: matter is not maybe really dark?
Speaker 2: Yeah, dark matter might be shining brightly from the center
Speaker 2: of our galaxy.
Speaker 1: Oh boy, And then can't we just point our telescope
Speaker 1: at another galaxy to see what kind of signal we
Speaker 1: get from.
Speaker 2: Then, yeah, we can do that, and we can see
Speaker 2: some other galaxies that are very clear, like galaxies with
Speaker 2: quasars in them or blazars, you know, quasars that are
Speaker 2: pointed right at us. Those are shooting really high energy
Speaker 2: photons right at us, and that we can tell like, Okay,
Speaker 2: it's definitely there. It's definitely there. So that's a part
Speaker 2: of the extragalactic background light that we can tell. But
Speaker 2: not every galaxy has an active nucleus, and we're interested
Speaker 2: in studying those that aren't active because those are the
Speaker 2: best ones for studying dark matter, but it's not always
Speaker 2: clear which photons are coming from our galaxy in which
Speaker 2: photons are coming from other galaxies, because remember we're inside
Speaker 2: the Milky Way, and not all of the gamma rays
Speaker 2: come directly from the center. Some of them are emitted
Speaker 2: along the galactic plane, and those are still brighter than
Speaker 2: the emissions from other.
Speaker 1: Galaxies emitted by what we are we this would be
Speaker 1: emitted by dark matter in the rim of the galaxy
Speaker 1: or what.
Speaker 2: Maybe dark matter in the room the galaxy. But anytime
Speaker 2: an electron is accelerated, it's going to emit a photon,
Speaker 2: and so there's some emission of photons from electrons in
Speaker 2: the galactic plane that cloud our observations.
Speaker 1: MM interesting. So looking at these gamma rays might reveal
Speaker 1: the what's going on with dark matter.
Speaker 2: Yeah, it would be super cool if we could make
Speaker 2: like a map of these gamma rays from other galaxies
Speaker 2: and then cross correlated with our understanding of like where
Speaker 2: the density of matter is, Like we have a pretty
Speaker 2: good understanding of where the galaxies are and the whole
Speaker 2: cosmic web. If we could cross correlate these gamma rays
Speaker 2: from like clumps of dark matter, then to be really
Speaker 2: powerful evidence that maybe these gamma rays really are coming
Speaker 2: from dark matter and not just from other sources of
Speaker 2: gamma rays.
Speaker 1: WHOA, would they have like a special signature if they
Speaker 1: came from dark matter?
Speaker 2: Unfortunately not, they're like energy distribution of these things is
Speaker 2: not that different from the energy distribution we see from
Speaker 2: other sources, which is what makes it so challenging and
Speaker 2: why it's so important to understand all the other sources
Speaker 2: of gamma rays so we can figure out which ones
Speaker 2: might be coming from dark matter. It's like you're trying
Speaker 2: to explain the spectrum with a few different blobs, but
Speaker 2: the blobs aren't that different, so it's hard to tell
Speaker 2: how much of each blob you need to use to
Speaker 2: explain the spectrum that you're seeing.
Speaker 1: WHOA. So, then if it turns out dark matter is shiny,
Speaker 1: would you need to change the name of it.
Speaker 2: Yes, and we'd come to you first.
Speaker 1: I'll call it dark light as ray star war Z.
Speaker 1: All right, Well, that's gamma rays, it might tell us
Speaker 1: about dark matter. What about the next range of frequencies
Speaker 1: in the spectrum of this background light?
Speaker 2: So taking a step down and energy, you get to
Speaker 2: X rays, and again there's just an arbitrary distinction between
Speaker 2: gamma rays and X rays, but X rays are lower energy,
Speaker 2: and here the technology is sort of like a bridge
Speaker 2: between particle physics detectors that see gamma rays and more
Speaker 2: traditional telescopes that see lower energy light. Here we have
Speaker 2: X ray telescopes, and these use like weird X ray
Speaker 2: optics because X rays are really energetic and really hard
Speaker 2: to bend using optics, So there's all sorts of weird
Speaker 2: tricks they use to try to gather and focus X rays.
Speaker 2: But we have a couple of cool space telescopes and
Speaker 2: New Star and Chandra up there observing the sky in
Speaker 2: the X ray What do.
Speaker 1: You mean they are hard to bend like that, You
Speaker 1: can't focus them. You can focus them with a lens.
Speaker 2: Yeah, exactly, because of the super high energy, they just
Speaker 2: don't bend very much through a lens, and so you
Speaker 2: need special techniques to shape these things, basically like wave
Speaker 2: guides and weird constructions. We had a whole episode about
Speaker 2: X ray telescopes and Chandra. Check it out for more
Speaker 2: details on how to build your own X ray telescope.
Speaker 1: Right, you can make them out of bones, right this,
Speaker 1: X rays don't go through bones.
Speaker 2: Yes, and we're calling the next one the Fred Flintstone Telescope. Exactly.
Speaker 1: Yeah, there is.
Speaker 2: We're gonna have a Stegosaurus.
Speaker 1: Operator after Hanna Barbara.
Speaker 2: Of course, if Hannah Barbera wanted a fund one, we
Speaker 2: would name it after them, for sure. There you go.
Speaker 1: I'm not sure there's still a.
Speaker 2: Round somebody owns that iph.
Speaker 1: Well you should follow the application.
Speaker 2: But the night sky in the X ray is really
Speaker 2: fascinating because this mostly comes from electrons. We were talking
Speaker 2: earlier about electrons emitting super high energy photons. They also
Speaker 2: emit X rays and this is a really cool German
Speaker 2: name for it. It's called bremstra Lung, which means breaking light.
Speaker 2: Since you have an electron it changes direction because it
Speaker 2: HiT's like a magnetic field or something, it has to
Speaker 2: give off a photon in order to do that, and
Speaker 2: based on the energy of those electrons the amount of curvature,
Speaker 2: how from, we give off X rays. So a lot
Speaker 2: of the night sky in X ray comes from these
Speaker 2: electrons giving off bremstralag.
Speaker 1: I mean, these are electrons that are just floating out
Speaker 1: there in space and if somehow they change direction, they
Speaker 1: emit an X ray.
Speaker 2: Yeah, exactly, electrons can change direction when they hit a
Speaker 2: magnetic field or if they like zoom around the black
Speaker 2: hole or something. Any sort of change of direction or
Speaker 2: change of velocity, an electron will emit a photon.
Speaker 1: And so the universe is just full of these electrons
Speaker 1: or what They're just floating out there like dust or
Speaker 1: are they like in galaxies or is this the stuff
Speaker 1: between galaxies?
Speaker 2: Electrons are everywhere, man, just thew way protons are. You know,
Speaker 2: most of the universe is hydrogen, but by that we
Speaker 2: mean protons and electrons, and often it's in plasma form.
Speaker 2: It's not neutral hydrogen, so there are also clouds of
Speaker 2: neutral hydrogen. But there's also just a lot of protons
Speaker 2: and electrons flying out there, both in galaxies and between galaxies.
Speaker 2: Remember that between galaxies is not as bright. There aren't stars,
Speaker 2: but a huge fraction of baryonic matter in the universe,
Speaker 2: meaning protons and electrons, is actually between the galaxies, not
Speaker 2: in the galaxies. So yeah, electrons are everywhere, and they're
Speaker 2: emitting X rays whenever they change direction.
Speaker 1: Do you consider this noise or is this part of
Speaker 1: what you want to see or is this getting in
Speaker 1: the way of the interesting things you want to see.
Speaker 2: This is definitely something you want to see because you
Speaker 2: want to understand all the sources of it. But it's
Speaker 2: really hard to pin down these sources. Some of it
Speaker 2: we can associate with the centers of galaxies, so like
Speaker 2: active galactic nuclei are pumping out these high energy X rays,
Speaker 2: but a lot of it we can't. I read one
Speaker 2: study that said that one percent of X rays can
Speaker 2: be associated with known objects. The rest of it, we're
Speaker 2: just like, we don't know what made this. So something
Speaker 2: out there in the universe is like shooting out X
Speaker 2: rays and we don't know what it is. Maybe it's
Speaker 2: just a bunch of more active galactic nuclei that have
Speaker 2: been like red shifted and are faint, but it could
Speaker 2: also be other weird stuff like early universe black holes,
Speaker 2: direct collapse black holes that formed during the early universe
Speaker 2: and emitted X rays.
Speaker 1: What do you mean only one percent, like one percent
Speaker 1: is coming from these electrons floating around, or we're just
Speaker 1: getting them from things that might have existed in the
Speaker 1: universe a long time ago that we can't sort of
Speaker 1: see it with their naked eye.
Speaker 2: Yeah, we can't associate them with anything we've seen, so
Speaker 2: We don't know what's making them. They could just be
Speaker 2: diffuse electrons. It could be a big chunk of it.
Speaker 2: There could also be a bunch of new astrophysical objects
Speaker 2: out there emitting X rays that we've just never seen
Speaker 2: before because we haven't been able to measure the X
Speaker 2: ray spectrum outside of our galaxy. So there could be
Speaker 2: these like direct collapse black holes that formed. Like remember
Speaker 2: we were talking about having the early universe with this
Speaker 2: famous moment when the universe became neutral. Protons and electrons
Speaker 2: came together to make hydrogen, and that was the Dark Ages.
Speaker 2: You have all this neutral hydrogen floating around, but there
Speaker 2: were no stars yet. At the end of that, there's
Speaker 2: a moment we call reionization, when the universe is then
Speaker 2: pulling those atoms apart again. That's when we think stars
Speaker 2: started to form. All possible that black holes formed at
Speaker 2: the same moment. They didn't just collapse into massive stars.
Speaker 2: Some clouds might have collapsed directly into black holes, and
Speaker 2: those formations we think left their imprint in the X
Speaker 2: ray spectrum. So if we could measure really really well,
Speaker 2: we might see hints of direct collapse black holes from
Speaker 2: the early universe.
Speaker 1: Oh I see like, this is stuff happened that happened
Speaker 1: a long time ago, but it happened so far away.
Speaker 1: We're only just now getting the evidence of these things
Speaker 1: that happened a long time ago, exactly.
Speaker 2: And it's very dim, much dimmer than X ray sources
Speaker 2: in our galaxy, and it's very hard to pull apart.
Speaker 2: So if we understood the X ray spectrum super duper well,
Speaker 2: we could ask questions like is there evidence in there
Speaker 2: for direct collapse black holes or not? But right now
Speaker 2: it's a big question mark. We don't know which photons
Speaker 2: come from our galaxy, which photons come from outside the galaxy.
Speaker 2: So we're like looking for a really tiny signal and
Speaker 2: we have big question.
Speaker 1: Marks, right, big ones. It ninety nine percent. We don't
Speaker 1: know what it is, question exactly. All right, let's go
Speaker 1: down to the next frequency. This is ultraviolet.
Speaker 2: Yeah, so ultraviolet photons are super interesting, and our best
Speaker 2: measurements of the ultraviolet extragalactic background light actually come from
Speaker 2: the voyager probes. Remember those probes we sent out into
Speaker 2: the Solar System to take pictures of the planets and
Speaker 2: then just continue on out into space. They had instruments
Speaker 2: on board for measuring ultraviolet photons because they were interested
Speaker 2: in the atmospheres of those planets and seeing ultraviolet light
Speaker 2: emitted from those planets to like study atmospheres and all
Speaker 2: sorts of planetary physics.
Speaker 1: WHOA. So even today we're still getting data from that spacecraft.
Speaker 2: I think actually Voyager just shut down. It ran out
Speaker 2: of power and we're no longer hearing from it. But
Speaker 2: until very recently we were getting measurements from Voyager. It's
Speaker 2: a really long lasting probe and it's one of our
Speaker 2: best ways to understand the UV night sky.
Speaker 1: Now what else can we see in this UV light?
Speaker 2: So we can't see very much unfortunately, because the galaxy
Speaker 2: is pretty bright in this UV light. Like planets emit
Speaker 2: in the UV. Neutral hydrogen in our galaxy absorbs this
Speaker 2: stuff and emits in the UV. But it's important for
Speaker 2: understanding the distribution of matter, Like we'd love to know
Speaker 2: more about the baryonic matter, the hydrogen that's between the galaxies.
Speaker 2: That's where most of the hydrogen in the universe is.
Speaker 2: If we could separate the UV light that's coming from
Speaker 2: within our galaxy from the UV light that's coming from
Speaker 2: outside the galaxy, then we could understand this better. But
Speaker 2: we don't really have great measurements here, Like Voyager was
Speaker 2: not set up to measure the extragalactic background light in
Speaker 2: the UV spectrum. It's just like the only thing we have.
Speaker 2: So it's if you look at the whole spectrum of
Speaker 2: extragalactic background light, there's like a big gap there in
Speaker 2: the UV because we really have almost no published studies
Speaker 2: at all. It's just like a big blank.
Speaker 1: But I guess what's making these you raise within our galaxy?
Speaker 2: So mostly it's clouds of neutral hydrogen. Hydrogen, Remember, is
Speaker 2: an atom, and electrons in the atom have certain energy levels,
Speaker 2: and one of those energy level transitions corresponds with the
Speaker 2: ultraviolet spectrum, and so neutral hydrogen tends to glow in
Speaker 2: many different spectrum but one of them is in the UV.
Speaker 1: UH just closed from being hot.
Speaker 2: Yeah, exactly. You think of space as cold, but a
Speaker 2: lot of this interstellar gas and intergalactic gas is actually
Speaker 2: quite hot in the sense that it has high velocity
Speaker 2: and each atom can have a significant amount of energy.
Speaker 2: Even if we know that if you went out there
Speaker 2: you'd freeze to death, you'd be surrounded by very sparse
Speaker 2: hot gas. And when we study this stuff, there's a
Speaker 2: lot that we don't understand, Like we can try to
Speaker 2: subtract the Milky Way contribution in the UV, and then
Speaker 2: there's all sorts of hints that other galaxies are emitting
Speaker 2: in the UV in ways that we don't understand, Like
Speaker 2: the Coma cluster is a famous puzzle. We don't understand
Speaker 2: the UV spectrum from the Coma cluster. What's going on there?
Speaker 2: Are those galaxies different from ours? Is there something else
Speaker 2: between us and that galaxy? Are we just not understanding
Speaker 2: the Milky Way contributions? It's a really open field to study.
Speaker 1: All right, pretty cool. I guess I wonder if it
Speaker 1: was healthy we stopped putting sunblock on all of our telscipes.
Speaker 2: Or directly on our eyeballs.
Speaker 1: Yeah, that never helps.
Speaker 2: I think that's a bad idea. Don't do that people.
Speaker 1: That's right, good health advice here on the Physics podcast.
Speaker 1: All right, let's get into our maybe more interesting frequency spectrum,
Speaker 1: the optical or visible light spectrum and infrared to see
Speaker 1: what is out there beyond the bounds of our galaxy.
Speaker 1: But first, let's take another quick break. Or we're talking
Speaker 1: about light that comes from outside of our galaxy, which
Speaker 1: turns out is like most of the light that we
Speaker 1: can see.
Speaker 2: Yeah, most of the photons in the universe didn't come
Speaker 2: from our galaxy. And yet those are the photons that
Speaker 2: are hardest to see because they're overwhelmed by the light
Speaker 2: from our galaxy.
Speaker 1: Right, because we're so close to our galaxy, we're in it.
Speaker 1: But also I feel like it's hard because like it's
Speaker 1: a big universe out there that's been around for a
Speaker 1: long time. So we're getting stuff at the same time,
Speaker 1: stuff that's closed, stuff that's far, stuff that's happening now,
Speaker 1: stuff that's happened, you know, fourteen billion years ago. So
Speaker 1: it's sort of like this big wash of light that
Speaker 1: we're getting that you're trying to sift through.
Speaker 2: Yeah, it's sort of amazing that you can understand any
Speaker 2: of it, you know, But the tricks we use are
Speaker 2: pretty basic. Look at the direction that came from, study
Speaker 2: it over time, study it as a function of energy.
Speaker 2: Using all those ideas, you can try to pull this
Speaker 2: apart to make a consistent picture of what's out there
Speaker 2: in the universe generating all these photons, and in the end,
Speaker 2: that's the goal. Come up with the whole history of
Speaker 2: the universe that can explain all the photons that we
Speaker 2: can see. But first you got to see those photons.
Speaker 1: Right, or I guess you can see them, you just
Speaker 1: don't know which what it is that you're looking at.
Speaker 2: Yeah, we see a bunch of photons. We'd love to
Speaker 2: explain them all, and we're hoping that some of those
Speaker 2: photons give us hints about what's outside our galaxy, not
Speaker 2: just what's inside our galaxy.
Speaker 1: Right. Well, so we've been going through different frequency rate
Speaker 1: and just in the light that we get from outside
Speaker 1: of our galaxy and how they can reveal different things,
Speaker 1: and we're down to the visible light spectrum, like what
Speaker 1: you could see with the naked eye.
Speaker 2: Yeah, exactly. And so in the optical of course, we're
Speaker 2: very curious about what's out there in the universe, and
Speaker 2: a lot of the light that's in the optical spectrum
Speaker 2: comes from stars. Right. The optical spectrum exists because we
Speaker 2: evolve to be able to see light from our sun,
Speaker 2: which makes us also able to see with our eyeballs
Speaker 2: directly light from other stars, and of course there are
Speaker 2: stars outside the Milky Way, and so a lot of
Speaker 2: the optical light that's in the extra galactic background light
Speaker 2: is emitted by stars in other galaxies.
Speaker 1: Right, because our sun is a pretty typical star in
Speaker 1: the universe. Like, what the kind of light that our
Speaker 1: sun puts out is pretty tychopical all stars in the universe.
Speaker 2: It's not that unusual. Our star is actually on the
Speaker 2: larger side compared to your typical star. The most common
Speaker 2: kind of star in the universe is a red dwarf,
Speaker 2: which is a little redder and dimmer than our star,
Speaker 2: but it's not a big deal. Red dwarfs are still
Speaker 2: mostly in.
Speaker 1: Optical Like, if we had been born under a red sun,
Speaker 1: we would maybe you have different eyeball right.
Speaker 2: Yeah, exactly, life could be very different if we evolved
Speaker 2: around a red dwarf. We have a whole episode about
Speaker 2: like how unlikely it is for life to have evolved
Speaker 2: around a weird star like the Sun.
Speaker 1: All right, So you're saying most of the visible light
Speaker 1: that we get from outside the galaxy comes from stars
Speaker 1: that are outside the galaxy, and these are mostly in
Speaker 1: other galaxies, right, Like, there aren't a lot of stars
Speaker 1: floating around, not in galaxies.
Speaker 2: Yeah, there aren't a lot of stars out there, but
Speaker 2: there are some, you know, there are stars in like
Speaker 2: extended halos of galaxies or that were stripped out from
Speaker 2: the galaxies during a merger or something and are now
Speaker 2: just like floating out there in space. And that's something
Speaker 2: we'd really like to understand. How much light is coming
Speaker 2: from outside our galaxy, but also outside the galaxies we
Speaker 2: can identify, you know, from between galaxies.
Speaker 1: WHOA wait, you mean there could be a sun out
Speaker 1: there in between galaxies all by itself with maybe a
Speaker 1: planet orbiting around it with life. And what would they
Speaker 1: see in the night sky?
Speaker 2: They would only see galaxies, right, so their night sky
Speaker 2: would be much much darker. They would only see smudges,
Speaker 2: no pinpoints of light. Oh, they wouldn't see stars. They
Speaker 2: wouldn't know that their sun is maybe just another star. Necessarily,
Speaker 2: it's maybe unlikely because a star that experiences those kind
Speaker 2: of forces would probably also lose its planets and then
Speaker 2: to the extreme gravity being like tossed out of a galaxy.
Speaker 2: But it's possible that the planets come along for the ride.
Speaker 1: Yeah, it makes me a little sad because they wouldn't
Speaker 1: be able to wish upon a star.
Speaker 2: So you might think that this is like the easiest
Speaker 2: extragalactic background like to see because you just like point
Speaker 2: the hubble or point your eyeball between galaxies and see
Speaker 2: what's there. But our galaxies actually really bright in this
Speaker 2: kind of light, not just from the stars, also from
Speaker 2: the scattering of dust. We talked about zodiacal light that
Speaker 2: you can see from Earth. It's like at sunset you
Speaker 2: can see this like a cone of light, like a
Speaker 2: hazy pyramid just above the sunrise or sunset. But this
Speaker 2: is the scattering of light off of dust in our
Speaker 2: Solar system, and it's exactly at the frequency we want
Speaker 2: to use to observe the extragalactic background light. So it's
Speaker 2: a big problem.
Speaker 1: You mean, like the dust that's out there in space
Speaker 1: reflects visible light exactly, but it must reflect other kinds
Speaker 1: of lights as well, doesn't it, Or only does it
Speaker 1: reflect visible light especially well.
Speaker 2: It reflects visible light especially well. It has to do
Speaker 2: with like the size of these dust grains that are
Speaker 2: like usually one to a few hundred microns, and these
Speaker 2: are a huge cloud of dust all through the center
Speaker 2: of the Solar system. It extends out we think, like
Speaker 2: just past Mars, and it might all be Mars's fault. Actually,
Speaker 2: dust storms on Mars might be kicking up a lot
Speaker 2: of this stuff, but it reflects light typically in the
Speaker 2: optical and also in the infrared, and it makes it
Speaker 2: really really hard to see the extragalactic background light.
Speaker 1: Well, this is it, I mean it. This makes it
Speaker 1: hard to see even the galactic light, right because all
Speaker 1: this dust pollution is within our solar system.
Speaker 2: Yeah, exactly, it's a big problem for seeing outside of
Speaker 2: our solar system. You're right, Even understanding our galaxy, the
Speaker 2: zodiacal light is a big.
Speaker 1: Issue, all right, Well, what can we tell from the
Speaker 1: light that is coming from outside of our galaxy in
Speaker 1: the visible spectrum?
Speaker 2: Well, if you look at all the light that's coming
Speaker 2: from outside of our galaxy, we can't explain it. Like
Speaker 2: there's a bunch of photons that are out there that
Speaker 2: just don't match our predictions. Like if you try to say, here,
Speaker 2: I understand what's in the universe. Let me predict all
Speaker 2: the photons we'll see in the optical spectrum, and then
Speaker 2: you compare that to what we do see, there's a
Speaker 2: big gap. Like there must be something wrong with our
Speaker 2: modeling of what's out there in the universe or how
Speaker 2: it emits. Like the data and our predictions do not agree.
Speaker 1: But what do you mean the gap like we're missing light.
Speaker 2: Yeah, there are more optical photons in the extragalactic background
Speaker 2: then we can explain. So that means either there's something
Speaker 2: else out there emitting photons we haven't accounted for it,
Speaker 2: or maybe we've underestimated the amount of scattering from this
Speaker 2: dust the zodiacal background light. But there are photons out
Speaker 2: there that we can't explain.
Speaker 1: Well, meaning like we look out there and we were
Speaker 1: getting light, but there's no object there to see.
Speaker 2: Yeah, a lot of this stuff is diffuse, right, We
Speaker 2: don't know what's out there emitting it. We can't associate
Speaker 2: it with any points source, and so we don't know
Speaker 2: what diffuse sources of this light there are out there
Speaker 2: in the galaxy. Maybe there are more of these like
Speaker 2: rogue stars out there in the middle of the galaxy
Speaker 2: and all their light like adds up to this big
Speaker 2: diffuse component to explain it. Or maybe it's something simpl
Speaker 2: or like misunderstanding the Milky Way.
Speaker 1: Whoa wait, So maybe this mysterious light is just Milky
Speaker 1: Way light. It's not necessarily extra galacti.
Speaker 2: Yeah, exactly. We can't quite pull it apart. But there's
Speaker 2: a really cool recent technique they're using to try to
Speaker 2: get a sense for what's outside of our galaxy. So
Speaker 2: they're can to help pull this thing apart, and that's
Speaker 2: by using even higher energy photons that come from quasars.
Speaker 2: So like the super high energy gamma rays we were
Speaker 2: talking about earlier that are emitted from the centers of
Speaker 2: very distant galaxies. We think we understand the spectrum that
Speaker 2: should be coming from them, but as they travel through
Speaker 2: the universe, sometimes they inter with lower energy photons they
Speaker 2: can get like scattered or reabsorbed or change to another energy.
Speaker 2: So if you look at the spectrum that comes from
Speaker 2: those distant blazers and you see how it's modified from
Speaker 2: what we expect, you can use that to try to
Speaker 2: like map how many extragalactic background photons they ran into
Speaker 2: along the way.
Speaker 1: Wait wait, wait, light can run into light. I thought
Speaker 1: light couldn't collide with other light particles.
Speaker 2: No, you're exactly right. In general, light does not interact
Speaker 2: with light because photons do not have electric charges, so
Speaker 2: they don't interact directly, but they can interact indirectly, like
Speaker 2: a photon can pair produce turn into an electron and
Speaker 2: a positron.
Speaker 1: Momentarily like randomly.
Speaker 2: Yeah, every photon has a probability to pair produce at
Speaker 2: any moment, and so there's a probability for two photons
Speaker 2: to interact. We did whole episode about light beams crossing
Speaker 2: and how you can actually study this. It's a rare process,
Speaker 2: but it does happen. It requires this indirect process through
Speaker 2: the electron field.
Speaker 1: Okay, So then by looking at a source like equasor
Speaker 1: that we know kind of pretty well and see how
Speaker 1: the light is modified when it gets to us. By
Speaker 1: the time it gets to us, you can sort of
Speaker 1: get a sense of what's out there in between.
Speaker 2: Yeah, you can try. It's tricky, like, first of all,
Speaker 2: you have to be very confident that you understand the
Speaker 2: unattenuated light, the light that was emitted by the quays are,
Speaker 2: and then compare it to what we see. Then you
Speaker 2: also have to convince yourself you understand everything else that
Speaker 2: could happen to those photons be absorbed or influenced by
Speaker 2: other things in the universe. That's why they like to
Speaker 2: use these very high energy sources because they tend to
Speaker 2: interact less than other photons, so they're more pristine.
Speaker 1: Oh. Interesting, And again it's sort of weird that we're
Speaker 1: getting all this light and we don't know where it's
Speaker 1: coming from.
Speaker 2: Yeah, exactly, But it's cool to be using these like
Speaker 2: pencil beams of super high energy photons to get a
Speaker 2: measure of like the other low energy photons along the way.
Speaker 2: It's like we're getting information about photons that would never
Speaker 2: have reached Earth.
Speaker 1: Pretty cool, all right.
Speaker 3: Now.
Speaker 1: The last frequency range is the low frequency range of light,
Speaker 1: and that's the infrared.
Speaker 2: Yeah. The lowest frequency range is interesting because you know,
Speaker 2: it's dominated by the microwaves, which we've studied very very
Speaker 2: well and talked about and it's sort of like a
Speaker 2: good example of what you can learn just by looking
Speaker 2: at the night sky in the microwave. We've learned so
Speaker 2: much about the early universe because there were microwaves emitted
Speaker 2: in the very early universe, this moment when protons and
Speaker 2: electrons came together in the universe became transparent again. Those
Speaker 2: photons are still around and we've captured them and measured
Speaker 2: things about the early universe. Really revolutionized all of cosmology.
Speaker 2: That's just like a taste of what we could learn
Speaker 2: from the other spectra. The microwaves, though, were like thirty
Speaker 2: or forty times brighter than every other wavelength. For the
Speaker 2: extragalactic background light, it's like the brightest part of the spectrum,
Speaker 2: which is why it's sort of like the easy thing.
Speaker 2: The microwaves are like the first by the d apple, But.
Speaker 1: We skipped over i infrared though, didn't we? In terms
Speaker 1: of frequency, infrared is higher frequency than microwaves.
Speaker 2: Yeah, absolutely, Infrared is higher energy than microwaves. And the
Speaker 2: cosmic infrared background is also super fascinating and we'd love
Speaker 2: to study it in more detail because it might have
Speaker 2: might be rich with information. Infrared light has lots of
Speaker 2: really interesting point sources, like star forming regions and galaxies
Speaker 2: at very high redshift that have been red shifted into
Speaker 2: the infrared. Super interesting to study. And this was actually
Speaker 2: studied by the same satellite that measured the cosmic microwave
Speaker 2: background light. There was an instrument on board that was
Speaker 2: capable of picking up infrared light, but it's much dimmer
Speaker 2: than the microwave light, and so it's much more difficult
Speaker 2: to study.
Speaker 1: Well, I wonder if all these things just kind of
Speaker 1: get smooched together, because you know, I know we've talked
Speaker 1: about that things that are really far away. They might
Speaker 1: emit light, but by the time that light gets to it,
Speaker 1: because of the expanding universe, that light gets red shifted,
Speaker 1: it becomes more red er. So like if you get
Speaker 1: red light, it could come from basically anything, right.
Speaker 2: Yeah, absolutely, you could have super high energy galactic nuclei
Speaker 2: emitting very high energy photons and by the time they
Speaker 2: get to us there infrared like even the cosmic microwave
Speaker 2: background light. Those photons are super long wavelength, but when
Speaker 2: they were emitted, they weren't. The plasma that made them
Speaker 2: was super high energy. They were admitted a very very
Speaker 2: high frequency. It's only the expansion of the universe that
Speaker 2: stretched them out all the way down to the microwave.
Speaker 2: So you're right, everything is piled on top of each
Speaker 2: other a minute at one way.
Speaker 1: Is this like the messiest kind of light we're getting?
Speaker 1: Do you know what I mean? Like, because everything piles
Speaker 1: onto that frequency spectrum as opposed to like the higher frequencies. Thinks,
Speaker 1: don't get bluer.
Speaker 2: Yeah, it's a beautiful mess down there at the long
Speaker 2: wave lengths because everything's red shifted down there into the
Speaker 2: dustbin of the universe.
Speaker 1: Now, this gets us into the microwave range, which we
Speaker 1: talked a lot about before. The cosmic microwave background, which
Speaker 1: you said is included in this idea of the extragalactic background.
Speaker 2: Exactly because it's generated by plasma that's outside the Milky Way,
Speaker 2: and so it's definitely extra galactic. It's also the brightest
Speaker 2: part of the spectrum, and so it's easiest to tackle,
Speaker 2: and it tells us a lot about the very early universe.
Speaker 2: So that's why it's sort of the best well known
Speaker 2: and the best well studied.
Speaker 1: So then the stuff we get that we call this
Speaker 1: CMB the cosmic microwave background, we know for sure what
Speaker 1: it is. We always say it's light from the beginning
Speaker 1: of the universe. How well do we actually know that?
Speaker 1: Wouldn't it be kind of confounded or mixed together with
Speaker 1: you know, distance, stars exploding and things like that.
Speaker 2: Absolutely, there are other sources in the microwave, including sources
Speaker 2: from the Milky Way, but everything is easier when you're
Speaker 2: looking for a bigger signal, right, it's easier to establish,
Speaker 2: it's easier to subtract a way. Uncertainties in the Milky
Speaker 2: Way can be larger without affecting your measurements because you
Speaker 2: have a larger signal you're looking for.
Speaker 1: What do you mean it's a bigger signal, Like it's
Speaker 1: more powerful, or just a wavelength is bigger.
Speaker 2: I mean it's more powerful. There are more photons, like
Speaker 2: there are forty times as many photons in the microwave
Speaker 2: than there are in the radio or in the UV
Speaker 2: or in the optical. The universe is brighter in the
Speaker 2: microwaves than they are any other spectrum.
Speaker 1: Yes, because of the Big Bang, because of the Big
Speaker 1: Bang was a huge source of it?
Speaker 2: Or is it because of this early universe plasma? Yeah,
Speaker 2: not technically the Big Bang, but there's a lot of
Speaker 2: emission in the very early universe, and that and then
Speaker 2: an and that got slid all the way down by
Speaker 2: the expansion of the universe to the microwave. And so
Speaker 2: it's sitting there as a very bright.
Speaker 1: Signal and we sort of know what it is. But
Speaker 1: then that means it also mean that one big source
Speaker 1: the beginning of the universe is drowning out anything else
Speaker 1: we might want to see in the microwave.
Speaker 2: Absolutely, and that's why we've been studying into great detail
Speaker 2: and trying to understand all the ripples in it and
Speaker 2: the other sources of it. Again, when you have a
Speaker 2: brighter signal, just everything is easier scientifically. You have more
Speaker 2: data to play with, so you can do more tricks
Speaker 2: like extrapolating from one region to the other. You have
Speaker 2: better ways to validate all of your models. It's much
Speaker 2: much more difficult when you're dealing with faint sources that
Speaker 2: you're not even sure you're seeing. So seeing that in
Speaker 2: the microwave and seeing it exactly the temperature we expected
Speaker 2: was a great confirmation of our understanding of that whole process.
Speaker 1: Mm interesting and it also heats up barburritos, all right.
Speaker 1: So then the last frequency range is the radio waves,
Speaker 1: which is like the lowest frigency, longest wavelength light that's
Speaker 1: out there exactly.
Speaker 2: And these are really cool experiments to see radio emissions
Speaker 2: from the deep universe. We have these balloon experiments. Well,
Speaker 2: you have like a radio antenna, but you cool it
Speaker 2: down using liquid helium, and then you'll launch it up
Speaker 2: like thirty seven kilometers above Texas or sometimes above the
Speaker 2: South Pole, so we can gather radio signals from space.
Speaker 2: You shield it so that it's not just like getting
Speaker 2: your local NPR station, and you cool it down so
Speaker 2: it can pick up the most faint signals, and then
Speaker 2: you try to gather radio signals from deep space. We
Speaker 2: measure a lot of these things, but we don't understand
Speaker 2: all the radio waves that we see. Maybe some of
Speaker 2: them are from dark matter colliding and emitting in the
Speaker 2: radio Maybe there's some other diffuse emission of radio. Maybe
Speaker 2: it's just something in the galactic foreground, something else in
Speaker 2: the galaxy that's emitting in the radio waves we don't
Speaker 2: yet understand. So we don't actually know if these photons
Speaker 2: that we're seeing are EBL radio photons or just vanilla
Speaker 2: milky way photons.
Speaker 1: If they have an actual like source generating them, or
Speaker 1: they're just kind of like noise. Is that what you mean.
Speaker 2: Yeah, if there's a point source generating all these radio
Speaker 2: frequency photons, we probably would have figured that out, could
Speaker 2: identify it with like the center of the galaxy or
Speaker 2: some black hole or some pulsar or something. So that
Speaker 2: would have been easy, and we haven't been able to
Speaker 2: do that, which means probably it's something diffuse, like maybe
Speaker 2: dark matter and the whole halo colliding with itself or
Speaker 2: something else. We don't really understand the sources here. WHOA.
Speaker 1: All right, So to recap we're getting a lot of
Speaker 1: lights from the universe. Only about one percent of that
Speaker 1: light comes from our galaxy. Ninety nine percent of the
Speaker 1: light that we see that when you look at the
Speaker 1: nice sky comes from outside of the galaxy. And it
Speaker 1: seems like ninety eight percent of that is all mystery light,
Speaker 1: Like we don't know what is making that light, where's
Speaker 1: it coming from? Right, That's kind of what it seems like.
Speaker 2: Well, most of the light in the universe is generated
Speaker 2: outside our galaxy, but most of the light that we
Speaker 2: see is generated inside our galaxy because we're inside our galaxy.
Speaker 2: So most of the light that we see in the
Speaker 2: night sky is coming from the Milky Way. But that's
Speaker 2: a tiny fraction of all the photons in the universe,
Speaker 2: and so the rest of the universe is quite dim
Speaker 2: in comparison to the Milky Way. But that's most of
Speaker 2: the interesting stuff, and these photons contain the whole history
Speaker 2: of the universe, but they're mostly outshined by the brightness
Speaker 2: of the Milky Way, which just you know, happens to
Speaker 2: be nearby.
Speaker 1: Oh, I see, there's a lot of light out there,
Speaker 1: but we don't get all of it is what you're saying.
Speaker 1: Because we're so close the Milky Way galaxy, most of
Speaker 1: the light that we get goes from the Milky Way.
Speaker 2: Yeah, it's like you're standing right next to a lighthouse,
Speaker 2: and so you can't really see anything that's far away.
Speaker 2: Even if those photons are coming to you, they're mostly
Speaker 2: outshined by the local sources.
Speaker 1: But well, we can't see the cosmic background outside of
Speaker 1: our galaxy. It's all sort of shrouded in mystery, it seems.
Speaker 2: Yeah, it's tricky to disentangle the Milky Way from the
Speaker 2: other sources. There's a lot of photons we don't understand,
Speaker 2: a lot of question marks. Over the next few years,
Speaker 2: we're hoping to turn on more sensitive instruments that can
Speaker 2: disentangle these things make at our measurements. Maybe this extragalactic
Speaker 2: background light's gonna come into sharper relief, and it might
Speaker 2: teach us things about the universe. It could reveal all
Speaker 2: sorts of weird stuff going on in the deep reaches
Speaker 2: of space and in the deep history of time.
Speaker 1: Yeah, and about the makeup of the universe as well.
Speaker 1: If it tells us about dark matter and maybe dark energy.
Speaker 1: I think Daniel. Maybe the only solution here is that
Speaker 1: you're gonna have to leave the galaxy to get a
Speaker 1: good look at this light. I'll pack a lot of children.
Speaker 1: Do we have an ex make an extra Daniel, extra
Speaker 1: galactic Daniel adventure here.
Speaker 2: That's going to take a very very long time.
Speaker 1: I'll report we got time, We got time, right.
Speaker 2: I gotta make it back before next week's episode. Man,
Speaker 2: I better build a fast ship.
Speaker 1: Yeah, there you go. Well, first of all, invent the
Speaker 1: faster than light space ship, and then then we're talking.
Speaker 1: Sounds good, all right? Well, another reminder of how much
Speaker 1: of the mystery we still have to observe and discover
Speaker 1: and figure out. There are still lots of mysteries out there,
Speaker 1: even in the light that bathes us every night, every day,
Speaker 1: all around the Earth.
Speaker 2: That's right, There is so much of the universe we
Speaker 2: have not yet observed or understood, so much left to
Speaker 2: discover for you young scientists out there, the next generation
Speaker 2: of curious explorers.
Speaker 1: Or the old ones, do right.
Speaker 2: I'm just going to retire and let everybody else figure
Speaker 2: it out and explain it to me at this point.
Speaker 1: Well, hopefully they do it in time, because come on,
Speaker 1: we're not getting any younger. Daniel, all right, well, we
Speaker 1: hope you enjoyed that. Thanks for joining us, See you
Speaker 1: next time.
Speaker 2: For more science and curiosity, come find us on social media,
Speaker 2: where we answer questions and post videos. We're on Twitter, This, Org,
Speaker 2: Instant and now TikTok. Thanks for listening and remember that
Speaker 2: Daniel and Jorge Explain the Universe is a production of iHeartRadio.
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