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How do galaxies die? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, how old are you? While I'm turning forty
• 0:12
  Speaker 1: seven pretty soon? Does that feel old? Too? Well? Whenever
• 0:16
  Speaker 1: I'm feeling old, I just decided to count my age differently.
• 0:20
  Speaker 1: Oh yeah, like a different base number, you know, Like
• 0:23
  Speaker 1: how old would I be if I lived in a
• 0:25
  Speaker 1: different place, For example, because Mars is years longer, I'd
• 0:29
  Speaker 1: be in my early twenties if I was a Martian.
• 0:31
  Speaker 1: But you would die in like a few hours, would
• 0:34
  Speaker 1: any or a few minutes if you lift the Mars
• 0:36
  Speaker 1: right now, depending if Elon Musk has money left from
• 0:39
  Speaker 1: his Twitter purchased to fund our oxygen on Mars. But
• 0:42
  Speaker 1: I guess even if you're a Martian, you would still
• 0:45
  Speaker 1: live the same amount of time, so you'd be just
• 0:47
  Speaker 1: as old. It'd just be a different year unit, that's right.
• 0:50
  Speaker 1: But it's all about how the number feels. Man. Yeah,
• 0:53
  Speaker 1: but also you have to take it to account your
• 0:55
  Speaker 1: life expectancy. That also gets changed on Mars, right yep.
• 0:59
  Speaker 1: On my cars we all die young anyway, or maybe not.
• 1:02
  Speaker 1: Maybe the air there is so much better that people
• 1:04
  Speaker 1: would live longer, long enough to tweet forever. Hi, I'm
• 1:22
  Speaker 1: Horrhand made cartoonists and the co author of frequently asked
• 1:24
  Speaker 1: questions about the universe. Hi, I'm Daniel. I'm a particle
• 1:28
  Speaker 1: physicist and a professor at U C Irvine, and I
• 1:31
  Speaker 1: plan to teach until my old old age. Do you
• 1:34
  Speaker 1: think you'll still have something to teach when you're that old?
• 1:37
  Speaker 1: I often teach freshman physics, which hasn't changed since Newton,
• 1:41
  Speaker 1: so I'm planning to teach it until I grade away.
• 1:46
  Speaker 1: What if you start forgetting things, even Newtonian physics? I'm
• 1:49
  Speaker 1: sorry would you ask? I forgot the first part of
• 1:50
  Speaker 1: your question, but anyways, Welcome to our podcast, Daniel and
• 1:55
  Speaker 1: Jorge Explain the Universe, a production of I Heart Radio,
• 1:58
  Speaker 1: in which we explore this incre rereadibly old and incredibly
• 2:02
  Speaker 1: young universe, which has lived for billions of years and
• 2:05
  Speaker 1: may survive for trillions and trillions more. We explore all
• 2:09
  Speaker 1: of the fast moving and slow paced processes that grow
• 2:13
  Speaker 1: and shape and define our universe and make it the
• 2:15
  Speaker 1: one that we love. Our goal is not to be
• 2:17
  Speaker 1: intimidated by the age of the universe or the billions
• 2:20
  Speaker 1: and trillions of years that are to come, but to
• 2:23
  Speaker 1: try to understand the processes that shape it, and to
• 2:26
  Speaker 1: explain all of them. To you, that's right, because we
• 2:29
  Speaker 1: live in an amazing universe that's been around for fourteen
• 2:32
  Speaker 1: billion years or maybe more, which sounds like a long time,
• 2:35
  Speaker 1: but who knows. We could be maybe in the universe's
• 2:38
  Speaker 1: infancy right now, and the universe might live on for
• 2:41
  Speaker 1: a long time, maybe even forever. Imagine if you had
• 2:44
  Speaker 1: lived in the universe in the first few hundred thousand years,
• 2:47
  Speaker 1: you might think, oh, this is what the universe is like,
• 2:50
  Speaker 1: not realizing that it was but the first flash of
• 2:52
  Speaker 1: the universe and that it would live for billions of
• 2:54
  Speaker 1: years with glowing stars. And now we live in the
• 2:56
  Speaker 1: starfield era of the universe. But it might just be
• 2:59
  Speaker 1: the first flash of a universe that lasts for trillions
• 3:02
  Speaker 1: of years and is mostly filled with black holes. So
• 3:05
  Speaker 1: this era of the universe could be very short lived
• 3:08
  Speaker 1: and very weird, weird, but also pretty cool. I think
• 3:12
  Speaker 1: you know right, Technically we are living in the cool
• 3:14
  Speaker 1: years of the universe, not the hot, crazy, insane years,
• 3:18
  Speaker 1: and also not the you know, super freezing years. Well,
• 3:21
  Speaker 1: because the universe is expanding, it's cooling, and so technically
• 3:24
  Speaker 1: it gets cooler every year, and not just because they
• 3:27
  Speaker 1: keep releasing more Marvel movies about more multiverses. Yeah, but
• 3:31
  Speaker 1: at some point you get it. I think it is
• 3:32
  Speaker 1: possible to be too cool, you know, like if the
• 3:36
  Speaker 1: entropy of the universe is set a maximum and the
• 3:38
  Speaker 1: temperature is like zero out there, Kelvin, that's not cool,
• 3:42
  Speaker 1: Doctor Strange, and the heat death of the universe coming soon.
• 3:47
  Speaker 1: I hear the last one is pretty dark, right, Everything
• 3:49
  Speaker 1: is getting darker. Everything in the Marvel universe is getting darker.
• 3:52
  Speaker 1: But in our universe, things continue to glow and brighten
• 3:56
  Speaker 1: and to fill the universe with photons that allow us
• 3:58
  Speaker 1: to see the incredible view was across billions and billions
• 4:01
  Speaker 1: of miles and allow our tiny human brains to grapple
• 4:05
  Speaker 1: with questions about how everything came to look the way
• 4:07
  Speaker 1: that it does and how long it will continue to shine. Yeah,
• 4:11
  Speaker 1: because unless you're Elon Musk, I guess the universe will
• 4:14
  Speaker 1: be bright and shiny and beautiful and amazing and an
• 4:18
  Speaker 1: open mystery pretty much until your old age. Right, is
• 4:21
  Speaker 1: Elon Musk going to figure it out? Is that what
• 4:22
  Speaker 1: you're saying. I think he's going to figure out how
• 4:24
  Speaker 1: to beat old age. He's going to download himself into
• 4:28
  Speaker 1: a Tesla and launch himself into space and live there forever.
• 4:31
  Speaker 1: That's right. With that, you know, special module for twittering,
• 4:36
  Speaker 1: Maybe he should combine all of his companies, you know, twistler,
• 4:40
  Speaker 1: put Twitter onto a Tesla in a boring tube and
• 4:43
  Speaker 1: launch that whole thing into space. That's right, Yeah, be
• 4:47
  Speaker 1: a Twistla X would be the company name, the Boring
• 4:52
  Speaker 1: twistle X. But we are not funded by Elon Musk,
• 4:56
  Speaker 1: although you know you know what to send your donations
• 4:58
  Speaker 1: Elon if you're a listener. But we are motor vated
• 5:00
  Speaker 1: by the desire to understand everything that's around us and
• 5:03
  Speaker 1: figure out exactly how long this flash in the pan
• 5:06
  Speaker 1: will last. Yeah, because as much as we know about
• 5:08
  Speaker 1: how the universe works right now, there are still open
• 5:11
  Speaker 1: questions about how the universe will keep on working and
• 5:14
  Speaker 1: how it's gonna work up until the end of the universe.
• 5:16
  Speaker 1: Remember that when you look up at the sky, it
• 5:19
  Speaker 1: seems sort of static and frozen. You don't notice from
• 5:21
  Speaker 1: day to day stars appearing or disappearing, or changing their location.
• 5:25
  Speaker 1: But that's just because we live in a tiny little
• 5:28
  Speaker 1: slice in the universe operates on much longer time scales.
• 5:32
  Speaker 1: If you could see a movie of the universe sped
• 5:34
  Speaker 1: up to see all fourteen billion years in just a minute,
• 5:37
  Speaker 1: it would seem like a crazy, chaotic, dynamic place. Even
• 5:41
  Speaker 1: our Solar System has had planets shift around in orbit.
• 5:44
  Speaker 1: Our galaxy is spinning and swirling and colliding. All sorts
• 5:48
  Speaker 1: of stuff is happening, and as we look out into
• 5:50
  Speaker 1: the universe on those cosmic scales, we ask questions about
• 5:53
  Speaker 1: how everything came to be and how long it will
• 5:55
  Speaker 1: continue to swirl and glow. Yeah, so your planet to
• 5:59
  Speaker 1: be a professor to you grow old or even die.
• 6:01
  Speaker 1: Apparently they're going to have to drag me out of
• 6:03
  Speaker 1: this office. I'm gonna be here until the Milky Way
• 6:05
  Speaker 1: stops spinning. I have a similar plan and plans to
• 6:09
  Speaker 1: be a cartoonis until rich I see, so another hundred
• 6:13
  Speaker 1: billion years. Yeah, so basically the same thing as growing
• 6:18
  Speaker 1: old till I die. Yeah. But anyways, it is an
• 6:21
  Speaker 1: interesting and growing universe, which makes me sort of wondering
• 6:25
  Speaker 1: how it's all going to end, If not the universe,
• 6:27
  Speaker 1: at least maybe our galaxy. That's right. On the podcast,
• 6:30
  Speaker 1: we have talked about the future of the Earth and
• 6:32
  Speaker 1: our Solar System, how the Earth will get subsumed eventually,
• 6:36
  Speaker 1: into the outer layers of the Sun. We've even talked
• 6:38
  Speaker 1: about the lifetime of stars, how long they will burn,
• 6:40
  Speaker 1: and how long our universe will keep making stars. But
• 6:44
  Speaker 1: the universe is not made just of planets or solar
• 6:47
  Speaker 1: systems or stars. One way to look at the universe
• 6:49
  Speaker 1: is that the basic unit are galaxies, these big swirling
• 6:53
  Speaker 1: swarms of billions of stars that flow through the universe.
• 6:56
  Speaker 1: How do galaxies form and how long do they live?
• 7:00
  Speaker 1: Do they just keep spinning until they become rich cartoonists?
• 7:03
  Speaker 1: So today we have an amazing and galactic question. Today
• 7:06
  Speaker 1: will be tackling how do galaxies die or how do
• 7:15
  Speaker 1: they fade away? I guess maybe maybe that's a nicer
• 7:17
  Speaker 1: way to say. How do they become emeritus galaxies? How
• 7:20
  Speaker 1: do they move on? How do they join the farm?
• 7:23
  Speaker 1: How do we make room for new galaxies because they're
• 7:26
  Speaker 1: just taken up space these old folks, Oh right, well,
• 7:29
  Speaker 1: well there's plenty of space and space right, that's true,
• 7:31
  Speaker 1: there is plenty of room for galaxies. But you know,
• 7:34
  Speaker 1: they are using the raw materials. They're occupying a whole
• 7:37
  Speaker 1: lot of carbon and iron, all sorts of other stuff
• 7:40
  Speaker 1: that could go to seed new stars and new galaxies
• 7:43
  Speaker 1: and new forms of life and intelligence. Well, the first
• 7:46
  Speaker 1: thing I thought when I saw this title was like,
• 7:48
  Speaker 1: what galaxies can die? I didn't know that they do.
• 7:51
  Speaker 1: They have like a beating heart. It depends a little
• 7:54
  Speaker 1: bit how you define death, right. Galaxies don't live, of course,
• 7:58
  Speaker 1: in the same sense that we're getic entities do the
• 8:01
  Speaker 1: way you and I do. But they do have a
• 8:02
  Speaker 1: really interesting life cycle, and at some point they stop
• 8:06
  Speaker 1: making stars, and eventually those stars will burn out, and
• 8:09
  Speaker 1: so the fate of galaxies is in fact to fade
• 8:12
  Speaker 1: away into darkness, which some could describe as the death
• 8:16
  Speaker 1: of a galaxy. I see you're defining the death of
• 8:19
  Speaker 1: a galaxy as when it I guess when it goes dark?
• 8:21
  Speaker 1: Is that what you're saying, Like when it stops shining
• 8:23
  Speaker 1: or we can't see it, or do you define it
• 8:25
  Speaker 1: there's like no activity, or when it breaks apart, or
• 8:28
  Speaker 1: when it turns into a black hole. There are many
• 8:30
  Speaker 1: possible ways to kill a galaxy. I guess there are
• 8:33
  Speaker 1: lots of milestones in galaxy retirement. But scientists call this
• 8:36
  Speaker 1: galaxy quenching. When it stops making stars, those stars burn
• 8:41
  Speaker 1: for a while and then eventually they will all fade out,
• 8:44
  Speaker 1: and you will indeed get just black holes with lumps
• 8:47
  Speaker 1: of stuff swirling around it. I see, So you're really
• 8:50
  Speaker 1: talking more about asking how do galaxies retire, like when
• 8:53
  Speaker 1: they start producing light? Kind of. Yah, it's not a
• 8:56
  Speaker 1: funeral home we're pushing these galaxies into. It's a nice
• 8:59
  Speaker 1: senior living facility with all sorts of other active galaxies
• 9:03
  Speaker 1: to hang out with. Interesting, there are cost bank retardant homes,
• 9:08
  Speaker 1: so you have early bird specials too. Well. Once you
• 9:10
  Speaker 1: no longer making stars, you're not really part of the
• 9:12
  Speaker 1: game anymore. Uh yeah, yeah, I guess you're you're off
• 9:18
  Speaker 1: the rat race kind of to see who's the Shinese galaxy.
• 9:21
  Speaker 1: You're definitely onto a new era in your life because
• 9:24
  Speaker 1: if you don't have stars, then you can't make new elements.
• 9:27
  Speaker 1: You're stuck with the composition that you have. Remember that
• 9:30
  Speaker 1: new elements are only born in the heart of stars
• 9:33
  Speaker 1: and supernovas and then the collisions of neutron stars. But
• 9:36
  Speaker 1: once you've stopped making stars, you're no longer initiating that
• 9:40
  Speaker 1: process that ends up with new elements, and so eventually
• 9:43
  Speaker 1: you're going to get stuck with whatever you have. You'd
• 9:45
  Speaker 1: be sort of frozen with your mixture of stuff. All right, Well,
• 9:50
  Speaker 1: we'll get into the details of galaxies retirement, both from
• 9:54
  Speaker 1: its job and mainly from its life. But as usually,
• 9:57
  Speaker 1: we were wondering how many people had had these dark
• 9:59
  Speaker 1: thoughts about galaxies and what happens to them in the future.
• 10:03
  Speaker 1: So it's usual Daniel went out there and ask people
• 10:05
  Speaker 1: a question, how do galaxies die? So thanks everybody who
• 10:08
  Speaker 1: participates in these one questions. If you'd like to hear
• 10:11
  Speaker 1: your speculations here on the podcast for everybody else to enjoy,
• 10:15
  Speaker 1: please don't be shy right to us two questions at
• 10:18
  Speaker 1: Daniel and Jorge dot com. Think about it for a second,
• 10:20
  Speaker 1: how do you think galaxies retired? Here's what people had
• 10:25
  Speaker 1: to say. I think I have a decent guest for this.
• 10:27
  Speaker 1: There are supermassive the black holes at the center of
• 10:30
  Speaker 1: every galaxy, and at some point black holes do die
• 10:36
  Speaker 1: or evaporate. Rather, it is possible. So maybe when that happens,
• 10:41
  Speaker 1: when the black hole kind of doesn't exist anymore than
• 10:44
  Speaker 1: the galaxies just disindigates. As far as I know, galaxies
• 10:49
  Speaker 1: die when their stares start dying and stop making more
• 10:55
  Speaker 1: stories and and it just becomes a cold cloud of
• 10:58
  Speaker 1: sadness and you can't see it. So I guess that's
• 11:04
  Speaker 1: a dead galaxy. I don't know how galaxies die. I
• 11:08
  Speaker 1: would guess that either the if it's a black hole
• 11:12
  Speaker 1: in the middle of the galaxy, that it as opposed
• 11:15
  Speaker 1: to swallows large parts of the of the galaxy, or
• 11:19
  Speaker 1: that stars either you should have died by not admitting
• 11:23
  Speaker 1: as much light as they previously would have been, and
• 11:25
  Speaker 1: so the galaxy becomes sort of darker and darker. So
• 11:28
  Speaker 1: I think galaxies are so large and have so much
• 11:31
  Speaker 1: angular momentum that all the stars within the galaxy will
• 11:35
  Speaker 1: reach the end stages of their life and we will
• 11:38
  Speaker 1: no longer have the starting conditions necessary to get new
• 11:42
  Speaker 1: stars created before the entire galaxy will be eaten up
• 11:46
  Speaker 1: by the supermassive black hole. Galaxies die in three ways,
• 11:51
  Speaker 1: first when they crash in and merge with a larger galaxy,
• 11:56
  Speaker 1: second when the stars fall in to the black hole
• 12:01
  Speaker 1: at the center of the galaxy, and third when the
• 12:05
  Speaker 1: galaxy creates no news stars and the stars and the
• 12:11
  Speaker 1: galaxy die from old age. The galaxies die if eventually
• 12:18
  Speaker 1: they lose the ability to make new stars. So this
• 12:23
  Speaker 1: can come from a different reasons, but one of them
• 12:27
  Speaker 1: that I know it might be that the black hole
• 12:31
  Speaker 1: from the center of the galaxy starts this process by
• 12:37
  Speaker 1: emitting probably powerful cluss or mm hmm, the supermassive black
• 12:44
  Speaker 1: holes in the center suck them up or something. All right,
• 12:48
  Speaker 1: dark answers for a dark question. I guess a lot
• 12:50
  Speaker 1: of people went for the black hole scenario. Yeah, that's
• 12:53
  Speaker 1: really interesting. People are aware that eventually everything will fall
• 12:57
  Speaker 1: into the black hole and imagine that that's what's going
• 12:59
  Speaker 1: to kill all the galaxy. But little do they know
• 13:04
  Speaker 1: there's something worth around the corner or what. Yeah, there's
• 13:07
  Speaker 1: sort of two competing timelines. They're like, will the stars
• 13:09
  Speaker 1: burn out, will it stop making stars before it falls
• 13:13
  Speaker 1: into the black hole? Or will the black hole like
• 13:15
  Speaker 1: eat up a bunch of otherwise lively elements of the
• 13:18
  Speaker 1: galaxy which could have made new stars if only they
• 13:21
  Speaker 1: hadn't fallen into that gravitational Well, all right, Well, before
• 13:25
  Speaker 1: we start to consider a galaxy's death, let's um, you know,
• 13:28
  Speaker 1: let's do a little retrospective here, and let's consider the
• 13:31
  Speaker 1: whole life of the galaxy. So let's start at getting Daniel,
• 13:34
  Speaker 1: how did galaxies even get born? Yeah, I like that
• 13:37
  Speaker 1: we begin with the nostalgic so like montage, musical mona,
• 13:42
  Speaker 1: all the good moments, remember those with an old timey
• 13:46
  Speaker 1: like film field, something soft focus, you know, fuzzy pictures
• 13:50
  Speaker 1: of the galaxy playing in the kiddie pool, like the
• 13:53
  Speaker 1: projector sounds quit, the galaxy being mean to the cat,
• 13:57
  Speaker 1: you know, that kind of stuff, but looking so cute.
• 14:01
  Speaker 1: No pictures of the galaxy crying or I think of
• 14:06
  Speaker 1: galaxies is one of the most fundamental units of the
• 14:09
  Speaker 1: universe because even though the stars are within them, it's
• 14:12
  Speaker 1: the galaxies that are formed by like the initial fluctuations.
• 14:17
  Speaker 1: So you know, you begin at the very very early
• 14:19
  Speaker 1: stages of the universe, everything is smooth and filled with matter.
• 14:22
  Speaker 1: There's no wiggles, there's no extra lumpy bits. And then
• 14:25
  Speaker 1: you get quantum mechanics that fluctuates and gives you like
• 14:28
  Speaker 1: a little bit more stuff here, maybe fewer particles over here.
• 14:31
  Speaker 1: That's really tiny, that's super microscopic. That kind of quantum
• 14:34
  Speaker 1: fluctuation is happening all the time, even today. But the
• 14:37
  Speaker 1: very early universe, those fluctuations were blown up to a
• 14:40
  Speaker 1: huge scale by early universe inflation. The way space itself
• 14:45
  Speaker 1: was stretched by like a factor of ten to the
• 14:47
  Speaker 1: thirty blew up these little tiny lumps, these little bits
• 14:51
  Speaker 1: where something was heavier and something was lighter into actual
• 14:54
  Speaker 1: macroscopic sizes. And then gravity took over and it said,
• 14:57
  Speaker 1: all right, this spot is a little bit denser than
• 15:00
  Speaker 1: that spot, so all the particles are going to get
• 15:02
  Speaker 1: attracted over there. So you begin with these very shallow
• 15:06
  Speaker 1: gravitational wells that slowly roll more particles into them, and
• 15:10
  Speaker 1: then they get stronger and stronger and stronger. We're talking
• 15:12
  Speaker 1: about it's mostly dark matter. It builds these gravitational wells
• 15:16
  Speaker 1: that then normal matter falls into, and where you have
• 15:19
  Speaker 1: these wells, you have big blobs of matter and that's
• 15:22
  Speaker 1: what forms galaxies. Yeah, I think we start talk about
• 15:25
  Speaker 1: it as the seeds of the galaxies, right, Like the seeds,
• 15:29
  Speaker 1: like where the galaxies came from, was all sort of
• 15:31
  Speaker 1: laid out in those first moments of the Big Bang,
• 15:34
  Speaker 1: so kind of in a way like galaxies were maybe
• 15:36
  Speaker 1: the first things that were defined in the universe, the
• 15:39
  Speaker 1: first kind of organization, right, Yeah, you can think about
• 15:41
  Speaker 1: it like that. We think that probably stars formed and
• 15:43
  Speaker 1: then they formed into galaxies, but they all formed out
• 15:46
  Speaker 1: of these big clouds of gas that were themselves formed
• 15:50
  Speaker 1: by these dark matter halos, which were themselves formed by
• 15:54
  Speaker 1: those initial seeds that we talked about. Something I think
• 15:57
  Speaker 1: is really cool is that the pattern of dark matter,
• 16:00
  Speaker 1: which controls the whole structure of the universe, is like
• 16:03
  Speaker 1: these filaments, you know, these like lines of through a
• 16:06
  Speaker 1: cosmic web, and where those filaments overlap with each other
• 16:09
  Speaker 1: are the dark matter halos. You can think of it
• 16:12
  Speaker 1: sort of like these filaments are funneling matter into these
• 16:15
  Speaker 1: lakes which are where they intersect, and so gas is
• 16:18
  Speaker 1: falling from these filaments into these halos and then swirling
• 16:22
  Speaker 1: together to form stars which form galaxies. Interesting. I guess
• 16:27
  Speaker 1: what I mean is like if you had been around
• 16:28
  Speaker 1: at that time, that early in the universe, you could
• 16:31
  Speaker 1: have been predicted maybe like, oh, there's gonna be a galaxy,
• 16:34
  Speaker 1: and there's gonna be a galaxy, you know what I mean,
• 16:36
  Speaker 1: Like the definition of a galaxy was sort of right
• 16:39
  Speaker 1: there at the beginning of the universe. Yeah, you definitely
• 16:41
  Speaker 1: would know that this is where complex structure is going
• 16:43
  Speaker 1: to form. I think it's super fascinating what you end
• 16:46
  Speaker 1: up getting. I mean, you start with just clouds of hydrogen,
• 16:49
  Speaker 1: and then like you get stars and you get galaxies.
• 16:52
  Speaker 1: It's really fascinating sort of the size and the scale
• 16:55
  Speaker 1: and the shape of what forms out of that. It's
• 16:57
  Speaker 1: one of the I think the deepest questions in modern physics.
• 17:00
  Speaker 1: Why these objects emerge at these scales? You know, why
• 17:03
  Speaker 1: do we get stars there about this size? Why do
• 17:05
  Speaker 1: we galaxies about this size? Why don't we get just
• 17:09
  Speaker 1: one humongous galaxy in the whole universe? Or why do
• 17:12
  Speaker 1: galaxies form at all? Why don't we just have stars
• 17:14
  Speaker 1: spread out everywhere? So I think it would have been
• 17:16
  Speaker 1: pretty hard to predict computationally. I think it's really interesting
• 17:19
  Speaker 1: to sort of see what emerges from structure in the universe.
• 17:22
  Speaker 1: And that's just what we're doing now. We're still seeing
• 17:24
  Speaker 1: structure emerged. Now we have clusters of galaxies and superclusters
• 17:28
  Speaker 1: of galaxies, and that process is still happening because we're
• 17:31
  Speaker 1: still fairly young in the universe. Yeah we are. Are
• 17:34
  Speaker 1: we still in there like the teenage years? You think,
• 17:36
  Speaker 1: the awkward teenage years full of pimples. Well, gravity is
• 17:39
  Speaker 1: really slow, so it takes a long time to form structure,
• 17:42
  Speaker 1: and so it's formed galaxies and galaxy clusters. But those
• 17:45
  Speaker 1: galaxy clusters themselves are like very loosely grouped into superclusters.
• 17:49
  Speaker 1: Gravity hasn't had time really to gather that together into
• 17:52
  Speaker 1: something greater. In the meantime, dark energy is doing the
• 17:55
  Speaker 1: opposite job. It's pushing everything apart, preventing gravity from making superstructures.
• 18:00
  Speaker 1: So we might be living in the moment when the
• 18:02
  Speaker 1: universe is the most organized. So I don't know if
• 18:04
  Speaker 1: those are really the teenage years or that's like it's
• 18:06
  Speaker 1: mid thirties. You know, it's really sort of on top
• 18:08
  Speaker 1: of its stuff before it starts to get old, before
• 18:11
  Speaker 1: it has its midlife crisis. We all do exactly all right, Well,
• 18:15
  Speaker 1: I guess that's the birth of a galaxy and so
• 18:18
  Speaker 1: and a surprising idea is that galaxies change and they
• 18:22
  Speaker 1: might even die one day. So let's dive right into that.
• 18:24
  Speaker 1: But first, let's take a quick break. All right, we're
• 18:40
  Speaker 1: talking about the death of galaxies, Daniel, Does that mean
• 18:43
  Speaker 1: our galaxy is gonna die? Scientists think that the Milky
• 18:46
  Speaker 1: Way itself is beginning to quench. We're entering a period
• 18:49
  Speaker 1: we're making fewer and fewer stars, and that's going to
• 18:52
  Speaker 1: happen faster and faster, so sort of the milky ways
• 18:55
  Speaker 1: peak might be behind us. Oh boy, well, for a second,
• 18:59
  Speaker 1: I thought you were going to give it a super
• 19:00
  Speaker 1: plot twist and say like, oh, our galaxy is already dead.
• 19:04
  Speaker 1: It's been dead the whole time, where it's just ghosts.
• 19:08
  Speaker 1: That's right, This whole galaxy is nothing but an m
• 19:10
  Speaker 1: Night shun Land film. Yeah, spoiler alert. I talked to
• 19:16
  Speaker 1: dead galaxies. I see that galaxies with my telescopes. Do
• 19:20
  Speaker 1: you kind of do maybe? Right, Like if you look
• 19:22
  Speaker 1: at into the far universe, you might be seeing galaxies
• 19:26
  Speaker 1: that have died already. Right, We do see a lot
• 19:28
  Speaker 1: of galaxies that aren't dead. And one of the surprising
• 19:31
  Speaker 1: things with something we struggle to understand is what's killing
• 19:34
  Speaker 1: all these galaxies? Why are there so many galaxies out
• 19:37
  Speaker 1: there that are already quenched and have stopped making new stars.
• 19:42
  Speaker 1: It just went from a thriller to a mystery. We
• 19:44
  Speaker 1: just switched genres here. It's a combination because somebody is
• 19:47
  Speaker 1: still out there killing galaxies. It's not over yet. We
• 19:50
  Speaker 1: still have a chance to catch the killer and maybe
• 19:53
  Speaker 1: save future galaxies and maybe do nothing about it because
• 19:59
  Speaker 1: because just physics, physics, are you saying we can't learn
• 20:03
  Speaker 1: things about the universe and intervene in its faith that
• 20:05
  Speaker 1: we have no control, no power to change the future
• 20:08
  Speaker 1: of humanity. That doesn't sound like an engineer, you tell me.
• 20:11
  Speaker 1: I guess you know, the physicist has to tell us
• 20:14
  Speaker 1: if it's possible. Well, the first thing to do is
• 20:15
  Speaker 1: to understand what it is that's clean these galaxies, and
• 20:18
  Speaker 1: then we can figure out if it's possible to intervene.
• 20:20
  Speaker 1: Then we have to start training some galaxy engineers. That
• 20:24
  Speaker 1: might be the sequel. I guess all right, Well, I
• 20:26
  Speaker 1: think one interesting bit of news is that, you know,
• 20:28
  Speaker 1: galaxies seem to change, and I guess they quention they
• 20:32
  Speaker 1: maybe eventually die. And so is that something that we've
• 20:35
  Speaker 1: always believed in, that galaxies can change or did we
• 20:38
  Speaker 1: think that we just you know, swirl around forever. We've
• 20:41
  Speaker 1: always thought that eventually galaxies would use up their raw
• 20:44
  Speaker 1: materials in terms of making stars. You know, you have
• 20:47
  Speaker 1: a certain amount of hydrogen in the universe, for example,
• 20:50
  Speaker 1: that's primarily what's burned to make stars. Eventually you will
• 20:53
  Speaker 1: run out of them. We have a podcast episode about
• 20:55
  Speaker 1: how many generations of stars will there be? Because you know,
• 20:58
  Speaker 1: stars don't live forever, and eventually they explode and spew
• 21:00
  Speaker 1: their materials back out in the universe, which gets gathered
• 21:03
  Speaker 1: back together. But we don't think that can happen forever.
• 21:06
  Speaker 1: So we've always known that eventually waves of star formation
• 21:09
  Speaker 1: will end. But we were surprised when we looked at
• 21:11
  Speaker 1: the galaxies and discovered how many are already dead, how
• 21:14
  Speaker 1: many have stopped making stars well before we expected them
• 21:17
  Speaker 1: too interesting. Yeah, I guess you know, there's a limited
• 21:20
  Speaker 1: supply of fuel in the universe, right, and at some
• 21:23
  Speaker 1: point we run out, like we have a gas tank
• 21:25
  Speaker 1: kind of and you know, hydrogen gets you stop, gets
• 21:28
  Speaker 1: turned into heavier element, and then to break it apart,
• 21:30
  Speaker 1: you need extra energy. Yeah, there's two things that drive this.
• 21:34
  Speaker 1: One is you need to have the fuel, and the
• 21:36
  Speaker 1: other is that the fuel has to have the right conditions.
• 21:39
  Speaker 1: Like it's not just enough to have hydrogen. You have
• 21:41
  Speaker 1: to have hydrogen in the right situation in order to
• 21:44
  Speaker 1: get a star to form. And the key thing to
• 21:47
  Speaker 1: making stars form is that you need huge blobs of
• 21:51
  Speaker 1: cold hydrogen. If you have hydrogen but it's really hot,
• 21:54
  Speaker 1: it's not going to form a star because remember that
• 21:57
  Speaker 1: gravity is super duper weak in order to tug things
• 21:59
  Speaker 1: together to make a star, the particles have to be
• 22:02
  Speaker 1: very slow moving. If your gas is heated up too much,
• 22:05
  Speaker 1: then gravity is not going to be able to pull
• 22:07
  Speaker 1: it together to make a star. All right, Well that's
• 22:09
  Speaker 1: the birth of a galaxy that I guess that's how
• 22:11
  Speaker 1: they start, right, They start for influctuations and the universe
• 22:14
  Speaker 1: and the dark matter sees which pull in stuff, and
• 22:18
  Speaker 1: then galaxies form. That's just kind of the beginning of
• 22:21
  Speaker 1: the story. Right. Eventually galaxies also merged, and our thinking
• 22:25
  Speaker 1: there has changed also. Originally, we looked out at the
• 22:28
  Speaker 1: universe and we saw really big galaxies, mostly because that's
• 22:31
  Speaker 1: all we could see because they were brighter, and we thought,
• 22:33
  Speaker 1: oh wow, maybe these really big galaxies sort of form
• 22:36
  Speaker 1: all at once, like monolithic collapse of a huge cloud
• 22:40
  Speaker 1: of gas into a swirling disc. But then as we
• 22:42
  Speaker 1: developed the technology look further and further into the universe
• 22:45
  Speaker 1: and earlier and earlier back in time, we discovered a
• 22:48
  Speaker 1: lot more galaxies that were much smaller, and we realized
• 22:51
  Speaker 1: that the story of galaxy formation is more complicated. Instead
• 22:54
  Speaker 1: of having really big galaxies form at once, it turns
• 22:57
  Speaker 1: out it's much more likely to form a bunch of
• 22:59
  Speaker 1: small galaxy then have those merged together. The galaxies are
• 23:03
  Speaker 1: doing a lot of merging. Most of the galaxies we
• 23:05
  Speaker 1: see how there are actually the products of lots of
• 23:07
  Speaker 1: little baby galaxies that came together to make a bigger galaxy.
• 23:11
  Speaker 1: I say, it's like we looked at into the universe
• 23:13
  Speaker 1: and saw more babies baby galaxies than we expect. It
• 23:17
  Speaker 1: is that kind of the idea, which I guess would
• 23:20
  Speaker 1: then logically mean the bigger galaxies don't inform. They they're
• 23:23
  Speaker 1: the result of two baby galaxies merging exactly where multiple
• 23:27
  Speaker 1: baby galaxies merging. It also helps explain the shape of
• 23:30
  Speaker 1: these galaxies, like when an individual galaxy forms, it tends
• 23:34
  Speaker 1: to form as a disk to give a big cloud
• 23:36
  Speaker 1: of gas, and it's spinning a certain way, and that
• 23:39
  Speaker 1: spin prevented from collapsing in one direction, but gravity can
• 23:42
  Speaker 1: collapse it in the other direction, which is why you
• 23:45
  Speaker 1: get a disc. So disc galaxies tend to have formed
• 23:48
  Speaker 1: from a single cloud of gas and dust with some spin.
• 23:51
  Speaker 1: But then when two of these things merge, they come
• 23:53
  Speaker 1: in with different shapes and different sizes and different disks,
• 23:56
  Speaker 1: and that's how you get like elliptical galaxies and things
• 23:59
  Speaker 1: that are are rounded because you're combining like the spin
• 24:02
  Speaker 1: of two disc galaxies together. So the idea is that
• 24:05
  Speaker 1: the bigger elliptical galaxies are formed from a bunch of
• 24:09
  Speaker 1: smaller disc galaxies that came together. For example, the Milky
• 24:12
  Speaker 1: Way and Andromeda are going to collide, but they're not
• 24:15
  Speaker 1: spinning in the same direction, So you're not going to
• 24:17
  Speaker 1: end up with one big disc galaxy. You're gonna endup
• 24:19
  Speaker 1: with something more elliptical that has like two spin axes. Interesting,
• 24:23
  Speaker 1: so a lot more of the galaxies foreign from merging
• 24:26
  Speaker 1: than we thought before. I guess why do we have
• 24:29
  Speaker 1: that wrong impression? But we had the wrong impression just
• 24:32
  Speaker 1: because we didn't see a lot of these baby galaxies
• 24:34
  Speaker 1: early on. You know, we didn't have the technology before
• 24:36
  Speaker 1: Hubble and those kinds of telescopes. We just couldn't see them.
• 24:39
  Speaker 1: But when Hubble looked really deep into the universe and
• 24:42
  Speaker 1: discovered how many galaxies are out there, that's when we
• 24:45
  Speaker 1: got the clue. Is from seeing these baby galaxies in
• 24:48
  Speaker 1: the early universe. You know, the number of galaxies out
• 24:51
  Speaker 1: there is sort of amazing. If you hold up your
• 24:54
  Speaker 1: pinky at arms length, then the part of your sky
• 24:57
  Speaker 1: blocked by your fingernail, and your pinky contains about a
• 25:01
  Speaker 1: million galaxies in the observable universe, you know, layered further
• 25:05
  Speaker 1: and further and back in space and time. And so
• 25:07
  Speaker 1: now like scan your pinky around the whole sky. Each
• 25:10
  Speaker 1: of those is a million galaxies. So there's an incredible
• 25:13
  Speaker 1: number out there, and now we have a huge population
• 25:15
  Speaker 1: to study, so we get a better sense of how
• 25:18
  Speaker 1: these things have evolved. Well, that's wild. What if I
• 25:20
  Speaker 1: let my pinky fingernailk and grow, then you're containing more
• 25:25
  Speaker 1: and more galaxies. You're becoming master of even more of
• 25:28
  Speaker 1: space and time, a pinky master. I guess, all right,
• 25:31
  Speaker 1: So that's uh, I guess that's the basic of the
• 25:34
  Speaker 1: life of a galaxy right there. Born as little baby galaxies,
• 25:37
  Speaker 1: they grow up to be a teenager as they start
• 25:39
  Speaker 1: hooking up doing inappropriate things, and then they merge into
• 25:42
  Speaker 1: bigger galaxies and then they keep burning for a while.
• 25:45
  Speaker 1: I guess, like, what's the average age of a galaxy?
• 25:48
  Speaker 1: Galaxies are really old, like the Milky Way is almost
• 25:51
  Speaker 1: as old as the universe. Galaxies formed very early on
• 25:54
  Speaker 1: a lot of them in the first billion years of
• 25:55
  Speaker 1: the universe. Many of them are more than ten billion
• 25:58
  Speaker 1: years old. But at some point, as you say, they die,
• 26:02
  Speaker 1: they stop shining, they stop making stars, they quench. So
• 26:05
  Speaker 1: I guess, Daniel, what's the process for a galaxy to die?
• 26:09
  Speaker 1: So for a galaxy to die, for to stop being
• 26:11
  Speaker 1: able to make stars, something has to heat up its gas,
• 26:15
  Speaker 1: something has to prevent its gas from staying cold, because
• 26:18
  Speaker 1: in order to form stars, you need that cold gas,
• 26:21
  Speaker 1: and some of the star formation comes from the original
• 26:24
  Speaker 1: gas that the galaxy started with, whatever was in that
• 26:26
  Speaker 1: dark matter halo. Remember, the galaxies are not really alone
• 26:30
  Speaker 1: in the universe. They're sort of like at the intersections
• 26:32
  Speaker 1: of these dark matter filaments. So there's also sometimes new
• 26:36
  Speaker 1: gas falling into these galaxies along those filaments, like tributaries
• 26:41
  Speaker 1: to a lake, and so galaxies can also continue to
• 26:43
  Speaker 1: form stars as they suck in more gas from these filaments.
• 26:47
  Speaker 1: So the way a galaxy dies is somehow losing access
• 26:51
  Speaker 1: to that new gas or blowing out the cold gas,
• 26:53
  Speaker 1: or heating up the cold gas that's inside it, because
• 26:56
  Speaker 1: remember you need cold gas to make star I see,
• 26:59
  Speaker 1: all right, So you're defining the death of a galaxy
• 27:01
  Speaker 1: is when it stops forming new stars. Is that, I
• 27:05
  Speaker 1: guess the definition. But even if it stops forming stars,
• 27:08
  Speaker 1: it's still shining though, right. Astronomers don't talk about galaxies dying.
• 27:12
  Speaker 1: They use the technical term quenching, which means it stopped
• 27:15
  Speaker 1: forming new stars. But you're right, even if it stopped
• 27:17
  Speaker 1: forming any stars at all, those stars themselves can burn
• 27:20
  Speaker 1: for billions and billions, for sometimes even trillions of years.
• 27:24
  Speaker 1: Remember that stars their lifetime depends on their size, so
• 27:28
  Speaker 1: really big stars don't burn very long, and really small
• 27:31
  Speaker 1: stars will burn for a very very long time. So
• 27:35
  Speaker 1: even a galaxy that's totally quenched won't make any new stars,
• 27:38
  Speaker 1: but those stars will continue to burn for a very
• 27:40
  Speaker 1: long time. Oh, I see, I see what's going on.
• 27:42
  Speaker 1: I think really, what you mean to ask is how
• 27:44
  Speaker 1: do galaxies peak? Or like, how do galaxies stop getting brighter? Maybe? Right? Yeah,
• 27:50
  Speaker 1: exactly how do they stop having any babies? I see,
• 27:54
  Speaker 1: I think I see what's going on here, Daniel. I
• 27:55
  Speaker 1: think you associate not working with dying. That's why never
• 28:00
  Speaker 1: gonna entire. Yeah, you're never gonna retire, like if you
• 28:07
  Speaker 1: retire or die, like if a professor stops making papers,
• 28:10
  Speaker 1: that's that's like that for them, who even are you
• 28:13
  Speaker 1: anymore in that situation? But yeah, just to becular, that's
• 28:17
  Speaker 1: kind of what we're saying, right, Like, the galaxies don't
• 28:20
  Speaker 1: maybe I guess they will at some point grow dark,
• 28:23
  Speaker 1: But here today, what we're talking about is like how
• 28:25
  Speaker 1: do galaxies peak or how when did they stop being
• 28:28
  Speaker 1: active exactly? And as we look out into the universe,
• 28:31
  Speaker 1: we can tell whether galaxies are still making stars or not.
• 28:36
  Speaker 1: And we do that, we see some pretty surprising things
• 28:38
  Speaker 1: about what's going on with all the galaxies in the universe. Wait,
• 28:42
  Speaker 1: that's kind of weird because I guess you were defining
• 28:46
  Speaker 1: death as stopping production of stars. You need to like
• 28:48
  Speaker 1: know how many news stars are being made and right,
• 28:52
  Speaker 1: but how do you tell that on a short human life.
• 28:55
  Speaker 1: So it's not that we look at the galaxies and
• 28:57
  Speaker 1: we count the stars and we say, oh, look, it's
• 28:59
  Speaker 1: still making more because these galaxies, a lot of them
• 29:02
  Speaker 1: are really far away. All we can see them is
• 29:04
  Speaker 1: like a pixel or two on the hubble. But what
• 29:07
  Speaker 1: we can do is look at the color of the
• 29:09
  Speaker 1: light that comes from these galaxies, and that tells us
• 29:13
  Speaker 1: about whether or not there are new young stars in
• 29:16
  Speaker 1: the galaxy. And the reason is that new young stars
• 29:19
  Speaker 1: means hot stars. When stars are formed, you get big
• 29:23
  Speaker 1: ones and you get small ones. But the big ones,
• 29:25
  Speaker 1: the ones that burn hot, that burn blue, they don't
• 29:28
  Speaker 1: last for very long. So if you're looking at a
• 29:30
  Speaker 1: galaxy and you're seeing blue light from it, that means
• 29:33
  Speaker 1: recently formed stars. You're looking at a galaxy and you
• 29:36
  Speaker 1: only see red light, that means cooler stars, only older stars,
• 29:41
  Speaker 1: no recently formed stars. So by looking at the color
• 29:44
  Speaker 1: of the light from the galaxy, you can tell if
• 29:46
  Speaker 1: there are recently formed stars in it, because they recently
• 29:49
  Speaker 1: formed ones burn hotter and burn bluer. Interesting, you're saying
• 29:54
  Speaker 1: that galaxies kind of peak they start producing new stars
• 29:58
  Speaker 1: when they turn from to it a red exactly blue
• 30:01
  Speaker 1: means you've recently produced some stars, because only young stars
• 30:05
  Speaker 1: are ever blue, because blue stars don't last very long,
• 30:07
  Speaker 1: and eventually all your blue stars burn out and all
• 30:10
  Speaker 1: you're left with our red stars. So if you're an
• 30:13
  Speaker 1: all red galaxy, it means you haven't made anything recently.
• 30:16
  Speaker 1: I feel like that's like people these days. You know,
• 30:18
  Speaker 1: everyone starts out as a democrat and liberal, but then
• 30:21
  Speaker 1: they get more conservative and red and the old age,
• 30:24
  Speaker 1: you know, maybe we should avoid the political analogy and
• 30:27
  Speaker 1: think about like musicians. You know, are you a musician
• 30:29
  Speaker 1: that keeps putting out new albums or you just like
• 30:32
  Speaker 1: the Beatles, you're just making money on your old catalog. Well,
• 30:37
  Speaker 1: the Beatles came out with the white album, so I
• 30:39
  Speaker 1: don't know you fit them into the color spectrum. Well,
• 30:41
  Speaker 1: the fascinating thing is when you look at this color spectrum,
• 30:44
  Speaker 1: you see some that are still making new stars. You
• 30:46
  Speaker 1: see a whole huge number that are already quenched, many
• 30:50
  Speaker 1: more than we expect, and you see very very few
• 30:53
  Speaker 1: in between, very few in this region they called the
• 30:56
  Speaker 1: green valley, between the blue and the red, the ones
• 30:59
  Speaker 1: that are like quenching. So there's a bunch that are
• 31:02
  Speaker 1: not quenched. Is a huge number that have been quenched,
• 31:05
  Speaker 1: and very few in the process of quenching. Interesting. Yeah,
• 31:09
  Speaker 1: as you said, it's a big mystery. And so let's
• 31:12
  Speaker 1: dive into that mystery of what is killing or I guess,
• 31:15
  Speaker 1: retiring all the stars in the universe. But first let's
• 31:19
  Speaker 1: take another quick break. Alright, we're talking about um, I
• 31:34
  Speaker 1: guess when Galaxy has changed their political views? Right, when
• 31:39
  Speaker 1: galaxies stopped recording albums when they stopped polishing papers, which
• 31:43
  Speaker 1: according to professors, is the same as death. Like give
• 31:46
  Speaker 1: me research or give me death. What if galaxies get
• 31:49
  Speaker 1: a hobby, then they won't see retirement as death anymore.
• 31:52
  Speaker 1: I do see a lot of Americans professors still around
• 31:55
  Speaker 1: the department, and I wonder, like, why do you still
• 31:56
  Speaker 1: come in every day? And I think, you know, maybe
• 31:59
  Speaker 1: their partners at home and don't really want them hanging
• 32:01
  Speaker 1: out doing their hobbies at home. You know, they're probably
• 32:04
  Speaker 1: thought the same thing you youth are thinking right now,
• 32:06
  Speaker 1: and you're like, they're gonna have to drag me out,
• 32:08
  Speaker 1: drag me out, or call the grim Reaper. I guess.
• 32:11
  Speaker 1: But apparently galaxies do retire. At some point, they peaked,
• 32:14
  Speaker 1: they stopped making news stars, and I guess it shows
• 32:17
  Speaker 1: on a galaxy, you know, like it's pretty. It sounds
• 32:20
  Speaker 1: like it's pretty obvious, Like if a galaxy is pretty blue,
• 32:23
  Speaker 1: then if it's pretty red, it's old exactly. And there
• 32:26
  Speaker 1: are very few galaxies in between, which suggests that this
• 32:29
  Speaker 1: process of quenchya galaxy happens rapidly. You're not in the
• 32:33
  Speaker 1: Green Valley for very long on your trip from blue
• 32:35
  Speaker 1: to red. Well, it's sort of like when you run
• 32:38
  Speaker 1: out of gas in your car. It's not a gradual process,
• 32:41
  Speaker 1: like once you run out of gas, your car stops. Yeah,
• 32:44
  Speaker 1: Like the galaxies are huge, right, you might imagine that
• 32:46
  Speaker 1: one part of it might quench, another part of it
• 32:48
  Speaker 1: might still be active. But it seems to happen on
• 32:51
  Speaker 1: sort of a galaxy wide scale, all at once. Well,
• 32:54
  Speaker 1: let's recap a little bit here. You said the galaxies peak,
• 32:57
  Speaker 1: they start producing new stars when they run out of gas.
• 33:01
  Speaker 1: And you said that it's not because you run out
• 33:03
  Speaker 1: of hydrogen gas, but because you run out of cold
• 33:05
  Speaker 1: hydrogen gas. In order to form stars, you need clouds
• 33:09
  Speaker 1: of cold gas that gravity can work on to pull
• 33:11
  Speaker 1: together to make those stars. And so it might be
• 33:14
  Speaker 1: that there's plenty of hydrogen left in those galaxies. We
• 33:17
  Speaker 1: suspect that there might be. It might just be that
• 33:19
  Speaker 1: it's too hot, So either you lose the gas or
• 33:22
  Speaker 1: the gas gets too hot. Those are the ideas for
• 33:25
  Speaker 1: how to quench a galaxy. Wait, what happens if it
• 33:27
  Speaker 1: gets too hot? Like why is it hard to make
• 33:29
  Speaker 1: a star from hot gas? Well, hot gas, the particles
• 33:31
  Speaker 1: are just flying around too much. You know, gravity is
• 33:34
  Speaker 1: not very strong and So if you have a gas
• 33:37
  Speaker 1: that's really hot, where the particles are flying around with
• 33:39
  Speaker 1: high speed, gravity just doesn't have the power to gather
• 33:42
  Speaker 1: them together into a star. Counterintuitive because gravity makes them hot, right,
• 33:46
  Speaker 1: but once it's already trapped them in a gravitational well,
• 33:49
  Speaker 1: think of it like a scape phlocity. The Earth is
• 33:51
  Speaker 1: boiling off molecules right now at the edge of our
• 33:53
  Speaker 1: atmosphere because they're too hot for the Earth's gravity to
• 33:57
  Speaker 1: contain them. And so it's certainly possible to have a
• 33:59
  Speaker 1: whole gas us where there's not enough gravity to contain it,
• 34:02
  Speaker 1: where it just disperses, it just keeps flying around instead
• 34:05
  Speaker 1: of clumping together to make a dense well at the
• 34:08
  Speaker 1: heart of it. But I guess you know, the temperature
• 34:11
  Speaker 1: is just one part of it. What if it do
• 34:12
  Speaker 1: you have hot gas but really dense or like a
• 34:15
  Speaker 1: lot enough of it or a lot of it, wouldn't
• 34:17
  Speaker 1: that also sort of create the intensity you need for
• 34:21
  Speaker 1: a start to form. Yeah, if you have a seed,
• 34:23
  Speaker 1: if you have a dense seed, but how does that form? Right?
• 34:25
  Speaker 1: So you need gravity to do that work. The only
• 34:27
  Speaker 1: way to get density is to have gravity pull things together.
• 34:31
  Speaker 1: So primarily is from having cold clouds of gas. There
• 34:35
  Speaker 1: are other rarer waves for it happen. You can have
• 34:37
  Speaker 1: supernova shock waves, for example, that compress things and then
• 34:41
  Speaker 1: make over densities, but primarily it's just from having clouds
• 34:44
  Speaker 1: of cold gas. Interesting, it's like a chill process star formation,
• 34:48
  Speaker 1: you know, like a NISA start amount of calm and
• 34:52
  Speaker 1: then for things to kind of build up to a start. Right. Yeah,
• 34:56
  Speaker 1: you can't rush it, man, It's like the slow food movement.
• 34:58
  Speaker 1: You want a hot start, it takes time. Yeah, yeah,
• 35:01
  Speaker 1: no fast food galaxies in this universe. It's all organic.
• 35:05
  Speaker 1: You can't microwave a star. It's all natural and organic.
• 35:11
  Speaker 1: All right. Well, um, you said there's a bit of
• 35:13
  Speaker 1: a mystery, right, let in the sense that there are
• 35:16
  Speaker 1: more stars sort of quenching or dying or retiring than
• 35:20
  Speaker 1: maybe we thought was possible or think is likely. Yeah,
• 35:23
  Speaker 1: we sort of can't explain all the quenching that's happening,
• 35:26
  Speaker 1: or at least, you know, twenty years ago, when we
• 35:28
  Speaker 1: discovered that this was happening, we were very surprised. We
• 35:31
  Speaker 1: thought that very gradually galaxies would start to lose the
• 35:35
  Speaker 1: ability to form stars, and we've seen this slow drop
• 35:38
  Speaker 1: off with a lot of galaxies in the Green Valley
• 35:40
  Speaker 1: and a few that are totally quenched. But it happens
• 35:43
  Speaker 1: faster than we thought. And so there's been a lot
• 35:45
  Speaker 1: of research in the last ten, fifteen, twenty years into
• 35:48
  Speaker 1: why galaxies quench, and now we have a few ideas.
• 35:50
  Speaker 1: All of course, stimulated by this observation, we dug deep
• 35:54
  Speaker 1: into the physics of and thought, how would you quench galaxies?
• 35:57
  Speaker 1: What are the processes we might have overlooked that could
• 35:59
  Speaker 1: be doing? Is I see? So what do you mean?
• 36:02
  Speaker 1: I guess it happens faster than you thought or earlier
• 36:05
  Speaker 1: than you thought. Do you know what I mean? Like,
• 36:06
  Speaker 1: are you saying that you know the process of retirement
• 36:09
  Speaker 1: is faster than you thought, or that it's happening earlier
• 36:11
  Speaker 1: than you thought, or that it happens you know, more
• 36:15
  Speaker 1: than you thought. All of that right. It definitely happens
• 36:18
  Speaker 1: earlier than we thought, because there are more galaxies that
• 36:21
  Speaker 1: are quenched that are in that red category than we expected.
• 36:24
  Speaker 1: And it also happens faster than we thought because there
• 36:27
  Speaker 1: are very few in the Green Valley. There's like two peaks.
• 36:29
  Speaker 1: If you look at a distribution of the color of galaxies.
• 36:32
  Speaker 1: You don't have a broad spectrum from red to blue.
• 36:34
  Speaker 1: If a peak in the blue and a peak in
• 36:36
  Speaker 1: the red and almost nothing in between, was suggest that
• 36:39
  Speaker 1: galaxies don't live in the green valley very long. When
• 36:41
  Speaker 1: they start to quench, they quench quickly and then it's over.
• 36:44
  Speaker 1: It's a fast process. You know. There's you can't go
• 36:47
  Speaker 1: it for like an independent third party. It's like you're
• 36:49
  Speaker 1: either blue or your red. I mean, I guess that's
• 36:51
  Speaker 1: good news if you like a quick death. You know,
• 36:52
  Speaker 1: it's not a long tortuous struggle against the eventual demise. Man.
• 36:58
  Speaker 1: I feel like a lot of the few need today
• 37:01
  Speaker 1: is that that to a professor, retirement is dead. They're like,
• 37:06
  Speaker 1: wait a minute, wire so many galaxies retiring, Why would
• 37:09
  Speaker 1: anyone retire? That's crazy. Yeah, Well, maybe they like it,
• 37:12
  Speaker 1: you know, maybe they're tired of making new stars and
• 37:14
  Speaker 1: they just want to enjoy the stars that they have
• 37:16
  Speaker 1: and hang out for a long time, trillions of years
• 37:18
  Speaker 1: until they eventually all fade. Maybe it's a happy retirement. Yeah,
• 37:22
  Speaker 1: there you go. You don't have to call it death.
• 37:25
  Speaker 1: It could be their their new life, their second life.
• 37:29
  Speaker 1: So that was the mystery. I guess and you said
• 37:31
  Speaker 1: that physicists have looked into this, and so they have
• 37:34
  Speaker 1: theories I guess about what's making all of these galaxies
• 37:37
  Speaker 1: retire so quickly. They have theories. None of them are
• 37:39
  Speaker 1: really perfect or comprehensive, but they have a few cool ideas.
• 37:42
  Speaker 1: One is related actually to the black hole at the
• 37:45
  Speaker 1: heart of the galaxies. And it's not the black hole
• 37:48
  Speaker 1: is gobbling up stars all that stars are falling into it,
• 37:51
  Speaker 1: but black holes actually admit a lot of radiation. The
• 37:54
  Speaker 1: black hole itself is not hot. Gas in the dust
• 37:56
  Speaker 1: that's swirling around the black hole before it falls in
• 38:00
  Speaker 1: gets really hot and emits a lot of radiation, and
• 38:03
  Speaker 1: so that blows out a lot of the gas and dust.
• 38:05
  Speaker 1: Once we talked about like how supermassive black holes form,
• 38:08
  Speaker 1: and there's a maximum rate at which they can grow
• 38:11
  Speaker 1: because the bigger they get, the more they push away
• 38:13
  Speaker 1: the gas that's feeding them. And so this could also
• 38:16
  Speaker 1: be spewing out radiation into the galaxy, which heats up
• 38:20
  Speaker 1: a lot of the gas in the galaxy or also
• 38:22
  Speaker 1: helps blow it out of the galaxy, and so that
• 38:25
  Speaker 1: can stifle star formation. I see you're saying, like you know,
• 38:28
  Speaker 1: because star formation depends on very chill, calm conditions. Anything
• 38:32
  Speaker 1: that kind of excites the galaxy is not good for
• 38:35
  Speaker 1: star formation. So one idea is that maybe it's the
• 38:38
  Speaker 1: black hole in the middle of galaxies that's I guess
• 38:40
  Speaker 1: agitating everything. Yeah, and we see that these black holes
• 38:43
  Speaker 1: grow at the same rate of galaxies is like a
• 38:46
  Speaker 1: ratio of the black hole mass to the galaxy mass,
• 38:49
  Speaker 1: which seems roughly constant. And so that might help explain
• 38:52
  Speaker 1: it because as these galaxies get big and get old,
• 38:55
  Speaker 1: the black hole gets big and powerful and not just
• 38:57
  Speaker 1: heats up the gas, but also helps prevent more gas
• 39:00
  Speaker 1: from falling into the galaxy from these filaments from the
• 39:04
  Speaker 1: intergalactic medium or the circum galactic medium. All right, but
• 39:08
  Speaker 1: so that's one idea, but it's not perfect. It's not
• 39:10
  Speaker 1: perfect because you know, the black hole is at the
• 39:12
  Speaker 1: center and they don't really understand how it could like
• 39:14
  Speaker 1: heat up the entire galaxy. You should still see star
• 39:17
  Speaker 1: formation sort of at the edges in that case. So
• 39:20
  Speaker 1: we can't explain everything. Okay, So then what's another possible
• 39:23
  Speaker 1: star retirer or another possibility is that stars themselves are
• 39:27
  Speaker 1: killing the galaxy. Remember that stars also generate radiation. Our
• 39:31
  Speaker 1: Sun doesn't just send out light. It sends out protons
• 39:34
  Speaker 1: and electrons and all sorts of stuff the solar wind.
• 39:37
  Speaker 1: And so if you have a lot of these stellar winds,
• 39:40
  Speaker 1: these particles, they can be blowing the gas out as
• 39:43
  Speaker 1: well or heating it up. It's sort of the same process.
• 39:46
  Speaker 1: This happens at the galactic center from the black hole,
• 39:49
  Speaker 1: but instead it's happening everywhere all at once. So in
• 39:52
  Speaker 1: some senses, it's a better candidate for what might be
• 39:55
  Speaker 1: stopping star formation because it's spread out across the whole galaxy.
• 39:59
  Speaker 1: N See, it's sort of like stars form, but then
• 40:01
  Speaker 1: they kind of spoil star formation for the area around them, right,
• 40:05
  Speaker 1: because it like it forms a star, and but then
• 40:07
  Speaker 1: it starts to heat up this gas around and so
• 40:10
  Speaker 1: nobody can make new stars. It's really nimbism, you know.
• 40:13
  Speaker 1: It's all these old professors. They've got these nice houses
• 40:15
  Speaker 1: they bought in the sixties, and they don't want anybody
• 40:17
  Speaker 1: building apartments in their backyard. Yeah, or like you know,
• 40:23
  Speaker 1: like you know, the old professors taken up office face exactly,
• 40:28
  Speaker 1: I got to make room for those young bloods, alright.
• 40:31
  Speaker 1: So and but that this one is also maybe not
• 40:33
  Speaker 1: a perfect idea or cost Yeah, because remember that the
• 40:36
  Speaker 1: galaxy is surrounded by gas, and that gas should be
• 40:39
  Speaker 1: falling in the dark matter halo should be pulling that
• 40:42
  Speaker 1: gas in and supplying us with fresh cold gas that's
• 40:45
  Speaker 1: been out there between galaxies that hasn't been heated up
• 40:48
  Speaker 1: from the stellar winds. So another area of research is
• 40:51
  Speaker 1: understanding why that might stop happening. And one suggestion there
• 40:56
  Speaker 1: is that the dark matter halo, the thing that forms
• 40:58
  Speaker 1: our galaxy and shapes it and provides the gravitational well
• 41:02
  Speaker 1: to suck in this new gas that might get a
• 41:04
  Speaker 1: little too big. If that gets really big, then it's
• 41:08
  Speaker 1: gravitational power is really strong. And then what happens when
• 41:11
  Speaker 1: the gas falls in from outside the galaxy into the
• 41:14
  Speaker 1: galaxy is that it gets heated up. This is called
• 41:16
  Speaker 1: shock heating. If this is like a gradient in the
• 41:19
  Speaker 1: density outside the galaxy, then when this gas falls in,
• 41:22
  Speaker 1: it basically collides with the other gas and then it
• 41:24
  Speaker 1: gets too hot, and so you get hot gas falling
• 41:27
  Speaker 1: in instead of cold gas, and then you can't use
• 41:30
  Speaker 1: that to make new stars interesting. But but what do
• 41:33
  Speaker 1: you mean the dark matter halo gets too big, like
• 41:36
  Speaker 1: dark matter is growing in galaxies. The dark matter halo
• 41:39
  Speaker 1: is definitely growing because you know, we have these filaments
• 41:42
  Speaker 1: and it's sucking in not just gas but also dark
• 41:44
  Speaker 1: matter gravity in the universe. Remember, its goal is to
• 41:47
  Speaker 1: gather everything together. So the reason you have these halos
• 41:50
  Speaker 1: and then these filaments is that the filaments are feeding
• 41:53
  Speaker 1: the halos, and not just baryonic matter, not just normal
• 41:56
  Speaker 1: matter like gas and dust, but also dark matter is
• 41:58
  Speaker 1: flowing along these filaments into our halo. So our halo
• 42:02
  Speaker 1: is getting bigger and bigger. Oh I see. So as
• 42:04
  Speaker 1: the galaxies get older, they pull in more of the
• 42:07
  Speaker 1: stuff around them, They get bigger and heavier, which makes
• 42:10
  Speaker 1: them more gravitationally powerful, which sucks in the gas maybe
• 42:15
  Speaker 1: too fast, and so when it sucks it in, it's
• 42:18
  Speaker 1: too hot and it can form anymore stars. And all
• 42:20
  Speaker 1: this just comes together to give you a picture of
• 42:22
  Speaker 1: star formation and galaxies as sort of fragile and special,
• 42:26
  Speaker 1: like you need these special conditions. Things have to be
• 42:28
  Speaker 1: just right, and it's not that hard to perturb it
• 42:31
  Speaker 1: and to make it to the galaxies while they're still
• 42:33
  Speaker 1: big swirling blobs of stuff, they're no longer have the
• 42:36
  Speaker 1: conditions to make these stars and you know, without stars, Wow,
• 42:40
  Speaker 1: the universe just wouldn't be the same place. Makes you
• 42:43
  Speaker 1: wonder if, like, if the universe has been a little
• 42:44
  Speaker 1: bit different, would we have not gotten stars at all,
• 42:47
  Speaker 1: just like big swirls of hot gas that never formed stars. Yeah,
• 42:51
  Speaker 1: it would be a much darker universe, right without stars,
• 42:54
  Speaker 1: but you couldn't see anything exactly. We certainly wouldn't be here, right,
• 42:58
  Speaker 1: You could have a literally dead universe that the capacity
• 43:00
  Speaker 1: to form life if it had been a little hotter,
• 43:03
  Speaker 1: if all the gas in the universe have been too
• 43:05
  Speaker 1: hot to form stars in the very beginning, well you
• 43:08
  Speaker 1: would still have I guess quasars, right, like black holes
• 43:11
  Speaker 1: maybe shining because of the gas swirling around them. You
• 43:14
  Speaker 1: still have some bright spots in the universe, right, Oh yeah,
• 43:17
  Speaker 1: absolutely quasars, You're right, and there were some of the
• 43:19
  Speaker 1: brightest things in the universe. I wonder if life could
• 43:21
  Speaker 1: form in the vicinity of a quaisar. Yeah, that would
• 43:24
  Speaker 1: be pretty cool, right, I mean to to that, to
• 43:26
  Speaker 1: that civilization, it would be indistinguishable, right, that would be
• 43:30
  Speaker 1: their son, that would be their son. Yeah, And in
• 43:32
  Speaker 1: podcasts around that quasar, they would wonder, you know, our
• 43:35
  Speaker 1: quays are still growing. They would wonder why quasars are
• 43:38
  Speaker 1: no longer forming, because you know, also quasars formed in
• 43:41
  Speaker 1: the early universe, and we're not really making quasars at
• 43:43
  Speaker 1: the same rate as we used to. That's the next episode.
• 43:46
  Speaker 1: I guess how to quasars retire? Who's killing off the quasar?
• 43:51
  Speaker 1: I'm just trying to tap into the paranormal true crime podcasts,
• 43:55
  Speaker 1: which seemed to be so popular. I see, I see,
• 43:57
  Speaker 1: I see, you're going for um murder. Is Bigfoot killing galaxies?
• 44:04
  Speaker 1: Next time? On Daniel and Jorge? That's right, although we
• 44:06
  Speaker 1: still want the science category. Amazically Bigfoot podcasts are in
• 44:09
  Speaker 1: the science category. Don't get me started on that. Oh really? Wow?
• 44:13
  Speaker 1: Well not maybe not the natural sciences category. Oh yes,
• 44:16
  Speaker 1: in the natural sciences. Well, I guess who killed Bigfoot
• 44:19
  Speaker 1: would be another topic. Why is Bigfoot retiring that his
• 44:23
  Speaker 1: foot or her foot stop growing? Maybe he stepped on
• 44:27
  Speaker 1: a galaxy and got quenched. All right, Well, so those
• 44:30
  Speaker 1: are three reasons why a galaxy might stop making new stars?
• 44:33
  Speaker 1: Is there are there any other reasons? There are lots
• 44:35
  Speaker 1: of other theories out there. Some of them involve like
• 44:38
  Speaker 1: how galaxies tug on each other. Like, we've noticed that
• 44:41
  Speaker 1: sometimes galaxies that are more by themselves that aren't part
• 44:44
  Speaker 1: of a cluster tend to get quenched less often. One
• 44:48
  Speaker 1: hypothesis is that galaxies are like harassing each other, that
• 44:51
  Speaker 1: gravity from other nearby galaxies might be preventing gas from
• 44:55
  Speaker 1: falling into those galaxies to help spurn new star formation.
• 45:00
  Speaker 1: So if you're in the middle of a cluster rather
• 45:01
  Speaker 1: than by yourselves, might be that your friends and the
• 45:04
  Speaker 1: cluster are interfering with a gas infall you need to
• 45:07
  Speaker 1: make those new stars. M wouldn't other galaxies will help
• 45:11
  Speaker 1: you in a way slow down or cool off your
• 45:14
  Speaker 1: own gas, makeing it better to make stars. Yeah, they
• 45:16
  Speaker 1: do slow it down, but they might slow down to
• 45:19
  Speaker 1: the point where it doesn't fall into your galaxy anymore.
• 45:22
  Speaker 1: So you still need that gas. It's a really fine
• 45:24
  Speaker 1: balance that you need the gas to fall in but
• 45:26
  Speaker 1: not too fast. You need it to be um like
• 45:30
  Speaker 1: in the Goldilocks zone kind of. Yeah, you need a
• 45:32
  Speaker 1: very slow delivery of frozen gas. All right, Well, I
• 45:36
  Speaker 1: guess that's the mystery. Although I wouldn't you just think
• 45:39
  Speaker 1: that you know a galaxy is born, it has owned
• 45:41
  Speaker 1: amount of a cold gas and at some point it
• 45:43
  Speaker 1: runs out, and so the whole thing just kind of
• 45:46
  Speaker 1: and all at the same time. Like, wouldn't that also
• 45:49
  Speaker 1: be an explanation why it seems to be so binary,
• 45:52
  Speaker 1: so red or blue, like it just runs all out
• 45:55
  Speaker 1: of gas at the same time, because all of it
• 45:58
  Speaker 1: was formed at the same time. Yeah, it's an question
• 46:00
  Speaker 1: why it's a surprise that galaxies quench all at once.
• 46:04
  Speaker 1: You're imagining that they start with some supply of gas,
• 46:08
  Speaker 1: and then the star forming that uses that gas is
• 46:11
  Speaker 1: like used up all at once, all over the galaxy.
• 46:14
  Speaker 1: But remember that it's not really about having the raw
• 46:17
  Speaker 1: materials the gas to make stars. You could keep making
• 46:20
  Speaker 1: stars for zillions of years with the gas supply that
• 46:23
  Speaker 1: we have. Stars burn and die, and you could just
• 46:26
  Speaker 1: keep making new stars for a long time. The issue
• 46:29
  Speaker 1: isn't the supply, it's the conditions to make new stars. Remember,
• 46:34
  Speaker 1: you need cold conditions. So it's a surprise if something
• 46:38
  Speaker 1: is spoiling those conditions and doing it quickly and in
• 46:42
  Speaker 1: a sort of coordinated banner across the galaxy all at once.
• 46:46
  Speaker 1: That's not what you expect to have from just the
• 46:48
  Speaker 1: sort of random distribution of conditions that you expect in galaxies.
• 46:54
  Speaker 1: You see, it doesn't match a random distribution, is It's
• 46:57
  Speaker 1: kind of the mystery. All right? Well, um, I hope
• 47:00
  Speaker 1: find a culprit, Daniel, you know, maybe before it kills
• 47:03
  Speaker 1: our galaxy or no, wait, our galaxes already dead. Well
• 47:05
  Speaker 1: I thought you were pro galaxy retirement anyway, so I
• 47:08
  Speaker 1: thought you'd want to find the culprit and then encourage it, like,
• 47:10
  Speaker 1: hey man, you let all these galaxies off the hook.
• 47:13
  Speaker 1: They've done enough. I have pro retirement in any situation.
• 47:18
  Speaker 1: All Right, you teenagers out there, Jorge suggesting that you
• 47:20
  Speaker 1: retire tonight, Yeah, if you can, you know, go for it.
• 47:26
  Speaker 1: You know, I feel like I retired in my early twenties,
• 47:29
  Speaker 1: so I definitely recommend you retired and then started three
• 47:32
  Speaker 1: or four new careers. Yeah. I've retired about three times.
• 47:37
  Speaker 1: That's how pro retirement I am, Daniel, exactly. So I
• 47:39
  Speaker 1: guess we shouldn't think of these galaxies is quenched. We
• 47:41
  Speaker 1: should think of them as beginning their next adventure. Yeah,
• 47:44
  Speaker 1: there you go, and we hope you also find your
• 47:46
  Speaker 1: next adventure pretty soon in the near future, hopefully in
• 47:49
  Speaker 1: this galaxy before it retires. And remember that we are
• 47:52
  Speaker 1: still learning about the process of galaxy formation and galaxy
• 47:55
  Speaker 1: quenching and galaxy retirement, and hopefully we will live long
• 47:59
  Speaker 1: enough to learn much much more. Yeah, and I guess
• 48:01
  Speaker 1: the main lesson is that the universe is still a
• 48:04
  Speaker 1: work in progress. It's changing, it's growing, it's changing its
• 48:08
  Speaker 1: color and showing its age. It's getting redder and redder. Well,
• 48:11
  Speaker 1: we hope you enjoyed that. Thanks for joining us, See
• 48:13
  Speaker 1: you next time. Thanks for listening, and remember that Daniel
• 48:24
  Speaker 1: and Jorge Explain the Universe is a production of I
• 48:26
  Speaker 1: Heart Radio. For More podcast. For my heart Radio, visit
• 48:30
  Speaker 1: the I heart Radio app, Apple Podcasts, or wherever you
• 48:33
  Speaker 1: listen to your favorite shows. Yea