Latest / Daniel and Kelly’s Extraordinary Universe / What mysteries are hiding in the extra-galactic background light?

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What mysteries are hiding in the extra-galactic background light? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel. When you look at the night sky on
• 0:10
  Speaker 1: a good night, how many stars can you see?
• 0:12
  Speaker 2: Well, if it's a really good dark night without any
• 0:15
  Speaker 2: light pollution, you might see several thousand.
• 0:18
  Speaker 1: Stars, thousands in Los Angeles.
• 0:22
  Speaker 2: Maybe forgotten Joshua Tree.
• 0:25
  Speaker 1: But that's it. Only a thousand stars and the night
• 0:28
  Speaker 1: sky aren't there like trillions of stars out there?
• 0:30
  Speaker 2: There are oodles of stars out there, but most of
• 0:33
  Speaker 2: them are too dim to see even though they're out.
• 0:36
  Speaker 1: There, like their photons are not reaching us, or they're
• 0:38
  Speaker 1: too weak.
• 0:39
  Speaker 2: Photons can travel in infinite distance without ever getting tired,
• 0:43
  Speaker 2: but their photons are just so rare that you need
• 0:46
  Speaker 2: like a really big eyeball, maybe Hubble or James Webb
• 0:50
  Speaker 2: to capture one of them.
• 0:51
  Speaker 1: Or just like a long exposure too.
• 0:53
  Speaker 2: Right, Yeah, if you set up your camera for months
• 0:55
  Speaker 2: and months, you'll probably see one of those dim stars you.
• 0:58
  Speaker 1: Need, like a slow eyeball.
• 1:00
  Speaker 2: A big or slow eyeball or both.
• 1:03
  Speaker 1: So I guess what else is out there? Like, what
• 1:05
  Speaker 1: else would you see? Would you see stars and anything else?
• 1:08
  Speaker 2: We don't know. What's out there could just be stars
• 1:10
  Speaker 2: and galaxies, or there could be like chocolate bars and
• 1:12
  Speaker 2: frozen bananas waiting for us to discover them.
• 1:14
  Speaker 1: WHOA, sounds delicious? But will we see those things? Wouldn't
• 1:18
  Speaker 1: they have to glow somehow? Would these be glowing frozen bananas?
• 1:22
  Speaker 2: Glowing or reflective?
• 1:24
  Speaker 1: Which case? Do you want to eat them?
• 1:25
  Speaker 2: I'm glowing with excitement to try them?
• 1:28
  Speaker 1: I do. I know it sounds a little slippery. Hi.
• 1:45
  Speaker 1: I'm Jorge make, cartoonist and the author of Oliver's Greek
• 1:47
  Speaker 1: Big Universe. Hi.
• 1:49
  Speaker 2: I'm Daniel. I'm a particle physicist and a professor at
• 1:52
  Speaker 2: UC Irvine, and I'm so excited to see what's hiding
• 1:55
  Speaker 2: out there in the universe waiting for us to discover it.
• 1:58
  Speaker 1: Yeah, there seems to be a lot out there. But
• 1:59
  Speaker 1: do you think it's hiding or we're just not good
• 2:02
  Speaker 1: at seeing I.
• 2:02
  Speaker 2: Don't think it knows or cares about whether we are
• 2:04
  Speaker 2: seeing it. But it has not yet been revealed, so
• 2:07
  Speaker 2: in that sense, it is concealed.
• 2:09
  Speaker 1: Oh yeah, yeah, But is it really the case that
• 2:12
  Speaker 1: it could anything could be out there? You think then
• 2:14
  Speaker 1: we have a pretty good sense from what we can
• 2:16
  Speaker 1: see around us.
• 2:17
  Speaker 2: There's still a lot of big questions about what's out there,
• 2:20
  Speaker 2: the stuff nearby, the stuff far away the stuff in between,
• 2:24
  Speaker 2: and we should never make the assumption that the stuff
• 2:26
  Speaker 2: that's close to us is typical and that it could
• 2:28
  Speaker 2: be used to explain the whole universe.
• 2:30
  Speaker 1: Well, I guess there could be things hiding within our
• 2:33
  Speaker 1: own galaxy that we can't see it, right, Like we
• 2:36
  Speaker 1: haven't seen all of the Milky Way galaxy.
• 2:38
  Speaker 2: Oh absolutely, And the center of the galaxy, the most
• 2:40
  Speaker 2: interesting place, is the hardest to see. But the Milky
• 2:43
  Speaker 2: Way itself is so bright and so big, and that
• 2:46
  Speaker 2: it makes it really hard to see beyond it to
• 2:48
  Speaker 2: other galaxies.
• 2:50
  Speaker 1: And these other galaxies could be totally different from.
• 2:52
  Speaker 2: Ours, right, They could be very different. They could have
• 2:55
  Speaker 2: a very different history. There could be all sorts of
• 2:57
  Speaker 2: stuff going on out there deep in the universe that
• 3:00
  Speaker 2: we haven't yet figured out. Most of the photons that
• 3:03
  Speaker 2: come to Earth we don't gather. We mostly ignore them.
• 3:07
  Speaker 1: You mean, like we're getting all this information from all
• 3:09
  Speaker 1: the cross the universe, but we're not doing anything with it.
• 3:12
  Speaker 1: We're not paying attention.
• 3:13
  Speaker 2: Yeah, the universe is screaming at us in photons. Everything
• 3:17
  Speaker 2: that's out there in the universe is telling us all
• 3:19
  Speaker 2: about itself. So we have like the whole history of
• 3:21
  Speaker 2: the universe is out there being literally beamed at us,
• 3:25
  Speaker 2: but mostly we're not paying attention. Mostly those photons just
• 3:28
  Speaker 2: like splash on the sidewalk.
• 3:30
  Speaker 1: I wonder if that could be a good thing. Like
• 3:31
  Speaker 1: I wonder if at some point it's like TMI Universe.
• 3:35
  Speaker 1: There's some things I don't want to never TMI.
• 3:38
  Speaker 2: With science, you always want more data so you can
• 3:41
  Speaker 2: know more about the universe. I want to know the
• 3:43
  Speaker 2: Universe's deepest, darkest, most embarrassing secrets.
• 3:47
  Speaker 1: I see, you're more of an any person.
• 3:49
  Speaker 2: Not enough information, Absolutely no information is too embarrassing.
• 3:54
  Speaker 1: Well I hope that's true. But anyways, welcome to our podcast,
• 3:56
  Speaker 1: Daniel and Jorge Explain the Universe, a production of Our
• 3:59
  Speaker 1: Heart Radio.
• 4:00
  Speaker 2: In which no question is too weird, too gross, too
• 4:03
  Speaker 2: icky for us to explore it. We want to know
• 4:06
  Speaker 2: all of the embarrassing details about how the universe was born,
• 4:09
  Speaker 2: how it grew up, and all the messes it made
• 4:11
  Speaker 2: along the way. We want to unravel the deep history
• 4:15
  Speaker 2: of time and understand how the universe got to be
• 4:18
  Speaker 2: the way that it is and why it operates in
• 4:20
  Speaker 2: such an incredible, amazing and beautiful fashion.
• 4:23
  Speaker 1: Man, you make it sound like crazy fans of the
• 4:25
  Speaker 1: universe we are, or as the kids say these days, stand.
• 4:30
  Speaker 2: I totally stand our universe absolutely. I will defend it
• 4:33
  Speaker 2: online against haters.
• 4:34
  Speaker 1: There you go until you turn on it.
• 4:39
  Speaker 2: If in season seven it does something really weird, then yes,
• 4:42
  Speaker 2: I will turn on it.
• 4:43
  Speaker 1: But so far, if it jumps the shark, if it
• 4:45
  Speaker 1: jumps the galactic shark, you're like, I like the earlier
• 4:49
  Speaker 1: seasons better.
• 4:50
  Speaker 2: So far, it's been pretty awesome, and everything we've learned
• 4:53
  Speaker 2: has blown our minds and revealed incredible things about the
• 4:57
  Speaker 2: way the universe works. Not only are the law that
• 5:00
  Speaker 2: it follows really fascinating and incredible and have weird philosophical
• 5:04
  Speaker 2: consequences for what the nature of reality is, but the
• 5:07
  Speaker 2: stuff that bubbles up from those tiny laws, the huge,
• 5:10
  Speaker 2: the big stuff, the black holes, the galaxies, the quasars,
• 5:13
  Speaker 2: the blazars, all that stuff is just so mind blowingly awesome.
• 5:17
  Speaker 1: Yeah, it's pretty amazing. As we said before, how much
• 5:20
  Speaker 1: we've been able to figure out about the larger universe,
• 5:23
  Speaker 1: even just about our galaxy and beyond, just from sitting
• 5:26
  Speaker 1: on this little tiny rock in one corner of the galaxy.
• 5:29
  Speaker 1: It's pretty incredible. If you think about the scale things
• 5:32
  Speaker 1: and how much we know about what's out there.
• 5:35
  Speaker 2: But we've only really just begun to observe our universe.
• 5:38
  Speaker 2: We have a few eyeballs capable of picking up really
• 5:41
  Speaker 2: distant objects, but most of the light that's out there,
• 5:45
  Speaker 2: billions and billions and trillions and trillions of photons that
• 5:48
  Speaker 2: contain super fascinating important information about the history of our universe,
• 5:53
  Speaker 2: we're not capturing them. They're mostly drowned out by bigger,
• 5:57
  Speaker 2: brighter stuff like the lights of Los Angeles.
• 6:00
  Speaker 1: We're sort of washed with information from the universe and
• 6:03
  Speaker 1: we're not really I guess we are capturing it, we're
• 6:06
  Speaker 1: just not recording it, is maybe what you mean. We're like,
• 6:09
  Speaker 1: we're getting night light, starlight, you know when I step
• 6:13
  Speaker 1: outside tonight, but I'm not going to be thinking about
• 6:16
  Speaker 1: it or recording it or trying to figure out what
• 6:18
  Speaker 1: it says.
• 6:18
  Speaker 2: Yeah, most of it just hits the earth, and unless
• 6:21
  Speaker 2: some like Gecko is looking up at the night sky,
• 6:23
  Speaker 2: there's no being that's gathering that information. It just like
• 6:26
  Speaker 2: gently warms some rock or some like gum wrapper that's
• 6:30
  Speaker 2: lying on the ground.
• 6:31
  Speaker 1: But do you think we're missing stuff? Like if I
• 6:34
  Speaker 1: just point a telescope there every once in a while.
• 6:37
  Speaker 1: Am I really going to miss anything?
• 6:38
  Speaker 2: I think there's an incredible amount of deep history out
• 6:41
  Speaker 2: there in the night sky. And if we build zillions
• 6:43
  Speaker 2: of telescopes and point to them in all those directions
• 6:46
  Speaker 2: and just let them accumulate information, we would learn so
• 6:49
  Speaker 2: much about the history of the universe from these really
• 6:52
  Speaker 2: faint sources.
• 6:53
  Speaker 1: But ironically, if you cover the night sky with telescopes
• 6:56
  Speaker 1: and you wouldn't be able to see the night sky.
• 7:00
  Speaker 2: See it in a different way.
• 7:02
  Speaker 1: Well, I guess that's true. We can see it on
• 7:03
  Speaker 1: our phones. It's a little less poetic, and we can
• 7:09
  Speaker 1: see it scientifically. But anyways, it's kind of interesting what
• 7:11
  Speaker 1: is out there and what kind of information we are getting,
• 7:14
  Speaker 1: including even the stuff we might consider being in the background.
• 7:18
  Speaker 2: Yeah, that's right. The night sky is chock full of
• 7:20
  Speaker 2: stars from our galaxy, but there's a lot of really
• 7:23
  Speaker 2: useful information in the background, information we're mostly missing.
• 7:27
  Speaker 1: So today on the program, we'll be tackling the question
• 7:35
  Speaker 1: what is extra galactic background light. It's a lot of
• 7:39
  Speaker 1: syllables there for one term.
• 7:42
  Speaker 2: Yeah, astronomers, you know this one. I think they actually
• 7:45
  Speaker 2: name pretty well.
• 7:46
  Speaker 1: We'll see, we'll see. You've always promised that, and most
• 7:50
  Speaker 1: of the time it disappoints.
• 7:52
  Speaker 2: You have a very unrealistic standard.
• 7:53
  Speaker 1: If I have to say so, I'm just saying, you know,
• 7:58
  Speaker 1: take a minute to think about it, all.
• 8:00
  Speaker 2: Right, all right, I'll suspend my judgment.
• 8:02
  Speaker 1: But yeah, it's an interesting question. A lot of words here,
• 8:05
  Speaker 1: extra galactic background light, which should have sounds self apparent,
• 8:10
  Speaker 1: but maybe the extra it throws me off a little bit,
• 8:13
  Speaker 1: like it's extra, like we don't need it. Or is
• 8:17
  Speaker 1: it extra like like you get a bonus, Like you know,
• 8:21
  Speaker 1: I pay for a certain amount of galactic background light
• 8:23
  Speaker 1: and I'm getting some extra serving of it.
• 8:27
  Speaker 2: Or maybe it's just like a bit much universe Like
• 8:29
  Speaker 2: why are you so extra? Yeah, that's how my teenage
• 8:33
  Speaker 2: daughter would interpret it.
• 8:34
  Speaker 1: Mmmm, you're being too extra?
• 8:35
  Speaker 2: Yeah, Dad, you're so extra.
• 8:37
  Speaker 1: Oh my god, that's better than me. Mid, that's like
• 8:40
  Speaker 1: the that's not the worst insult from a teen right now,
• 8:44
  Speaker 1: that's kind of Mid. Oh boy university, Mid, I've seen
• 8:51
  Speaker 1: better life existence, Mid. But yeah, I guess we'll dig
• 8:54
  Speaker 1: into what all these turns mean and why it's interesting
• 8:57
  Speaker 1: to think about the extra galactic background light us you'll
• 9:00
  Speaker 1: be for wondering how many people out there know about
• 9:03
  Speaker 1: this or have any thoughts about what it might be.
• 9:06
  Speaker 2: Thanks very much to our panel of volunteers who comment
• 9:09
  Speaker 2: on these well named astronomical phenomena and offer their opinions
• 9:15
  Speaker 2: without the chance to google about it. If you would
• 9:17
  Speaker 2: like to play for a future episode, please don't be shy.
• 9:20
  Speaker 2: Write to me two questions at Danielandjorge dot com.
• 9:23
  Speaker 1: So think about it for a second. What do you
• 9:25
  Speaker 1: think is the extra galactic background light? Here's what people
• 9:31
  Speaker 1: have to say. It must be all the light coming
• 9:33
  Speaker 1: from outside our galaxy, either that or one of those
• 9:36
  Speaker 1: LED sets that you can use to make your bedroom
• 9:38
  Speaker 1: lighting extra galactic.
• 9:40
  Speaker 3: Extra galactic background light is the light produced by Club
• 9:43
  Speaker 3: Andromeda when the Aliens are having a rave. I don't
• 9:46
  Speaker 3: know something to do with our galaxy and there's too
• 9:48
  Speaker 3: much light for what there should be by some theory
• 9:51
  Speaker 3: that's calculating, Uh, you know how it fits within the
• 9:55
  Speaker 3: light it should get from other galaxies or our sun,
• 9:58
  Speaker 3: and somehow it's being amplified.
• 10:00
  Speaker 4: I've heard of it, but I'm thinking that our other
• 10:03
  Speaker 4: galaxies out of our galaxy and there it's a large background.
• 10:08
  Speaker 4: Most of the stars we see is the background galaxies,
• 10:11
  Speaker 4: and that's the range of galaxies we see in their lives.
• 10:15
  Speaker 1: Yes, all right, a lot of creative answers here. Some
• 10:19
  Speaker 1: people think extra means party.
• 10:21
  Speaker 2: I was wondering if anybody was going to say, it's
• 10:23
  Speaker 2: like breaking news, like extra extra, read all about it.
• 10:27
  Speaker 1: Oh right, that's another use of the word extra. Yeah,
• 10:29
  Speaker 1: if you're from the nineteen twenties, I.
• 10:32
  Speaker 2: Got a jaunty cap on. I'm standing on a so
• 10:35
  Speaker 2: far I'm selling newspaper.
• 10:37
  Speaker 1: You get a vest on. Yeah, yeah, yeah, you're on
• 10:39
  Speaker 1: the street. I got on my cheeks hanging out scientific papers.
• 10:44
  Speaker 2: That's how we distribute science these days. We passed it
• 10:46
  Speaker 2: out to OHI g mister, have you read white since
• 10:49
  Speaker 2: latest paper? It's a hoot.
• 10:55
  Speaker 1: But yeah, that's one way to think about it. But
• 10:57
  Speaker 1: a lot of creative answers here. I guess it's sort
• 10:59
  Speaker 1: of stelf a. But like I said, there's some ambiguity
• 11:01
  Speaker 1: in the terms.
• 11:02
  Speaker 2: Yes, as always.
• 11:03
  Speaker 1: All right, well let's dig into it, Daniel. What is
• 11:06
  Speaker 1: the extragalactic background light?
• 11:08
  Speaker 2: Basically, the extragalactic background light, or EBL, as astronomers like
• 11:13
  Speaker 2: to call it, is all the light emitted by everything
• 11:17
  Speaker 2: else in the universe except for the Milky Way, during
• 11:20
  Speaker 2: the entire history of the universe. So it's like all
• 11:24
  Speaker 2: of the photons in the universe not emitted by our galaxy.
• 11:28
  Speaker 1: Wait what but also not emitted by the background microwave
• 11:32
  Speaker 1: background radiation.
• 11:33
  Speaker 2: Now the CMB, the cosmic microwave background is part of
• 11:37
  Speaker 2: the EBL. The EBL is like the super general version
• 11:40
  Speaker 2: of the CMB. CMB is only at one wavelength, that's
• 11:44
  Speaker 2: the cosmic microwave background. The EBL is like, well, what's
• 11:48
  Speaker 2: the background in all of the wavelengths?
• 11:50
  Speaker 1: Oh wait, wait, so the EBL is part of this CMB.
• 11:53
  Speaker 2: No, the CMB is part of the EBL.
• 11:55
  Speaker 1: Oh okay, but it doesn't come from galaxies, does it.
• 11:58
  Speaker 2: It doesn't have to come from galaxy. This is the
• 12:01
  Speaker 2: light emitted by everything else in the universe other than
• 12:04
  Speaker 2: our galaxy. So extragalactic means not the Milky Way. So
• 12:08
  Speaker 2: any other star, any other object, any frozen banana floating
• 12:12
  Speaker 2: out there in space, any primordial soup of gas and plasma,
• 12:16
  Speaker 2: everything in the whole history of the universe except for
• 12:19
  Speaker 2: the Milky Way.
• 12:20
  Speaker 1: Oh I see, this is not galactic light. It's like
• 12:23
  Speaker 1: extra galactic in the sense of being outside of our gutsy.
• 12:26
  Speaker 2: Yes, take all the photons in the universe and subtract
• 12:29
  Speaker 2: the ones emitted by our galaxy the ones left over,
• 12:33
  Speaker 2: that's the extragalactic background light. So like most of the
• 12:36
  Speaker 2: photons in the universe are the EBL photons I see.
• 12:40
  Speaker 1: So any photon that we see out there in space
• 12:42
  Speaker 1: that doesn't come from a star within or any other
• 12:45
  Speaker 1: source within our Milky Way galaxy exactly.
• 12:49
  Speaker 2: And the estimate is that there's like ten to the
• 12:51
  Speaker 2: eighty four extragalactic background photons in the universe. So there's
• 12:56
  Speaker 2: a whole lot of them.
• 12:57
  Speaker 1: Ten to the eighty four.
• 13:00
  Speaker 2: Or ten with eighty four zeros in front of it.
• 13:03
  Speaker 2: It's a lot.
• 13:04
  Speaker 1: It sounds like a lot, But I don't know how
• 13:07
  Speaker 1: many come from our galaxy.
• 13:08
  Speaker 2: I mean, the whole history of the universe, the tiniest
• 13:11
  Speaker 2: fraction come from our galaxy. Almost every single photon in
• 13:15
  Speaker 2: the universe doesn't come from our galaxy. So almost every
• 13:19
  Speaker 2: single photon in the universe is an EBL photon.
• 13:23
  Speaker 1: Interesting, and this comes from other galaxies, or as we
• 13:26
  Speaker 1: talked about, maybe directly from the Big Bang, right.
• 13:28
  Speaker 2: It comes from the whole history of the universe, and
• 13:30
  Speaker 2: that's what's super fascinating about it. Some of these photons
• 13:33
  Speaker 2: were made before there were galaxies. Some of them were
• 13:35
  Speaker 2: made before there were stars. Some of them were made
• 13:38
  Speaker 2: during reionization, when these big clouds of neutral gas first
• 13:42
  Speaker 2: started to clump together to form stars. The whole history
• 13:46
  Speaker 2: of the universe is written in these photons. They're out there,
• 13:49
  Speaker 2: they're floating around. They contain all of this information, but
• 13:53
  Speaker 2: they're very, very tricky to see.
• 13:54
  Speaker 1: Right, So, like we're getting the lighte from Andromeda, which
• 13:57
  Speaker 1: is close to us, but we're also getting light from
• 14:00
  Speaker 1: super far away, which also happens to be really a
• 14:03
  Speaker 1: long time ago. That's all mixed in with this in
• 14:05
  Speaker 1: this micro background.
• 14:07
  Speaker 2: Like that's right, all that counts as EBL because it's
• 14:10
  Speaker 2: not coming from the Milky Way.
• 14:12
  Speaker 1: Maybe should be like extra Milky Way background light, right,
• 14:16
  Speaker 1: because it's not just like extra galactic like in the
• 14:19
  Speaker 1: general sense, it's just like our anything outside of our galaxy.
• 14:23
  Speaker 2: Yeah, extra our galactic background light.
• 14:25
  Speaker 1: Yeah, there you go. Yeah, because do so many in Andromeda,
• 14:28
  Speaker 1: the EBL would be different.
• 14:29
  Speaker 2: Right, Yeah, that's true. Those astronomers would argue with our
• 14:32
  Speaker 2: astronomers about how to name this thing. That would be fun.
• 14:36
  Speaker 1: Yeah, there a cartoonists would call my cartoonists. We would
• 14:40
  Speaker 1: duke it out, and then I, you know, I would
• 14:42
  Speaker 1: take them out and then I get to name it
• 14:44
  Speaker 1: for the whole universe.
• 14:45
  Speaker 2: You know, you wouldn't be the first cartoonist to actually
• 14:48
  Speaker 2: name something scientific. You know, Gary Larson actually had an
• 14:50
  Speaker 2: impact on science.
• 14:52
  Speaker 1: Oh yeah, what did he name?
• 14:54
  Speaker 2: Yes, this hilarious cartoon of a caveman giving a name
• 14:57
  Speaker 2: to those four pokey spikes on the back of a
• 14:59
  Speaker 2: status He calls it the phagomizer, after thag who was
• 15:03
  Speaker 2: killed by a stegosaurus. And it turns out that nobody
• 15:06
  Speaker 2: had actually named that before. So then scientists actually started
• 15:09
  Speaker 2: using phagomizer in their science papers. And now it's the
• 15:12
  Speaker 2: official name for the stegosaurus table.
• 15:14
  Speaker 1: Whoa. See, Now that's a well named thing in science.
• 15:18
  Speaker 1: You should put cartoonis in charge of everything.
• 15:21
  Speaker 2: Right, Yes, we should name everything after the caveman that
• 15:24
  Speaker 2: was killed by it.
• 15:26
  Speaker 1: Yeah. No, I'm just saying trust cartoonists, you know, with
• 15:31
  Speaker 1: anything science.
• 15:32
  Speaker 2: That's the lesson I learned from that. Yeah for sure.
• 15:35
  Speaker 1: All right, So this seems like a really broad concept
• 15:37
  Speaker 1: all the light. Basically, it's all the light and universe
• 15:39
  Speaker 1: we're getting from outside our galaxy. It seems like a lot.
• 15:43
  Speaker 1: Can we make any sense of it? Or is it
• 15:45
  Speaker 1: all just like a wash?
• 15:46
  Speaker 2: It's a lot, and it contains an incredible amount of information,
• 15:50
  Speaker 2: and it's really varied. It varies across the spectrum from
• 15:52
  Speaker 2: like super high energy gamma rays produced by really distant
• 15:57
  Speaker 2: active galactic nuclei like blazers, all the way down to
• 16:01
  Speaker 2: like really long wavelength radio that might be produced by
• 16:05
  Speaker 2: like dark matter decaying. There's an incredible amount of information
• 16:08
  Speaker 2: across the spectrum, but we want to see across the spectrum.
• 16:11
  Speaker 2: But we want to see it all the way across
• 16:13
  Speaker 2: the spectrum, and we also want to see where it's
• 16:15
  Speaker 2: coming from in the universe. So it's not just like
• 16:17
  Speaker 2: let's just see it. Let's see where it's coming from
• 16:19
  Speaker 2: and what the energy distributions are. Let's use that to
• 16:22
  Speaker 2: learn about the history of the universe.
• 16:25
  Speaker 1: Because I guess I wonder, like, how do you tell
• 16:27
  Speaker 1: the difference if it's how do you know it's coming
• 16:29
  Speaker 1: if it's coming from our galaxy or from outside of
• 16:31
  Speaker 1: our galaxy.
• 16:32
  Speaker 2: Yeah, that's really tricky because our galaxy turns out to
• 16:34
  Speaker 2: be really bright. You might think, well, can't you just
• 16:37
  Speaker 2: point your telescope in the night sky and gather some
• 16:39
  Speaker 2: EBL photons. Yeah, but it's sort of like trying to
• 16:42
  Speaker 2: see the Milky Way. If you're in times square. Times
• 16:45
  Speaker 2: Square is so bright you can't even see any stars
• 16:47
  Speaker 2: in the sky. So our Milky Way is so bright
• 16:51
  Speaker 2: that we can't see the distant dim things that are
• 16:53
  Speaker 2: hiding behind it. So there's a lot of competition from
• 16:56
  Speaker 2: the Milky Way, and the Milky Way is not something
• 16:58
  Speaker 2: we understand super well, so it's difficult to disentangle which
• 17:02
  Speaker 2: photons come from the extragalactic background light and which photons
• 17:05
  Speaker 2: come from our own Milky Way. There's an interplay there
• 17:08
  Speaker 2: where if we knew really, really well what the extragalactic
• 17:11
  Speaker 2: background light was, it would help us understand the Milky Way,
• 17:14
  Speaker 2: Or if we understood the Milky Way better, we could
• 17:16
  Speaker 2: subtract it from what we see in the night sky
• 17:18
  Speaker 2: and understand better what the extragalactic light is. We would
• 17:21
  Speaker 2: learn so much either way, But right now, the Milky
• 17:24
  Speaker 2: Way outshines everything, and the whole thing is kind of entangled.
• 17:27
  Speaker 2: It's a big mess.
• 17:28
  Speaker 1: So when you look at the stars of the nice Guy,
• 17:30
  Speaker 1: every star he sees is in the Milky Way galaxy, right,
• 17:32
  Speaker 1: you can really see stars that are outside of the
• 17:35
  Speaker 1: Milky Way. So any pinpoint you see out there is
• 17:37
  Speaker 1: in the Milky Way galaxy. So if I wanted to
• 17:39
  Speaker 1: see something outside of our galaxy, I would maybe point
• 17:42
  Speaker 1: my discope at a spot between the stars. Absolutely, do
• 17:46
  Speaker 1: you get raw light from outside of our galaxy or
• 17:49
  Speaker 1: is there still like dust from our galaxy there that
• 17:52
  Speaker 1: is maybe polluting that light?
• 17:54
  Speaker 2: So the answer depends on the frequency of light that
• 17:56
  Speaker 2: you're looking for. You're always looking through the Milky Way,
• 17:59
  Speaker 2: and there's always going to be dust that interferes, But
• 18:02
  Speaker 2: it depends on the wavelength. Some wavelengths of light can
• 18:04
  Speaker 2: penetrate that dust, some wavelengths of light can't. So it's
• 18:07
  Speaker 2: really a different puzzle at different wavelengths, from GAMER rays
• 18:10
  Speaker 2: to X rays to ultraviolet light. There's different sources of
• 18:14
  Speaker 2: photons that get confused between the Milky Way and the
• 18:17
  Speaker 2: extragalactic background light. A big factor is the zodiacal light
• 18:21
  Speaker 2: scattering of dust from within our solar system, which makes
• 18:23
  Speaker 2: everything very tricky, all right.
• 18:24
  Speaker 1: It sounds like we need to parse it by frequency,
• 18:26
  Speaker 1: and so let's break down the spectrum of light from
• 18:29
  Speaker 1: outside of our galaxy and see what it tells us
• 18:32
  Speaker 1: about what's out there. But first let's take a quick break.
• 18:48
  Speaker 1: All right, we're talking about the extra Milky Way background light.
• 18:51
  Speaker 1: I just renamed it. I called the extra tag tag
• 18:56
  Speaker 1: is a sizer background light.
• 18:59
  Speaker 2: I'm not even gonnam. It's all up to you at
• 19:01
  Speaker 2: this point.
• 19:02
  Speaker 1: I'll just called the extra larsen or here here we go,
• 19:06
  Speaker 1: we'll call it the extra far Side background line because
• 19:10
  Speaker 1: and that is both true and a homage to Gary Larson. Yeah,
• 19:13
  Speaker 1: absolutely right, because anything outside of our galaxy is technically
• 19:18
  Speaker 1: on the far side of the universe.
• 19:19
  Speaker 2: That's probably what he meant when he was talking about
• 19:21
  Speaker 2: the far side. He meant the other galaxies.
• 19:23
  Speaker 1: Yes, well, no, I don't think so. I'm saying in
• 19:26
  Speaker 1: this context it seems like an appropriateate. But anyways, let's
• 19:29
  Speaker 1: break it down by frequency, you said, because we're getting
• 19:31
  Speaker 1: a wash by all this life from outside of our galaxy.
• 19:34
  Speaker 1: We don't even know if it's coming from outside of
• 19:35
  Speaker 1: our galaxy. But if you break it down by frequency,
• 19:38
  Speaker 1: you can get a better handle on what's going on.
• 19:41
  Speaker 2: And the main challenges are that most of this stuff
• 19:43
  Speaker 2: is really dim because the sources are really distant. Everything
• 19:47
  Speaker 2: we're talking about is coming from outside our galaxy, which
• 19:50
  Speaker 2: means it's probably millions or billions of light years away,
• 19:54
  Speaker 2: So it's photons have been spread out. They don't get tired,
• 19:56
  Speaker 2: but they do get spread out. So everything out there
• 19:59
  Speaker 2: is really really dim, or there are sources in the
• 20:01
  Speaker 2: milky Way that are brighter than it, or there's like
• 20:04
  Speaker 2: dust and stuff interfering. The challenge is change as you
• 20:08
  Speaker 2: go through the frequency, just the same way that the
• 20:09
  Speaker 2: night sky changes in frequency. If you look at the
• 20:12
  Speaker 2: night sky in the optical or in the infrared or
• 20:14
  Speaker 2: in the UV, you see a very different picture.
• 20:17
  Speaker 1: All right, let's break it down, and let's start maybe
• 20:18
  Speaker 1: with the higher frequencies. What do we see at the
• 20:21
  Speaker 1: high frequencies.
• 20:22
  Speaker 2: So at the very highest frequencies, the highest energy photons,
• 20:25
  Speaker 2: we call these things gamma rays for silly historical reasons.
• 20:28
  Speaker 1: Related to the Hulk, right, That's where it started.
• 20:33
  Speaker 2: It started because in the early part of the century
• 20:36
  Speaker 2: we didn't really understand quantum mechanics or radiation, so we
• 20:38
  Speaker 2: just started naming things like alpha rays, beta rays, gamma rays.
• 20:43
  Speaker 2: We had no idea what gamma rays were. Then later
• 20:45
  Speaker 2: we understood, oh, they're just high energry photons, but they
• 20:47
  Speaker 2: already had this name. So you can't just say, oh,
• 20:50
  Speaker 2: gamma rays and X rays are really the same thing.
• 20:52
  Speaker 2: There's artificial distinction between them. We just sort of.
• 20:55
  Speaker 1: Stuck with it, and so they had the Hulk back
• 20:57
  Speaker 1: at the Court of the century.
• 21:00
  Speaker 2: Hulk was actually the one doing these science experiments. Remember,
• 21:02
  Speaker 2: Bruce Banner is a scientist man. You should read his
• 21:04
  Speaker 2: paper right now. They're great stuff.
• 21:07
  Speaker 1: It's the famous correspondence between Bruce Banner and Albert Einstein
• 21:10
  Speaker 1: right where they're like, pal come them.
• 21:14
  Speaker 2: It was in German, though, right, wasn't it in German?
• 21:17
  Speaker 1: I'm just messing with you, all right? Sorry, I keep going,
• 21:18
  Speaker 1: keep going. What do we see in the gamma rays?
• 21:20
  Speaker 2: So the gamma ray night sky is mostly surveyed by
• 21:23
  Speaker 2: a space probe called Fermei LAT, which is basically a
• 21:26
  Speaker 2: particle physics detector in space. Photons hid it and they
• 21:29
  Speaker 2: convert into a pair of electrons and positrons and we
• 21:32
  Speaker 2: track those. We can use that to measure the energy.
• 21:35
  Speaker 2: This is our best way to see the night sky.
• 21:38
  Speaker 2: In gamma rays. And if you look out in the
• 21:40
  Speaker 2: night sky you do see a bunch of gamma rays.
• 21:42
  Speaker 2: You see a huge source coming from the center of
• 21:45
  Speaker 2: our galaxy. Like the center of the galaxy emits a
• 21:47
  Speaker 2: very large number.
• 21:48
  Speaker 1: And that's from the black hole there, right.
• 21:50
  Speaker 2: That's from all sorts of crazy processes happening in the
• 21:53
  Speaker 2: center of the Milky Way. There's pulsars that emit gamma rays.
• 21:55
  Speaker 2: These are point sources. There's also just a lot of
• 21:57
  Speaker 2: diffuse emission of gamma rays from like really high energy processes,
• 22:02
  Speaker 2: like electrons getting accelerated really really hard and emitting gamma
• 22:06
  Speaker 2: ray photons. The problem is that we don't really understand
• 22:09
  Speaker 2: the center of the galaxy. So if you just look
• 22:11
  Speaker 2: at like the distribution of gamma rays in the center
• 22:13
  Speaker 2: of our galaxy, we don't understand it. We can't explain it.
• 22:16
  Speaker 2: There's lots of mysteries there, and so that's a real problem.
• 22:19
  Speaker 2: If you want to subtract out the Milky Ways contribution
• 22:22
  Speaker 2: and look at the rest of the universe, you don't
• 22:24
  Speaker 2: really understand what to subtract out.
• 22:27
  Speaker 1: You just like point your telescope at the center of
• 22:29
  Speaker 1: the galaxy and then point it away from the center
• 22:31
  Speaker 1: of the galaxy to kind of get a sense of
• 22:33
  Speaker 1: what's coming from the center and what is not.
• 22:35
  Speaker 2: Yeah, you can measure what's happening at the center, but
• 22:38
  Speaker 2: then you also want to understand how that varies across
• 22:41
  Speaker 2: the night sky. To do that, you really need some
• 22:43
  Speaker 2: sort of model so you can interpolate, so then when
• 22:45
  Speaker 2: you're looking at some other place, you can say how
• 22:48
  Speaker 2: much of this is from the center of the galaxy
• 22:49
  Speaker 2: and how much isn't. You can't just like turn off
• 22:52
  Speaker 2: the center of the galaxy in nature and see the
• 22:53
  Speaker 2: rest of it, or turn it back on or vary it.
• 22:56
  Speaker 2: So we need some sort of like understanding of the
• 22:57
  Speaker 2: center of the galaxy in or to extrapolate away from
• 23:00
  Speaker 2: it and like subtract it out from underneath the extragalactic background.
• 23:04
  Speaker 1: Light, so we can study the extragalactic background, or so
• 23:07
  Speaker 1: we can study the center of the galaxy or both both.
• 23:10
  Speaker 2: Because mostly the extragalactic background light in the gamma rays
• 23:13
  Speaker 2: we think is coming from the centers of other galaxies.
• 23:17
  Speaker 2: Right other galaxies we think are also emitting gamma rays
• 23:20
  Speaker 2: at high rates, and the extragalactic background light in the
• 23:23
  Speaker 2: gamma rays is then mostly from those other galactic nuclei,
• 23:26
  Speaker 2: and so if we could see those, we could understand
• 23:28
  Speaker 2: our own galaxy better, or if we understood our own
• 23:30
  Speaker 2: galaxy better, we could subtract it out and then understand
• 23:33
  Speaker 2: those other galaxies. So it's sort of a chicken and
• 23:36
  Speaker 2: egg problem. We don't really know how to pull this.
• 23:38
  Speaker 1: Apart, all right, So then what can we see in
• 23:40
  Speaker 1: the gamma rays when we look around us.
• 23:42
  Speaker 2: Well, there's this long standing mystery, but the center of
• 23:44
  Speaker 2: our galaxy, whether it's sending us signals of dark matter
• 23:47
  Speaker 2: like dark matter, we know this a lot of it
• 23:49
  Speaker 2: in the center of the galaxy, and we wonder if
• 23:51
  Speaker 2: sometimes when two dark matter particles bounce off each other
• 23:55
  Speaker 2: they actually annihilate, Like there might be dark matter and
• 23:57
  Speaker 2: anti dark matter, and it might be possible for it
• 24:00
  Speaker 2: to annihilate and actually produce photons. This is counterintuitive because
• 24:03
  Speaker 2: you think of dark matter as dark, not shining in
• 24:06
  Speaker 2: any electromagnetic spectrum. But there are theories where dark matter
• 24:09
  Speaker 2: will annihilate itself and make very high energy photons. And
• 24:13
  Speaker 2: in fact, there's a signal from the center of the
• 24:14
  Speaker 2: galaxy that we don't understand that a bunch of people
• 24:17
  Speaker 2: think is from dark matter. So we don't understand that
• 24:20
  Speaker 2: very well, and we'd love to look for that signal
• 24:22
  Speaker 2: in the centers of other galaxies, basically see if we
• 24:24
  Speaker 2: can reproduce this in the extra galactic background light. But
• 24:27
  Speaker 2: so far we have themen able to pull those things
• 24:29
  Speaker 2: apart and understand which photons come from other galaxies.
• 24:34
  Speaker 1: Wait, wait, wait, First of all, are you saying dark
• 24:36
  Speaker 1: matter is not maybe really dark?
• 24:38
  Speaker 2: Yeah, dark matter might be shining brightly from the center
• 24:40
  Speaker 2: of our galaxy.
• 24:41
  Speaker 1: Oh boy, And then can't we just point our telescope
• 24:44
  Speaker 1: at another galaxy to see what kind of signal we
• 24:46
  Speaker 1: get from.
• 24:47
  Speaker 2: Then, yeah, we can do that, and we can see
• 24:48
  Speaker 2: some other galaxies that are very clear, like galaxies with
• 24:51
  Speaker 2: quasars in them or blazars, you know, quasars that are
• 24:55
  Speaker 2: pointed right at us. Those are shooting really high energy
• 24:57
  Speaker 2: photons right at us, and that we can tell like, Okay,
• 25:00
  Speaker 2: it's definitely there. It's definitely there. So that's a part
• 25:03
  Speaker 2: of the extragalactic background light that we can tell. But
• 25:06
  Speaker 2: not every galaxy has an active nucleus, and we're interested
• 25:09
  Speaker 2: in studying those that aren't active because those are the
• 25:11
  Speaker 2: best ones for studying dark matter, but it's not always
• 25:14
  Speaker 2: clear which photons are coming from our galaxy in which
• 25:17
  Speaker 2: photons are coming from other galaxies, because remember we're inside
• 25:20
  Speaker 2: the Milky Way, and not all of the gamma rays
• 25:23
  Speaker 2: come directly from the center. Some of them are emitted
• 25:25
  Speaker 2: along the galactic plane, and those are still brighter than
• 25:28
  Speaker 2: the emissions from other.
• 25:29
  Speaker 1: Galaxies emitted by what we are we this would be
• 25:34
  Speaker 1: emitted by dark matter in the rim of the galaxy
• 25:37
  Speaker 1: or what.
• 25:38
  Speaker 2: Maybe dark matter in the room the galaxy. But anytime
• 25:40
  Speaker 2: an electron is accelerated, it's going to emit a photon,
• 25:43
  Speaker 2: and so there's some emission of photons from electrons in
• 25:46
  Speaker 2: the galactic plane that cloud our observations.
• 25:50
  Speaker 1: MM interesting. So looking at these gamma rays might reveal
• 25:54
  Speaker 1: the what's going on with dark matter.
• 25:56
  Speaker 2: Yeah, it would be super cool if we could make
• 25:58
  Speaker 2: like a map of these gamma rays from other galaxies
• 26:01
  Speaker 2: and then cross correlated with our understanding of like where
• 26:03
  Speaker 2: the density of matter is, Like we have a pretty
• 26:06
  Speaker 2: good understanding of where the galaxies are and the whole
• 26:08
  Speaker 2: cosmic web. If we could cross correlate these gamma rays
• 26:12
  Speaker 2: from like clumps of dark matter, then to be really
• 26:15
  Speaker 2: powerful evidence that maybe these gamma rays really are coming
• 26:19
  Speaker 2: from dark matter and not just from other sources of
• 26:21
  Speaker 2: gamma rays.
• 26:22
  Speaker 1: WHOA, would they have like a special signature if they
• 26:25
  Speaker 1: came from dark matter?
• 26:26
  Speaker 2: Unfortunately not, they're like energy distribution of these things is
• 26:29
  Speaker 2: not that different from the energy distribution we see from
• 26:32
  Speaker 2: other sources, which is what makes it so challenging and
• 26:34
  Speaker 2: why it's so important to understand all the other sources
• 26:38
  Speaker 2: of gamma rays so we can figure out which ones
• 26:41
  Speaker 2: might be coming from dark matter. It's like you're trying
• 26:43
  Speaker 2: to explain the spectrum with a few different blobs, but
• 26:45
  Speaker 2: the blobs aren't that different, so it's hard to tell
• 26:48
  Speaker 2: how much of each blob you need to use to
• 26:50
  Speaker 2: explain the spectrum that you're seeing.
• 26:52
  Speaker 1: WHOA. So, then if it turns out dark matter is shiny,
• 26:54
  Speaker 1: would you need to change the name of it.
• 26:57
  Speaker 2: Yes, and we'd come to you first.
• 27:00
  Speaker 1: I'll call it dark light as ray star war Z.
• 27:04
  Speaker 1: All right, Well, that's gamma rays, it might tell us
• 27:06
  Speaker 1: about dark matter. What about the next range of frequencies
• 27:10
  Speaker 1: in the spectrum of this background light?
• 27:12
  Speaker 2: So taking a step down and energy, you get to
• 27:15
  Speaker 2: X rays, and again there's just an arbitrary distinction between
• 27:18
  Speaker 2: gamma rays and X rays, but X rays are lower energy,
• 27:21
  Speaker 2: and here the technology is sort of like a bridge
• 27:23
  Speaker 2: between particle physics detectors that see gamma rays and more
• 27:26
  Speaker 2: traditional telescopes that see lower energy light. Here we have
• 27:31
  Speaker 2: X ray telescopes, and these use like weird X ray
• 27:34
  Speaker 2: optics because X rays are really energetic and really hard
• 27:36
  Speaker 2: to bend using optics, So there's all sorts of weird
• 27:40
  Speaker 2: tricks they use to try to gather and focus X rays.
• 27:43
  Speaker 2: But we have a couple of cool space telescopes and
• 27:44
  Speaker 2: New Star and Chandra up there observing the sky in
• 27:48
  Speaker 2: the X ray What do.
• 27:49
  Speaker 1: You mean they are hard to bend like that, You
• 27:51
  Speaker 1: can't focus them. You can focus them with a lens.
• 27:53
  Speaker 2: Yeah, exactly, because of the super high energy, they just
• 27:55
  Speaker 2: don't bend very much through a lens, and so you
• 27:58
  Speaker 2: need special techniques to shape these things, basically like wave
• 28:01
  Speaker 2: guides and weird constructions. We had a whole episode about
• 28:04
  Speaker 2: X ray telescopes and Chandra. Check it out for more
• 28:06
  Speaker 2: details on how to build your own X ray telescope.
• 28:09
  Speaker 1: Right, you can make them out of bones, right this,
• 28:11
  Speaker 1: X rays don't go through bones.
• 28:16
  Speaker 2: Yes, and we're calling the next one the Fred Flintstone Telescope. Exactly.
• 28:20
  Speaker 1: Yeah, there is.
• 28:21
  Speaker 2: We're gonna have a Stegosaurus.
• 28:23
  Speaker 1: Operator after Hanna Barbara.
• 28:24
  Speaker 2: Of course, if Hannah Barbera wanted a fund one, we
• 28:27
  Speaker 2: would name it after them, for sure. There you go.
• 28:30
  Speaker 1: I'm not sure there's still a.
• 28:31
  Speaker 2: Round somebody owns that iph.
• 28:35
  Speaker 1: Well you should follow the application.
• 28:37
  Speaker 2: But the night sky in the X ray is really
• 28:39
  Speaker 2: fascinating because this mostly comes from electrons. We were talking
• 28:43
  Speaker 2: earlier about electrons emitting super high energy photons. They also
• 28:47
  Speaker 2: emit X rays and this is a really cool German
• 28:50
  Speaker 2: name for it. It's called bremstra Lung, which means breaking light.
• 28:54
  Speaker 2: Since you have an electron it changes direction because it
• 28:56
  Speaker 2: HiT's like a magnetic field or something, it has to
• 28:59
  Speaker 2: give off a photon in order to do that, and
• 29:00
  Speaker 2: based on the energy of those electrons the amount of curvature,
• 29:03
  Speaker 2: how from, we give off X rays. So a lot
• 29:05
  Speaker 2: of the night sky in X ray comes from these
• 29:08
  Speaker 2: electrons giving off bremstralag.
• 29:10
  Speaker 1: I mean, these are electrons that are just floating out
• 29:12
  Speaker 1: there in space and if somehow they change direction, they
• 29:16
  Speaker 1: emit an X ray.
• 29:18
  Speaker 2: Yeah, exactly, electrons can change direction when they hit a
• 29:20
  Speaker 2: magnetic field or if they like zoom around the black
• 29:23
  Speaker 2: hole or something. Any sort of change of direction or
• 29:26
  Speaker 2: change of velocity, an electron will emit a photon.
• 29:29
  Speaker 1: And so the universe is just full of these electrons
• 29:31
  Speaker 1: or what They're just floating out there like dust or
• 29:34
  Speaker 1: are they like in galaxies or is this the stuff
• 29:36
  Speaker 1: between galaxies?
• 29:37
  Speaker 2: Electrons are everywhere, man, just thew way protons are. You know,
• 29:40
  Speaker 2: most of the universe is hydrogen, but by that we
• 29:43
  Speaker 2: mean protons and electrons, and often it's in plasma form.
• 29:47
  Speaker 2: It's not neutral hydrogen, so there are also clouds of
• 29:50
  Speaker 2: neutral hydrogen. But there's also just a lot of protons
• 29:52
  Speaker 2: and electrons flying out there, both in galaxies and between galaxies.
• 29:57
  Speaker 2: Remember that between galaxies is not as bright. There aren't stars,
• 30:01
  Speaker 2: but a huge fraction of baryonic matter in the universe,
• 30:04
  Speaker 2: meaning protons and electrons, is actually between the galaxies, not
• 30:08
  Speaker 2: in the galaxies. So yeah, electrons are everywhere, and they're
• 30:11
  Speaker 2: emitting X rays whenever they change direction.
• 30:13
  Speaker 1: Do you consider this noise or is this part of
• 30:16
  Speaker 1: what you want to see or is this getting in
• 30:18
  Speaker 1: the way of the interesting things you want to see.
• 30:20
  Speaker 2: This is definitely something you want to see because you
• 30:22
  Speaker 2: want to understand all the sources of it. But it's
• 30:25
  Speaker 2: really hard to pin down these sources. Some of it
• 30:27
  Speaker 2: we can associate with the centers of galaxies, so like
• 30:30
  Speaker 2: active galactic nuclei are pumping out these high energy X rays,
• 30:34
  Speaker 2: but a lot of it we can't. I read one
• 30:36
  Speaker 2: study that said that one percent of X rays can
• 30:39
  Speaker 2: be associated with known objects. The rest of it, we're
• 30:42
  Speaker 2: just like, we don't know what made this. So something
• 30:44
  Speaker 2: out there in the universe is like shooting out X
• 30:47
  Speaker 2: rays and we don't know what it is. Maybe it's
• 30:49
  Speaker 2: just a bunch of more active galactic nuclei that have
• 30:51
  Speaker 2: been like red shifted and are faint, but it could
• 30:54
  Speaker 2: also be other weird stuff like early universe black holes,
• 30:58
  Speaker 2: direct collapse black holes that formed during the early universe
• 31:02
  Speaker 2: and emitted X rays.
• 31:04
  Speaker 1: What do you mean only one percent, like one percent
• 31:07
  Speaker 1: is coming from these electrons floating around, or we're just
• 31:11
  Speaker 1: getting them from things that might have existed in the
• 31:13
  Speaker 1: universe a long time ago that we can't sort of
• 31:16
  Speaker 1: see it with their naked eye.
• 31:17
  Speaker 2: Yeah, we can't associate them with anything we've seen, so
• 31:20
  Speaker 2: We don't know what's making them. They could just be
• 31:22
  Speaker 2: diffuse electrons. It could be a big chunk of it.
• 31:24
  Speaker 2: There could also be a bunch of new astrophysical objects
• 31:26
  Speaker 2: out there emitting X rays that we've just never seen
• 31:29
  Speaker 2: before because we haven't been able to measure the X
• 31:32
  Speaker 2: ray spectrum outside of our galaxy. So there could be
• 31:35
  Speaker 2: these like direct collapse black holes that formed. Like remember
• 31:39
  Speaker 2: we were talking about having the early universe with this
• 31:41
  Speaker 2: famous moment when the universe became neutral. Protons and electrons
• 31:45
  Speaker 2: came together to make hydrogen, and that was the Dark Ages.
• 31:48
  Speaker 2: You have all this neutral hydrogen floating around, but there
• 31:50
  Speaker 2: were no stars yet. At the end of that, there's
• 31:52
  Speaker 2: a moment we call reionization, when the universe is then
• 31:55
  Speaker 2: pulling those atoms apart again. That's when we think stars
• 31:58
  Speaker 2: started to form. All possible that black holes formed at
• 32:02
  Speaker 2: the same moment. They didn't just collapse into massive stars.
• 32:05
  Speaker 2: Some clouds might have collapsed directly into black holes, and
• 32:08
  Speaker 2: those formations we think left their imprint in the X
• 32:11
  Speaker 2: ray spectrum. So if we could measure really really well,
• 32:14
  Speaker 2: we might see hints of direct collapse black holes from
• 32:17
  Speaker 2: the early universe.
• 32:18
  Speaker 1: Oh I see like, this is stuff happened that happened
• 32:20
  Speaker 1: a long time ago, but it happened so far away.
• 32:23
  Speaker 1: We're only just now getting the evidence of these things
• 32:26
  Speaker 1: that happened a long time ago, exactly.
• 32:28
  Speaker 2: And it's very dim, much dimmer than X ray sources
• 32:31
  Speaker 2: in our galaxy, and it's very hard to pull apart.
• 32:34
  Speaker 2: So if we understood the X ray spectrum super duper well,
• 32:37
  Speaker 2: we could ask questions like is there evidence in there
• 32:40
  Speaker 2: for direct collapse black holes or not? But right now
• 32:43
  Speaker 2: it's a big question mark. We don't know which photons
• 32:45
  Speaker 2: come from our galaxy, which photons come from outside the galaxy.
• 32:48
  Speaker 2: So we're like looking for a really tiny signal and
• 32:51
  Speaker 2: we have big question.
• 32:52
  Speaker 1: Marks, right, big ones. It ninety nine percent. We don't
• 32:55
  Speaker 1: know what it is, question exactly. All right, let's go
• 32:58
  Speaker 1: down to the next frequency. This is ultraviolet.
• 33:02
  Speaker 2: Yeah, so ultraviolet photons are super interesting, and our best
• 33:06
  Speaker 2: measurements of the ultraviolet extragalactic background light actually come from
• 33:10
  Speaker 2: the voyager probes. Remember those probes we sent out into
• 33:13
  Speaker 2: the Solar System to take pictures of the planets and
• 33:15
  Speaker 2: then just continue on out into space. They had instruments
• 33:19
  Speaker 2: on board for measuring ultraviolet photons because they were interested
• 33:22
  Speaker 2: in the atmospheres of those planets and seeing ultraviolet light
• 33:26
  Speaker 2: emitted from those planets to like study atmospheres and all
• 33:29
  Speaker 2: sorts of planetary physics.
• 33:31
  Speaker 1: WHOA. So even today we're still getting data from that spacecraft.
• 33:35
  Speaker 2: I think actually Voyager just shut down. It ran out
• 33:37
  Speaker 2: of power and we're no longer hearing from it. But
• 33:40
  Speaker 2: until very recently we were getting measurements from Voyager. It's
• 33:44
  Speaker 2: a really long lasting probe and it's one of our
• 33:47
  Speaker 2: best ways to understand the UV night sky.
• 33:50
  Speaker 1: Now what else can we see in this UV light?
• 33:52
  Speaker 2: So we can't see very much unfortunately, because the galaxy
• 33:55
  Speaker 2: is pretty bright in this UV light. Like planets emit
• 33:58
  Speaker 2: in the UV. Neutral hydrogen in our galaxy absorbs this
• 34:02
  Speaker 2: stuff and emits in the UV. But it's important for
• 34:05
  Speaker 2: understanding the distribution of matter, Like we'd love to know
• 34:09
  Speaker 2: more about the baryonic matter, the hydrogen that's between the galaxies.
• 34:13
  Speaker 2: That's where most of the hydrogen in the universe is.
• 34:16
  Speaker 2: If we could separate the UV light that's coming from
• 34:19
  Speaker 2: within our galaxy from the UV light that's coming from
• 34:21
  Speaker 2: outside the galaxy, then we could understand this better. But
• 34:24
  Speaker 2: we don't really have great measurements here, Like Voyager was
• 34:26
  Speaker 2: not set up to measure the extragalactic background light in
• 34:29
  Speaker 2: the UV spectrum. It's just like the only thing we have.
• 34:32
  Speaker 2: So it's if you look at the whole spectrum of
• 34:34
  Speaker 2: extragalactic background light, there's like a big gap there in
• 34:37
  Speaker 2: the UV because we really have almost no published studies
• 34:40
  Speaker 2: at all. It's just like a big blank.
• 34:43
  Speaker 1: But I guess what's making these you raise within our galaxy?
• 34:47
  Speaker 2: So mostly it's clouds of neutral hydrogen. Hydrogen, Remember, is
• 34:51
  Speaker 2: an atom, and electrons in the atom have certain energy levels,
• 34:54
  Speaker 2: and one of those energy level transitions corresponds with the
• 34:57
  Speaker 2: ultraviolet spectrum, and so neutral hydrogen tends to glow in
• 35:01
  Speaker 2: many different spectrum but one of them is in the UV.
• 35:04
  Speaker 1: UH just closed from being hot.
• 35:06
  Speaker 2: Yeah, exactly. You think of space as cold, but a
• 35:08
  Speaker 2: lot of this interstellar gas and intergalactic gas is actually
• 35:12
  Speaker 2: quite hot in the sense that it has high velocity
• 35:14
  Speaker 2: and each atom can have a significant amount of energy.
• 35:17
  Speaker 2: Even if we know that if you went out there
• 35:18
  Speaker 2: you'd freeze to death, you'd be surrounded by very sparse
• 35:21
  Speaker 2: hot gas. And when we study this stuff, there's a
• 35:24
  Speaker 2: lot that we don't understand, Like we can try to
• 35:26
  Speaker 2: subtract the Milky Way contribution in the UV, and then
• 35:30
  Speaker 2: there's all sorts of hints that other galaxies are emitting
• 35:33
  Speaker 2: in the UV in ways that we don't understand, Like
• 35:36
  Speaker 2: the Coma cluster is a famous puzzle. We don't understand
• 35:39
  Speaker 2: the UV spectrum from the Coma cluster. What's going on there?
• 35:42
  Speaker 2: Are those galaxies different from ours? Is there something else
• 35:45
  Speaker 2: between us and that galaxy? Are we just not understanding
• 35:48
  Speaker 2: the Milky Way contributions? It's a really open field to study.
• 35:52
  Speaker 1: All right, pretty cool. I guess I wonder if it
• 35:54
  Speaker 1: was healthy we stopped putting sunblock on all of our telscipes.
• 36:00
  Speaker 2: Or directly on our eyeballs.
• 36:02
  Speaker 1: Yeah, that never helps.
• 36:03
  Speaker 2: I think that's a bad idea. Don't do that people.
• 36:06
  Speaker 1: That's right, good health advice here on the Physics podcast.
• 36:10
  Speaker 1: All right, let's get into our maybe more interesting frequency spectrum,
• 36:14
  Speaker 1: the optical or visible light spectrum and infrared to see
• 36:18
  Speaker 1: what is out there beyond the bounds of our galaxy.
• 36:22
  Speaker 1: But first, let's take another quick break. Or we're talking
• 36:38
  Speaker 1: about light that comes from outside of our galaxy, which
• 36:41
  Speaker 1: turns out is like most of the light that we
• 36:44
  Speaker 1: can see.
• 36:45
  Speaker 2: Yeah, most of the photons in the universe didn't come
• 36:48
  Speaker 2: from our galaxy. And yet those are the photons that
• 36:50
  Speaker 2: are hardest to see because they're overwhelmed by the light
• 36:54
  Speaker 2: from our galaxy.
• 36:55
  Speaker 1: Right, because we're so close to our galaxy, we're in it.
• 36:59
  Speaker 1: But also I feel like it's hard because like it's
• 37:01
  Speaker 1: a big universe out there that's been around for a
• 37:03
  Speaker 1: long time. So we're getting stuff at the same time,
• 37:05
  Speaker 1: stuff that's closed, stuff that's far, stuff that's happening now,
• 37:08
  Speaker 1: stuff that's happened, you know, fourteen billion years ago. So
• 37:11
  Speaker 1: it's sort of like this big wash of light that
• 37:14
  Speaker 1: we're getting that you're trying to sift through.
• 37:16
  Speaker 2: Yeah, it's sort of amazing that you can understand any
• 37:18
  Speaker 2: of it, you know, But the tricks we use are
• 37:20
  Speaker 2: pretty basic. Look at the direction that came from, study
• 37:24
  Speaker 2: it over time, study it as a function of energy.
• 37:27
  Speaker 2: Using all those ideas, you can try to pull this
• 37:29
  Speaker 2: apart to make a consistent picture of what's out there
• 37:31
  Speaker 2: in the universe generating all these photons, and in the end,
• 37:34
  Speaker 2: that's the goal. Come up with the whole history of
• 37:36
  Speaker 2: the universe that can explain all the photons that we
• 37:39
  Speaker 2: can see. But first you got to see those photons.
• 37:42
  Speaker 1: Right, or I guess you can see them, you just
• 37:44
  Speaker 1: don't know which what it is that you're looking at.
• 37:47
  Speaker 2: Yeah, we see a bunch of photons. We'd love to
• 37:49
  Speaker 2: explain them all, and we're hoping that some of those
• 37:51
  Speaker 2: photons give us hints about what's outside our galaxy, not
• 37:55
  Speaker 2: just what's inside our galaxy.
• 37:56
  Speaker 1: Right. Well, so we've been going through different frequency rate
• 38:00
  Speaker 1: and just in the light that we get from outside
• 38:01
  Speaker 1: of our galaxy and how they can reveal different things,
• 38:04
  Speaker 1: and we're down to the visible light spectrum, like what
• 38:06
  Speaker 1: you could see with the naked eye.
• 38:08
  Speaker 2: Yeah, exactly. And so in the optical of course, we're
• 38:12
  Speaker 2: very curious about what's out there in the universe, and
• 38:15
  Speaker 2: a lot of the light that's in the optical spectrum
• 38:18
  Speaker 2: comes from stars. Right. The optical spectrum exists because we
• 38:21
  Speaker 2: evolve to be able to see light from our sun,
• 38:23
  Speaker 2: which makes us also able to see with our eyeballs
• 38:26
  Speaker 2: directly light from other stars, and of course there are
• 38:29
  Speaker 2: stars outside the Milky Way, and so a lot of
• 38:32
  Speaker 2: the optical light that's in the extra galactic background light
• 38:35
  Speaker 2: is emitted by stars in other galaxies.
• 38:37
  Speaker 1: Right, because our sun is a pretty typical star in
• 38:40
  Speaker 1: the universe. Like, what the kind of light that our
• 38:42
  Speaker 1: sun puts out is pretty tychopical all stars in the universe.
• 38:46
  Speaker 2: It's not that unusual. Our star is actually on the
• 38:49
  Speaker 2: larger side compared to your typical star. The most common
• 38:52
  Speaker 2: kind of star in the universe is a red dwarf,
• 38:54
  Speaker 2: which is a little redder and dimmer than our star,
• 38:57
  Speaker 2: but it's not a big deal. Red dwarfs are still
• 38:59
  Speaker 2: mostly in.
• 39:00
  Speaker 1: Optical Like, if we had been born under a red sun,
• 39:03
  Speaker 1: we would maybe you have different eyeball right.
• 39:07
  Speaker 2: Yeah, exactly, life could be very different if we evolved
• 39:09
  Speaker 2: around a red dwarf. We have a whole episode about
• 39:12
  Speaker 2: like how unlikely it is for life to have evolved
• 39:15
  Speaker 2: around a weird star like the Sun.
• 39:16
  Speaker 1: All right, So you're saying most of the visible light
• 39:19
  Speaker 1: that we get from outside the galaxy comes from stars
• 39:22
  Speaker 1: that are outside the galaxy, and these are mostly in
• 39:24
  Speaker 1: other galaxies, right, Like, there aren't a lot of stars
• 39:27
  Speaker 1: floating around, not in galaxies.
• 39:29
  Speaker 2: Yeah, there aren't a lot of stars out there, but
• 39:31
  Speaker 2: there are some, you know, there are stars in like
• 39:34
  Speaker 2: extended halos of galaxies or that were stripped out from
• 39:37
  Speaker 2: the galaxies during a merger or something and are now
• 39:40
  Speaker 2: just like floating out there in space. And that's something
• 39:43
  Speaker 2: we'd really like to understand. How much light is coming
• 39:46
  Speaker 2: from outside our galaxy, but also outside the galaxies we
• 39:49
  Speaker 2: can identify, you know, from between galaxies.
• 39:53
  Speaker 1: WHOA wait, you mean there could be a sun out
• 39:55
  Speaker 1: there in between galaxies all by itself with maybe a
• 39:59
  Speaker 1: planet orbiting around it with life. And what would they
• 40:03
  Speaker 1: see in the night sky?
• 40:04
  Speaker 2: They would only see galaxies, right, so their night sky
• 40:06
  Speaker 2: would be much much darker. They would only see smudges,
• 40:09
  Speaker 2: no pinpoints of light. Oh, they wouldn't see stars. They
• 40:12
  Speaker 2: wouldn't know that their sun is maybe just another star. Necessarily,
• 40:17
  Speaker 2: it's maybe unlikely because a star that experiences those kind
• 40:21
  Speaker 2: of forces would probably also lose its planets and then
• 40:24
  Speaker 2: to the extreme gravity being like tossed out of a galaxy.
• 40:26
  Speaker 2: But it's possible that the planets come along for the ride.
• 40:29
  Speaker 1: Yeah, it makes me a little sad because they wouldn't
• 40:31
  Speaker 1: be able to wish upon a star.
• 40:34
  Speaker 2: So you might think that this is like the easiest
• 40:36
  Speaker 2: extragalactic background like to see because you just like point
• 40:39
  Speaker 2: the hubble or point your eyeball between galaxies and see
• 40:42
  Speaker 2: what's there. But our galaxies actually really bright in this
• 40:45
  Speaker 2: kind of light, not just from the stars, also from
• 40:48
  Speaker 2: the scattering of dust. We talked about zodiacal light that
• 40:52
  Speaker 2: you can see from Earth. It's like at sunset you
• 40:55
  Speaker 2: can see this like a cone of light, like a
• 40:57
  Speaker 2: hazy pyramid just above the sunrise or sunset. But this
• 41:00
  Speaker 2: is the scattering of light off of dust in our
• 41:03
  Speaker 2: Solar system, and it's exactly at the frequency we want
• 41:06
  Speaker 2: to use to observe the extragalactic background light. So it's
• 41:09
  Speaker 2: a big problem.
• 41:11
  Speaker 1: You mean, like the dust that's out there in space
• 41:13
  Speaker 1: reflects visible light exactly, but it must reflect other kinds
• 41:17
  Speaker 1: of lights as well, doesn't it, Or only does it
• 41:20
  Speaker 1: reflect visible light especially well.
• 41:22
  Speaker 2: It reflects visible light especially well. It has to do
• 41:24
  Speaker 2: with like the size of these dust grains that are
• 41:27
  Speaker 2: like usually one to a few hundred microns, and these
• 41:31
  Speaker 2: are a huge cloud of dust all through the center
• 41:34
  Speaker 2: of the Solar system. It extends out we think, like
• 41:36
  Speaker 2: just past Mars, and it might all be Mars's fault. Actually,
• 41:39
  Speaker 2: dust storms on Mars might be kicking up a lot
• 41:42
  Speaker 2: of this stuff, but it reflects light typically in the
• 41:44
  Speaker 2: optical and also in the infrared, and it makes it
• 41:47
  Speaker 2: really really hard to see the extragalactic background light.
• 41:51
  Speaker 1: Well, this is it, I mean it. This makes it
• 41:53
  Speaker 1: hard to see even the galactic light, right because all
• 41:55
  Speaker 1: this dust pollution is within our solar system.
• 41:59
  Speaker 2: Yeah, exactly, it's a big problem for seeing outside of
• 42:02
  Speaker 2: our solar system. You're right, Even understanding our galaxy, the
• 42:05
  Speaker 2: zodiacal light is a big.
• 42:06
  Speaker 1: Issue, all right, Well, what can we tell from the
• 42:10
  Speaker 1: light that is coming from outside of our galaxy in
• 42:12
  Speaker 1: the visible spectrum?
• 42:13
  Speaker 2: Well, if you look at all the light that's coming
• 42:15
  Speaker 2: from outside of our galaxy, we can't explain it. Like
• 42:18
  Speaker 2: there's a bunch of photons that are out there that
• 42:20
  Speaker 2: just don't match our predictions. Like if you try to say, here,
• 42:23
  Speaker 2: I understand what's in the universe. Let me predict all
• 42:26
  Speaker 2: the photons we'll see in the optical spectrum, and then
• 42:28
  Speaker 2: you compare that to what we do see, there's a
• 42:30
  Speaker 2: big gap. Like there must be something wrong with our
• 42:33
  Speaker 2: modeling of what's out there in the universe or how
• 42:36
  Speaker 2: it emits. Like the data and our predictions do not agree.
• 42:40
  Speaker 1: But what do you mean the gap like we're missing light.
• 42:42
  Speaker 2: Yeah, there are more optical photons in the extragalactic background
• 42:46
  Speaker 2: then we can explain. So that means either there's something
• 42:49
  Speaker 2: else out there emitting photons we haven't accounted for it,
• 42:52
  Speaker 2: or maybe we've underestimated the amount of scattering from this
• 42:55
  Speaker 2: dust the zodiacal background light. But there are photons out
• 42:58
  Speaker 2: there that we can't explain.
• 43:00
  Speaker 1: Well, meaning like we look out there and we were
• 43:03
  Speaker 1: getting light, but there's no object there to see.
• 43:06
  Speaker 2: Yeah, a lot of this stuff is diffuse, right, We
• 43:08
  Speaker 2: don't know what's out there emitting it. We can't associate
• 43:10
  Speaker 2: it with any points source, and so we don't know
• 43:13
  Speaker 2: what diffuse sources of this light there are out there
• 43:16
  Speaker 2: in the galaxy. Maybe there are more of these like
• 43:18
  Speaker 2: rogue stars out there in the middle of the galaxy
• 43:20
  Speaker 2: and all their light like adds up to this big
• 43:22
  Speaker 2: diffuse component to explain it. Or maybe it's something simpl
• 43:26
  Speaker 2: or like misunderstanding the Milky Way.
• 43:27
  Speaker 1: Whoa wait, So maybe this mysterious light is just Milky
• 43:32
  Speaker 1: Way light. It's not necessarily extra galacti.
• 43:34
  Speaker 2: Yeah, exactly. We can't quite pull it apart. But there's
• 43:37
  Speaker 2: a really cool recent technique they're using to try to
• 43:40
  Speaker 2: get a sense for what's outside of our galaxy. So
• 43:42
  Speaker 2: they're can to help pull this thing apart, and that's
• 43:44
  Speaker 2: by using even higher energy photons that come from quasars.
• 43:48
  Speaker 2: So like the super high energy gamma rays we were
• 43:50
  Speaker 2: talking about earlier that are emitted from the centers of
• 43:52
  Speaker 2: very distant galaxies. We think we understand the spectrum that
• 43:55
  Speaker 2: should be coming from them, but as they travel through
• 43:58
  Speaker 2: the universe, sometimes they inter with lower energy photons they
• 44:02
  Speaker 2: can get like scattered or reabsorbed or change to another energy.
• 44:06
  Speaker 2: So if you look at the spectrum that comes from
• 44:08
  Speaker 2: those distant blazers and you see how it's modified from
• 44:11
  Speaker 2: what we expect, you can use that to try to
• 44:13
  Speaker 2: like map how many extragalactic background photons they ran into
• 44:18
  Speaker 2: along the way.
• 44:19
  Speaker 1: Wait wait, wait, light can run into light. I thought
• 44:22
  Speaker 1: light couldn't collide with other light particles.
• 44:24
  Speaker 2: No, you're exactly right. In general, light does not interact
• 44:27
  Speaker 2: with light because photons do not have electric charges, so
• 44:30
  Speaker 2: they don't interact directly, but they can interact indirectly, like
• 44:34
  Speaker 2: a photon can pair produce turn into an electron and
• 44:37
  Speaker 2: a positron.
• 44:37
  Speaker 1: Momentarily like randomly.
• 44:39
  Speaker 2: Yeah, every photon has a probability to pair produce at
• 44:41
  Speaker 2: any moment, and so there's a probability for two photons
• 44:44
  Speaker 2: to interact. We did whole episode about light beams crossing
• 44:47
  Speaker 2: and how you can actually study this. It's a rare process,
• 44:50
  Speaker 2: but it does happen. It requires this indirect process through
• 44:53
  Speaker 2: the electron field.
• 44:54
  Speaker 1: Okay, So then by looking at a source like equasor
• 44:58
  Speaker 1: that we know kind of pretty well and see how
• 45:00
  Speaker 1: the light is modified when it gets to us. By
• 45:02
  Speaker 1: the time it gets to us, you can sort of
• 45:04
  Speaker 1: get a sense of what's out there in between.
• 45:06
  Speaker 2: Yeah, you can try. It's tricky, like, first of all,
• 45:09
  Speaker 2: you have to be very confident that you understand the
• 45:11
  Speaker 2: unattenuated light, the light that was emitted by the quays are,
• 45:14
  Speaker 2: and then compare it to what we see. Then you
• 45:16
  Speaker 2: also have to convince yourself you understand everything else that
• 45:19
  Speaker 2: could happen to those photons be absorbed or influenced by
• 45:22
  Speaker 2: other things in the universe. That's why they like to
• 45:24
  Speaker 2: use these very high energy sources because they tend to
• 45:27
  Speaker 2: interact less than other photons, so they're more pristine.
• 45:30
  Speaker 1: Oh. Interesting, And again it's sort of weird that we're
• 45:33
  Speaker 1: getting all this light and we don't know where it's
• 45:35
  Speaker 1: coming from.
• 45:36
  Speaker 2: Yeah, exactly, But it's cool to be using these like
• 45:38
  Speaker 2: pencil beams of super high energy photons to get a
• 45:41
  Speaker 2: measure of like the other low energy photons along the way.
• 45:45
  Speaker 2: It's like we're getting information about photons that would never
• 45:47
  Speaker 2: have reached Earth.
• 45:48
  Speaker 1: Pretty cool, all right.
• 45:49
  Speaker 3: Now.
• 45:50
  Speaker 1: The last frequency range is the low frequency range of light,
• 45:53
  Speaker 1: and that's the infrared.
• 45:54
  Speaker 2: Yeah. The lowest frequency range is interesting because you know,
• 45:57
  Speaker 2: it's dominated by the microwaves, which we've studied very very
• 46:00
  Speaker 2: well and talked about and it's sort of like a
• 46:02
  Speaker 2: good example of what you can learn just by looking
• 46:04
  Speaker 2: at the night sky in the microwave. We've learned so
• 46:07
  Speaker 2: much about the early universe because there were microwaves emitted
• 46:10
  Speaker 2: in the very early universe, this moment when protons and
• 46:14
  Speaker 2: electrons came together in the universe became transparent again. Those
• 46:17
  Speaker 2: photons are still around and we've captured them and measured
• 46:20
  Speaker 2: things about the early universe. Really revolutionized all of cosmology.
• 46:24
  Speaker 2: That's just like a taste of what we could learn
• 46:26
  Speaker 2: from the other spectra. The microwaves, though, were like thirty
• 46:29
  Speaker 2: or forty times brighter than every other wavelength. For the
• 46:33
  Speaker 2: extragalactic background light, it's like the brightest part of the spectrum,
• 46:36
  Speaker 2: which is why it's sort of like the easy thing.
• 46:39
  Speaker 2: The microwaves are like the first by the d apple, But.
• 46:42
  Speaker 1: We skipped over i infrared though, didn't we? In terms
• 46:45
  Speaker 1: of frequency, infrared is higher frequency than microwaves.
• 46:49
  Speaker 2: Yeah, absolutely, Infrared is higher energy than microwaves. And the
• 46:52
  Speaker 2: cosmic infrared background is also super fascinating and we'd love
• 46:56
  Speaker 2: to study it in more detail because it might have
• 46:59
  Speaker 2: might be rich with information. Infrared light has lots of
• 47:02
  Speaker 2: really interesting point sources, like star forming regions and galaxies
• 47:06
  Speaker 2: at very high redshift that have been red shifted into
• 47:09
  Speaker 2: the infrared. Super interesting to study. And this was actually
• 47:12
  Speaker 2: studied by the same satellite that measured the cosmic microwave
• 47:16
  Speaker 2: background light. There was an instrument on board that was
• 47:18
  Speaker 2: capable of picking up infrared light, but it's much dimmer
• 47:21
  Speaker 2: than the microwave light, and so it's much more difficult
• 47:23
  Speaker 2: to study.
• 47:24
  Speaker 1: Well, I wonder if all these things just kind of
• 47:26
  Speaker 1: get smooched together, because you know, I know we've talked
• 47:29
  Speaker 1: about that things that are really far away. They might
• 47:32
  Speaker 1: emit light, but by the time that light gets to it,
• 47:34
  Speaker 1: because of the expanding universe, that light gets red shifted,
• 47:37
  Speaker 1: it becomes more red er. So like if you get
• 47:39
  Speaker 1: red light, it could come from basically anything, right.
• 47:43
  Speaker 2: Yeah, absolutely, you could have super high energy galactic nuclei
• 47:47
  Speaker 2: emitting very high energy photons and by the time they
• 47:50
  Speaker 2: get to us there infrared like even the cosmic microwave
• 47:54
  Speaker 2: background light. Those photons are super long wavelength, but when
• 47:57
  Speaker 2: they were emitted, they weren't. The plasma that made them
• 47:59
  Speaker 2: was super high energy. They were admitted a very very
• 48:02
  Speaker 2: high frequency. It's only the expansion of the universe that
• 48:05
  Speaker 2: stretched them out all the way down to the microwave.
• 48:08
  Speaker 2: So you're right, everything is piled on top of each
• 48:10
  Speaker 2: other a minute at one way.
• 48:11
  Speaker 1: Is this like the messiest kind of light we're getting?
• 48:14
  Speaker 1: Do you know what I mean? Like, because everything piles
• 48:15
  Speaker 1: onto that frequency spectrum as opposed to like the higher frequencies. Thinks,
• 48:19
  Speaker 1: don't get bluer.
• 48:20
  Speaker 2: Yeah, it's a beautiful mess down there at the long
• 48:22
  Speaker 2: wave lengths because everything's red shifted down there into the
• 48:25
  Speaker 2: dustbin of the universe.
• 48:26
  Speaker 1: Now, this gets us into the microwave range, which we
• 48:30
  Speaker 1: talked a lot about before. The cosmic microwave background, which
• 48:34
  Speaker 1: you said is included in this idea of the extragalactic background.
• 48:38
  Speaker 2: Exactly because it's generated by plasma that's outside the Milky Way,
• 48:42
  Speaker 2: and so it's definitely extra galactic. It's also the brightest
• 48:46
  Speaker 2: part of the spectrum, and so it's easiest to tackle,
• 48:48
  Speaker 2: and it tells us a lot about the very early universe.
• 48:51
  Speaker 2: So that's why it's sort of the best well known
• 48:53
  Speaker 2: and the best well studied.
• 48:54
  Speaker 1: So then the stuff we get that we call this
• 48:56
  Speaker 1: CMB the cosmic microwave background, we know for sure what
• 48:59
  Speaker 1: it is. We always say it's light from the beginning
• 49:02
  Speaker 1: of the universe. How well do we actually know that?
• 49:04
  Speaker 1: Wouldn't it be kind of confounded or mixed together with
• 49:08
  Speaker 1: you know, distance, stars exploding and things like that.
• 49:11
  Speaker 2: Absolutely, there are other sources in the microwave, including sources
• 49:14
  Speaker 2: from the Milky Way, but everything is easier when you're
• 49:18
  Speaker 2: looking for a bigger signal, right, it's easier to establish,
• 49:21
  Speaker 2: it's easier to subtract a way. Uncertainties in the Milky
• 49:24
  Speaker 2: Way can be larger without affecting your measurements because you
• 49:26
  Speaker 2: have a larger signal you're looking for.
• 49:28
  Speaker 1: What do you mean it's a bigger signal, Like it's
• 49:30
  Speaker 1: more powerful, or just a wavelength is bigger.
• 49:32
  Speaker 2: I mean it's more powerful. There are more photons, like
• 49:35
  Speaker 2: there are forty times as many photons in the microwave
• 49:38
  Speaker 2: than there are in the radio or in the UV
• 49:41
  Speaker 2: or in the optical. The universe is brighter in the
• 49:44
  Speaker 2: microwaves than they are any other spectrum.
• 49:47
  Speaker 1: Yes, because of the Big Bang, because of the Big
• 49:49
  Speaker 1: Bang was a huge source of it?
• 49:51
  Speaker 2: Or is it because of this early universe plasma? Yeah,
• 49:54
  Speaker 2: not technically the Big Bang, but there's a lot of
• 49:56
  Speaker 2: emission in the very early universe, and that and then
• 49:59
  Speaker 2: an and that got slid all the way down by
• 50:01
  Speaker 2: the expansion of the universe to the microwave. And so
• 50:04
  Speaker 2: it's sitting there as a very bright.
• 50:06
  Speaker 1: Signal and we sort of know what it is. But
• 50:08
  Speaker 1: then that means it also mean that one big source
• 50:12
  Speaker 1: the beginning of the universe is drowning out anything else
• 50:14
  Speaker 1: we might want to see in the microwave.
• 50:16
  Speaker 2: Absolutely, and that's why we've been studying into great detail
• 50:18
  Speaker 2: and trying to understand all the ripples in it and
• 50:20
  Speaker 2: the other sources of it. Again, when you have a
• 50:22
  Speaker 2: brighter signal, just everything is easier scientifically. You have more
• 50:26
  Speaker 2: data to play with, so you can do more tricks
• 50:28
  Speaker 2: like extrapolating from one region to the other. You have
• 50:31
  Speaker 2: better ways to validate all of your models. It's much
• 50:34
  Speaker 2: much more difficult when you're dealing with faint sources that
• 50:36
  Speaker 2: you're not even sure you're seeing. So seeing that in
• 50:39
  Speaker 2: the microwave and seeing it exactly the temperature we expected
• 50:42
  Speaker 2: was a great confirmation of our understanding of that whole process.
• 50:45
  Speaker 1: Mm interesting and it also heats up barburritos, all right.
• 50:50
  Speaker 1: So then the last frequency range is the radio waves,
• 50:53
  Speaker 1: which is like the lowest frigency, longest wavelength light that's
• 50:58
  Speaker 1: out there exactly.
• 51:00
  Speaker 2: And these are really cool experiments to see radio emissions
• 51:04
  Speaker 2: from the deep universe. We have these balloon experiments. Well,
• 51:08
  Speaker 2: you have like a radio antenna, but you cool it
• 51:10
  Speaker 2: down using liquid helium, and then you'll launch it up
• 51:14
  Speaker 2: like thirty seven kilometers above Texas or sometimes above the
• 51:17
  Speaker 2: South Pole, so we can gather radio signals from space.
• 51:21
  Speaker 2: You shield it so that it's not just like getting
• 51:23
  Speaker 2: your local NPR station, and you cool it down so
• 51:26
  Speaker 2: it can pick up the most faint signals, and then
• 51:28
  Speaker 2: you try to gather radio signals from deep space. We
• 51:31
  Speaker 2: measure a lot of these things, but we don't understand
• 51:33
  Speaker 2: all the radio waves that we see. Maybe some of
• 51:36
  Speaker 2: them are from dark matter colliding and emitting in the
• 51:39
  Speaker 2: radio Maybe there's some other diffuse emission of radio. Maybe
• 51:43
  Speaker 2: it's just something in the galactic foreground, something else in
• 51:46
  Speaker 2: the galaxy that's emitting in the radio waves we don't
• 51:48
  Speaker 2: yet understand. So we don't actually know if these photons
• 51:52
  Speaker 2: that we're seeing are EBL radio photons or just vanilla
• 51:56
  Speaker 2: milky way photons.
• 52:00
  Speaker 1: If they have an actual like source generating them, or
• 52:03
  Speaker 1: they're just kind of like noise. Is that what you mean.
• 52:05
  Speaker 2: Yeah, if there's a point source generating all these radio
• 52:08
  Speaker 2: frequency photons, we probably would have figured that out, could
• 52:12
  Speaker 2: identify it with like the center of the galaxy or
• 52:14
  Speaker 2: some black hole or some pulsar or something. So that
• 52:17
  Speaker 2: would have been easy, and we haven't been able to
• 52:19
  Speaker 2: do that, which means probably it's something diffuse, like maybe
• 52:22
  Speaker 2: dark matter and the whole halo colliding with itself or
• 52:25
  Speaker 2: something else. We don't really understand the sources here. WHOA.
• 52:29
  Speaker 1: All right, So to recap we're getting a lot of
• 52:32
  Speaker 1: lights from the universe. Only about one percent of that
• 52:36
  Speaker 1: light comes from our galaxy. Ninety nine percent of the
• 52:38
  Speaker 1: light that we see that when you look at the
• 52:40
  Speaker 1: nice sky comes from outside of the galaxy. And it
• 52:44
  Speaker 1: seems like ninety eight percent of that is all mystery light,
• 52:48
  Speaker 1: Like we don't know what is making that light, where's
• 52:50
  Speaker 1: it coming from? Right, That's kind of what it seems like.
• 52:52
  Speaker 2: Well, most of the light in the universe is generated
• 52:55
  Speaker 2: outside our galaxy, but most of the light that we
• 52:57
  Speaker 2: see is generated inside our galaxy because we're inside our galaxy.
• 53:01
  Speaker 2: So most of the light that we see in the
• 53:02
  Speaker 2: night sky is coming from the Milky Way. But that's
• 53:05
  Speaker 2: a tiny fraction of all the photons in the universe,
• 53:08
  Speaker 2: and so the rest of the universe is quite dim
• 53:10
  Speaker 2: in comparison to the Milky Way. But that's most of
• 53:12
  Speaker 2: the interesting stuff, and these photons contain the whole history
• 53:16
  Speaker 2: of the universe, but they're mostly outshined by the brightness
• 53:19
  Speaker 2: of the Milky Way, which just you know, happens to
• 53:21
  Speaker 2: be nearby.
• 53:23
  Speaker 1: Oh, I see, there's a lot of light out there,
• 53:25
  Speaker 1: but we don't get all of it is what you're saying.
• 53:28
  Speaker 1: Because we're so close the Milky Way galaxy, most of
• 53:30
  Speaker 1: the light that we get goes from the Milky Way.
• 53:32
  Speaker 2: Yeah, it's like you're standing right next to a lighthouse,
• 53:35
  Speaker 2: and so you can't really see anything that's far away.
• 53:37
  Speaker 2: Even if those photons are coming to you, they're mostly
• 53:39
  Speaker 2: outshined by the local sources.
• 53:41
  Speaker 1: But well, we can't see the cosmic background outside of
• 53:44
  Speaker 1: our galaxy. It's all sort of shrouded in mystery, it seems.
• 53:47
  Speaker 2: Yeah, it's tricky to disentangle the Milky Way from the
• 53:50
  Speaker 2: other sources. There's a lot of photons we don't understand,
• 53:53
  Speaker 2: a lot of question marks. Over the next few years,
• 53:55
  Speaker 2: we're hoping to turn on more sensitive instruments that can
• 53:58
  Speaker 2: disentangle these things make at our measurements. Maybe this extragalactic
• 54:02
  Speaker 2: background light's gonna come into sharper relief, and it might
• 54:05
  Speaker 2: teach us things about the universe. It could reveal all
• 54:07
  Speaker 2: sorts of weird stuff going on in the deep reaches
• 54:10
  Speaker 2: of space and in the deep history of time.
• 54:13
  Speaker 1: Yeah, and about the makeup of the universe as well.
• 54:16
  Speaker 1: If it tells us about dark matter and maybe dark energy.
• 54:20
  Speaker 1: I think Daniel. Maybe the only solution here is that
• 54:21
  Speaker 1: you're gonna have to leave the galaxy to get a
• 54:25
  Speaker 1: good look at this light. I'll pack a lot of children.
• 54:27
  Speaker 1: Do we have an ex make an extra Daniel, extra
• 54:30
  Speaker 1: galactic Daniel adventure here.
• 54:33
  Speaker 2: That's going to take a very very long time.
• 54:37
  Speaker 1: I'll report we got time, We got time, right.
• 54:39
  Speaker 2: I gotta make it back before next week's episode. Man,
• 54:41
  Speaker 2: I better build a fast ship.
• 54:43
  Speaker 1: Yeah, there you go. Well, first of all, invent the
• 54:45
  Speaker 1: faster than light space ship, and then then we're talking.
• 54:48
  Speaker 1: Sounds good, all right? Well, another reminder of how much
• 54:51
  Speaker 1: of the mystery we still have to observe and discover
• 54:54
  Speaker 1: and figure out. There are still lots of mysteries out there,
• 54:57
  Speaker 1: even in the light that bathes us every night, every day,
• 55:00
  Speaker 1: all around the Earth.
• 55:02
  Speaker 2: That's right, There is so much of the universe we
• 55:04
  Speaker 2: have not yet observed or understood, so much left to
• 55:07
  Speaker 2: discover for you young scientists out there, the next generation
• 55:10
  Speaker 2: of curious explorers.
• 55:12
  Speaker 1: Or the old ones, do right.
• 55:15
  Speaker 2: I'm just going to retire and let everybody else figure
• 55:17
  Speaker 2: it out and explain it to me at this point.
• 55:20
  Speaker 1: Well, hopefully they do it in time, because come on,
• 55:22
  Speaker 1: we're not getting any younger. Daniel, all right, well, we
• 55:26
  Speaker 1: hope you enjoyed that. Thanks for joining us, See you
• 55:29
  Speaker 1: next time.
• 55:34
  Speaker 2: For more science and curiosity, come find us on social media,
• 55:37
  Speaker 2: where we answer questions and post videos. We're on Twitter, This, Org,
• 55:41
  Speaker 2: Instant and now TikTok. Thanks for listening and remember that
• 55:45
  Speaker 2: Daniel and Jorge Explain the Universe is a production of iHeartRadio.
• 55:49
  Speaker 2: For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts,
• 55:55
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