It's a real pleasure to introduce today's speaker, Professor Elliott Dart,
who holds the Thomas and Alison Schneider chair in the physics department at the University of California at Berkeley.
I have known Elliott personally for many years and watched him grow from a student to,
in my view, one of the world's leading theoretical astrophysicists.
And it's a real pleasure to have him here for this lecture. His interests, I have to I have to have an index card for his interests alone.
You know, usually I put the career on, but this is just his interest.
He's on his pay web page. He lists black holes, stellar physics, plasma astrophysics, cosmology and galaxy formation.
And he's made seminal contributions in all of these areas.
So Elliott did his Ph.D. in 1999 at Harvard University with Professor Ramesh Narayan
and then a two year postdoc at the Institute for Advanced Study in Princeton.
And he was quickly snapped up at the University of California at Berkeley and rose rapidly through the ranks.
And at the moment, in addition to his position in the physics department,
he is also the head of the Theoretical Astrophysics Centre in the Astronomy Department and has led it to become,
I would say, one of the most important schools of theoretical astrophysics in the world.
So it's a real pleasure once again to have him here.
And he is going to tell us about how the universe evolved from smooth to that looks like pretty small scale lumpy going.
Thanks, Steve. So thanks very much.
It's a real pleasure to be here. So what I'd like to do today is give you an overview of, I think, what one of the.
Yeah, right. One of the key astronomical parts of our understanding of our origins.
It's one of, I think the great contributions to of astronomy to the broader culture is the impact
that the field has had on understanding our place in the universe and in particular,
our origins. And I'm going to do that by first giving you some context about our understanding of the history of the universe,
and then focusing in on some of the cutting edge areas where we're still struggling to understand how we went from the smooth beginnings that we
see in the cosmic microwave background to the very rich and complicated population of galaxies and stars and black holes that we see in the sky.
And so I want to start just by telling you a little bit about some of the objects that we see, since I know there is a diverse background.
So this is actually a picture of our own galaxy.
This is a picture of the Milky Way galaxy taken by satellite above the atmosphere of the earth.
To give you a sense of scale, our galaxy is about 100,000 light years across.
That's something like 10 million times bigger than the distance between the Sun and Pluto or Neptune, roughly the size of the solar system.
The galaxy contains something like 100 billion stars, a total mass of a trillion times the mass of the sun.
And most of the mass in our galaxy is actually not in anything that you can see in this picture,
but it's in the form of dark matter or some kind of other fundamental particle not gas, not stars.
And dark matter plays a very important role in the story of the evolution of the history of the universe.
This is our kind of nearest big neighbour, the Andromeda Galaxy.
That's about 3 million light years away, actually about 4 billion years from now.
Our Galaxy and Andromeda are going to slam into each other by that point.
That really won't matter much to the Earth because the Earth will have already boiled away as the sun expands to become a red giant late in its life.
But this fact that there are two galaxies near each other in this case that will collide in the age of the universe.
That's actually not that uncommon.
That's a kind of an insight into how structure forms in the universe that you have galaxies orbiting around each other,
just like planets orbit around stars and stars orbit around each other.
In fact, galaxies are very often found in collections held together by gravity,
such as this example of a cluster of galaxies where you have many different galaxies all orbiting around the gravity of their mutual dark matter.
This picture is one of the most famous pictures in astronomy of galaxies in the distant universe.
Actually, the famous part is right there. This just gives you a sense of scale.
So this is an image that I'll show on the next slide is an image taken with the Hubble Space Telescope.
It'll zoom in to this little region here, which for scale is small compared to the moon on the sky.
And in that tiny little patch of sky, when we stare for a very long time with the Hubble Space Telescope,
we see something like 10,000 galaxies in this image here.
And then this region zooms in yet further to show in more detail some of the particular galaxies observed in this one little patch of sky.
And because these the Hubble Space Telescope is showing us galaxies that are very faint, those are galaxies that are very far away.
And it's taken light a very long time to travel to us.
And so we're actually seeing the galaxies as they were something like 12 billion years ago when the universe was much younger than it is today.
And that's, of course, one of the great wonders of astronomy that's sitting here on Earth.
We can use the fact that it takes light to travel a certain amount of time to reach us,
to figure out the entire history of the universe without going anywhere.
And that was really started pioneered by Hubble in the early 1920s by observing distant
galaxies and measuring how they're moving relative to our own Milky Way galaxy.
And what Hubble discovered was the expansion of the universe, the fact that if we sit on our galaxy here looking up at the sky,
we measure all the galaxies around us seeming to move away from us.
And that's not something special about our galaxy.
Everything's not running away from us.
In fact, we think this is a property of space itself, that all galaxies are moving away from each other as shown in this kind of schematic image.
If you sit on any one of these galaxies and watch all of the other ones, you'll see all of the galaxies moving away from each other.
And in fact, one of the great goals in astronomy since the discovery of the expansion of the universe has been to understand
really the history of that expansion was the distance between galaxies always expanding at the same rate,
where things going faster in the past, slower in the past, etc.
And the real breakthrough actually came only relatively recently, certainly in the history of astronomy in the late 1990s,
with the recognition that in fact, the expansion of the universe is currently accelerating.
So things are starting to move apart faster and faster and faster.
And we've kind of given that a name, although we don't really understand why that is, why the expansion is accelerating.
But the name we've given it is that most of the energy in the universe has to be in something that we call dark energy.
And this really is a label for a form of energy that produces the acceleration that's observed.
And this dark energy is completely different from the dark matter that we think makes up most of the mass of our galaxy.
And in fact, that leaves very little room. Only about 4% of the stuff in the universe is the stuff that's in this room,
the iron in our blood, the oxygen we breathe, or in fact, mostly the hydrogen in water.
So the next big clue to thinking about our astronomical origins came from what might seem to be a slightly unexpected source.
It came basically from taking pictures like this one, pictures of the sky.
So I already said this is a picture of the night sky taken with an infrared telescope orbiting around the earth.
You can take pictures, though, of the sky with different types of telescopes that observe different parts of the electromagnetic spectrum.
This is a picture of the sky taken with an X-ray telescope.
What you see with an X-ray telescope is very different than what you see with an infrared telescope.
You see these kind of circles of X-ray emission, some point sources of X-ray radiation.
Those point sources of X-ray radiation in some cases are, in fact, gas swirling on to black holes as the gas spirals on to the black hole.
It produces huge amounts of radiation. That's something I'll come back to a little bit later.
But in fact, the picture of the sky that turns out to be the most important for thinking about our origins is actually a really boring picture.
It's this. This is a picture of this night sky taken with a radio telescope that observes in the microwave
part of the electromagnetic spectrum the same microwaves that you use for heating up your food.
And actually, this picture of the sky corresponds to a source of radiation, a source of light microwaves.
In this case, that has a very particular temperature, about 2.7 and Kelvin, or -454 Fahrenheit.
Or now I realise I should have put it up in centigrade and can't do the conversion immediately.
So this is in fact a signature of the expansion of the universe.
This is I as a theorist. I'm particularly proud that this is an example where the theorists made a prediction before the observations were made.
And what Gamma and others predicted was that if you take this expansion seriously and you win back the clock,
things in the distant past were much closer together, much denser.
The universe was much hotter.
And at some earlier phase, all of the gas in the universe itself should produce a bath of radiation that now fills the universe.
And we think that this cosmic microwave background is indeed the glow of an early phase in the universe's history,
when things were much denser and much hotter than they are today.
And over time, as the universe has expanded, that very hot early universe has cooled to produce the microwave glow that we see around us now.
When we look up at the sky and this was first detected with one of the early radio telescopes by Penzias and Wilson in the 1960s.
And since then, really the study of the microwave sky has been one of the most important areas of astronomy.
It's provided much of our detailed quantitative information about the energy and mass content of the universe,
about the expansion history of the universe, and about the properties of the universe when it was much younger.
And our kind of best picture now of what the infant universe looks like was taken by the European Planck telescope just a few years ago,
and it's illustrated here. And so the difference between these two pictures is that in this original picture,
which is really just a PowerPoint oval that I drew myself, but in this in this original picture,
the telescope wasn't good enough to see that there are actually tiny differences
in the amount of microwave light coming from one part of the sky to another part.
And it took the development of technology better telescopes,
better instrumentation to actually be able to see that different parts of the sky produce very slightly different amounts of microwave light.
And in fact, the differences are really small.
The difference in amount of microwave light from one part of the sky to another is something like
.01 percent differences in the amount of light that we get from one part of the sky to the other.
And we know how to interpret differences in the amount of this kind of thermal radiation produced by hot objects.
And what it tells us is that in the early universe at this time when this radiation was produced,
there were only tiny differences at this level of 0.01%,
one part in 100,000 differences in the temperature and density of the matter that fills the universe at every place that we can see on the sky.
So that part of the universe and that part of the universe were almost exactly the same in their density and temperature,
with differences that amount to only about 0.01%.
And this time is about 380,000 years after the Big Bang.
The Big Bang is sometimes what we call T equals zero.
But that's really just the time when you rewind the clock,
when things get so dense and so hot that the laws of physics as we know them, start to break down.
And we don't really know what happened before that time.
And so in that sense, when we talk about the universe being 13.8 billion years old,
what we really mean is the universe is 13.8 billion years old since we don't know what happened.
So this is the universe then. And that's an incredibly different universe than what we see around us today in this room.
There are enormous differences in the property of matter from one place to another.
The difference in my density and temperature from the difference of the density and
temperature of the air to the density and temperature of the centre of the earth,
of the centre of the sun is many, many, many orders of magnitude, much larger than these tiny differences of 0.01% in the early universe.
So how did we go then,
from this early phase to this incredibly rich diversity of structure that we now see around us with billions of planets in our own galaxy,
billions of galaxies in the visible universe.
And so that's really the rest of the story that I want to tell you.
This is in some sense been just trying to explain the meaning of the title Smooth to Lumpy is this transition
from the very early universe we see in the microwave background to all of the complicated structure we see today?
And there is actually a single, simple one word answer for how this happened.
And so that one word answer is gravity.
Gravity is the force that dominates the behaviour of the universe on the largest scales.
The other forces that we know of in nature, electromagnetism, the weak and strong nuclear forces pale in comparison to gravity.
When considering the large scales of the universe as a whole, and in particular what gravity does, is it takes very small differences.
Those very small differences that you see in the microwave background, and it makes them bigger and bigger as time goes on.
And the kind of general argument goes like this.
We see in the microwave background that if you look in different places, the position is meant to be different points along this axis.
Different places in the universe have slightly different densities and regions
where there happens to be more stuff are regions where gravity is stronger.
Those regions pull in more surrounding material,
and so those regions grow in mass more effectively than regions where there isn't initially very much matter.
And so gravity takes small differences and makes them bigger and bigger as time goes on.
This is a form of instability called the gravitational instability studied by genes in the early 1900s.
And it's really the critical mechanism for taking these tiny initial differences and making them bigger and bigger and bigger as time goes on.
And this is where in detail the fact that the universe is dominated by dark matter becomes important.
Most of the mass in the universe is not the normal matter in this room.
It's dark matter. And so most of the mass producing the gravity that causes these kind of lumps to grow with time is dark matter.
And in fact,
this is the fundamental difference between dark matter and dark energy is that dark energy doesn't really participate in this clumping process.
It's uniform. It's relatively uniformly spread throughout space.
So dark energy is relatively smooth, while dark matter is relatively inhomogeneous,
and it dominates over normal matter in producing the growth of structure.
So this is kind of a cartoon to give you a physical picture of what this looks like.
I want to now show you some examples from more realistic numerical simulations of what this looks like.
So this is a movie that starts when the universe was quite young and goes forward in time following just the dark matter.
And the box here is about the size of a region that will make a galaxy like our own Milky Way galaxy.
And the kind of little bumps represent the dark matter.
And so initially, I'm going to start over again.
Initially, you see things are moving out and then gravity kind of winds and causes things to collapse back down.
And so initially, the expansion of the universe drives things that things apart.
But then gravity is strong enough to overcome the expansion of the universe and cause our galaxy to collapse in on itself.
And this then is the formation of the dark matter backbone to our galaxy.
And this basic process of gravity winning out over the expansion of the universe and causing things
to collapse in on themselves is really the kind of central process that gets galaxy formation going.
And you'll also notice one of the things you see visually in this movie that's quite important to how we think
galaxies form is that initially the little small lumps that represent the first galaxies are quite small.
And then as time goes on, they get bigger and bigger and bigger.
So we think, in fact, that galaxies started off relatively small, maybe only consisting of a few stars, actually.
And as time goes on, you build up bigger and bigger galaxies as illustrated by this movie.
Now, to give you a sense of scale, I'm going to add to this movie the actual size of the stars in our galaxy,
and that's just this tiny little region at the centre.
So most of what we call our galaxy actually consists of this large region that's dominated by the dark matter.
The region where the star is in the gas and the planets are is confined really just to the central part.
So this is a few million light years big.
This now shows the distribution of dark matter spread out over a region that something like a billion times bigger.
So this is about a few billion light years across now.
And this shows the distribution of dark matter in a computer simulation in the present day.
And again, the kind of whitish regions are regions where there's lots of dark matter.
And the dark regions are regions where there is very little dark matter.
And this really is the dark matter backbone that dictates where galaxies form.
Regions that have lots of dark matter have enough gravitational pull to bring in gas and stars from their surroundings and build up galaxies.
And so it's the gravity of the dark matter,
pulling in the gas and stars that leads the galaxies that we can actually see in stars, in gas to form where the dark matter is.
And so, in fact, in a very real sense, when we look up at the distribution of galaxies on the sky.
So this is now an image of where galaxies are positioned, observed on the sky.
So we're sitting at the centre and galaxies out here further away from us, ones in here closer to us.
And this shows the angle on the sky.
And so you see this kind of lumpy, inhomogeneous pattern of galaxies on the sky.
And that really is, to first approximation, that distribution of galaxies is tracing the distribution of dark matter.
Because the dark matter dominates the gravity. And so it tells the gas in the stars where to go.
If that were it. If that were the whole story, our understanding of how galaxies form would really be a solved problem.
But it's not. And an indication of that is that.
When we look at the properties that galaxies actually have, how big are they?
How massive are they?
It turns out that the properties of galaxies are related in a rather complicated way to the properties of the dark matter around them.
And so a famous way that astronomers like to illustrate this is to show how many objects are there versus the mass of the object.
So really massive objects are over here, low mass objects are here.
And the solid line represents the distribution of these dark matter clumps that you see in this image.
So the regions like this one are lots of dark matter and those are rare.
And so those are objects out here.
Rare, very massive objects. The solid circles represent observed galaxies, the stellar mass of observed galaxies.
And the point of this is just to drive home that when you actually look at the masses you see in stars and galaxies,
it's not the case that the mass of stars is just equal to the mass of dark matter,
or it's not equal to the mass of dark matter times point one or some simple number like that.
There's a complicated relationship between the distribution of galaxies and the properties of galaxies that we see and the properties of dark matter.
And in some ways, this is the challenge that the modern theory of galaxy formation is trying to get
at what drives these variations in the sizes and shapes of galaxies that we see,
what sets the masses of galaxies relative to the masses of the dark matter around.
And so what I want to do is try to describe to you a couple of the areas of forefront research
trying to get at this question and just give you some of the big ideas that we're grappling with.
And I'm going to emphasise really two key questions.
So one is trying to understand a little bit more about how gas actually gets into galaxies,
because that determines then how the mass of normal matter, the stuff that we can actually see grows in time.
And then the second question that I'll address is what happens once you start actually forming stars and galaxies?
What happens once you start actually forming black holes at the centres of galaxies?
How does that actually change this picture of gravity?
The gravity of dark matter dominates. It pulls stuff in.
That's the end of the story. And so these are the questions that I want to address.
And these really are questions that get at the heart of a lot of research that's being done today.
So there are really two major ways that galaxies grow in mass with tall.
One is that just like you saw how dark matter lumps start out small and then they
collide with each other and they build up bigger and bigger dark matter lumps.
The same thing happens with the stars and gas in galaxies.
So galaxies slam into each other. The Milky Way in Andromeda, as I said, will collide with each other in 4 billion years.
And that's not an accident. That's actually a property of things start out small and they build up bigger and bigger as time goes on.
And this, in fact, is a series of images taken with the Hubble Space Telescope showing galaxies in the act of colliding with each other.
And what they look like is kind of train wrecks, right, when galaxies slam together.
You get this very complicated, disordered shape. The other way that we think galaxies grow.
And that may actually be the dominant way that galaxies grow, is that they actually just pull in surrounding gas.
And that's shown in this computer simulation.
The dashed line represents about the size of that dark matter region I showed you in the earlier simulation.
And the red shows gas that's being pulled in by the gravity of the dark matter.
And that gas gets pulled in to the central part of the galaxy.
And as it gets pulled into the centre,
eventually that gas can turn into stars forming the stars that we see in these images with the Hubble Space Telescope.
And understanding which of these is actually the dominant way that galaxies grow.
This has been a major set of questions that people have been working on over the last 5 to 10 years.
And in particular, one of the real forefront areas today is trying to directly observe this gas as it flows into galaxies.
And that turns out to be quite challenging for a variety of observational reasons.
So this again is our picture of how galaxies assemble.
They start out as these very small lumps and as time marches on, they become bigger and bigger structures.
As a theorist, I like to simplify things.
This is a little complicated. It's very messy.
It's not symmetric. I like circles. So this is kind of a simpler theoretical picture of how a galaxy grows.
The white region shows the gas that has actually fallen into the gravitational clutches of the dark matter associated with that galaxy.
Outside this blue stuff is the gas that's getting pulled into the galaxy by the gravity of the dark matter.
As time goes on, you can see that the size of the galaxy is actually growing in time.
And again, as I already illustrated in one of the earlier pictures, we think that where the stars actually sit is at the very, very centre here.
That's where things are dense enough that this gas that fills the space in the dark matter halo, the gas can actually cool to form new stars.
And so one of the big questions is trying to understand what is the behaviour of this gas that fills up the halo of dark matter?
How does it behave? How does the gas actually get to the centre of the galaxy?
How does it cool to form stars? And so one of the discoveries that's come out really over the last five or ten years, work done by Steve,
this here and then myself has been demonstrating that gas in galaxy, just like water on your stove undergoes a very vigorous kind of boiling motion.
It's not this simple static picture that you might have from this kind of image, but it's a much more dynamic process than we earlier thought.
The gas is actually churning around, boiling very much.
The physics here is in fact, quite analogous to what happens in a pot of water.
And we're, in fact, still struggling to understand how does this change our picture of what determines
the properties of gas in the very central regions where the stars actually form,
where the galaxy actually grows? And so this is one area that my group is doing a lot of work, is trying to understand this particular question.
Okay. So I want to turn now to what is really I think probably the most important question in our modern understanding of
galaxy formation is trying to understand observationally and theoretically what happens once stars start forming,
once black holes start forming? What effect does that have on the rest of the process of forming galaxies?
And this goes under the general name of feedback.
And the idea behind that name is that once you start forming stars and black holes,
that the process of star formation and black hole growth feeds back on or impacts the rest of the gas in galaxies,
the rest of the process of galaxy formation.
And the kind of simple moral of the story is that once gas gets into galaxies,
it starts being affected by processes and forces in addition to the gravity of the dark matter that brought it into the galaxy in the first place.
So on very large scales in the universe, the gravity of dark matter is the dominant force and it dictates what happens.
But once you get inside galaxies, the gravity of dark matter is actually no longer the dominant force.
There are other processes like stellar explosions, the effects of black holes on their environment,
which can be much more important than the gravity of dark matter in dictating how the gas behaves.
And the kind of dramatic illustration of this is that we have this picture of gas
flowing into galaxies as illustrated in this computer simulation shown here.
But we actually think that essentially 90% of the gas that at any one time came into a galaxy
is later blown back out of that galaxy by the combined effects of stellar explosions,
black holes, etc. And that's actually illustrated.
This is not just something theorists make up. That's illustrated in these two images.
This is a Hubble space telescope image.
This is an image with the Chandra X-ray telescope which actually show these dramatic outflows of gas driven by star formation.
And so what I want to do is try to give you a simple feel physically for why is it that once stars start forming, once black holes start growing,
why is it that that has such a dramatic effect that it can actually blow out
most of the stuff that at any time made it into the galaxy in the first place?
And the way we like to think about this as physicists is in thinking about the forces on gas or the energetics of gas.
And so the way I'm going to explain this to you is really in both ways, forces in energy.
And so if you turn if you have a certain amount of mass in a galaxy, a certain amount of gas,
and it turns into stars, the process of forming stars actually dumps energy back into its surroundings.
And the way that happens is a couple of different ways.
One is star shone. They produce light.
Nuclear fusion in stars produces enormous amounts of light, and that heats up and pushes on the surrounding gas.
This is a picture of one of the regions in our own galaxy where massive stars are forming.
And you see these regions lit up by the newly formed stars within.
The other thing that happens is that massive stars at the end of their lives
actually explode after they collapse to form either a neutron star or a black hole.
And those explosions release enormous amounts of energy.
These are an example of this. This is an X-ray image of the explosion of a massive star.
So these processes of the radiation produced by nuclear fusion in stars or the explosion
associated with the formation of neutron stars and black holes at the ends of their lives.
Take some of the energy associated with forming stars and it dumps it back into the surroundings.
And that amount of energy is about 0.1 percent of M.C. Square.
So I take a certain amount of gas and I turn it into stars.
I get about point 1% of EMC squared back out in the form of radiation and stellar explosions.
And this is a nice astronomical application of equals MC squared.
This amount of energy actually pales in comparison to the amount of energy we get out of black holes.
So we think of nuclear fusion in stars or nuclear fusion here on the earth as being this very efficient source of energy.
It's the source of energy that keeps the earth hot and allows life to proceed.
It's the source of energy that we think we hope one day will solve the energy needs here on Earth.
But in fact,
turning a mass of gas into a black hole is a far more efficient way of getting energy out than nuclear fusion in stars up to 100 times more efficient.
And the way this happens is that as gas falls in to form a black hole,
it orbits around, it gets hotter and hotter and hotter due to friction in that gas.
Friction mediated by magnetic fields. And that friction actually releases enormous amounts of energy.
And something like 10% of M.c squared comes out in the form of energy when you form black holes in galaxies.
And that energy, again, it comes out in different forms.
Some of it comes out in the form of radiation light.
This is an image of a accreting black hole at the centre of a galaxy, a quasar,
where this single region at the centre of the galaxy with a massive black hole that maybe weighs a billion times the mass of the sun.
This tiny little region, no bigger than our solar system, actually produces more light than the entire billions of stars in the galaxy.
And the only way you can even see the stars in this galaxy is if you use the exquisite resolution
of the Hubble Space Telescope to subtract off the light from the accreting black hole,
revealing the faint distribution of stars making up the galaxy around the black hole.
Another way that the energy released forming black holes comes out is in the form of
powerful outflows that carry away energy and energy of motion of relativistic particles,
much like the stellar explosions at the ends of the life of massive stars.
Forming black holes releases energy in the form of powerful outflows shown here in this radio image.
The pink stuff is material moving at nearly the speed of light ejected from the vicinity of the black hole.
This is actually one of the real surprises in our thinking about how galaxies form.
Over the last ten years in particular, has been the recognition that black holes at the centres of galaxies are actually important for that process.
And at first glance, that sounds like a kind of crazy idea.
And so let me first tell you why it sounds like a crazy idea and then tell you why it's actually right.
So the reason it sounds a little bit crazy is that the mass of the black hole at the centre
of a galaxy is something like 100,000 times smaller than the mass of its surrounding galaxy.
So the gravity of the black hole on the scale of the galaxy as a whole is completely irrelevant.
All that matters is the gravity of the dark matter and the gravity of the star.
Moreover, the size of the black hole, the radius of the event horizon, the Schwarzschild radius is really just 100 times or so bigger than the sun.
That's about the distance between the earth and the sun.
And so that's again, that's a region that's 10 billion times smaller than the size of the galaxy.
So how can this tiny little region that is completely negligible in mass and completely negligible in size?
How can it affect the galaxy as a whole? And the explanation for that is, again,
this fact that throwing stuff into a black hole is the most efficient way of getting energy out that we know of in astronomy.
And so, in fact, black holes at the centres of galaxies are these extraordinarily powerful sources of light that impact their surroundings.
When the light and outflows produced by the black hole travels out from the black hole and runs into surrounding material.
And we think, in fact, that the amount of energy and the amount of radiation produced by gas spiralling on to a black hole at the centre of a galaxy
can be enough that the force produced as gas in a galaxy gets hit by the radiation produced in the vicinity of the black hole.
The force produced on that surrounding gas can be stronger than the gravity of everything else in the galaxy.
All of the dark matter. All of the stars, all of the gas.
So black holes are indeed irrelevant in terms of the store, their size, their gravitational influence on the galaxy as a whole.
But they, in fact, have a can have a really outsized impact because of their extraordinarily efficient production of energy.
And so another way to kind of frame this issue is that if you, again,
take a certain amount of mass and ask how much energy do you get out turning it into stars
or turning it into a black hole where you got about a 10th of a percent of the energy out?
If you turn some mass into stars, you get about 10% out.
If you turn that mass into a black hole, but it only takes about a million years or 10 billion years, depending on the size of the galaxy.
It only takes a tiny amount of energy being dumped into the gas orbiting around in the galaxy to completely push the gas out of the galaxy.
And that essential difference between the amount of energy generated by nuclear fusion,
the amount of energy generated by accretion of gas on to black holes,
and the comparatively small amount of energy needed to blow gas out of the galaxy.
That's really the essential reason why the process of forming stars and the process of growing black
holes at the centres of galaxies has such a big impact on the evolution and life of galaxies,
because these processes release enormous amounts of energy and substantially
affect how gas how gas actually behaves on the scales of galaxies as a whole.
And so our modern picture of kind of the life cycle of stuff in galaxies is one in which gravity pulls matter into galaxies.
Gravity causes matter to collapse and form stars.
Stars then shine by nuclear fusion for millions or billions of years.
They eventually end their life as white dwarfs, neutron stars or black holes.
And then, in fact, most of the gas in galaxies gets blown back out again,
only to later get pulled back into galaxies and be part of this large scale cycle.
And so we have this picture right from the time of Carl Sagan that were all formed from the kind of debris of stars.
And that picture is right. But it really operates not on the scales of stars, but really on the scales of galaxies as a whole.
Stuff gets blown out of galaxies, then falls back in, turns into stars, gets black, blown out again, and participates in this large scale cycle.
And so the challenge then, in understanding how galaxies form, why a galaxy like our Milky Way has the stellar mass it does.
Why? It's mostly a disk with a little bit of a spherical thing.
Spherical bulge at the centre lies in understanding the interplay between all of these different processes.
And so a lot of effort is going into identifying what physical processes are the most important blowing gas out of galaxies.
How does it get back into galaxies? And then trying to incorporate that into more and more realistic numerical simulations.
And so I want to show you a few examples of work done by our group where we try to incorporate this physics into,
at least in physically reasonable but still simplified ways into understanding not how dark matter behaves when galaxies form,
but into understanding how gas and stars behave when galaxies.
And so this is a numerical simulation about a region about the size of the stars in our own galaxy.
The simulation is showing you the gas in a galaxy.
The movie is only a few hundred million years long.
That's actually pretty short for the scale of the universe as a whole.
The white regions are dense regions where gravity causes gas and galaxies to collapse and form stars.
And then you see little explosions going off, which are the processes of stellar death, stellar radiation, stellar explosions pushing gas around.
And in fact, in the kind of side projection, you can see stuff being flung up out of the galaxy.
And so this is attempting to realise now in a computer simulation these processes that actually dictate how gas turns into stars,
how gas gets blown out of galaxies. Since those are the critical processes that govern the type of galaxy that will result.
And again, this movie is about 30,000 light years across.
That's about the size of the stars that make up our galaxy and now show you a similar computer simulation.
But over a region that's about a million light years across.
That's the region that makes up our the entire dark matter distribution that makes up our galaxy.
So this computer simulation starts when the universe was much younger, goes to the present day.
It's about a million light years across. This shows the density of gas.
So dark regions, lots of gas. This shows the temperature.
The red is hot regions. And you can see literally like explosions driven by gas getting blown out of galaxies, heating up the surroundings.
That process actually shuts off star formation for a while in the galaxy.
Things settle down a little bit. New gas falls in fuelling a new generation of star formation, which blows out gas again.
Right. And this is again, this is kind of an illustration of this cycle of gas flowing into galaxies and getting blown out.
And really only in the last few years have somewhat first principles,
calculations incorporating this physics been successfully able to even explain why galaxies roughly have the stellar masses that they do.
And so this is just an illustration of that. So this shows the stellar mass of a galaxy as a function of how much dark matter there is.
The dotted line would be a simple story in which stars just track dark matter.
About 10% of the mass of dark matter ends up in stars.
Reality observations are indicated by the solid and dashed lines.
So in small galaxies, the amount of stars is actually very small compared to the amount of dark matter,
because most of the stuff has actually been blown out.
And so really, only in the last few years have computer simulations reached the point where we
can kind of successfully produce galaxies of roughly the right stellar masses.
And that's shown by the different symbols. Here are different computer simulations by a couple of different groups trying to incorporate this physics.
These ideas had been around for decades,
but really incorporating them in the first principles way into our understanding of how galaxies form is a relatively, relatively recent.
And so some of the current frontiers then are trying to understand what makes some galaxies disc like rather than elliptical or spherical.
That remains a rather thorny theoretical problem.
There's a lot of good ideas for what determines that distinction, but which of those ideas is right?
We still don't really know. Okay.
So I'll end there. I hope I've given you a little bit of a feel for how Gravity and the action star
formation in black hole growth has caused the universe to evolve from its very simple,
smooth initial conditions that we see in the Big Bang, in the cosmic microwave background,
to the much more rich inhomogeneous structure that we see in the present day universe.
So thanks a lot.