How do galaxies form and evolve? Is the universe still making new ones? What will happen to the current galaxies in the universe? I discuss these questions and more in today’s Ask a Spaceman!

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Hosted by Paul M. Sutter.

 

EPISODE TRANSCRIPTION (AUTO GENERATED)

So a couple episodes ago, I talked on and on about how to destroy galaxies, and we had a lot of fun. We tried to blow them up, tried to starve them out, tried to smash them together, but at the end of the episode, it felt a little sad for the galaxies. I mean, it wasn't their fault that they exist. They are the natural consequence of the various forces that play in our universe, and so they did nothing wrong. Certainly not anything worth destroying them over.

So I wanted to revisit galaxies, and instead of tearing them down, I wanted to build them up. And as I was browsing the list of unanswered questions, of which there are over 600 unanswered questions. Thank you so much for all the questions you keep sending me. I came across a very interesting one, one that I've never encountered before. Are new galaxies still forming in the universe?

As in today, right now. At this very moment, if we stared at a random patch of space in the cosmos and waited long enough, would we see a galaxy emerge? Now it turns out that the answer to this question is delightfully complicated, and it's delightful because it has no clear yes or no answer, which I know what you're thinking. Paul, it seems like rather straightforward. Yes or no?

Are galaxies forming today or not? Just answer the question. But listen. Listen. I didn't make the universe.

I just work here. And I'm going to stick to my guns that the answer is nuanced, and we have to be careful. I mean, I'll give away the game for you because I'm in a generous mood. The ultimate answer is that galaxies kind of are still appearing, but not in the way you might think of. The ultimate answer is that galaxies are still appearing, but only kind of, and not in the way you might think when you think of galaxies appearing.

I I told you it was complicated. So let's start with the not in the way you might think part because that's going to be a through line for the rest of the discussion. Galaxies as objects are pretty easy to identify. They are large collections of stars, gas, and dark matter. They are largely distinct from each other in the cosmic scene.

A typical galaxy is roughly a 100000 light years across, while the typical distance between galaxies is roughly a 1000000 light years. Sometimes galaxies merge together or clump together inside of clusters, but except for a minority of edge cases, we can largely separate one galaxy from another. It's like cities or towns out, you know, in the rural parts of a country. The distance between towns is larger than the towns themselves, so the towns are pretty easy to spot and define. Yeah.

Sometimes towns bump up next to each other, and sometimes sprawling cities consume their neighbors, but by and large, a town is just a town, and we're all cool with that. Similarly, galaxies are just a galaxy, and we're all cool with that. You can look in space and you can spot a galaxy. But what about the origins of galaxies? You know, it's like the origins of a of a city.

Like, how do you define the beginning of a city? Is it when the first survey marker is planted? Is it when the first house is built? Do you have to wait until there's a large enough community? It's a little bit fuzzier even though you can spot an existing city.

An existing town, it's a little bit harder to define when exactly a town gets started. Similarly, the start of a galaxy is way harder to define than spotting an already existing galaxy. Now we can outline the general process, and don't worry, I'm about to. And at the end of that description, you might be lulled into a false sense of security because now that you understand galaxy formation, you think you can say when a galaxy starts, but I urge you not to give in to temptation. And really, the story of galaxies begins with the story of dark matter.

Dark matter is by far the most important part of a galaxy. A typical galaxy is around 70 to 90% dark matter depending on the size of the galaxy. Dark matter is this invisible form of matter that seems to fill up every galaxy, dominate the universe. We have no idea what the dark matter is, but we have all these sorts of lines of evidence that point to its existence. It's not the greatest idea we have, especially since we don't know what it is.

We don't have a 100% confirmed detection of what the dark matter particle actually is. So it's not the greatest of ideas, but it's the best we got so far. We're gonna so we're gonna roll with it. There is something like dark matter operating at the very largest of scales, and it is the most important part of the galaxy. When you look at a galaxy, you're only seeing a very, very small percentage of the actual mass in the galaxy.

Less than 1% is in the form of stars. And if even if you add up all the regular matter, all the clouds of gas and dust, you're at less than 10%, usually, of the mass of the galaxy. And even if you see the disc of a galaxy and the spiral arms of galaxy and all these beautiful structures, the actual object of the galaxy that's defined by the dark matter is much, much larger than that. The galaxy is this tiny little jewel embedded in a, what we call, a halo of dark matter. This gigantic sphere that completely surrounds and subsumes its host galaxy.

And so when we look at the formation of galaxies, we have to look at the evolution of the underlying dark matter. And that starts out as tiny little ripples in density in the incredibly early universe. Dark matter starts flowing towards those tiny little ripples of density right away because there's nothing to stop it. Gas eventually falls in after the dark matter, but it has a much harder time because gas has a really hard time getting crunchier and smaller and denser because when it does, it heats up. And then when it gets hot, it doesn't want to compress anymore, and so it has to release radiation.

It has to go through all this giant mess of a process for gas to squeeze down into small volumes, but the dark matter just flows on in, just piles on in like it's no big deal. When that initial ball of gas first begins to compress into the hole that the dark matter has already dug for it, we call that a proto galaxy. That gas is initially hot. It eventually cools off and fragments and begins forming stars. It's not necessarily in that order.

We're not exactly sure. The gas may form stars as it collapses in. There may be many many mergers of smaller proto galaxies involved in this process, but that's the basic sketch. Dark matter piles in, gas follows, cools off, stars form, voila, galaxy. Seems fine.

Right? This is the story, but this is where your illusion of certainty bursts. When do we start calling it a galaxy? When did the galaxy actually form? Was it when the dark matter first started collecting?

Well, that started happening like a second after the big bang. That seems a little bit off. Was it when the first stars ignited? I don't know. It's just a loose collection of stars, like a blob of stars.

I wouldn't necessarily call that a galaxy. Is it when it develops spiral arms? But not all galaxies develop spiral arms. Is it when it reaches a certain threshold in mass? Well, that's totally arbitrary.

Is it when the dark matter has finished having its most prolific epoch of mergers and it's settled down all that? I don't know. It's tough to say. When does the galaxy start? There are a lot of unknowns.

And it's tough to say when a galaxy starts. And we do know that galaxies first formed in the first few 100000000 years after the big bang. And the key idea to building galaxies is that you first need a lot of dark matter concentrated in one spot, and then you need some loose gas to collect into that spot and start forming stars. And so in this sense, if we're asking, do brand new galaxies appear in the modern present day universe? We know it happened 1 or 200000000 years after the big bang.

But does it happen now 13,770,000,000 years after the big bang? The answer is no. New galaxies are not forming. If you just look at an empty patch of space and there's nothing there, there is no amount of time that you can wait where, eventually, dark matter will flow into that spot, dig a hole for itself, then allow some gas to follow in and the gas compresses and fragments and turns into stars. That is not going to happen.

And the reason for that is that the process of structure formation, this building of galaxies started in the incredibly early universe and then it finished. It did its job. It built the galaxies. It collected the dark matter. Those density differences grew.

They grew large enough to collect enough dark matter. The dark matter grew large enough to collect the gas, and then that was it. There are no new pockets in the universe that have not already existed. We can't make new pockets of density in the universe, like, little ripples in space time itself for dark matter to collect in. That was laid down just, like a second into the big bang.

And then any pockets that were already there, which is all of them, grew up to become galaxies. There are no new pockets emerging. No new seeds of galaxies emerging in the cosmic scene. Now we can't observe this directly, the statement that I just made that the pockets of dark matter that lay the groundwork for future galaxies have done their thing. We can't observe this because dark matter is is kind of hard to look at, but simulations tell us the story.

We can build cosmological simulations that play through the physics of the past 14000000000 years, that they play through the gravity of the dark matter, the interaction with the gas, the formation of the stars, these simulations, which is just taking our mathematical equations and solving them on a computer and then spitting out pretty pictures for us to look at, which deserves an entire episode on its own on how we do computer simulations of physics. I'd love to do that because it's kind of my specialty, and so feel free to ask. The simulations tell us about the behavior of the underlying dark matter. And in the simulations, when we can make the dark matter light up for our amusement, the number of small dark matter halos, these, like, seeds of galaxies peaked about 10000000000 years ago. The universe has not been producing new ones.

There are no new seeds of dark matter structures appearing in the modern day universe. Instead, all we have today in the modern universe is the merger of smaller dark matter halos crashing into each other to form larger ones. And so we see this, like, the Milky Way is on a collision course with Andromeda. We have a dark matter halo. Andromeda has a dark matter halo.

They are merging together, and there's no new Milky Way size halo coming on the scene today. The only way you can get larger dark matter halos is through the mergers of smaller ones today. And before I continue, I do need to mention that this show is sponsored by BetterHelp, and I want to talk quickly about your self care nonnegotiables. What are the things that you don't ever let yourself skip? You know, whatever craziness is happening in your life, whatever insanity is happening in your life, what do you make sure you do?

Is it a workout? Is it therapy? Is it that slice of cheese every single evening? Is it ask a space man? I don't know.

Whatever it is, I want you to think of it, and I want you to consider therapy as a nonnegotiable something that you're going to prioritize no matter what is happening in your life. Therapy is an incredibly powerful tool. And if you're thinking of starting therapy, I want you to give BetterHelp a try. It's entirely online. It's convenient, flexible, suited to your schedule.

You find the therapist, a licensed therapist that works with you. Never skip therapy day with BetterHelp. Visit betterhelp.com/spaceman today to get 10% off your 1st month. That's better help, h e l p, dot com slash spaceman. So the universe is done making new dark matter halos.

It's done making galaxies. But that's not the whole story. Because that's one way to define a galaxy. You know, the the seed of a dark matter halo, that's one way to do it. Or once the dark matter halo has reached a certain critical mass, that's one way to do it.

But there's a lot more to a galaxy than the dark matter. Well, okay. Not a lot more. But there's more to a galaxy than the dark matter. They're the stars.

I mean, that's the most visually obvious component. Yeah. It's it's we're biased with our human vision, and we see stars even though they're less than 1% of the mass of a galaxy. That's what's striking and apparent about galaxies to us today with our human eyes, and and so it's valid. It's a way.

I mean, if you want to define a distant seashore, you can define it with the rocks and outcroppings. And in the daylight, that works just fine. At night, it's a little bit harder. So instead, we use lighthouses to define the shore. And that's just as valid for that particular application, which is not crashing into the shore.

Galaxies look nothing like any other kind of astronomical object. They are distinct. These these large populations of stars do make them stand out, do make them different than other things. So if we lean away from when did the dark matter halo first collapse as our benchmark for creating galaxy, we get some more flexibility. For example, is a city created when it is first planned?

Is it created when the first survey markers are put in? Is it created when the first roads are laid down? Or is it created when the people start moving in? I'm pretty sure that the humans in New York City only make up a small fraction of the total mass of all the objects in New York City, but they're the most important part. That's what makes it a city.

It's the people. And so in this sense, galaxy formation is alive and well. For certain, there are no more proto galaxies. There are no more loose collections of gas that are just beginning to assemble. That's done.

That process finished billions of years ago. But there are already existing pockets of gas, already collapsed, dark matter halo already formed, that are just now lighting up in a significant way. And the evidence for this has really only come to light, in the past few years, by mapping out something called the Stellar Mass Function. This is the basic demographic census of galaxy. It maps how many stars are lighting up in each galaxy, or put another way, it asks, how much total mass of stars is living in galaxies in any one time?

And so this mapping, it's it's like mapping cities not by their town borders or their survey markers, but by their population. Or at least, how much of that population is turning on their lights at night. We care about quantities like this because they are easy to measure. It's hard to measure the dark matter. It's hard to observe it.

It's hard to measure it. It's hard to detect it. It's easier to measure the stars, the brightness of these galaxies, the total mass that's, in the form of stars in a galaxy. We can actually measure that really, really, really easy. Just like getting an estimate for the total mass of a city is going to be hard.

There are buildings. There are sewage systems, there are electrical, there are cars, that's a hard number to pin down. But getting a census of the population is pretty easy. Even though the population only makes up a small fraction of the mass, they're a pretty important part, and they're an easy to measure part. And so we can study this.

We can measure this. We can put together this stellar mass function by doing surveys across big chunks of the universe, both broad and deep, going far back in time as we can. And when we do this, we see something amazing. Most of the galaxies, 10000000000 years ago, when the dark matter was done making new small halos, when all the proto galaxies had already finished collapsing, at around that same time, about 10000000000 years ago, most of the galaxies were medium sized. And as time went on, we ended up getting more of both big and small galaxies.

We get the big galaxies through mergers. You take a couple medium galaxies, you smash them together, you get a big galaxy. So we get more of those. You take 2 small galaxies, you merge them together, you get a medium galaxy. But we're also seeing more small galaxies.

In terms, not in mass, not in terms of dark matter concentration, but in terms of starlight. We're seeing more populations of galaxies with, you know, a lot of stars. Like a 1000000 stars, 10,000,000 stars, a 100,000,000 stars. We're seeing more of those. And there are more of those today than there were 1,000,000,000 of years ago.

How does this work? It works through Patreon. That's patreon.com/pmsutter. If you contribute to this show, more galaxies appear in the universe. How great is that?

It's patreon.com/pmsutter. Thank you so much for all of your support. It means the world to me. This is an observational reality. There are more of every kind of galaxy in the present day universe than there was 1,000,000,000 of years ago.

There are more small galaxies, more medium galaxies, more large galaxies. How can this possibly work? If there are no new dark matter halos, if there are no new planned cities, how can there be more cities that we actually observe when we go out to do a population census? The answer is that making stars is hard. Star formation is a ridiculously inefficient process.

It's really really really hard to make stars. And because you need to take a whole bunch of gas that's spread out over thousands of light years, and you need to compress that gas, which it really doesn't like to do because when you compress it, it heats up, which resists further compression. So it has to cool off, and then you compress it, heats up, you need to cool off, and you can go through all these cycles, and you need to squeeze, like, a 1000 light years worth of gas into a tiny, tiny, tiny little ball that can ignite nuclear fusion. That's really hard. And so only a small percentage of the total mass of a galaxy lights up as stars at any one time.

And not even including the dark matter. Let's just leave that to the side. Of all the gas reservoirs in a galaxy, at any one time, only a small fraction of it is in the process of forming a star or being a star. It's only a tiny percentage. It's a very inefficient process.

Most of the mass is just tucked away in these gas clouds and because star formation is an inefficient process, it can take a while for the smallest galaxies to light up. So what this means is that the early young universe can form a galaxy. It can collect the dark matter, can collect the gas, evolve past a proto galaxy, have a few stars here and there, and then just hang out and take 10,000,000,000 years before star formation really kicks in and it really shines and it gets really bright. It can take 10,000,000,000 years for people to finally move into the planned city. And that process is ongoing today.

It's happening right now. New galaxies are appearing. They were already there. They were have everything in place. They have the infrastructure.

They have the buildings. They have the sewage systems. They have everything. They're just missing the people. And the people are starting to move in.

The star formation is starting to ramp up to the point where we can finally observe it. So in that sense, yes. Galaxy formation is happening today. There are no new planned surveyed cities, but people are still moving in. There are no new dark matter halos emerging on the cosmic scene, but already existing dark matter halos with collections of gas that have been quiet and dim for 1000000000 of years are just now starting to light up.

But sadly, the state of affairs will not last long. The peak of galaxy formation, the fastest rate of the appearance of new galaxies, was around 10000000000 years ago. Around the same time that the universe stopped making new dark matter halos that was all done with that. That was also the time when we were making lighting up galaxies at the fastest rate. Yes.

We're still getting new galaxies today in the sense that they are converting their mass into stars. They are lighting up. We're still having that today. But that process is slowing down. The past 1,000,000,000 years saw fewer new galaxies lighting up than the previous 1,000,000,000 years and so on and so on.

Plus, the data are telling us that we're seeing a precipitous drop in star forming galaxies, ones with high rates of active star formation. We're seeing a huge rise in quiet galaxies that I've lit up, but they're not making babies. You know, it's just an older population that's still there, still shining, but there's you know, they're running into a demographic crisis here. There are no new, young, stars appearing on the scene. That those populations are are getting huge while the populations of star forming galaxies where they're pumping out a lot of new baby stars all the time, that is dropping.

The ultimate culprit behind this is dark energy. We live in an expanding universe. Dark energy tells us that the expansion of the universe is accelerating. It's getting faster and faster every day. The process of structure formation is winding down.

Galaxies aren't merging as often. Gas is getting spread out through the universe. You know, a a galaxy isn't 100% isolated. It's surrounded by streams of very, very thin gas. You can barely see it.

But that gas falls onto the galaxies. It enriches the galaxies, gives it new fuel, you know, migrants into a city. Every galaxy has new migrants coming in every year that gives it some more fuel reserves that it can keep turning into more and more stars as time goes on. That process is slowing down because our universe's expansion is accelerating. Galaxies are getting farther apart on average.

The gas surrounding galaxies is getting more dilute. It's having a harder time coming on to galaxies. And so that's why we observe. We know for a fact that star formation is shutting down in our universe. That galaxy formation in terms of lighting up in stars is shutting down in our universe and has been for 1,000,000,000 of years.

We suspect the culprit is dark energy. Unfortunately, it's a very grim fate. Galaxies are running out of usable material to make new stars. A galaxy like the Milky Way, it can last quite a long time. We're talking 100 of 1,000,000,000 of years, even 1,000,000,000,000 of years of continued star formation.

But right now, the Milky Way galaxy converts about 1 solar mass of material into stars every single year. You know, sometimes that's one star the size of the sun. Sometimes that's many stars smaller than the sun. Sometimes, you know, you you put a few years together and you get one giant star appearing. It's about one solar mass of material goes into forming stars every single year in the Milky Way galaxy.

That's pretty typical. Our galaxy is pretty typical that way. But that rate is slowing down. 1000000000 of years ago, it was 10 times higher. Why?

Because there were more small dwarf galaxies colliding with ours, merging with ours, allowing us to cannibalize it, stirring stuff up, getting new material. There's more raw loose gas flowing in. All of that isn't happening as much anymore. The appearance of new dark matter halos shut off 10,000,000,000 years ago. The peak of galaxy formation happened 10,000,000,000 years ago.

Since then, everything's just slowly winding down. It's got a long runway. The last star will appear about a 1000000000000 years from now, and it will die about 1000000000 years after that. It will be incredibly small red dwarf stars at that incredibly distant epoch. So the party isn't over, but it's not going to last forever.

And we should enjoy it while the music is still playing. Thank you to Carrie g for the question that led to today's episode. Thank you for all your questions, all your support. I absolutely love it. I I thrive on it.

This show could not be possible without you, and it couldn't be possible with all of my generous Patreon supporters. All of you are gems, are stars glowing in dark matter halos. I'd like to thank my top Patreon contributors this month. Justin g, Chris l, Alberto m, Duncan m, Corey d, Stargazer, Robert b, Tom g, Nyla, Sam r, John s, Joshua, Scott m, Rob h, Scott m, Lewis m, John w, Alexis, Gilbert m, Rob w, Denise a, Jules r, Mike g, Jim l, Scott j, David s, Scott r, Heather, Mike s, Michelle r, Pete h, Seves, Wat Wat Bird, Lisa r, and Koozie. That's patreon.com/pmsutter where you can support this show.

If you ever wonder, how can I support this show? Patreon.com/pmsutter is your answer. Keep those questions coming. Ask a spaceman@gmail.com or the website askaspaceman.com, and I will see you next time for more complete knowledge of time and space.

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