1
00:00:08,440 --> 00:00:11,200
Speaker 1: Hey, Daniel, most of the stuff in the universe is

2
00:00:11,520 --> 00:00:12,560
Speaker 1: dark matter, isn't that right?

3
00:00:12,640 --> 00:00:15,760
Speaker 2: Yeah, it's about eighty percent of the matter in the universe.

4
00:00:16,079 --> 00:00:18,720
Speaker 1: And it's all around us, like right here with us

5
00:00:18,960 --> 00:00:20,119
Speaker 1: immersed in dark matter.

6
00:00:20,239 --> 00:00:21,880
Speaker 2: Yeah, we are swimming in it.

7
00:00:22,040 --> 00:00:24,239
Speaker 1: And it's also inside of us, is it?

8
00:00:24,320 --> 00:00:27,360
Speaker 2: Yeah? It passes through us, it doesn't bounce off your skin.

9
00:00:27,480 --> 00:00:30,479
Speaker 1: Does that mean that we're partly made out of dark matter? Oh?

10
00:00:30,640 --> 00:00:33,000
Speaker 2: I guess. So that's kind of dark to think about.

11
00:00:33,159 --> 00:00:36,360
Speaker 1: So if I discover myself, I'm discovering dark matter.

12
00:00:36,360 --> 00:00:39,200
Speaker 2: Know thyself. Win a Nobel Prize.

13
00:00:38,880 --> 00:00:41,720
Speaker 1: Or at least a dark Nobel Prize. Technically all Nobel

14
00:00:41,720 --> 00:00:43,560
Speaker 1: prizes have dark matter inside of them too.

15
00:00:43,640 --> 00:00:45,760
Speaker 2: Maybe they'll give you the cash in dark money.

16
00:00:45,880 --> 00:00:47,000
Speaker 1: Ooh, that got dark quick.

17
00:01:02,280 --> 00:01:02,440
Speaker 3: Hi.

18
00:01:02,480 --> 00:01:05,080
Speaker 1: I'm Hoeham and cartoonists and the author of Oliver's Great

19
00:01:05,120 --> 00:01:05,880
Speaker 1: Big Universe.

20
00:01:06,160 --> 00:01:08,959
Speaker 2: Hi. I'm Daniel. I'm a particle physicist at CERN and

21
00:01:09,040 --> 00:01:12,559
Speaker 2: a professor at UC Irvine. And I'll take your money

22
00:01:12,760 --> 00:01:13,479
Speaker 2: dark or light.

23
00:01:14,000 --> 00:01:16,480
Speaker 1: Really, you'll take dark money. Isn't that dangerous for a

24
00:01:16,560 --> 00:01:17,399
Speaker 1: tenured professor?

25
00:01:17,480 --> 00:01:20,160
Speaker 2: No, that's what ten years for man. I can take money.

26
00:01:19,880 --> 00:01:26,000
Speaker 1: From us visuals. I guess technically, the university takes the

27
00:01:26,040 --> 00:01:27,479
Speaker 1: money and you get a cut of it.

28
00:01:27,920 --> 00:01:30,319
Speaker 2: I never really had to worry about that. Like, nobody

29
00:01:30,319 --> 00:01:33,720
Speaker 2: who got wealthy with dubious techniques ever offered me a

30
00:01:33,760 --> 00:01:36,440
Speaker 2: slush fund, So I've never had to really grapple with

31
00:01:36,480 --> 00:01:36,960
Speaker 2: that question.

32
00:01:37,200 --> 00:01:39,080
Speaker 1: Wait, how do you know? I mean, you've taken money.

33
00:01:39,480 --> 00:01:42,800
Speaker 1: Have you done your due diligence? How deep did you go?

34
00:01:44,160 --> 00:01:46,840
Speaker 2: You know, almost all my research money comes from the government,

35
00:01:46,880 --> 00:01:49,200
Speaker 2: and we all know the various crimes that the government

36
00:01:49,240 --> 00:01:50,000
Speaker 2: has committed.

37
00:01:50,280 --> 00:01:53,040
Speaker 1: Exactly all government money is dark money.

38
00:01:53,200 --> 00:01:55,320
Speaker 2: M do you believe taxes are theft? Is that where

39
00:01:55,320 --> 00:01:55,840
Speaker 2: this is going?

40
00:01:57,560 --> 00:02:00,560
Speaker 1: Well, technically, or funding comes from taxpayers and there must

41
00:02:00,600 --> 00:02:03,160
Speaker 1: be some sketchy tax payers out there, so your work

42
00:02:03,200 --> 00:02:03,720
Speaker 1: is painted.

43
00:02:04,200 --> 00:02:07,480
Speaker 2: So you're saying drug dealers who are paying taxes and

44
00:02:07,560 --> 00:02:11,320
Speaker 2: indirectly funding my research have made me complicit in their crimes.

45
00:02:11,520 --> 00:02:15,120
Speaker 1: That's right. Yes, assuming drug dealers pay taxes, which I

46
00:02:15,160 --> 00:02:17,520
Speaker 1: guess they do. If they're laundering money, I don't know.

47
00:02:17,680 --> 00:02:19,760
Speaker 1: I feel like we need a whole podcast episode just

48
00:02:19,840 --> 00:02:21,520
Speaker 1: to cover dark money.

49
00:02:21,720 --> 00:02:23,840
Speaker 2: Yeah, and then I suppose everybody's guilty.

50
00:02:23,960 --> 00:02:26,520
Speaker 1: That's right, We're all guilty of paying taxes, I guess.

51
00:02:27,400 --> 00:02:30,160
Speaker 1: But anyways, Welcome to our podcast Daniel and Jorge Explain

52
00:02:30,200 --> 00:02:33,160
Speaker 1: the Universe, a production of iHeartRadio.

53
00:02:32,520 --> 00:02:35,080
Speaker 2: In which we try not to overtax your brain while

54
00:02:35,120 --> 00:02:38,399
Speaker 2: explaining all of the mysteries of the universe. We want

55
00:02:38,440 --> 00:02:41,360
Speaker 2: to reveal the light universe and the dark universe, the

56
00:02:41,440 --> 00:02:44,320
Speaker 2: visible and the invisible. We want to show you how

57
00:02:44,320 --> 00:02:47,040
Speaker 2: the universe is not just what you see around you

58
00:02:47,120 --> 00:02:50,440
Speaker 2: and experience day to day, but so much more, so

59
00:02:50,639 --> 00:02:54,280
Speaker 2: much deeper, so many more mysteries waiting to be solved.

60
00:02:54,440 --> 00:02:57,000
Speaker 1: That's why we explore the universe, the dark corners of it,

61
00:02:57,080 --> 00:03:00,400
Speaker 1: the light corners, and the sketchy corners of it, because

62
00:03:00,480 --> 00:03:04,359
Speaker 1: our understanding of the universe is still a little bit sketchy, like.

63
00:03:04,320 --> 00:03:05,960
Speaker 2: The Mafia corners of the universe.

64
00:03:06,160 --> 00:03:08,120
Speaker 1: No, like, not clearly drawn.

65
00:03:08,560 --> 00:03:10,600
Speaker 2: Oh I see, I thought we're gonna be talking about

66
00:03:10,600 --> 00:03:12,160
Speaker 2: like the physics of New Jersey or something.

67
00:03:12,680 --> 00:03:16,360
Speaker 1: Yeah, there's the only some dark matters going on out there.

68
00:03:17,240 --> 00:03:19,280
Speaker 2: How do you eat all those rich cookies and not

69
00:03:19,360 --> 00:03:20,760
Speaker 2: gain weight? Really, it's amazing.

70
00:03:20,919 --> 00:03:24,359
Speaker 1: Yeah, Well, I think the secret is dark chocolate. It's

71
00:03:24,400 --> 00:03:25,239
Speaker 1: lower and fat.

72
00:03:25,000 --> 00:03:26,080
Speaker 2: Isn't it?

73
00:03:26,120 --> 00:03:26,400
Speaker 1: Is it?

74
00:03:26,480 --> 00:03:27,320
Speaker 2: Really? I don't know.

75
00:03:27,840 --> 00:03:31,840
Speaker 1: You're like, what, what, Why aren't I eating more dark chocolate.

76
00:03:33,440 --> 00:03:35,760
Speaker 2: I'm not really stopping myself from eating dark chocolate. But

77
00:03:35,760 --> 00:03:38,040
Speaker 2: that wasn't the reason I don't see dark chocolate is

78
00:03:38,080 --> 00:03:39,240
Speaker 2: like diet chocolate.

79
00:03:40,680 --> 00:03:43,119
Speaker 1: It's chocolate light. Oh no, wait, it's chocolate dark.

80
00:03:43,320 --> 00:03:46,960
Speaker 2: It's dark chocolate exactly. But there are lots of fun

81
00:03:47,000 --> 00:03:49,880
Speaker 2: questions out there in the universe, not just about how

82
00:03:49,960 --> 00:03:53,120
Speaker 2: waste management organizations in New Jersey are getting their money

83
00:03:53,200 --> 00:03:56,400
Speaker 2: and the calories that Daniel's eating. We're wondering about the

84
00:03:56,400 --> 00:03:59,560
Speaker 2: biggest questions in the universe, like where is all the stuff?

85
00:03:59,600 --> 00:04:01,440
Speaker 2: What is most of the universe made out of? But

86
00:04:01,520 --> 00:04:03,839
Speaker 2: how has it shaped the night sky that we see

87
00:04:03,880 --> 00:04:05,600
Speaker 2: today in the galaxy that we live in.

88
00:04:05,800 --> 00:04:08,160
Speaker 1: That's right, because what the universe is made out of

89
00:04:08,280 --> 00:04:10,280
Speaker 1: is maybe one of the biggest questions we can ask

90
00:04:10,320 --> 00:04:13,000
Speaker 1: about the universe. What is this whole place made out of?

91
00:04:13,320 --> 00:04:15,600
Speaker 1: What are we made out of? Are we made out

92
00:04:15,640 --> 00:04:18,279
Speaker 1: of dark matter? Can we exist without dark matter?

93
00:04:18,320 --> 00:04:20,320
Speaker 2: And we talk on this podcast a lot about dark matter.

94
00:04:20,360 --> 00:04:22,600
Speaker 2: We have lots of episodes about what it is and

95
00:04:22,640 --> 00:04:25,200
Speaker 2: where it is and how it works. And one thing

96
00:04:25,240 --> 00:04:28,000
Speaker 2: we often stress is that dark matter is part of

97
00:04:28,040 --> 00:04:31,400
Speaker 2: our galaxy, that most of our galaxy is actually dark

98
00:04:31,440 --> 00:04:33,159
Speaker 2: matter that has played a big role in the formation

99
00:04:33,240 --> 00:04:35,800
Speaker 2: of our galaxy. We wouldn't be here without it.

100
00:04:35,960 --> 00:04:38,799
Speaker 1: Yeah, we do talk a lot about dark matter. Daniel,

101
00:04:38,800 --> 00:04:41,960
Speaker 1: What percentage of our episodes would you say we talk

102
00:04:42,000 --> 00:04:44,960
Speaker 1: about dark matter? Is it representative of the amount of

103
00:04:45,040 --> 00:04:47,720
Speaker 1: dark matter in the universe? Are we ignoring dark matter

104
00:04:47,760 --> 00:04:48,120
Speaker 1: in a way?

105
00:04:48,320 --> 00:04:52,120
Speaker 2: Is dark matter underrepresented matter on podcasts? That's a good question.

106
00:04:52,320 --> 00:04:55,080
Speaker 1: Yeah, well, especially our podcast. If we're trying to explain

107
00:04:55,120 --> 00:04:58,720
Speaker 1: the universe and the universe is twenty seven percent dark matter.

108
00:04:58,600 --> 00:05:00,480
Speaker 2: You know, we might reach twenty seven percent of our

109
00:05:00,520 --> 00:05:02,760
Speaker 2: podcast being about dark matter. It's been a lot of them,

110
00:05:03,160 --> 00:05:05,120
Speaker 2: and I'm pretty sure we don't have two thirds of

111
00:05:05,160 --> 00:05:06,880
Speaker 2: them about dark energy. Maybe we should.

112
00:05:07,160 --> 00:05:10,479
Speaker 1: Yeah, yeah, that's what everyone is asking for more dark

113
00:05:10,560 --> 00:05:13,080
Speaker 1: energy episodes. But it is a pretty important part of

114
00:05:13,120 --> 00:05:16,039
Speaker 1: the universe. It's a pretty important part of our existence

115
00:05:16,080 --> 00:05:19,520
Speaker 1: because without dark matter, maybe galaxies would not have formed

116
00:05:19,880 --> 00:05:22,120
Speaker 1: the same way that they formed. The Milky Way would

117
00:05:22,200 --> 00:05:23,760
Speaker 1: might not be the same way it is now.

118
00:05:23,839 --> 00:05:26,080
Speaker 2: But we're always tempted to over general life to say

119
00:05:26,120 --> 00:05:28,640
Speaker 2: that the way we live in our certain situation over

120
00:05:28,680 --> 00:05:31,479
Speaker 2: here is the way the whole universe works. It's important

121
00:05:31,480 --> 00:05:33,840
Speaker 2: to take a step back and to ask whether our

122
00:05:33,920 --> 00:05:36,880
Speaker 2: way of life and our way of galaxying is the

123
00:05:36,920 --> 00:05:38,120
Speaker 2: only way that it can be.

124
00:05:38,480 --> 00:05:40,800
Speaker 1: And so today on the podcast, we'll be asking the question,

125
00:05:45,960 --> 00:05:50,080
Speaker 1: are there galaxies without dark matter? Would you call these

126
00:05:50,279 --> 00:05:53,839
Speaker 1: light galaxies then? Or galaxies light diet galaxies?

127
00:05:54,080 --> 00:05:56,880
Speaker 2: I call them tragic galaxies because they're probably galaxies where

128
00:05:56,880 --> 00:05:58,880
Speaker 2: everybody only eats white chocolate.

129
00:05:59,560 --> 00:06:03,479
Speaker 1: Or milk chocolate, some chocolate snobs. I also call that

130
00:06:03,520 --> 00:06:04,520
Speaker 1: a tragedy.

131
00:06:04,400 --> 00:06:06,159
Speaker 2: Also known as crimes against chocolate.

132
00:06:06,320 --> 00:06:08,400
Speaker 1: Are you saying Hershey's is dark money as well?

133
00:06:10,520 --> 00:06:11,840
Speaker 2: Her She's his garbage man?

134
00:06:12,080 --> 00:06:20,120
Speaker 1: Oh my gosh. And there goes our sponsorship deal with Hershe's.

135
00:06:20,200 --> 00:06:23,000
Speaker 1: We were so close to funding this thing for the

136
00:06:23,040 --> 00:06:25,279
Speaker 1: next twenty years, and you have to go and insult them.

137
00:06:25,720 --> 00:06:28,720
Speaker 2: Maybe you could hear my hesitation there. I'm being torn

138
00:06:28,760 --> 00:06:31,680
Speaker 2: between being honest and truthful on a hard science podcast

139
00:06:32,080 --> 00:06:34,640
Speaker 2: and pandering to our sponsors, and I just had to

140
00:06:34,680 --> 00:06:37,160
Speaker 2: be honest about it. I passed on hershe'es. I'd rather

141
00:06:37,160 --> 00:06:38,480
Speaker 2: have no chocolate than Hershe's.

142
00:06:38,560 --> 00:06:41,800
Speaker 1: That was pretty harsh to garbage. Oh my gosh. Well,

143
00:06:41,800 --> 00:06:43,800
Speaker 1: here here's the thing, Like when you eat it. Let's

144
00:06:43,800 --> 00:06:45,960
Speaker 1: say you eat like a chocolate cake at a restaurant.

145
00:06:46,000 --> 00:06:48,240
Speaker 1: How do you know they didn't use Hershey's chocolate.

146
00:06:48,360 --> 00:06:51,360
Speaker 2: You can taste it. Man, that's sour affront to chocolate.

147
00:06:51,640 --> 00:06:54,080
Speaker 2: It's turned so many people off of chocolate when chocolate

148
00:06:54,120 --> 00:06:57,279
Speaker 2: is this wonderful, amazing thing. Even milk chocolate can be

149
00:06:57,400 --> 00:07:01,640
Speaker 2: high quality, can be amazing. What they may in Pennsylvania. O, man,

150
00:07:01,880 --> 00:07:02,480
Speaker 2: it's a crime.

151
00:07:03,200 --> 00:07:05,919
Speaker 1: Now you're insulting the whole state of Pennsylvania.

152
00:07:07,000 --> 00:07:08,920
Speaker 2: Let's keep going. See how many people I can offend.

153
00:07:09,160 --> 00:07:12,760
Speaker 1: Yeah, yeah, let's moves and let's insult the whole universe.

154
00:07:13,080 --> 00:07:15,760
Speaker 2: Why don't you No, Fortunately, most of the universe is

155
00:07:15,840 --> 00:07:18,760
Speaker 2: dark matter and therefore appreciates dark chocolate, and so we're good.

156
00:07:19,280 --> 00:07:21,200
Speaker 2: We have the majority firmly on our side.

157
00:07:21,320 --> 00:07:24,440
Speaker 1: But doesn't Hershey's own some fancy brands like Scharfenberger or

158
00:07:24,480 --> 00:07:25,320
Speaker 1: something like that.

159
00:07:25,440 --> 00:07:26,400
Speaker 2: I don't know, I hope not.

160
00:07:26,600 --> 00:07:29,120
Speaker 1: Well, maybe you've been eating Hershey' chocolates all this time.

161
00:07:29,440 --> 00:07:35,080
Speaker 1: A plot twist. Anyways, we're talking about dark matter and

162
00:07:35,200 --> 00:07:39,400
Speaker 1: galaxies and could there exist galaxies without dark matter. These

163
00:07:39,480 --> 00:07:42,680
Speaker 1: would be like galaxies that don't have any dark matter

164
00:07:42,760 --> 00:07:43,760
Speaker 1: in them or around them.

165
00:07:43,960 --> 00:07:47,600
Speaker 2: Yeah, exactly, just stars and gas and dust and black holes,

166
00:07:47,760 --> 00:07:49,040
Speaker 2: no dark matter, all.

167
00:07:49,000 --> 00:07:51,440
Speaker 1: Right, Well, whether they are tragic or not. We were

168
00:07:51,440 --> 00:07:53,800
Speaker 1: wondering how many people out there had wondered about this

169
00:07:53,920 --> 00:07:56,760
Speaker 1: question and if they have any ideas about the answer.

170
00:07:56,800 --> 00:07:59,800
Speaker 2: Thanks very much to everybody out there who answers these questions,

171
00:08:00,080 --> 00:08:03,160
Speaker 2: they or not there supporters of Hershey's Crimes against Chocolate

172
00:08:03,360 --> 00:08:05,960
Speaker 2: would really appreciate everything you do. If you would like

173
00:08:06,000 --> 00:08:08,520
Speaker 2: to join this group, just write to me two questions

174
00:08:08,560 --> 00:08:10,480
Speaker 2: at Danielandjorge dot com.

175
00:08:10,560 --> 00:08:12,760
Speaker 1: I feel like maybe there's a Hurshi's employee out there

176
00:08:12,760 --> 00:08:16,320
Speaker 1: who listens to our podcast and it's now very very sad.

177
00:08:16,520 --> 00:08:18,720
Speaker 2: There's an easy fix for that, find a new job

178
00:08:19,880 --> 00:08:21,880
Speaker 2: or a new podcast. No, you've got to go to

179
00:08:21,880 --> 00:08:22,760
Speaker 2: the root of the problem.

180
00:08:22,880 --> 00:08:24,960
Speaker 1: That's right anyway. So think about it for a second.

181
00:08:24,960 --> 00:08:27,920
Speaker 1: Do you think there can be galaxies without dark matter?

182
00:08:28,600 --> 00:08:29,800
Speaker 1: Here's what people had to say.

183
00:08:30,520 --> 00:08:34,040
Speaker 4: I feel like there probably aren't, only because I know

184
00:08:34,120 --> 00:08:36,760
Speaker 4: that dark matter, as far as I know, is distributed

185
00:08:36,760 --> 00:08:38,600
Speaker 4: pretty much evenly throughout the universe. I think it tends

186
00:08:38,600 --> 00:08:41,320
Speaker 4: to be clustered in galaxies, but I think it tends

187
00:08:41,320 --> 00:08:42,760
Speaker 4: to be pretty uniform. So I would be surprised if

188
00:08:42,760 --> 00:08:43,960
Speaker 4: there were galaxies without dark matter.

189
00:08:44,040 --> 00:08:47,200
Speaker 5: I'm going to say why not, because, like going back

190
00:08:47,240 --> 00:08:51,360
Speaker 5: to an episode about uranium on Uranus, there could be

191
00:08:51,400 --> 00:08:54,440
Speaker 5: a tiny bit here and there in a galaxy. So

192
00:08:55,559 --> 00:08:58,760
Speaker 5: I'm going to say I don't know why there wouldn't

193
00:08:58,800 --> 00:09:01,520
Speaker 5: be dark matter in a galaxy. I'm also going to

194
00:09:01,520 --> 00:09:02,880
Speaker 5: say I don't know why there would.

195
00:09:03,160 --> 00:09:06,360
Speaker 3: I don't believe there are galaxies without dark matter because

196
00:09:06,520 --> 00:09:10,160
Speaker 3: dark matter is, in my understanding, a general term for

197
00:09:11,120 --> 00:09:15,160
Speaker 3: unknown matter, which is this and makes up the overwhelming

198
00:09:15,240 --> 00:09:19,440
Speaker 3: majority of the universe. Therefore, I think it is not

199
00:09:19,600 --> 00:09:22,400
Speaker 3: possible for galaxies not to have at least some dark

200
00:09:22,440 --> 00:09:23,080
Speaker 3: matter in them.

201
00:09:23,160 --> 00:09:25,800
Speaker 6: I'm not sure that we've observed any galaxies without dark matter,

202
00:09:25,840 --> 00:09:28,520
Speaker 6: but I suppose anything could be possible in this crazy universe.

203
00:09:29,080 --> 00:09:31,840
Speaker 6: It's also possible that all galaxies are without dark matter

204
00:09:31,880 --> 00:09:33,880
Speaker 6: and we just don't understand gravity all right.

205
00:09:33,960 --> 00:09:35,720
Speaker 1: Most people are skeptical about this question.

206
00:09:35,920 --> 00:09:38,680
Speaker 2: Yeah, people have the idea that dark matter is everywhere.

207
00:09:38,679 --> 00:09:41,320
Speaker 1: It's inescapable, you can't get away from it. It seems

208
00:09:41,559 --> 00:09:43,800
Speaker 1: in our question, although one person has kind of said,

209
00:09:43,800 --> 00:09:45,360
Speaker 1: why not, that's a good attitude to have.

210
00:09:45,640 --> 00:09:48,360
Speaker 2: Yeah, that's the whole attitude about physics, Like, well, maybe

211
00:09:48,360 --> 00:09:51,240
Speaker 2: everything is different from what we've boughter. Maybe there's something

212
00:09:51,280 --> 00:09:53,440
Speaker 2: really weird out there that could teach us something new

213
00:09:53,480 --> 00:09:54,280
Speaker 2: about the universe.

214
00:09:54,679 --> 00:09:56,720
Speaker 1: I do feel like that is a guiding question in

215
00:09:56,840 --> 00:10:00,520
Speaker 1: theoretical physics at least. Why not? Yes? Why not?

216
00:10:00,880 --> 00:10:04,000
Speaker 2: Sure? Maybe everything is just tiny cats at the quantum scale.

217
00:10:04,120 --> 00:10:06,360
Speaker 1: That's right, Maybe everything's just made out of Hersy's charga.

218
00:10:06,600 --> 00:10:11,040
Speaker 2: No, I got to hop somewhere else in the multiverse.

219
00:10:11,040 --> 00:10:13,480
Speaker 1: If that's the case, all right, Well let's get going

220
00:10:13,520 --> 00:10:17,560
Speaker 1: before this podcast gets too dark. Daniel, give us the basics.

221
00:10:17,600 --> 00:10:20,000
Speaker 1: What is dark matter? For those of us who haven't

222
00:10:20,040 --> 00:10:22,600
Speaker 1: listened to the twenty seven percent of our podcast.

223
00:10:22,280 --> 00:10:26,160
Speaker 2: Episodes, dark matter is fascinating because we simultaneously know a

224
00:10:26,200 --> 00:10:29,960
Speaker 2: lot about it and very little. Like we know that

225
00:10:30,000 --> 00:10:32,320
Speaker 2: there's a lot of dark matter in the universe, and

226
00:10:32,360 --> 00:10:35,200
Speaker 2: we know that it's matter. We know there's something out

227
00:10:35,240 --> 00:10:39,000
Speaker 2: there that's creating gravity or curvature of space time, but

228
00:10:39,120 --> 00:10:41,880
Speaker 2: that it's invisible. It doesn't glow, it doesn't give off light.

229
00:10:41,920 --> 00:10:45,240
Speaker 2: It doesn't reflect light. We sense it only because we

230
00:10:45,320 --> 00:10:49,080
Speaker 2: see its gravitational effects on stuff. It's curving space, which

231
00:10:49,160 --> 00:10:53,000
Speaker 2: changes how things are moving through that space. We see

232
00:10:53,040 --> 00:10:56,240
Speaker 2: galaxies rotating much faster than their gravity would be able

233
00:10:56,280 --> 00:10:59,000
Speaker 2: to hold them together if there wasn't also dark matter

234
00:10:59,080 --> 00:11:01,280
Speaker 2: in them holding them together. On the other hand, we

235
00:11:01,320 --> 00:11:03,720
Speaker 2: don't know what stuff it is. We know that it

236
00:11:03,760 --> 00:11:06,160
Speaker 2: has gravity, so it's matter, but we don't know it's

237
00:11:06,200 --> 00:11:08,760
Speaker 2: some weird new kind of neutrino or a totally different

238
00:11:08,840 --> 00:11:11,520
Speaker 2: kind of particle we've never seen before, or a thousand

239
00:11:11,559 --> 00:11:14,880
Speaker 2: new kinds of particles, or something that's not even a particle.

240
00:11:15,360 --> 00:11:17,000
Speaker 2: So we know a lot about it on the sort

241
00:11:17,000 --> 00:11:20,520
Speaker 2: of cosmological scale, but very very little or almost nothing

242
00:11:20,559 --> 00:11:22,440
Speaker 2: about it at the particle level. Right.

243
00:11:22,559 --> 00:11:24,440
Speaker 1: Dark matter is this kind of mysterious stuff in the

244
00:11:24,520 --> 00:11:27,240
Speaker 1: universe that we kind of feel its presence. We can

245
00:11:27,280 --> 00:11:30,240
Speaker 1: see its presence through gravity, but as you said, you

246
00:11:30,280 --> 00:11:33,760
Speaker 1: can't see it because it doesn't interact with elechormagnetic light

247
00:11:33,920 --> 00:11:36,120
Speaker 1: or maybe any of the other forces in the universe,

248
00:11:36,160 --> 00:11:37,319
Speaker 1: and so you can't see it. And that's why you

249
00:11:37,360 --> 00:11:38,640
Speaker 1: call it dark exactly.

250
00:11:38,800 --> 00:11:40,640
Speaker 2: And a bunch of listeners write in with the idea

251
00:11:40,679 --> 00:11:43,959
Speaker 2: that maybe dark matter is matter in another universe that's

252
00:11:43,960 --> 00:11:47,440
Speaker 2: somehow leaking in to ours. Remember that dark matter is

253
00:11:47,440 --> 00:11:50,880
Speaker 2: creating gravity in our universe, or changing the curvature of

254
00:11:50,920 --> 00:11:54,400
Speaker 2: space in our universe, which means that it's in that space. Right,

255
00:11:54,400 --> 00:11:57,360
Speaker 2: it shares that space with us, which kind of means

256
00:11:57,400 --> 00:12:00,680
Speaker 2: that it's in our universe. We really do know that

257
00:12:00,760 --> 00:12:03,920
Speaker 2: dark matter is something in our universe that's changing the

258
00:12:03,960 --> 00:12:06,760
Speaker 2: shape of space. We can only see sort of indirectly

259
00:12:06,840 --> 00:12:09,800
Speaker 2: through gravity, which is really frustrating because gravity is the

260
00:12:09,840 --> 00:12:12,720
Speaker 2: worst way to see things. It's so weak that it

261
00:12:12,760 --> 00:12:14,040
Speaker 2: makes it very, very difficult.

262
00:12:14,720 --> 00:12:17,760
Speaker 1: But wait, couldn't it bend our space and not be

263
00:12:17,880 --> 00:12:18,640
Speaker 1: part of our space.

264
00:12:18,920 --> 00:12:21,440
Speaker 2: That's possible if you overthrow general relativity in our entire

265
00:12:21,520 --> 00:12:25,240
Speaker 2: understanding of space time. General relativity says that matter tells

266
00:12:25,320 --> 00:12:28,120
Speaker 2: space how to bend, and space tells matter how to move,

267
00:12:28,640 --> 00:12:31,920
Speaker 2: and that means matter in our space. Like in general relativity,

268
00:12:32,120 --> 00:12:34,800
Speaker 2: the curvature or space comes from the energy density in

269
00:12:35,000 --> 00:12:38,240
Speaker 2: that space. So if you have some like parallel space

270
00:12:38,400 --> 00:12:42,160
Speaker 2: overlaid on top of it, which can also bend that space,

271
00:12:42,280 --> 00:12:43,680
Speaker 2: then it seems to me like it would be part

272
00:12:43,679 --> 00:12:46,160
Speaker 2: of our space, you know, sort of like the by definition,

273
00:12:46,720 --> 00:12:49,240
Speaker 2: but yet you could augment or throw out general relativity

274
00:12:49,320 --> 00:12:51,839
Speaker 2: replace it with something totally different. But the simpler idea

275
00:12:52,000 --> 00:12:53,679
Speaker 2: is that it's just some kind of mass we can't

276
00:12:53,720 --> 00:12:56,240
Speaker 2: see that explains almost everything we see out there in

277
00:12:56,240 --> 00:12:58,959
Speaker 2: the universe. So it's sort of the best going explanation.

278
00:12:59,000 --> 00:13:02,120
Speaker 2: You can always make more or baroque complicated explanations if

279
00:13:02,120 --> 00:13:02,480
Speaker 2: you like.

280
00:13:02,640 --> 00:13:05,520
Speaker 1: Well, it kind of might as well be in another universe, right,

281
00:13:05,559 --> 00:13:07,720
Speaker 1: because if it doesn't feel a lot of the same

282
00:13:07,760 --> 00:13:10,120
Speaker 1: force as we don't, it's just kind of like ghostly

283
00:13:10,160 --> 00:13:12,679
Speaker 1: matter that's kind of living on top of us. There

284
00:13:12,760 --> 00:13:14,880
Speaker 1: might be beings made out of dark matter, right.

285
00:13:14,960 --> 00:13:17,360
Speaker 2: Physicists call the different sectors of the universe. If you

286
00:13:17,400 --> 00:13:19,880
Speaker 2: have like two different sets of particles that don't interact

287
00:13:19,880 --> 00:13:22,520
Speaker 2: at all except for through gravity, then we call those

288
00:13:22,559 --> 00:13:24,640
Speaker 2: like the lights of the visible sector and the dark

289
00:13:24,679 --> 00:13:27,440
Speaker 2: sector of the universe. And that's totally possible that you

290
00:13:27,480 --> 00:13:30,280
Speaker 2: could have a whole complicated physics happening in the dark

291
00:13:30,280 --> 00:13:33,480
Speaker 2: sector that we can't see. Now. Mostly we know that

292
00:13:33,600 --> 00:13:36,600
Speaker 2: dark matter can't interact with itself. If it did interact

293
00:13:36,600 --> 00:13:39,160
Speaker 2: with itself, it would form all sorts of complicated structure

294
00:13:39,240 --> 00:13:41,839
Speaker 2: and do all sorts of interesting things. We think that

295
00:13:41,960 --> 00:13:45,319
Speaker 2: dark matter is pretty spread out. However, there could be

296
00:13:45,360 --> 00:13:47,680
Speaker 2: a little component dark dark matter. Dark matter may be

297
00:13:47,720 --> 00:13:50,720
Speaker 2: lots of different kinds of things, and one little component

298
00:13:50,760 --> 00:13:53,319
Speaker 2: of it might be more complicated and do complicated things

299
00:13:53,360 --> 00:13:56,839
Speaker 2: like form life or ice cream, cones or cats or

300
00:13:56,960 --> 00:13:59,880
Speaker 2: good chocolate without violating what we've seen dark matter do,

301
00:14:00,040 --> 00:14:01,680
Speaker 2: which is mostly spread out smoothly.

302
00:14:01,880 --> 00:14:04,840
Speaker 1: All right, Well, the question here today is can there

303
00:14:04,880 --> 00:14:08,800
Speaker 1: be galaxies without dark better? Why is this even a question? Like,

304
00:14:08,840 --> 00:14:10,800
Speaker 1: are most of the galaxies that we see out there?

305
00:14:10,840 --> 00:14:12,080
Speaker 1: Do they all have dark matter?

306
00:14:12,200 --> 00:14:14,040
Speaker 2: Yeah, the galaxies that you see out there in the

307
00:14:14,080 --> 00:14:17,280
Speaker 2: sky are like tracers. They're basically telling you where the

308
00:14:17,400 --> 00:14:20,760
Speaker 2: dark matter is in the universe. Remember that dark matter

309
00:14:20,840 --> 00:14:23,360
Speaker 2: is not something we can see, but it also dominates

310
00:14:23,360 --> 00:14:26,600
Speaker 2: the universe. Our estimates are that four fifths of the

311
00:14:26,640 --> 00:14:29,760
Speaker 2: matter in the universe is dark matter. So if you

312
00:14:29,920 --> 00:14:32,120
Speaker 2: like spin the wheel and pick a random object in

313
00:14:32,120 --> 00:14:34,320
Speaker 2: the universe, most likely you're going to get dark matter.

314
00:14:34,440 --> 00:14:38,000
Speaker 2: It's like overwhelmingly dark matter. So when the universe is

315
00:14:38,040 --> 00:14:41,000
Speaker 2: forming its structure and the gravity that determines like where

316
00:14:41,000 --> 00:14:43,600
Speaker 2: things are going to be. It's mostly the gravity of

317
00:14:43,680 --> 00:14:46,920
Speaker 2: dark matter that decides where things are going to clump

318
00:14:46,920 --> 00:14:49,640
Speaker 2: together and where things are not going to clump together. Remember,

319
00:14:49,640 --> 00:14:52,480
Speaker 2: the very early universe is mostly smooth, with a few

320
00:14:52,680 --> 00:14:56,160
Speaker 2: little blobs that are denser than others. The gravity of

321
00:14:56,160 --> 00:14:59,240
Speaker 2: those over dense pieces pull things together to form structure,

322
00:14:59,240 --> 00:15:00,960
Speaker 2: and that's where you get like galaxies in one part

323
00:15:01,000 --> 00:15:03,200
Speaker 2: of space and not galaxies in another part of space.

324
00:15:03,560 --> 00:15:06,440
Speaker 2: Where you have galaxies is where you had more dark

325
00:15:06,480 --> 00:15:09,920
Speaker 2: matter to pull that stuff together to form those galaxies.

326
00:15:10,120 --> 00:15:13,280
Speaker 1: Yeah, that's pretty wild to think that something we can't see,

327
00:15:13,320 --> 00:15:17,000
Speaker 1: that is invisible to us basically kind of dictates the

328
00:15:17,160 --> 00:15:19,720
Speaker 1: entire structure of the universe, right at least at the

329
00:15:19,760 --> 00:15:24,400
Speaker 1: galaxy level. Does dark matter also dictate things like superclusters

330
00:15:24,440 --> 00:15:26,320
Speaker 1: and beyond exactly?

331
00:15:26,720 --> 00:15:29,400
Speaker 2: And so you can imagine like these invisible wells, like

332
00:15:29,480 --> 00:15:32,880
Speaker 2: dark matter's curving space with Shepherd's the other kind of

333
00:15:32,920 --> 00:15:34,680
Speaker 2: matter together. So every time you look up at the

334
00:15:34,760 --> 00:15:37,280
Speaker 2: night sky and you see a galaxy, you should imagine

335
00:15:37,280 --> 00:15:40,840
Speaker 2: there's an invisible blob of dark matter surrounding that galaxy.

336
00:15:40,880 --> 00:15:44,000
Speaker 2: There's a whole halo that's created the conditions to form

337
00:15:44,080 --> 00:15:46,920
Speaker 2: that galaxy. We run simulations, for example, of a universe

338
00:15:46,920 --> 00:15:50,440
Speaker 2: without dark matter, and it doesn't form galaxies after fourteen

339
00:15:50,440 --> 00:15:53,360
Speaker 2: billion years. So quite literally, we would not be alive

340
00:15:53,400 --> 00:15:54,280
Speaker 2: without dark matter.

341
00:15:54,680 --> 00:15:57,320
Speaker 1: And now is the same true for things like superclusters

342
00:15:57,640 --> 00:16:01,080
Speaker 1: and those giant bubbles of galaxies they're in the universe.

343
00:16:01,320 --> 00:16:03,920
Speaker 1: Is that dictated by dark matter as well? Or is

344
00:16:03,960 --> 00:16:07,040
Speaker 1: that more of the quantum fluctuations of the universe.

345
00:16:07,160 --> 00:16:10,360
Speaker 2: Well, both, because there's this cosmic web that tells us

346
00:16:10,360 --> 00:16:12,800
Speaker 2: where dark matter will be denser and where dark matter

347
00:16:12,840 --> 00:16:15,200
Speaker 2: will not be dense. These filaments in some places they

348
00:16:15,240 --> 00:16:18,480
Speaker 2: overlap and you get these wells where things pool together.

349
00:16:18,600 --> 00:16:21,600
Speaker 2: And so the whole cosmic web is dictated by dark

350
00:16:21,640 --> 00:16:24,240
Speaker 2: matter and the light matter. The normal matter, baryons and

351
00:16:24,280 --> 00:16:27,640
Speaker 2: quarks and electrons just follows that. And so it's not

352
00:16:27,680 --> 00:16:30,920
Speaker 2: like a supercluster has a super halo. A supercluster is

353
00:16:30,960 --> 00:16:35,000
Speaker 2: made of galaxies connected together by these filaments.

354
00:16:34,600 --> 00:16:36,760
Speaker 1: So it also has filaments of dark matter.

355
00:16:37,000 --> 00:16:40,400
Speaker 2: Absolutely, yes, there are filaments of dark matter connecting these

356
00:16:40,440 --> 00:16:42,840
Speaker 2: halos of around each galaxy.

357
00:16:43,080 --> 00:16:43,320
Speaker 3: Hmmm.

358
00:16:43,840 --> 00:16:46,280
Speaker 1: Interesting. All right, Well, let's dig into the question of

359
00:16:46,320 --> 00:16:50,840
Speaker 1: whether a galaxy can be clean of dark matter, or

360
00:16:50,880 --> 00:16:53,800
Speaker 1: whether it's kind of a requirement for a galaxy to form.

361
00:16:53,960 --> 00:16:55,760
Speaker 1: So let's dig into that, but first let's take a

362
00:16:55,800 --> 00:17:10,840
Speaker 1: quick break. All Right, we're asking the question, can you

363
00:17:10,880 --> 00:17:13,639
Speaker 1: have a galaxy without dark matter? Could you maybe have

364
00:17:13,720 --> 00:17:15,760
Speaker 1: a scripy little galaxy out there? That was like, no,

365
00:17:15,840 --> 00:17:17,920
Speaker 1: I don't care about dark matter. I'm just going to

366
00:17:18,240 --> 00:17:21,880
Speaker 1: gather all these all this gas and dust and on

367
00:17:21,920 --> 00:17:24,760
Speaker 1: my own without any help. That's kind of what we're asking.

368
00:17:24,440 --> 00:17:26,320
Speaker 2: Today, right, Yeah, exactly.

369
00:17:26,720 --> 00:17:29,560
Speaker 1: And so you talked about how most of the galaxies

370
00:17:30,119 --> 00:17:33,400
Speaker 1: that we see out there probably have dark matter, right,

371
00:17:33,480 --> 00:17:35,399
Speaker 1: we think they have dark matter, right, because they couldn't

372
00:17:35,400 --> 00:17:37,000
Speaker 1: be holding together without dark matter.

373
00:17:37,080 --> 00:17:39,880
Speaker 2: That's right. And it's even more than just most galaxies

374
00:17:39,920 --> 00:17:42,520
Speaker 2: have dark matter. It turns out galaxies are sort of

375
00:17:42,560 --> 00:17:46,479
Speaker 2: like extra rich in dark matter, Like most galaxies have

376
00:17:46,760 --> 00:17:49,639
Speaker 2: more dark matter than the average dark matter density in

377
00:17:49,680 --> 00:17:50,320
Speaker 2: the universe.

378
00:17:50,640 --> 00:17:52,479
Speaker 1: Wait, what what do you mean? Like, what are some numbers?

379
00:17:52,680 --> 00:17:54,960
Speaker 2: So if you average over the whole universe, like what

380
00:17:55,119 --> 00:17:58,120
Speaker 2: fraction of matter in the universe is dark matter? That's

381
00:17:58,160 --> 00:18:01,520
Speaker 2: eighty four percent by counting. We don't know how many

382
00:18:01,600 --> 00:18:04,479
Speaker 2: dark matter particles there are that's by mass, Like, what

383
00:18:04,560 --> 00:18:07,280
Speaker 2: fraction of the mass of stuff in the universe is

384
00:18:07,359 --> 00:18:09,800
Speaker 2: dark matter? That's about eighty four percent. But when you

385
00:18:09,840 --> 00:18:12,520
Speaker 2: look at galaxies and you ask, like, what fraction of

386
00:18:12,520 --> 00:18:15,640
Speaker 2: the mass in a galaxy is dark matter, that's more

387
00:18:15,760 --> 00:18:19,879
Speaker 2: like ninety one percent. So galaxies have like about half

388
00:18:19,960 --> 00:18:23,680
Speaker 2: as much normal matter as the average normal matter density

389
00:18:23,680 --> 00:18:27,280
Speaker 2: in the universe. Galaxies are like concentrated blobs of dark matter.

390
00:18:27,520 --> 00:18:31,040
Speaker 1: And we get these numbers by measuring how fast the

391
00:18:31,040 --> 00:18:33,920
Speaker 1: galaxies are rotating and kind of guessing how much dark

392
00:18:33,920 --> 00:18:35,520
Speaker 1: matter you need to hold it all together.

393
00:18:36,359 --> 00:18:38,840
Speaker 2: Yeah, not so much guessing, measuring right, But you're right.

394
00:18:38,840 --> 00:18:41,879
Speaker 2: It's looking at how the galaxy rotates. We can measure

395
00:18:41,960 --> 00:18:44,560
Speaker 2: the speed of those stars in the galaxy as they

396
00:18:44,560 --> 00:18:47,560
Speaker 2: whizz around the center by looking at their light and

397
00:18:47,600 --> 00:18:50,160
Speaker 2: seeing how it's red shifted or blue shifted. You're looking

398
00:18:50,160 --> 00:18:52,120
Speaker 2: at a galaxy, some of the stars will be moving

399
00:18:52,160 --> 00:18:54,600
Speaker 2: away from you and some moving towards you, so to

400
00:18:54,720 --> 00:18:57,800
Speaker 2: be red or blue shifted their light from the Doppler shift,

401
00:18:57,840 --> 00:19:00,440
Speaker 2: so you can measure their velocities. So you can look

402
00:19:00,440 --> 00:19:03,080
Speaker 2: at the velocity of stars as they get further and

403
00:19:03,320 --> 00:19:05,560
Speaker 2: further away from the center, and in order to hold

404
00:19:05,560 --> 00:19:08,159
Speaker 2: a star at a certain velocity a certain radius, you

405
00:19:08,280 --> 00:19:11,120
Speaker 2: need a gravitational force there, so you can calculate exactly

406
00:19:11,119 --> 00:19:13,919
Speaker 2: how much gravity is needed to hold a star at

407
00:19:13,960 --> 00:19:16,240
Speaker 2: a certain radius. They have all these stars at different

408
00:19:16,280 --> 00:19:19,920
Speaker 2: distances from the center, telling you exactly how much gravity

409
00:19:19,960 --> 00:19:22,600
Speaker 2: you need to keep those stars going at that speed,

410
00:19:23,080 --> 00:19:25,520
Speaker 2: and then you can add up how much you can see,

411
00:19:25,720 --> 00:19:28,280
Speaker 2: like count all the visible stars and the rest you

412
00:19:28,359 --> 00:19:31,159
Speaker 2: suppose is dark matter. I guess that's what you mean

413
00:19:31,240 --> 00:19:32,120
Speaker 2: by guessing.

414
00:19:31,840 --> 00:19:33,440
Speaker 1: Yeah, inferring guessing.

415
00:19:34,480 --> 00:19:37,000
Speaker 2: There's a whole field of statistical inference that we should

416
00:19:37,040 --> 00:19:37,719
Speaker 2: just call guessing.

417
00:19:37,840 --> 00:19:41,440
Speaker 1: Yeah, Well, I mean you don't actually know how many

418
00:19:41,480 --> 00:19:43,840
Speaker 1: stars there are in that galaxy right, so far away

419
00:19:43,880 --> 00:19:46,960
Speaker 1: you can't see the individual stars, so you're also sort

420
00:19:47,000 --> 00:19:50,119
Speaker 1: of inferring how many stars there are there. You're guessing

421
00:19:50,160 --> 00:19:50,919
Speaker 1: a little bit, aren't you.

422
00:19:51,160 --> 00:19:53,960
Speaker 2: There's always uncertainty in these measurements. Absolutely, and you're right

423
00:19:53,960 --> 00:19:57,600
Speaker 2: that we cannot resolve individual stars, especially near the center

424
00:19:57,640 --> 00:20:00,800
Speaker 2: where things get very dense. But we can see this streams, right,

425
00:20:00,840 --> 00:20:02,879
Speaker 2: we can see streams of stars. We have models for

426
00:20:02,920 --> 00:20:06,600
Speaker 2: how these galaxies work. But absolutely there's always uncertainty, but

427
00:20:06,680 --> 00:20:09,919
Speaker 2: the uncertainty in these calculations is tiny compared to the

428
00:20:09,960 --> 00:20:12,760
Speaker 2: size of dark matter. So there's no uncertainty that there's

429
00:20:12,800 --> 00:20:15,600
Speaker 2: a lot of dark matter in these galaxies because remember

430
00:20:15,680 --> 00:20:18,560
Speaker 2: the fractions we're talking about here, like ninety percent, which

431
00:20:18,600 --> 00:20:21,400
Speaker 2: means you're looking at a star, you're measuring its velocity.

432
00:20:21,400 --> 00:20:24,120
Speaker 2: You figure out how much gravity is needed to hold

433
00:20:24,119 --> 00:20:26,840
Speaker 2: it there so it doesn't fly out into intergalactic space.

434
00:20:27,240 --> 00:20:28,600
Speaker 2: And when you add up all the stars, you get

435
00:20:28,600 --> 00:20:31,280
Speaker 2: like ten percent of the gravity you need. So there's

436
00:20:31,280 --> 00:20:33,040
Speaker 2: a huge missing chunk.

437
00:20:34,200 --> 00:20:36,880
Speaker 1: And you're sure it's not just you know, a lot

438
00:20:36,880 --> 00:20:38,640
Speaker 1: of asteroids or rocks that don't close.

439
00:20:38,840 --> 00:20:41,119
Speaker 2: Yeah, that's a great question. Could dark matter just be

440
00:20:41,359 --> 00:20:44,080
Speaker 2: normal matter that we're not seeing right, just like dark

441
00:20:44,200 --> 00:20:47,200
Speaker 2: chunks of matter. So people have looked for that directly.

442
00:20:47,440 --> 00:20:52,600
Speaker 2: Those are called MACHOs massive compact halo objects, and we

443
00:20:52,640 --> 00:20:54,960
Speaker 2: think we would see those occasionally, like they would pass

444
00:20:55,040 --> 00:20:56,959
Speaker 2: in front of stars if there was a lot of them.

445
00:20:57,000 --> 00:20:58,919
Speaker 2: If they were really big, we would have spotted them.

446
00:20:58,960 --> 00:21:00,960
Speaker 2: So people have looked for that kind of stuff and

447
00:21:01,000 --> 00:21:03,680
Speaker 2: not seen it. Plus we know something about how much

448
00:21:03,840 --> 00:21:06,960
Speaker 2: normal matter there was in the very early universe because

449
00:21:07,000 --> 00:21:10,240
Speaker 2: it dictates the fraction of elements that were produced, like

450
00:21:10,280 --> 00:21:13,320
Speaker 2: the hydrogen and helium and lithium very sensitive to the

451
00:21:13,359 --> 00:21:17,240
Speaker 2: density of quarks and electrons in the early universe. We

452
00:21:17,320 --> 00:21:20,359
Speaker 2: talked about that once Big Bang nucleosynthesis. So we have

453
00:21:20,400 --> 00:21:23,400
Speaker 2: a pretty good handle on how much normal matter there

454
00:21:23,480 --> 00:21:26,000
Speaker 2: was around, and we can explain where most of that

455
00:21:26,200 --> 00:21:27,720
Speaker 2: is now and the rest of it's got to be

456
00:21:27,800 --> 00:21:28,280
Speaker 2: dark matter.

457
00:21:28,480 --> 00:21:30,880
Speaker 1: And so basically every galaxy out there that we've seen,

458
00:21:32,000 --> 00:21:35,440
Speaker 1: we see that it's spinning faster than it should, or

459
00:21:35,440 --> 00:21:37,880
Speaker 1: it's holding together more than it should, so we think

460
00:21:38,040 --> 00:21:39,560
Speaker 1: it has dark matter. And now is that true for

461
00:21:39,600 --> 00:21:40,920
Speaker 1: every galaxy we've seen out there?

462
00:21:40,960 --> 00:21:43,440
Speaker 2: It turns out there's a pretty wide variety. Like when

463
00:21:43,480 --> 00:21:45,879
Speaker 2: you look at galaxies out there, some of them have

464
00:21:46,040 --> 00:21:48,159
Speaker 2: a lot of dark matter, and some of them have

465
00:21:48,200 --> 00:21:51,080
Speaker 2: a huge amount of dark matter. There is a variety.

466
00:21:51,119 --> 00:21:54,399
Speaker 2: It turns out that smaller galaxies tend to have more

467
00:21:54,520 --> 00:21:57,879
Speaker 2: dark matter than really massive galaxies.

468
00:21:57,400 --> 00:22:00,480
Speaker 1: Not by absolute amounts, but just relative town these stars

469
00:22:00,480 --> 00:22:00,760
Speaker 1: they have.

470
00:22:00,880 --> 00:22:04,040
Speaker 2: Yeah, exactly higher dark matter fractions, I should say, And

471
00:22:04,080 --> 00:22:07,359
Speaker 2: that's because galaxies are better at holding onto their dark

472
00:22:07,440 --> 00:22:11,080
Speaker 2: matter than they are their normal matter. Galaxies are crazy places.

473
00:22:11,119 --> 00:22:13,560
Speaker 2: There's winds from all the stars, right every star is

474
00:22:13,600 --> 00:22:17,640
Speaker 2: a fusion furnace and pushing out protons and electrons. These

475
00:22:17,840 --> 00:22:21,600
Speaker 2: cellar winds are pushing gas out of the galaxies. Then

476
00:22:21,640 --> 00:22:24,040
Speaker 2: there are supernovas going off all the time, blowing things

477
00:22:24,119 --> 00:22:27,800
Speaker 2: up and pushing things out. There's radiation, really intense radiation

478
00:22:27,920 --> 00:22:30,880
Speaker 2: from the center of the galaxy that's pushing gas out.

479
00:22:31,040 --> 00:22:34,240
Speaker 2: So galaxies are basically exploding and they're pushing a lot

480
00:22:34,240 --> 00:22:37,400
Speaker 2: of their matter out. And so the smaller galaxy is,

481
00:22:37,720 --> 00:22:40,879
Speaker 2: the less it's capable of holding onto its normal matter,

482
00:22:41,160 --> 00:22:44,359
Speaker 2: the less it's capable of resisting these forces that push

483
00:22:44,480 --> 00:22:46,680
Speaker 2: gas out of the galaxies.

484
00:22:46,200 --> 00:22:48,119
Speaker 1: Because the bigger galaxies have more gravity.

485
00:22:48,160 --> 00:22:50,760
Speaker 2: Basically right exactly, the bigger galaxies are still doing this,

486
00:22:50,800 --> 00:22:52,680
Speaker 2: but they have more gravity so they can hold onto

487
00:22:52,680 --> 00:22:55,720
Speaker 2: their normal matter. So smaller galaxies, which have weaker gravity,

488
00:22:55,840 --> 00:22:58,440
Speaker 2: lose more of this normal matter. So you look out

489
00:22:58,480 --> 00:23:01,000
Speaker 2: there at dwarf galaxies, really tiny ones. They can be

490
00:23:01,080 --> 00:23:03,119
Speaker 2: like ninety nine percent dark matter.

491
00:23:03,320 --> 00:23:06,040
Speaker 1: But wouldn't the larger galaxies also be better at holding

492
00:23:06,040 --> 00:23:09,919
Speaker 1: onto their dark matter, Like wouldn't smaller galaxies lose some

493
00:23:10,000 --> 00:23:12,600
Speaker 1: dark matter eventually, like it might evaporate or something.

494
00:23:12,680 --> 00:23:15,439
Speaker 2: Yeah, that depends on what dark matter does. And in

495
00:23:15,480 --> 00:23:18,840
Speaker 2: this theory, dark matter does nothing but gravity, and so

496
00:23:18,880 --> 00:23:21,200
Speaker 2: you can't really lose your dark matter. Like to lose

497
00:23:21,200 --> 00:23:23,160
Speaker 2: your dark matter, you need some force that's pushing out

498
00:23:23,160 --> 00:23:25,680
Speaker 2: on it. But gravity is just attractive. So all these

499
00:23:25,720 --> 00:23:28,200
Speaker 2: forces like the solar winds and the radiation and the

500
00:23:28,240 --> 00:23:30,920
Speaker 2: supernova basically have no impact on the dark matter. Dark

501
00:23:30,920 --> 00:23:33,480
Speaker 2: matter just like brushes it right off, Like supernova could

502
00:23:33,480 --> 00:23:35,639
Speaker 2: happen right next to you, and a dark matter particle

503
00:23:35,680 --> 00:23:36,920
Speaker 2: would be like whatever, dude.

504
00:23:37,080 --> 00:23:39,480
Speaker 1: And so we haven't seen any galaxies without dark matter,

505
00:23:39,560 --> 00:23:41,520
Speaker 1: So then why are we asking the question are there

506
00:23:41,560 --> 00:23:44,199
Speaker 1: galaxies without dark matter? Is it more of like, is

507
00:23:44,240 --> 00:23:46,520
Speaker 1: it possible to have a galaxy without dark matter? Or

508
00:23:47,160 --> 00:23:49,439
Speaker 1: are we asking like could there be galaxies where we

509
00:23:49,520 --> 00:23:51,160
Speaker 1: haven't noticed it doesn't have dark matter?

510
00:23:51,240 --> 00:23:53,840
Speaker 2: Yeah, so great question. We're curious about this for lots

511
00:23:53,880 --> 00:23:56,520
Speaker 2: of reasons, Like number one, we have a theory about

512
00:23:56,600 --> 00:23:58,840
Speaker 2: how the structure of the universe came to be and

513
00:23:58,880 --> 00:24:01,680
Speaker 2: how it made galaxy and this nice story we told

514
00:24:01,720 --> 00:24:05,840
Speaker 2: you about over densities clumping together to form galaxies et cetera,

515
00:24:05,880 --> 00:24:07,720
Speaker 2: et cetera. But we'd like to test that. We'd like

516
00:24:07,800 --> 00:24:11,040
Speaker 2: to make sure that's correct. We're often surprised when we

517
00:24:11,040 --> 00:24:13,920
Speaker 2: look out in the universe and see how things actually work,

518
00:24:14,560 --> 00:24:16,440
Speaker 2: and so what we'd like to do is check our

519
00:24:16,440 --> 00:24:20,000
Speaker 2: predictions about like the dark matter fractions of galaxies against

520
00:24:20,040 --> 00:24:23,359
Speaker 2: reality and see is this really the way things work. Also,

521
00:24:23,400 --> 00:24:26,280
Speaker 2: this really helps us understand what dark matter is, because

522
00:24:26,320 --> 00:24:29,080
Speaker 2: seeing how dark matter varies across the universe can tell

523
00:24:29,119 --> 00:24:31,120
Speaker 2: us something about the nature of dark matter and help

524
00:24:31,200 --> 00:24:34,800
Speaker 2: us test various alternative theories about what dark matter might

525
00:24:35,000 --> 00:24:37,040
Speaker 2: or might not be. But it's not exactly true that

526
00:24:37,080 --> 00:24:39,639
Speaker 2: we've never seen a galaxy without dark matter. People are

527
00:24:39,680 --> 00:24:41,960
Speaker 2: out there looking for these and they found some pretty

528
00:24:41,960 --> 00:24:42,720
Speaker 2: weird cases.

529
00:24:43,680 --> 00:24:46,720
Speaker 1: Interesting, all right, what are some of these cases?

530
00:24:46,880 --> 00:24:49,800
Speaker 2: So this galaxy group kind of nearby on cosmic scales,

531
00:24:49,840 --> 00:24:52,560
Speaker 2: that's sixty three million light years away. It's called in

532
00:24:52,720 --> 00:24:57,320
Speaker 2: GC one zero five two, and basically it's an elliptical

533
00:24:57,359 --> 00:25:00,199
Speaker 2: galaxy in the Cetus constellation. We've known about if like

534
00:25:00,240 --> 00:25:02,399
Speaker 2: two hundred and fifty years or so, but there's actually

535
00:25:02,400 --> 00:25:05,119
Speaker 2: a little group of galaxies. It's like a major galaxy

536
00:25:05,440 --> 00:25:08,040
Speaker 2: with a bunch of little galaxies nearby. They call these

537
00:25:08,119 --> 00:25:10,479
Speaker 2: dwarf galaxies, so it's a whole group, so they call

538
00:25:10,560 --> 00:25:13,800
Speaker 2: the group of galaxies. And these little galaxies are actually

539
00:25:13,920 --> 00:25:17,359
Speaker 2: ultra diffuse galaxies. That means they're galaxies that are not

540
00:25:17,480 --> 00:25:20,080
Speaker 2: very bright, they have very few stars in them. And

541
00:25:20,160 --> 00:25:24,160
Speaker 2: these ultra diffuse galaxies near this NGC group they think

542
00:25:24,280 --> 00:25:26,720
Speaker 2: might have no dark matter in them at all.

543
00:25:27,040 --> 00:25:27,879
Speaker 1: Why do they think that?

544
00:25:28,160 --> 00:25:30,359
Speaker 2: So they look at the rotations of these galaxies and

545
00:25:30,359 --> 00:25:33,520
Speaker 2: they do that calculation and they estimate zero dark matter.

546
00:25:34,119 --> 00:25:35,960
Speaker 2: Like every time you're doing this, you're not assuming the

547
00:25:36,040 --> 00:25:39,160
Speaker 2: dark matter. You're measuring it. And sometimes it comes out

548
00:25:39,240 --> 00:25:42,520
Speaker 2: ninety percent, sometimes eighty four percent, sometimes ninety nine percent.

549
00:25:42,600 --> 00:25:44,800
Speaker 2: In this case, it comes out close to zero or

550
00:25:44,920 --> 00:25:48,560
Speaker 2: consistent with zero. So they think these are little galaxies

551
00:25:48,560 --> 00:25:50,400
Speaker 2: that have no dark matter in them at all.

552
00:25:50,480 --> 00:25:52,240
Speaker 1: So that's pretty wild. That means that you can have

553
00:25:52,280 --> 00:25:53,520
Speaker 1: a galaxy without dark matter.

554
00:25:53,720 --> 00:25:57,360
Speaker 2: Yeah, it's fascinating because remember, our theory of galaxy formation

555
00:25:57,600 --> 00:26:00,960
Speaker 2: is that basically every big galaxy is a merger of

556
00:26:01,000 --> 00:26:03,960
Speaker 2: a bunch of small galaxies. Big galaxies don't like form

557
00:26:04,000 --> 00:26:05,920
Speaker 2: all at once in a single collapse. You have a

558
00:26:05,960 --> 00:26:08,520
Speaker 2: bunch of baby galaxies then merge to make bigger and

559
00:26:08,520 --> 00:26:11,560
Speaker 2: bigger galaxies, so like a bottoms up approach. And so

560
00:26:11,600 --> 00:26:13,520
Speaker 2: if your big galaxy ends up with a lot of

561
00:26:13,600 --> 00:26:16,040
Speaker 2: dark matter in it, that means that the little galaxies

562
00:26:16,040 --> 00:26:17,960
Speaker 2: that made it should each have their own dark matter.

563
00:26:18,080 --> 00:26:19,920
Speaker 2: And we look out a dwarf galaxies and we mostly

564
00:26:19,920 --> 00:26:21,840
Speaker 2: see them having dark matter. In fact, some of them

565
00:26:21,880 --> 00:26:23,960
Speaker 2: have a lot. So it is really weird to see

566
00:26:23,960 --> 00:26:27,000
Speaker 2: these little galaxies without any dark matter at all. And

567
00:26:27,000 --> 00:26:29,240
Speaker 2: the question is like, did they form this way or

568
00:26:29,280 --> 00:26:32,639
Speaker 2: did something happen to strip them of their dark matter.

569
00:26:32,600 --> 00:26:34,600
Speaker 1: Or maybe they formed later in the universe.

570
00:26:34,840 --> 00:26:37,959
Speaker 2: Yeah, exactly, And so that's a fascinating question. And so

571
00:26:38,000 --> 00:26:39,720
Speaker 2: there's a group that's done a study of these and

572
00:26:39,720 --> 00:26:42,240
Speaker 2: they have a theory about how these little diffuse galaxies

573
00:26:42,600 --> 00:26:44,440
Speaker 2: ended up without any dark matter in them.

574
00:26:44,600 --> 00:26:45,240
Speaker 1: What's a theory.

575
00:26:45,359 --> 00:26:47,840
Speaker 2: So the theory is basically a mini version of the

576
00:26:47,840 --> 00:26:51,120
Speaker 2: Bullet cluster. You remember. The Bullet cluster is this famous

577
00:26:51,160 --> 00:26:53,440
Speaker 2: example that really convinced a lot of people that dark

578
00:26:53,480 --> 00:26:55,679
Speaker 2: matter was a real thing. It was a cluster of

579
00:26:55,720 --> 00:26:58,840
Speaker 2: galaxies that collided with another cluster of galaxies and we

580
00:26:58,920 --> 00:27:00,679
Speaker 2: saw that what happened to them the gas and the

581
00:27:00,760 --> 00:27:03,679
Speaker 2: dust and the dark matter was very different. So the

582
00:27:03,720 --> 00:27:06,680
Speaker 2: gas and the dust interacted and created collisions. The dark

583
00:27:06,720 --> 00:27:09,080
Speaker 2: matter passed right through because it doesn't they direct at

584
00:27:09,080 --> 00:27:13,080
Speaker 2: that level, gravity's not strong enough, so basically separated the

585
00:27:13,200 --> 00:27:16,160
Speaker 2: dark matter from the normal matter. So the bullet cluster,

586
00:27:16,240 --> 00:27:17,679
Speaker 2: and now you have a blob in the middle with

587
00:27:17,720 --> 00:27:19,199
Speaker 2: a bunch of normal matter in it, and then you

588
00:27:19,280 --> 00:27:21,840
Speaker 2: have dark matter on both sides, so we can see

589
00:27:21,840 --> 00:27:24,840
Speaker 2: through gravitational lensing. So they think that might be similar

590
00:27:24,880 --> 00:27:26,919
Speaker 2: to what happened in this case, that maybe there was

591
00:27:26,960 --> 00:27:30,360
Speaker 2: a big collision between two other objects, and these two

592
00:27:30,359 --> 00:27:32,360
Speaker 2: things that we're seeing now they call them DF two

593
00:27:32,560 --> 00:27:35,440
Speaker 2: and DF four are basically the results of that, like

594
00:27:35,840 --> 00:27:39,040
Speaker 2: chunks of stars and gas and dust that got stripped

595
00:27:39,080 --> 00:27:41,800
Speaker 2: of their dark matter and a collision and then tossed aside.

596
00:27:42,280 --> 00:27:45,280
Speaker 1: Well, why wouldn't some little bit of dark matter go

597
00:27:45,400 --> 00:27:45,960
Speaker 1: with them.

598
00:27:45,800 --> 00:27:47,680
Speaker 2: Because the dark matter and the normal man have very

599
00:27:47,680 --> 00:27:51,000
Speaker 2: different experiences in a collision, Like dark matter basically passes

600
00:27:51,040 --> 00:27:53,159
Speaker 2: the right through. There's not really a collision when it

601
00:27:53,200 --> 00:27:55,640
Speaker 2: comes to dark matter. It's like two ghosts just phasing

602
00:27:55,680 --> 00:27:58,440
Speaker 2: through each other whereas two people bumping into a hallway

603
00:27:58,480 --> 00:28:01,119
Speaker 2: are going to change their direction. So imagine you have

604
00:28:01,240 --> 00:28:03,680
Speaker 2: like a ghost inside you and somebody else is a

605
00:28:03,720 --> 00:28:06,280
Speaker 2: ghost inside them, and you have a collision in the hallway.

606
00:28:06,440 --> 00:28:08,920
Speaker 2: The ghosts just keep on going and the living people

607
00:28:08,920 --> 00:28:11,560
Speaker 2: bounce off each other. Now you know you're separated from

608
00:28:11,600 --> 00:28:12,000
Speaker 2: your ghost.

609
00:28:12,160 --> 00:28:14,280
Speaker 1: But there's so many that they discovered. Did they all

610
00:28:14,320 --> 00:28:15,600
Speaker 1: get that way from the collision?

611
00:28:15,680 --> 00:28:18,080
Speaker 2: So they've only found these two, and they have this

612
00:28:18,160 --> 00:28:21,720
Speaker 2: reconstruction of the collision that suggests that these two things

613
00:28:21,760 --> 00:28:25,639
Speaker 2: happen somewhere near NGC and created its collision, and it

614
00:28:25,680 --> 00:28:28,480
Speaker 2: should have also created a bunch of other ultra diiffuse

615
00:28:28,560 --> 00:28:31,719
Speaker 2: galaxies that they should be able to spot. That there

616
00:28:31,720 --> 00:28:33,880
Speaker 2: should be like five or six of these that came

617
00:28:33,920 --> 00:28:36,720
Speaker 2: out of the collision that also have no dark matter.

618
00:28:36,960 --> 00:28:38,280
Speaker 2: So they're going to go and look for those.

619
00:28:39,000 --> 00:28:41,160
Speaker 1: So these are you said, these are dwarf galaxies.

620
00:28:41,440 --> 00:28:44,760
Speaker 2: Yeah, they're ultra diffuse galaxies. They're also dwarf galaxies, so

621
00:28:44,800 --> 00:28:46,080
Speaker 2: they're small and they're not.

622
00:28:46,200 --> 00:28:48,120
Speaker 1: Very bright, so in a way they kind of got

623
00:28:48,120 --> 00:28:50,840
Speaker 1: made later or not? Are they as old as the universe?

624
00:28:50,920 --> 00:28:54,640
Speaker 2: Well, they think this collision happened about eight billion years ago,

625
00:28:54,680 --> 00:28:57,160
Speaker 2: and so how you age these things, I guess depends,

626
00:28:57,280 --> 00:29:01,520
Speaker 2: like the way they are now started about billion years ago. Now,

627
00:29:01,560 --> 00:29:04,440
Speaker 2: of course, have some progenitor or something that they came from. Right,

628
00:29:04,480 --> 00:29:06,800
Speaker 2: there was a larger object they were a part of

629
00:29:07,120 --> 00:29:09,760
Speaker 2: which definitely had dark matter in it, and their dark

630
00:29:09,800 --> 00:29:12,360
Speaker 2: matter is now sprayed in some other direction. So they

631
00:29:12,360 --> 00:29:15,440
Speaker 2: become separated from their dark matter. And this must have

632
00:29:15,480 --> 00:29:18,080
Speaker 2: been a pretty mammoth event. I mean, they reconstruct this

633
00:29:18,120 --> 00:29:21,000
Speaker 2: thing and it's like a collision at three hundred kilometers

634
00:29:21,040 --> 00:29:23,880
Speaker 2: per second of these huge cosmic objects.

635
00:29:24,000 --> 00:29:27,840
Speaker 1: Well, so these are galaxies that had dark matter, but

636
00:29:27,880 --> 00:29:30,160
Speaker 1: then they got stripped away of their dark matter, and

637
00:29:30,200 --> 00:29:32,040
Speaker 1: so that shows that, hey, you can have a galaxy

638
00:29:32,640 --> 00:29:34,920
Speaker 1: without dark matter. But I guess maybe the larger question

639
00:29:35,160 --> 00:29:38,200
Speaker 1: is can you form a galaxy without dark matter?

640
00:29:38,440 --> 00:29:40,440
Speaker 2: Right? And so as you say, that's the deep question

641
00:29:40,520 --> 00:29:43,360
Speaker 2: about the nature of the formation of structure in the universe,

642
00:29:43,920 --> 00:29:46,000
Speaker 2: and so far the answer to that is no, we

643
00:29:46,080 --> 00:29:48,400
Speaker 2: do not think it's possible to form a galaxy without

644
00:29:48,480 --> 00:29:50,600
Speaker 2: dark matter. We think you need that dark matter around

645
00:29:50,600 --> 00:29:53,000
Speaker 2: to gather enough gas and dust to make stars and

646
00:29:53,120 --> 00:29:56,640
Speaker 2: to make a galaxy that without dark matter, normal matter

647
00:29:56,680 --> 00:29:59,600
Speaker 2: doesn't have enough gravity to have formed galaxies this early

648
00:29:59,600 --> 00:30:02,160
Speaker 2: in the UNI. If you had a universe without dark matter,

649
00:30:02,400 --> 00:30:04,600
Speaker 2: or big section of it without dark matter and just

650
00:30:04,680 --> 00:30:07,840
Speaker 2: normal matter, it would form galaxies eventually, but it would

651
00:30:07,840 --> 00:30:09,080
Speaker 2: take a lot longer to do so.

652
00:30:09,280 --> 00:30:11,280
Speaker 1: But you know, as I understand it, and during the

653
00:30:11,280 --> 00:30:14,520
Speaker 1: Big Bang, things were really hot and dense, and there

654
00:30:14,520 --> 00:30:17,320
Speaker 1: were pockets of things, and there were quantum fluctuations which

655
00:30:17,360 --> 00:30:21,560
Speaker 1: maybe created pockets of extra densities here and there. Couldn't

656
00:30:21,560 --> 00:30:24,440
Speaker 1: there have been a pocket of extra density of normal

657
00:30:24,480 --> 00:30:27,200
Speaker 1: stuff but not dark matter. That then when the universe

658
00:30:27,240 --> 00:30:30,400
Speaker 1: blew up, it became a galaxy without dark matter, like

659
00:30:30,480 --> 00:30:33,320
Speaker 1: during the Big Bang. Why does the normal matter have

660
00:30:33,440 --> 00:30:34,760
Speaker 1: to follow the dark matter?

661
00:30:34,880 --> 00:30:38,120
Speaker 2: I guess it does in general because dark matter dominates

662
00:30:38,160 --> 00:30:40,320
Speaker 2: because it's just so much more of it. So it

663
00:30:40,320 --> 00:30:42,840
Speaker 2: basically like sets the scene for everything. But you're right,

664
00:30:42,880 --> 00:30:46,120
Speaker 2: it's theoretically possible to have a downward fluctuation in the

665
00:30:46,200 --> 00:30:49,120
Speaker 2: dark matter and an upward fluctuation in the normal matter.

666
00:30:49,480 --> 00:30:51,480
Speaker 2: So you get some region of space where you have

667
00:30:51,600 --> 00:30:54,920
Speaker 2: like extra super dense normal matter and almost no dark matter.

668
00:30:55,280 --> 00:30:58,080
Speaker 2: That's possible. Yeah, and so in principle that could happen,

669
00:30:58,080 --> 00:31:00,520
Speaker 2: and if it had enough matter, then it would form

670
00:31:00,560 --> 00:31:03,640
Speaker 2: a galaxy on its own. So in principle that's not impossible,

671
00:31:03,840 --> 00:31:05,880
Speaker 2: but we've never seen that, and I don't know what

672
00:31:05,960 --> 00:31:08,640
Speaker 2: the chances are of that happening theoretically.

673
00:31:08,120 --> 00:31:10,200
Speaker 1: Like, how many galaxies have we've done this calculation to

674
00:31:10,200 --> 00:31:11,800
Speaker 1: make sure that it has dark matter in it.

675
00:31:12,000 --> 00:31:15,200
Speaker 2: Yeah, that's a great question. We've measured the rotation velocity

676
00:31:15,240 --> 00:31:18,600
Speaker 2: of thousands and thousands of galaxies, but that's a tiny

677
00:31:18,640 --> 00:31:21,120
Speaker 2: fraction of the number of galaxies that are out there.

678
00:31:21,120 --> 00:31:23,160
Speaker 2: In the number of galaxies we can see, most of

679
00:31:23,200 --> 00:31:25,320
Speaker 2: the galaxies we can see, we can't measure their rotation

680
00:31:25,440 --> 00:31:28,840
Speaker 2: velocity because you're looking at like one pixel or two pixels.

681
00:31:29,160 --> 00:31:30,720
Speaker 2: You need to be able to sort of resolve the

682
00:31:30,760 --> 00:31:33,520
Speaker 2: whole galaxies you can see like from one side versus

683
00:31:33,640 --> 00:31:36,200
Speaker 2: light from the other side. So it's tricky. But yeah,

684
00:31:36,200 --> 00:31:38,880
Speaker 2: we haven't looked at that many galaxies as possible. There

685
00:31:38,880 --> 00:31:41,920
Speaker 2: are galaxies out there that did really form without dark matter.

686
00:31:42,480 --> 00:31:44,200
Speaker 1: All right, Well, it seems like we kind of answered

687
00:31:44,200 --> 00:31:47,000
Speaker 1: the question of the episode, which is can you have

688
00:31:47,040 --> 00:31:49,760
Speaker 1: a galaxy without dark matter? The answer is yes, you

689
00:31:49,760 --> 00:31:52,840
Speaker 1: can have maybe galaxies that had dark matter, but then

690
00:31:52,840 --> 00:31:56,080
Speaker 1: they lose it or they get left behind by the

691
00:31:56,160 --> 00:31:59,760
Speaker 1: dark matter and so they're dark matter less. Or maybe

692
00:31:59,800 --> 00:32:01,720
Speaker 1: they could have forded at the beginning of the universe

693
00:32:01,800 --> 00:32:04,600
Speaker 1: in theory, but we haven't seen one yet. All right, Well,

694
00:32:04,680 --> 00:32:07,800
Speaker 1: let's dig into what this all means about our understanding

695
00:32:07,880 --> 00:32:10,840
Speaker 1: of dark matter and also gravity and whether or not

696
00:32:10,960 --> 00:32:14,520
Speaker 1: it needs to be overhauled. But first, let's take another

697
00:32:14,600 --> 00:32:29,920
Speaker 1: quick break. Orright, we're asking the question can there be

698
00:32:30,120 --> 00:32:35,600
Speaker 1: a galaxy with no dark chocolate? And the answer is probably,

699
00:32:35,640 --> 00:32:37,880
Speaker 1: But that's not a universe Daniel wants to live in.

700
00:32:38,400 --> 00:32:41,720
Speaker 2: No, that's right, Transport me somewhere else in the multiverse asap.

701
00:32:42,000 --> 00:32:43,560
Speaker 1: What if you end up in the universe where there's

702
00:32:43,640 --> 00:32:45,120
Speaker 1: only milk or white chocolate?

703
00:32:46,000 --> 00:32:48,240
Speaker 2: Just keep smashing that button until I get somewhere good.

704
00:32:48,640 --> 00:32:49,800
Speaker 1: No, No, you only get one trim.

705
00:32:49,880 --> 00:32:53,920
Speaker 2: Would you take the risk? I'm pretty happy with our universe,

706
00:32:53,960 --> 00:32:54,200
Speaker 2: you know.

707
00:32:54,240 --> 00:32:57,440
Speaker 1: It's a pretty good one. We're talking about whether galaxies

708
00:32:57,480 --> 00:33:00,560
Speaker 1: can exist without dark matter, and the answers yes, they

709
00:33:00,560 --> 00:33:03,160
Speaker 1: can be stripped away of their dark matter, or theoretically

710
00:33:03,280 --> 00:33:05,360
Speaker 1: they could form in the early universe, but we haven't

711
00:33:05,400 --> 00:33:08,400
Speaker 1: seen one yet, and so maybe probably not. What do

712
00:33:08,400 --> 00:33:10,840
Speaker 1: you think happened? Why haven't we seen any? If they

713
00:33:10,880 --> 00:33:13,080
Speaker 1: can form without dark matter, why haven't we seen any.

714
00:33:13,240 --> 00:33:15,280
Speaker 2: I think that'd be really unlikely. I mean, the kind

715
00:33:15,320 --> 00:33:18,360
Speaker 2: of fluctuations we're talking about are very, very large. In

716
00:33:18,440 --> 00:33:20,800
Speaker 2: the early universe, you had just sort of like energy,

717
00:33:20,960 --> 00:33:24,360
Speaker 2: and then it decays into matter. As the universe expands

718
00:33:24,360 --> 00:33:26,920
Speaker 2: in cools, and every kind of matter is sort of

719
00:33:26,960 --> 00:33:29,600
Speaker 2: made uniformly, so you get more dark matter made and

720
00:33:29,680 --> 00:33:32,120
Speaker 2: less normal matter. But in order to have no dark

721
00:33:32,120 --> 00:33:35,200
Speaker 2: matter made, you'd need a really big fluctuation. You expect

722
00:33:35,200 --> 00:33:37,400
Speaker 2: to get like eighty five percent and you get zero.

723
00:33:37,560 --> 00:33:39,400
Speaker 2: It's like flipping a coin one hundred times in a

724
00:33:39,480 --> 00:33:42,160
Speaker 2: row and getting only heads instead of half heads and

725
00:33:42,200 --> 00:33:45,440
Speaker 2: half tails. It's pretty unlikely. Now the universe is really big,

726
00:33:45,440 --> 00:33:47,600
Speaker 2: of course, so that means that eventually it's going to happen,

727
00:33:47,680 --> 00:33:51,360
Speaker 2: especially if the universe is infinite. But it's so unlikely

728
00:33:51,720 --> 00:33:53,080
Speaker 2: that it's not going to be the first kind of

729
00:33:53,080 --> 00:33:55,480
Speaker 2: galaxy we see out there, or even in the first tranch.

730
00:33:56,000 --> 00:33:57,440
Speaker 2: Eventually we might spot one.

731
00:33:57,520 --> 00:33:59,240
Speaker 1: Well, as you said, it is possible to have a

732
00:33:59,240 --> 00:34:02,720
Speaker 1: galaxy without dark matter. We've seen it in some ultra

733
00:34:02,760 --> 00:34:06,600
Speaker 1: diffuse galaxies and they've done a measurement on these galaxies, right,

734
00:34:06,920 --> 00:34:09,319
Speaker 1: They've measured how fast it's spinning, and they're pretty sure

735
00:34:09,320 --> 00:34:10,640
Speaker 1: there's no dark matter in them.

736
00:34:10,719 --> 00:34:13,480
Speaker 2: Yeah, there were a series of papers where people said, oh,

737
00:34:13,560 --> 00:34:15,840
Speaker 2: there's no dark matter, and another group did a different

738
00:34:15,840 --> 00:34:18,200
Speaker 2: measurement said no, there is some dark matter. And then

739
00:34:18,200 --> 00:34:20,840
Speaker 2: there were follow up papers arguing, and now they're pretty

740
00:34:20,840 --> 00:34:23,200
Speaker 2: sure there's no dark matter in these but there's always

741
00:34:23,200 --> 00:34:25,759
Speaker 2: somebody out there who disagrees. I mean, it's astronomy, after all.

742
00:34:25,800 --> 00:34:27,000
Speaker 1: That's right, they're all just guessing.

743
00:34:28,920 --> 00:34:31,719
Speaker 2: They're all just doing their best statistical inference.

744
00:34:33,160 --> 00:34:37,680
Speaker 1: That's right. That's a great word for guessing. I'm just kidding.

745
00:34:38,719 --> 00:34:41,640
Speaker 1: A best guessing. How about that best guessing. Nobody has

746
00:34:41,640 --> 00:34:42,280
Speaker 1: a better guess.

747
00:34:42,360 --> 00:34:44,400
Speaker 2: You know, those whole departments of people who do nothing

748
00:34:44,440 --> 00:34:48,759
Speaker 2: but statistics for a living. I'm trashing Hershey's, but you're

749
00:34:48,800 --> 00:34:50,080
Speaker 2: trashing statistics. Man.

750
00:34:50,800 --> 00:34:53,720
Speaker 1: No, there's nothing wrong with guessing wrong.

751
00:34:55,400 --> 00:34:56,880
Speaker 2: How do you know the universe is the way it is.

752
00:34:57,000 --> 00:34:57,800
Speaker 2: We're just guessing.

753
00:34:58,040 --> 00:35:00,760
Speaker 1: A best guess doesn't mean that you're making thing. So randomly.

754
00:35:00,840 --> 00:35:03,200
Speaker 1: You're just using the best information you have to make

755
00:35:03,320 --> 00:35:05,760
Speaker 1: a best estimate or inference, right.

756
00:35:05,640 --> 00:35:08,040
Speaker 2: I suppose so. I think if it's very well informed,

757
00:35:08,040 --> 00:35:09,719
Speaker 2: it's not really a guess, you know.

758
00:35:09,800 --> 00:35:12,960
Speaker 1: But you're one hundred percent sure it's also not a fact.

759
00:35:13,040 --> 00:35:15,840
Speaker 2: Yeah, that's true. That's why we use statistics to describe

760
00:35:15,920 --> 00:35:19,960
Speaker 2: our uncertainties. Anyway. One of the things we are uncertain

761
00:35:20,000 --> 00:35:22,799
Speaker 2: about is the nature of dark matter. Like, a lot

762
00:35:22,840 --> 00:35:24,600
Speaker 2: of the stuff we talk about for dark matter is

763
00:35:24,719 --> 00:35:27,560
Speaker 2: kind of unsatisfyingly indirect, and a lot of people out

764
00:35:27,600 --> 00:35:30,920
Speaker 2: there treat dark matter like it's some placeholder, not a

765
00:35:30,960 --> 00:35:34,960
Speaker 2: real theory of the universe because we never see it directly.

766
00:35:35,000 --> 00:35:37,600
Speaker 2: We don't can't really grapple with it and grasp.

767
00:35:37,280 --> 00:35:39,600
Speaker 1: It directly, right right. Well, I think it's interesting that

768
00:35:39,680 --> 00:35:44,120
Speaker 1: we have or that astronomers have found galaxies without dark matter,

769
00:35:44,160 --> 00:35:46,280
Speaker 1: because it almost gives you kind of like a test

770
00:35:46,360 --> 00:35:49,000
Speaker 1: case to confirm that the other galaxies that we have

771
00:35:49,040 --> 00:35:51,480
Speaker 1: seen with dark matter actually have dark matter and it's

772
00:35:51,560 --> 00:35:55,080
Speaker 1: not just some weird, you know, fluke or mistake in

773
00:35:55,120 --> 00:35:56,440
Speaker 1: our theory of gravity.

774
00:35:56,719 --> 00:35:59,200
Speaker 2: Mm hmm. Yeah, it's a cool test case. It's like

775
00:35:59,239 --> 00:36:02,839
Speaker 2: a control, right, would you see galaxies without dark matter

776
00:36:02,920 --> 00:36:04,719
Speaker 2: if they were there. So it's nice to have some

777
00:36:04,880 --> 00:36:07,920
Speaker 2: verification that we're seeing that. It's also, as you say,

778
00:36:07,960 --> 00:36:11,640
Speaker 2: a great test bed for comparing various theories of dark matter,

779
00:36:11,920 --> 00:36:14,800
Speaker 2: which make different predictions about what would happen in these scenarios.

780
00:36:14,960 --> 00:36:15,400
Speaker 3: Mm.

781
00:36:15,480 --> 00:36:17,520
Speaker 1: Yeah, different guesses about dark.

782
00:36:17,320 --> 00:36:21,239
Speaker 2: Matter, different ideas, different theoretical.

783
00:36:22,840 --> 00:36:25,520
Speaker 1: All right, well, how do these galaxies help us decide

784
00:36:25,600 --> 00:36:26,880
Speaker 1: what dark matter is made out of.

785
00:36:27,040 --> 00:36:29,520
Speaker 2: Well, one of the most popular alternatives to dark matter

786
00:36:29,640 --> 00:36:32,080
Speaker 2: as a theory of matter, some kind of stuff in

787
00:36:32,120 --> 00:36:35,920
Speaker 2: the universe is an alternative theory of gravity to say, well,

788
00:36:35,920 --> 00:36:38,240
Speaker 2: there's no other stuff in the universe. We're seeing everything

789
00:36:38,280 --> 00:36:41,520
Speaker 2: there is. It's just that gravity works differently from what

790
00:36:41,600 --> 00:36:44,480
Speaker 2: we expected. Because remember, the argument for dark matter is like,

791
00:36:44,560 --> 00:36:47,160
Speaker 2: we understand gravity, and there's a lot more gravity than

792
00:36:47,200 --> 00:36:49,319
Speaker 2: we can explain with the visible stuff, So there must

793
00:36:49,320 --> 00:36:52,480
Speaker 2: be more stuff. There must be invisible stuff creating that gravity.

794
00:36:53,000 --> 00:36:55,920
Speaker 2: But what if instead we just don't understand how gravity

795
00:36:55,960 --> 00:36:59,560
Speaker 2: works and it can be explained by all the visible

796
00:36:59,560 --> 00:37:02,080
Speaker 2: stuff if you tweak your theory of gravity.

797
00:37:01,800 --> 00:37:04,800
Speaker 1: Meaning like what if gravity just gets stronger, The bigger

798
00:37:04,840 --> 00:37:08,480
Speaker 1: the distances that might account for why galaxies are holding

799
00:37:08,520 --> 00:37:12,240
Speaker 1: on together without needing dark matter. That's kind of the idea, right.

800
00:37:12,120 --> 00:37:15,000
Speaker 2: That's kind of the idea. More specifically, there's this theory

801
00:37:15,040 --> 00:37:20,600
Speaker 2: called mond modified Newtonian dynamics that suggests that gravity's mostly

802
00:37:20,719 --> 00:37:23,720
Speaker 2: like Newton described, but there are some tweaks. It depends

803
00:37:23,760 --> 00:37:27,680
Speaker 2: on the acceleration of these objects, and for some accelerations,

804
00:37:27,719 --> 00:37:30,480
Speaker 2: gravity gets stronger or weaker, and you know, it's a

805
00:37:30,480 --> 00:37:33,880
Speaker 2: little baroquely like added these terms and these tweaks basically

806
00:37:33,920 --> 00:37:36,839
Speaker 2: to explain these rotation curves, to say like, oh, these

807
00:37:36,880 --> 00:37:39,279
Speaker 2: stars are accelerating more than those stars. So if we

808
00:37:39,680 --> 00:37:42,600
Speaker 2: change the way gravity works, can we describe the rotation

809
00:37:42,719 --> 00:37:45,719
Speaker 2: curves that we see. The answer is yes, you can

810
00:37:45,960 --> 00:37:49,720
Speaker 2: devise a theory to describe the rotation curves that explain

811
00:37:49,760 --> 00:37:53,000
Speaker 2: how these galaxies are rotating without needing dark matter. If

812
00:37:53,040 --> 00:37:55,719
Speaker 2: you tweak gravity, right, you have to tweak something, either

813
00:37:55,800 --> 00:37:57,719
Speaker 2: change the amount of matter that's there, or you change

814
00:37:57,719 --> 00:37:58,640
Speaker 2: the way gravity works.

815
00:37:58,800 --> 00:38:00,680
Speaker 1: Well, first of all, you just saw of the whole

816
00:38:00,680 --> 00:38:02,560
Speaker 1: period of human history, the Baroque period.

817
00:38:03,520 --> 00:38:06,600
Speaker 2: I meant that in a positive way, right right.

818
00:38:07,520 --> 00:38:09,760
Speaker 1: The second of lie, I know that we've talked about before.

819
00:38:09,840 --> 00:38:13,680
Speaker 1: How you know, maybe Mond modified Newtonian dynamics, it could

820
00:38:13,719 --> 00:38:16,799
Speaker 1: replace dark matter. But we confirm dark matter in other

821
00:38:16,840 --> 00:38:17,920
Speaker 1: ways right exactly.

822
00:38:17,960 --> 00:38:20,600
Speaker 2: So Mond is a success in describing the rotations of

823
00:38:20,640 --> 00:38:26,520
Speaker 2: galaxies by making these beautiful baroque extensions to Newton's theory.

824
00:38:26,640 --> 00:38:28,680
Speaker 2: But there's lots of other ways we've seen dark matter,

825
00:38:28,920 --> 00:38:31,680
Speaker 2: like in the ripples of the cosmic microwave background from

826
00:38:31,719 --> 00:38:34,359
Speaker 2: the very early universe. We can see how that early

827
00:38:34,440 --> 00:38:38,560
Speaker 2: universe plasma sloshing around, and that depends very sensitively on

828
00:38:38,600 --> 00:38:41,920
Speaker 2: the amount of dark matter, which slashes differently in that

829
00:38:42,040 --> 00:38:46,120
Speaker 2: plasma than normal matter did, and Mond cannot explain that. Also,

830
00:38:46,160 --> 00:38:48,520
Speaker 2: the Bullet cluster shows us that dark matter can be

831
00:38:48,640 --> 00:38:52,120
Speaker 2: separated from normal matter. It's not just a different way

832
00:38:52,120 --> 00:38:55,320
Speaker 2: that gravity works for normal matter. It really is something

833
00:38:55,360 --> 00:38:58,440
Speaker 2: else with its own gravity. So Mond really struggles to

834
00:38:58,560 --> 00:39:01,520
Speaker 2: explain everything that the theory of dark matter can explain,

835
00:39:01,640 --> 00:39:04,319
Speaker 2: but it's still a popular alternative, and this is another way.

836
00:39:04,360 --> 00:39:06,879
Speaker 2: These dark matter free galaxies are another way to draw

837
00:39:07,000 --> 00:39:10,200
Speaker 2: contrast between what Mond predicts and what dark matter predict

838
00:39:10,200 --> 00:39:12,400
Speaker 2: because according to Mond, there is no dark matter and

839
00:39:12,440 --> 00:39:15,239
Speaker 2: gravity only depends on the visible matter. And so these

840
00:39:15,239 --> 00:39:18,000
Speaker 2: altered diffuse galaxies with no dark matter should behave the

841
00:39:18,000 --> 00:39:20,480
Speaker 2: same way all the other galaxies do because there's no

842
00:39:20,560 --> 00:39:22,839
Speaker 2: dark matter in any of them. But we do see

843
00:39:22,840 --> 00:39:27,040
Speaker 2: a difference. We see that these guys are rotating more slowly, right,

844
00:39:27,080 --> 00:39:30,680
Speaker 2: and so Mond struggles to explain how slowly rotating these

845
00:39:30,719 --> 00:39:34,120
Speaker 2: galaxies are without any dark matter, whereas dark matter can

846
00:39:34,160 --> 00:39:35,839
Speaker 2: explain all of it. It's like, well, this has more

847
00:39:35,920 --> 00:39:38,399
Speaker 2: dark matter, that has less dark matter. So because dark

848
00:39:38,440 --> 00:39:41,759
Speaker 2: matter can be variable in the universe, some galaxies have

849
00:39:41,840 --> 00:39:44,800
Speaker 2: more and some have less. Whereas the rules of gravity

850
00:39:44,840 --> 00:39:47,359
Speaker 2: have to be the same, Mind is sort of hamstrung

851
00:39:47,640 --> 00:39:51,319
Speaker 2: and can't really explain the variation of all these galaxies.

852
00:39:51,560 --> 00:39:53,200
Speaker 1: Well, I feel like Mond was already kind of dead

853
00:39:53,200 --> 00:39:55,720
Speaker 1: in the water for all these other reasons for a while.

854
00:39:55,920 --> 00:39:58,440
Speaker 1: But it is kind of interesting that seeing a galaxy

855
00:39:58,480 --> 00:40:01,800
Speaker 1: without dark matter almost kind of helps prove that it exists.

856
00:40:01,880 --> 00:40:03,680
Speaker 2: Yeah, that is really interesting, and I agree with you

857
00:40:03,719 --> 00:40:06,280
Speaker 2: that Mond is not a theory we should take terribly seriously.

858
00:40:06,280 --> 00:40:10,160
Speaker 2: In dark matter is overwhelming the better guess for what's

859
00:40:10,200 --> 00:40:11,120
Speaker 2: going on in the universe.

860
00:40:11,200 --> 00:40:13,480
Speaker 1: Yeah, there you go. See, I brought you on board.

861
00:40:15,239 --> 00:40:16,120
Speaker 2: I'm loving that word.

862
00:40:16,200 --> 00:40:18,040
Speaker 1: Now you're like, I guess, I guess so.

863
00:40:18,320 --> 00:40:20,640
Speaker 2: But you know, full caveats. There are some things that

864
00:40:20,719 --> 00:40:22,920
Speaker 2: dark matter can't explain. There are a few galaxies out

865
00:40:22,920 --> 00:40:24,959
Speaker 2: there that don't make any sense that no dark matter

866
00:40:25,000 --> 00:40:27,839
Speaker 2: can really explain. Some people think that some hybrid like

867
00:40:28,160 --> 00:40:30,640
Speaker 2: mostly dark matter with a little bit of mind is

868
00:40:30,640 --> 00:40:32,680
Speaker 2: what we need to explain everything that's out there in

869
00:40:32,680 --> 00:40:35,200
Speaker 2: the universe, and so it's best to keep an open mind.

870
00:40:35,239 --> 00:40:37,400
Speaker 2: It's also always nice to find a new way to

871
00:40:37,480 --> 00:40:40,360
Speaker 2: test our understanding of dark matter and gravity in general,

872
00:40:40,760 --> 00:40:43,239
Speaker 2: and so these galaxies without dark matter are a nice

873
00:40:43,280 --> 00:40:43,920
Speaker 2: test bed.

874
00:40:43,719 --> 00:40:46,719
Speaker 1: For that interesting you could call the new theory darkmond

875
00:40:48,280 --> 00:40:51,720
Speaker 1: all right, well, another interesting example of how the universe

876
00:40:51,800 --> 00:40:54,319
Speaker 1: just always has surprises. Like you think that maybe you

877
00:40:54,520 --> 00:40:58,279
Speaker 1: need dark matter to have a galaxy, but only one

878
00:40:58,320 --> 00:41:00,600
Speaker 1: day you find galaxies without dark matter, and it makes

879
00:41:00,600 --> 00:41:03,319
Speaker 1: you think, and it actually maybe helps you confirm the

880
00:41:03,360 --> 00:41:05,960
Speaker 1: existence of something as mysterious as dark.

881
00:41:05,840 --> 00:41:07,920
Speaker 2: Matter, and it goes to show you that the universe

882
00:41:07,960 --> 00:41:10,719
Speaker 2: does all these experiments for us. We can just look

883
00:41:10,800 --> 00:41:13,080
Speaker 2: up in the night sky and find the examples of

884
00:41:13,120 --> 00:41:17,000
Speaker 2: galaxies smashing into other galaxies or black holes colliding. All

885
00:41:17,120 --> 00:41:20,600
Speaker 2: these things are wonderful experiments that help reveal the nature

886
00:41:20,640 --> 00:41:22,799
Speaker 2: of the universe, the rules that it follows, and how

887
00:41:22,840 --> 00:41:23,799
Speaker 2: it all works.

888
00:41:24,200 --> 00:41:25,719
Speaker 1: Yeah, I guess.

889
00:41:27,880 --> 00:41:31,879
Speaker 2: Or you guess in the end, aren't we all just guessing? Man?

890
00:41:32,400 --> 00:41:35,360
Speaker 1: All right, Well, we hope you enjoyed that. Thanks for

891
00:41:35,440 --> 00:41:37,399
Speaker 1: joining us, See you next time.

892
00:41:42,600 --> 00:41:45,759
Speaker 2: For more science and curiosity, come find us on social media,

893
00:41:45,880 --> 00:41:49,360
Speaker 2: where we answer questions and post video. We're on Twitter

894
00:41:49,440 --> 00:41:52,360
Speaker 2: at this word instant and now TikTok. And remember that

895
00:41:52,520 --> 00:41:56,320
Speaker 2: Daniel and Jorge Explain the Universe is a production of iHeartRadio.

896
00:41:56,840 --> 00:42:00,520
Speaker 2: More podcasts from iHeart Radio visit the iHeartRadio you Apple

897
00:42:00,800 --> 00:42:04,200
Speaker 2: Apple Podcasts, or wherever you listen to your favorite shows.

898
00:42:09,120 --> 00:42:09,440
Speaker 2: M HM.