Mapping the universe: dark energy, black holes, and gravity – with Chris Clarkson

thank you so much thank you for coming tonight so yeah thank you for the introduction and for inviting me to come speak here so tonight what I thought I would do is take a look at an exploration of the

universe

that we live in like this tonight I would like to just zoom out from Earth to give you a perspective of what the

universe

looks like on the largest scales both in space and time and then what I would like to do is put all of that together and show you what our model is. of the universe and talk a little about how we build a model of the universe itself.

Well, I guess the place to start is our planet Earth, which you will soon see. just a simple speck of dust in the whole of the cosmos that exists um and in fact it is not just a speck of dust but the small part that we inhabit in the few kilometers in the atmosphere above the surface of the Earth where we live, obviously there is a very , very special place our image of the Earth is a planet that rotates with photographs like this one taken by the Copernicus satellite is obviously a very modern way of looking at the Earth and it was not always like this, about 500 years ago this was the The image of the Earth We Had is a mock map from Walt and it's the first map with all the continents in the right place, so back then people knew that the world was obviously round and it told you where the continents were, where the oceans were and that kind of things. of things and I would say that our understanding of the universe as a whole is roughly similar to our understanding of the Earth about 500 years ago, so we have an idea of ​​where things are, but we don't know all the details, so Moving on to astronomy, I'll move away from our position on Earth, obviously the most important astrophysical body to us is the sun, which obviously creates the

energy

that powers all of our lives, so the sun is about a million times the The size of the Earth is quite massive, it is also very dynamic, there is a lot going on in the Sun and its heart is a fusion reactor essentially surrounded by a giant plasma with all kinds of crazy things happening, as you can see in the image below right where Earth is shown to scale against a close-up image of the sun's surface.

We would fall into the sun with barely a splash, so we're not the only planet, obviously we live in a solar system with other planets, um and just. A little historical perspective again, it is very easy for us to see ourselves in our solar system. It is the Third Rock from the Sun surrounding it with the other planets again. This is a fairly modern perspective on the history of. We humans understand that the Earth is not the center of the universe and the Copernican revolution that led to enlightenment essentially in the Scientific Revolution took us out of the center of the universe to be just an average place in it, I guess that's how it was. an important part in our understanding of our position in our universe and really the first understanding of cosmology, so when we look at the night sky, this is what the sky looks like outside of London if you ever get a chance to go outside, we see stars everywhere, we see the Milky Way, which I'll get to in a minute, but stars, if we really focus on what kind of stars exist, they exist everywhere the Milky Way is full of them.

There are about 300 billion stars in the Milky Way and our sun is a fairly typical star. It is called a main sequence star, which means it is in the middle of its life cycle. It is about five billion years old and will live another 5 billion. Approximately years until it swells and becomes a red giant star like the ones we see in this graph below. Stars are not only grouped into galaxies, but within galaxies they can be grouped into things called globular clusters, which is a picture at the top. It's a sting of many thousands of stars in a pretty dense sort of region.

The stars themselves are alive. Dynamic objects. They are born. They are born from a gas that collapses under

gravity

and that we can see in different areas. of the Milky Way that we see, that's what we've zoomed in on here on the right, so the stars themselves are living objects with a finite life surrounded by a living solar system, in many cases bringing together everything we see in the night sky. It forms the Milky Way we live in, so here is a photograph that an amateur astronomer took traveling around the world putting together many different photos of the night sky compressed into this ellipse to show a complete sphere from which the Milky Way would be seen. all over the world, so it's quite spectacular that there is a band of stars in the middle, which implies that the Milky Way itself is a kind of flat disk and, as I say, consists of about 300 billion stars, which is a pretty average number of stars for a galaxy.

Andromeda, our closest neighbor, has about three times as many stars and is a bit massive, so this is the number in kilometers. It doesn't really make much sense, so we measure it in light years, its size and it's about a hundred thousand light years wide, so when we look at the other side of the galaxy, the light that was emitted from there took about a hundred thousand years in getting here, so just to get a little perspective, it's from the time when Neanderthals We also share the planet, the Milky Way, as we learn more and more about it, we get closer to its center.

This is another common characteristic of galaxies. When we approach its center, there is actually a

black

supermassive in the very center. hole that will become

black

holes

a little later, but at the center is a black hole that is about 10 to 6 million times more massive than our sun compressed into an area about a million kilometers in diameter and what I've enlarged here a photograph taken by the Event Horizon telescope, which is a network of radio telescopes around Earth that have created the first direct images of a black hole and what we see here is a giant accretion disk around it.

Our galaxy then seen from the outside is the cartoon type on the right, it is a bad spiral galaxy, a fairly normal type of galaxy with the sun halfway in a rotating spiral arm, so it is a dynamic system and evolving itself, so it rotates every about 200 million years and as we get closer to the center, this is another graphic made by the European Space Agency for people to spend their time looking at the stars in the center of the galaxy and then we can see them moving on our time scale. over years, so for several years we can see the stars in the center of the galaxy moving around the central black hole that exists there, so there are these two pieces of evidence of this central black hole in the center of our galaxy , both a direct image of its secretion disk, which is essentially just stars that have been shaken up and are being sucked into it, and the dynamics of the stars moving around it indicate the same type of black hole mass in the center, so simulations that people can do now.

The bottom left also shows a simulation of what will happen in the future, a cloud of gas that is falling towards that black hole, so the simulations of what will happen in the next few years in the center, that is our galaxy, where we live, we are still nowhere. Almost cosmological scales, but still, I'm still looking at a galaxy, there are all kinds of different galaxies, they come in many different shapes and sizes, there is a kind of random selection here, so our milk quality control is something similar to upper central galaxy, but they come in different types and they also come in pairs and triples and colliding galaxies and all kinds of other things, so here are some examples of top right, bottom left and bottom right, they are all examples of galaxies that are in the middle of giant collisions and these galaxy collisions.

It takes many hundreds of millions of years to continue so in fact we see galaxies that have undergone collisions or are undergoing collisions, so we see that with the galaxies called mice in the lower left image, the two galaxies are actually in the process of colliding and now we can do simulations of these gigantic computer simulations of galaxy formation, so on the left we have the galaxy forming over several billion years flying through the center and then to the Right is the galaxy seen today. this simulation and this is a pretty typical galaxy in a simulation that turns out to be similar to our galaxy, the Milky Way, so by studying galaxy simulations we can understand a lot about how galaxies form and how they live and, in fact, the Simulations are getting pretty sophisticated, so here's an example of one where, instead of looking at the stars, we look just at the gas spinning around the center and falling in on itself under the gravitational force and then, in the image Bottom right, there is an enlargement of the central region. where all the star formation is happening, with the kind of violence and rotation of this gas, it heats up, stars form under gravitational collapse and over the billions of years that this simulation is taking place, many stars are born in this in this process, so understanding galaxies is a whole area of ​​research.

I'm not going to say much more about that. What I would like to do now is zoom out from the individual galaxies to see how they behave in the neighborhood. is our local neighborhood in the universe, so our local galactic group is highlighted in red. It is part of a small galaxy cluster that we are gravitationally falling into. Falling towards the Virgo cluster on the right side of this image. I'm forming what's known as a Virgo Supercluster of galaxies, a few million galaxies that are gravitationally bound, that's our neck of the woods of the universe, it's about 50 million light years from the Virgo cluster, so the light from then would be emitted around the time that the dinosaurs became extinct when we look at when we look at the Virgo cluster and that's really our closest collection of galaxies in our local cosmological neighborhood and now let's zoom out a little bit from that as well and see what We see now on the scale of cosmology, so this is a flyby of a study that is slow and digital.

The sky survey sampled a bit of the sky. I'll say more about that in a second and flying through all these galaxies we see that there are just millions of them everywhere the scale now we're genuinely on cosmological scales where we like to look at things on the time scale of billions of years, billions of light years now, so the light emitted from these galaxies was emitted billions of years in the past and during reference: the earth formed about four and a half billion years ago, the sun five billion years ago, life began about three billion years ago and dinosaurs became extinct about 100 million years ago, so to put this into context, how far back in time are we?

We're looking when we look at these distant galaxies and we can see just from this fly over here that they come in different shapes and sizes, different colors, many have spirals like ours, many are just elliptical blobs, some are undergoing collisions, etc. Just to give you a perspective of the scale of what we're seeing here, on a practical level, what we cosmologists do is part of what we do anyway, is try to map the universe in the same way. In the same way that 500 years ago people tried to make a map of the Earth, the map of our local neighborhood, today we try to make a map of our local Universe using galaxy surveys, so at the top right there is a kind of twist. parts of the universe that we have studied so far, so when studies of galaxies are carried out, usually a telescope points at the sky observes everything that passes by and finds where the galaxies are, records their angular position and their distance from us and then from there You can essentially make a map and one thing to note about this map on the bottom left, this is a 2df study, this was quite a while ago, now you can see that there is some structure to the way the images are laid out. galaxies, so each point in the bottom left image is a galaxy that has been observed and you can see from the distribution that there is some kind of structure there, it's not entirely random, so as I say, we're still in the process of

mapping

the entire universe, this is what we have.

So far there are several satellites in operation. There are satellite and some ground experiments in the next few years that will hopefully fill in many of the gaps so we have a more complete picture of our universe. around us, so that goes back to those studies go back about five billion years in the past, what happens if we try to look further? Here is a beautiful photograph taken by the Hubble Space Telescope over several years called the Hubble Deep Field. Basically, it is to startlooking at a very

dark

part of the sky with a very long exposure, a photograph essentially of which we have a flight simulation here, obviously, you don't see, you don't see this, but you were able to see about 10 billion, like 10 billion years . back in time, so looking at galaxies that were emitted like 10 billion years ago, long before our galaxy was forming, it seems like there are galaxies absolutely everywhere, everywhere we look, it's just galaxies upon galaxies upon galaxies and it is believed to be around one. to two trillion galaxies in our observable universe of which, of course, we've only measured a small fraction of so far, so what would the universe look like if we could see it all right?

Here is the result of a computer simulation, the Millennium simulation, which reveals what I think the large-scale distribution of matter will be that of galaxies, so in this image it is a simulation of what is called a

dark

matter simulation where each point of light will roughly represent where there will be some galaxies and where you can see particularly bright spots there will be galaxy clusters like our Virgo supercluster for example, we can see that they are spread out in a non-random manner, it is not a random scattering where the galaxies of the Path are distributed and it is called The Cosmic Network because it looks like just a large network of galaxies, so super clusters are very bright points joined by filamentary structures and very empty regions known. like cosmic voids, this simulation is about 2 billion light years wide, generally speaking, it's not how we would see the universe, we will never see the universe like this because light takes time to travel to us, we will never be able to look at a part of it. um, all at once, if you want, then it's completely inaccessible to us just because light travels towards us at a finite speed, so this is what we think the universe looks like if we could imagine it, um, being able to see it all at once. time, I can imagine it in computer simulations.

I'll say a little more about that later, but for now let's continue our exploration of the universe and say what happens if we try to look further than Hubble did, beyond what Hubble's Deep Field could measure. Are we continuing to see more and more galaxies in the distant past or is something else appearing? This is what the universe looks like as far as we can see it. I will explain why later, so if we look at 13.5 billion light years away, the universe changes completely from what we have seen from Hubble, where there are many galaxies, it becomes completely smooth, there is practically no structure of any kind. type, so the galaxies disappear and, in fact, 13 and a half billion light years away. away, um, the universe was completely smooth and just pure radiation and what we see in this image is that radiation, the radiation that reaches us today is 2,725 degrees Kelvin and the last decimal fluctuates over the sky, so we look at all the directions.

We can see this microwave background radiation of 2,725 degrees Kelvin. The next decimal is a fluctuation that is represented on this sphere and that temperature fluctuation that we will see gives us a lot of information about the Big Bang and actually also about the late temporal structure. The universe then was very soft 13.5 billion years ago and 10 billion years ago it had many, many galaxies and today, of course, we know that it has zillions of galaxies forming this network of connected cosmic structure, so let's put ourselves in the place with This, then, the next slide moves away from us with the type of galaxies placed in the right position based on what we've seen, so let's go over that moving away from us with galaxies that have been observed in the right place in a three-dimensional format.

In that sense, in a brief second this is going to turn around and then we will see only the very narrow portions of the universe that we have been able to study so far, which is why galaxy studies work by focusing on areas of the sky where the Milky Way is located. particularly dark because we have to look far beyond the Milky Way to see these galaxies and essentially just map out in thin slices where all the galaxies are that we can see, so that as we rotate we can see that there are relatively few slices that have been In fact, there's a lot to fill in on this image and then as we get further away from our position we also go back in time here and as we get further away we come to this cosmic microwave background which you can imagine is a giant sphere around . us in a sense in the sense that we have put together our observations in this way and that is the current state of what we know about where things are in the universe, so obviously there is a little bit more and we come now to the next part of what we know about the universe is that it is expanding, so this is where modern cosmology really began about 100 years ago with Edwin Hubble and George Lemaitre shown here in the two images at the top left and what they did was they did, They were just studying galaxies, so they were measuring the positions of galaxies and their velocities, so measuring their positions and velocities is a natural thing to do is make a graph of the position of a galaxy and its velocity, so The speed of the galaxy you would think if it were getting closer or farther away from us would be quite random, but in fact what they found was that there is actually a systematic trend that the further a galaxy is from us, the faster it is moving away from us. and so on this graph on the right where If we have the distance plotted horizontally and the velocity vertically, we can see that the straight line that roughly goes through all of these data points tells us that the farther away a galaxy is, the faster it is moving away from us, so the implications of this are quite profound, it means that the universe is expanding around us and since we assume that we are not in a special place in the universe, we can assume that the universe is also expanding for everyone. other observers, so a commonly used analogy is if you imagine a balloon being inflated. above with some points drawn, those points are moving away from each other at the same rate, so any point will see all other points moving away from its particular position, so this very important fact that the universe is expanding today It means that as we go back to the past all the images we saw go back to the past, the universe was not only younger but also smaller, so when we look at the past, whether we are billions of light years in the past or thousands Millions of light years away from us means billions of years in the past, the universe was much smaller and if we look to the cosmic microwave background for reference, the farthest we can see is thirteen and a half billion ago. years the universe was approximately one thousandth of its size. size, then it's something that was a kilometer or let's say something that was a meter at the time would have now expanded to be a kilometer in size, so that's the last key piece of information that we really need before we can start to build a model of the universe to understand how it came to be that way and why there is structure at all these different scales, so before we talk about how to build a model, I thought I'd go over a sort of timeline of the evolution of the University, so in this cartoon we have 13.7 billion years of evolution with the horizontal direction timed since the Big Bang, it went through different phases, phase called inflation, which I will mention a little later, then we have this cosmic microwave background that It formed when the universe was about three 380,000 years old, so very early in University history this formed, the universe was very, very smooth at that time with these small fluctuations in temperature over time.

Those fluctuations grew under

gravity

to form the first galaxies, the first stars, so essentially the type of universe we see today began with many stars, planets, etc., and then, over 13 billion years of evolution, basically more and more stars were formed. Obviously, there are a lot of things that go into that model. Let me explain it. in words, so we started with a big explosion, a kind of big bang, which is when the universe experienced an inflationary period, um, during that time, quantum fluctuations in the infoton field seeded the entire structure, so the little ones fluctuations that we observe in the cosmic microwave background are essentially an explosion of quantum fluctuations that occur during the Big Bang, which went through, then went through two types of phases, went through an era of radiation for the first few hundred thousand years, where radiation was a dominant gravitational component and the universe then was just a plasma essentially of protons, electrons and radiation and ended with the constant formation of microwave background.

Now I'll say a little more about that later. It's just to put things into some sort of timeline to understand what's coming. Then it went through a Mata era where galaxies and things. formed and appears to be transitioning today into an era dominated by dark

energy

where the rate of expansion of the universe is actually not slowing down but accelerating, so I'll address each of these things in turn, but let's start briefly with how we begin to look for this information, so we start by looking at the universe and saying where everything is and, in the language of young people on Love Island, they might say: "Well, it is what it is, but maybe we would like to try to do a bit." better than that with the scientific method, which is to generate ideas about why it might be so, then we test those ideas by confronting them with the data that we have observed, for example, in cosmology, can we make a prediction from our model about the distribution of galaxies, can we make a prediction from the model about what the cosmic microwave background will look like, for example, in reality, of course, the scientific method is a long-standing discussion with people with competing theories, competing observations, trying to determine what is actually a reality, so in fact in cosmology there has been a long-standing discussion about what the true expansion rate of the universe is today, people have not been able to get We have agreed on this for the last 60 years or so and they all converge into their own separate world. separate answers and that's the way we approach it, so you start from observations that we just take for granted, we come up with theories, we argue about it and we try to come to some kind of scientific consensus about what's really there. happening, so we continue When modeling the universe, the first thing is to identify what is the most important force that acts on cosmological scales and the most important force once we approach the galactic scales is gravity, the universe is neutral, electromagnetism It only plays a very minor role, making it the main force that is acting.

Gravity and things fall on themselves under gravity, so how do we do it? Well, we use Einstein's theory of gravity, so Einstein came up with a theory of gravity and excuse me around 19, 1915, known as the general theory of relativity, um equations for it's in the bottom corner and visualize gravity not as a force according to the Newtonian image of gravity, it is that there is a force that pushes us towards the ground at this moment there is a force that attracts the Earth towards the Sun, the Moon towards the Earth. and everything in the galaxy with each other.

Einstein's theory completely eliminates that image and reimagines it as spacetime itself being curved, a curved manifold that we all live in, and objects falling on that curved manifold in effectively straight lines, so that's pretty much it. to digest, but a good analogy that is often used is if you imagine that you are an ant walking on the surface of an apple just because you are an ant, you don't know that the apple is a curved surface, so two ants walking from, Let's say From the Equator to the Core, we would find ourselves walking towards each other even though they are both walking in a straight line in their minds, whatever Minds they have, but you could interpret that as being dragged together through a force of nature that they do not have.

They don't quite understand it and as they move around the core of Apple they could interpret it as a peculiar Force doing all sorts of very strange things when in reality we can see that they are just ants walking on a curved surface and hence the imagination of space- While it occurred to Einstein was to imagine that space-time is similarly curved and that objects like the Earth fall freely into that space-time, the key for us when we think about cosmology is that it allows for a dynamic, expanding universe. and it gives us a framework in which to build a model of the entire Cosmos, so what does it look like?

Well, these are Einstein's field equations.Obviously, it's a little complicated to try and understand. BSc in Physics to understand them, essentially there are equations that balance the energy density of all the matter in the universe on the right side, so take everything, turn it all around, put it into the equation on the right side, the left hand. The side represents a space-time geometry that reacts to the matter within it, so that as we move through this room, the space-time curvature reacts to our mass and to that energy contained within us, obviously, a very large amount. , very small for tiny objects like ours, but for objects. the size of the sun solar system the galaxy becomes a fairly significant space-time curvature that changes so genus relativity has very important predictions that when confronted with observations have led us to believe that it is genuinely a theory of nature almost correct or almost correct.

So some of their predictions are that spacetime bends and moves under mass motions, which is why the conventional image is at the bottom right. Imagine a sheet of rubber with some heavy balls on it. It's pretty much the same kind of idea as the rubber sheet. will change in reaction to the weight of the balls. The 3D image looks more like the mesh shown on the left to see that this is a force. Objects traveling in that curved space-time travel on a curved path that is actually a straight or shortest path. possible path in space-time, so our Earth falling around the Sun in free fall actually travels the shortest possible path in space-time, given that the Sun is there curving space-time around it , so that's the type of image for massive objects.

A really important prediction of Einstein's theory that was later observed by um Eddington and shown here at the bottom was that light itself is also bent by a gravitational field, so this is a big divergence from Newtonian gravity. which does not have this prediction, for example, in fact. The experiment Eddington did was that stars that are just behind the Sun or near the Sun's position in the sky will actually have their light bent by the Sun's gravitational field, the curvature of spacetime around them, and the star .it will actually move its position when the sun moves around it and this was observed in 1919 more or less verifying Einstein's theory.

In fact, I made Einstein a bit of a star at the time, so that's a key prediction that light is deflected by a gravitational field and, in fact, or when we go back to cosmological scales, it gives us a phenomenon known as gravitational lensing, so gravitational lensing is a kind of collective process of large objects like galaxy clusters, so the top two images here are images taken. by the Hubble telescope and the James Webb Space Telescope and what we can see in the center of these images is a galaxy cluster and then the galaxies behind that cluster.

In reality, its light is completely distorted by the sheer gravitational mass of that cluster in the middle and so at the bottom right is a simulation of a cluster moving past a distant star field and a galactic galaxy field is seen distant and the shape of those galaxies is distorted by the mass of the cluster in front, so this is another important prediction of Einstein's theory and, as we can see on the top right, it creates all kinds of strange effects by stretching the galaxies in fun ways. Another prediction of Einstein's theory that was developed over many years was the prediction that black

holes

must exist if objects are massive enough and begin to collapse under their own mass, it has been shown that there is no way to stop that collapse until a singular point, so these were predictions by Roger Penrose and Stephen Hawking back in the '60s, essentially stating that black holes have to exist if Einstein's theory is correct, so if a sufficiently massive star dies, the pressure can no longer hold it against its gravitational mass and it begins to collapse and nothing can stop it until it reaches a singular point.

These objects are surrounded by what is called the event horizon, the gravitational field of these objects as you get closer the singularity becomes so strong that light can't get away from it basically so you can get in but never again. you can get out, the point where there is a point of no return. It is the event horizon of that object and is known as a black hole. These things come in all kinds of different sizes, from about the mass of our sun to those found in the centers of galaxies, which measure between a million and a million. billion times the mass of our sun and that's why they are devouring all kinds of things that are shown in the simulation and observed by the Event Horizon telescope in those two top circular images, these are real images made of black holes that we observe in the top left. is from our center of our galaxy, the other is from the center of Andromeda, so these are pretty amazing observations that have been made in recent years, so the final prediction of Einstein's theory is that gravitational waves, The gravitational field does not exist.

It is no longer a kind of static force, but actually a dynamic variety in which things happen when two black holes meet and start to spiral around each other and collide with each other, as shown in the simulation, they release enormous amounts of gravitational radiation, essentially space. The weather that responds to those moving objects radiates that information outward and contains enormous amounts of energy as it does so. At the top right there is something like a creepy moving image. Moving shows how the actual wave travels through space-time, so if it was coming towards you, it would.

Make your head swing in opposite directions and, in fact, gravitational waves were first detected a few years ago by the Ligo Observatory. This is an image of Virgo in Italy that is part of the network of gravitational detectors being built around the planet. The Earth and what they have been able to observe is the following situation: This is a simulation of two black holes that were thought to have been observed by someone, rotating around each other and shown against a field of stars. We can see space. -the curvature of time is so strong that it shakes the stars behind these two black holes as they spiral towards each other.

They are both about 30 solar masses, so they are very large black holes, and yet they are only a few kilometers wide, a few kilometers. They have a mass of 30 or 40 suns each and as they spiral together they emit gravitational radiation which as you can see is a jolt of the star field behind it, so in this simulation it slowed down enormously and, In fact, this happened over a period of less than a second, so what they observed was two separate detections in different parts of America, where they have these two detectors that indicated that they had found the first black hole merger and really solidified the theory. of Einstein as a correct theory of gravity.

It's really impressive, really impressive. The achievement and now gravitational wave binaries, as they are called, are being observed all the time. Well, let's go back to cosmology, let's see how we can use the fact that we now have a theory of gravity that we can rely on to build a model. of the universe and understanding the large scale dynamics of the universe itself, so a simple question is is the universe expanding, how fast is it expanding, how long has it been expanding. Well, let's go back to Einstein's equations, we are balancing the energy density of matter on the one hand, the dynamics of the curvature of space-time on the other hand and what this means is that we need to know what is in the universe, what are the proportions of cosmic matter in the universe, so what is in the universe, well, putting it all together.

If you want, you can have a kind of pie chart of things in the universe, if you mix everything up and calculate their energy density. What is striking about this image is the ratio of heavy elements, so that is all we know in our world. the smallest fraction of a percentage of all things in the universe, so elements heavier than helium basically have no energy content for the universe, so we are actually a very insignificant part, then they come Stars, stars that you might think make up most of the mass, in fact, they don't, they make up about half a percent of the mass of things in the universe.

Next up is free hydrogen helium, which has actually been left over from the Big Bang and hasn't yet collapsed into stars, but could still in the future that makes up about five percent of the energy density of things in the universe and the rest is made up of things we don't know what it is one component is called Dark Matter another component is called dark energy dark matter which we have a pretty good idea of ​​what dark energy is like minus so let me go over the evidence for both to persuade you of that we're not completely crazy, um, never mind, the evidence for that matter comes in many different forms, uh. many different ways, the first discovery really was about the way galaxies rotate, so we saw in simulations of galaxies that rotate over hundreds of millions of years that we can measure that rotation quite accurately in galaxies of our neighborhood in the expected way. rotate if they only contain the stellar matter we see is shown in the yellow curve, but the observed way in which they rotate is shown in the green curve and what this suggests is that the galaxies themselves do not consist only of what we can see in they. those are the stars, but in reality, because they live in a halo of dark matter that is larger than the galaxy itself, the galaxy we see extends much further away, but with material that is completely dark, we also see dark matter in lenses so this is kind of an important image of one of the first gravitational lensing events of a cluster that was taken so that the arcs that you can see in that image of the lensed image, the central part contains a large galaxy cluster and by examining arcs and curvature.

Of the galaxies in the background you can calculate the mass of the cluster and the mass of the cluster from gravitational lensing, which is a purely gravitational effect, it is not equal to the mass of all the visible matter in that system and this is true for the cumulus clouds that we see throughout the universe and this is another evidence of why we believe that Dark Matter exists on the scale of clusters and I will get to other evidence later. Dark energy is much more mysterious, appearing in the very large-scale dynamics of the universe and showing up because it appears to be causing the expansion rate of the universe to accelerate rather than slow down.

If you imagine, if you took a bunch of galaxies and made them fly, their combined gravitational pull would slow down, you imagine. that expansion over time and in fact it seems that the expansion of those galaxies that are separating is getting faster over time and not slower and this has been happening for the last five or six billion years in the history of the University, so dark energy is very peculiar. It acts in an anti-gravity way, so it doesn't work in the normal way that we think gravity works and it also has very strange properties, like if you compress it it doesn't get denser, so it's very peculiar.

Again, we have a lot of separate evidence for that, one of the first was measuring the supernova, so supernovae are special types of exploding stars that we can see at very great distances and we have a good idea of ​​how bright they are. We can calculate how far away they are and continuing with the Hubble diagram allows us to see the kind of expansion rate of the universe over time, so this key piece of this key information actually led to the Nobel Prize a few years ago. It has taken years to understand that the rate of expansion of the universe is accelerating over time, so many efforts are being made to continue observing these exploding stars known as supernovae now that we know what is in the universe, the composition of the cosmos, dark energy, dark matter.

A little bit of all the other things that we really understand, we can put them all together in Einstein's equations, solve them and calculate the dynamics of the universe over time, so here is what is known as the relative scale of the current universe. So as we go back in time the universe gets smaller, going back in time how fast does it get smaller and at what point did that contraction reach size zero? So these curves here show us different models of the Universe for different amounts of these different Cosmics. materials that are used to create a model and you can see that you can have universities that do all kinds of things that we collapse on each otherThey expand forever the rate of expansion becomes faster and faster our curve is the one that is a little green just the transition between orange and blue in the background, which implies that there is a big explosion.

Singularity about 13 billion years in the past, so this is a very large-scale simplified model of the universe that smoothes out everything in it. What is the expansion? Dynamics: How old will the universe be? it expands faster into the future, which is what we think it is. If we understand how the general dynamics work, we can explain this cosmic network of galaxies that we see and we can explain the cosmic microwave background at the same time because they are related to each other. This cosmic web is known as a large-scale structure in the universe. How does this large-scale structure evolve?

So the seeds of the structure were laid during inflation. Type of initial conditions. The Big Bang triggered this. Quantum fluctuations. which exploded on classical scales, so it's a bit speculative that that area, but it seems to be the case, so the early Universe was dominated by radiation until about 380,000 years after the Big Bang, they went through an era dominated by Kill during the radiation era. universe, things in the universe oscillated, you can actually imagine it a little bit like the inside of the sun, mostly hydrogen, a little bit of helium with sound waves propagating through it and gravity acting on that.

Basically what happened is that dense regions collapse into galaxies and the material that led to their collapse comes from these void-like regions at late times, so on a very simplified level, really all that's happening is a slight excess density in the early times, they simply become heavier and heavier and collapse gravitationally at the expense of the slightly underdense regions obviously there is a bit more of that so let's look at the radiation era the radiation here was relatively simple the universe It was incredibly soft because it's soft it's actually very easy to model from a physics point of view, so the physics of what was happening at this early time, less than 300,000 years or so, after the big bang we just have soup of photons, mainly radiation, dark matter particles, protons and electrons, the universe is very hot, it is above 3000 degrees Kelvin, it is much smaller than it is. today and because it is above 3000 degrees Kelvin, the ionization temperature of hydrogen means that there are no atoms, the protons, the electrons are free bouncing around in a large plasma like in the Sun, this also means that the photons are trapped in that plasma and they can't.

In the free stream, they bounce off charged particles and that is what is happening in the early universe, there are sound waves propagating and those sound waves are oscillations in the plasma and the Dark Matter particles respond to the gravitational pull of those oscillations Even though Dark Matter itself does not interact directly with protons, electrons and photons electromagnetically, so they are pulled by gravity and we actually observe the end of the radiation in the cosmic microwave background, so what What happened then is that as the universe expands, it cools. goes down all the time if you take a gas, you expand it, it cools, and as it cools below 3000 degrees Kelvin, the ionization temperature of hydrogen suddenly throughout the universe, the protons and electrons combine to form neutral hydrogen the Neutral hydrogen no longer scatters the photons, so the photons were free to flow freely through the universe, suddenly, almost instantly, the entire universe went from being completely opaque to completely clear and a way to think about how we can Now let's look at the Because of the microwave background, those free photons can flow freely for 13.5 billion years until we can observe them today, it's a bit like when we look at a cloud within a cloud, the photons are bouncing between all the water molecules. it reaches the edge of a cloud and can flow freely towards our eyes, so we perceive it as a surface and therefore a very similar type of physical process occurs when the temperature of the University cools below 3000 Kelvin , so it is released in is in the microwave background of the closet, so what we have here at the bottom right is the map of the cosmic microwave background created by the Planck satellite shown at the bottom left and it is a complete sky but compressed into an ellipse so we can see it. in an image, the fluctuations in this map are one part in 10 to five, so it's an incredibly smooth surface and much smaller, much smoother than the smoothest billable that you would find, for example, and when we take a mathematical transformation From that, we show In the graph at the top left now, when we look at the map, we can see that the red dots representing slightly warmer regions and the blue bits representing slightly colder regions are roughly the same size throughout. the map and you can see with the naked eye that there is a type of characteristic scale in that map when we do a mathematical transformation of it, we see the big peak in the middle of that transformation, the function um represents those scales that we can see in approximately one degree scale on the sky, so roughly that size on the sky is a characteristic scale on that map and that characteristic scale actually tells us a lot of information about what the universe was like in those early times, so it tells us It says first of all the shape of the universe, whether it was curved or not, it appears to be flat.

It tells us the age of the universe, it tells us the proportions of dark matter, hydrogen and helium, so if you vary all this in the models that we make of the other Universe, it changes the appearance of this cosmic background and one can simply do an analysis of best fit to see what it looks like and it also contains information about what happened in the Big Bang, so we've been able to get huge amounts of information from this map of the early universe and it's called the micro background. And then, how do we get from that to this cosmic network?

Well, one can do analytical work, usually it has to be done in simulations, so the simulations look like this. Take a box factor to factor the expansion of the universe, give the initial condition to the custom microwave. In the background you watch everything collapse and when you see everything collapse you can see that gravity does all the work for us and the simulation just shows us this, it produces a cosmic web just because that's what it is and then we can take the results of those simulations and then compare them with the galaxy studies that we measure, so here is an example where in blue I think they are real maps of the Universe that are made in red are analogous things that are made purely from simulations and one can try to match the statistics . of those two things and decide which model best fits the observations and so, by looking at the structure at all these different scales, you can calculate what is called the power spectrum of the density fluctuations, so essentially these functions encode the statistics in the cosmic web that we can It is difficult to see with the naked eye the differences that arise from all these changes, but in mathematical functions we can do so, so these simulations become quite sophisticated.

The previous one was just a Dark Matter simulation, we just threw out everything we actually see. I think it's important, but modern simulations can now model the gas in these, so hydrogen and helium star formation rates look at things blowing up as they go, etc., and get quite sophisticated on these. small scales, so on the left is Dark Matter in the Right is all the gas, which is hydrogen, helium, which produces stars and things that explode, so dark matter is absolutely crucial to this picture of the formation of structures. It's the final piece of the puzzle, so it's not just gravitational lensing, it's not just galaxy rotations, it's the whole thing. distribution of the cosmic web The cosmic microwave background requires dark matter for the model to work, but we still have very little idea of ​​what dark matter itself is because we only interact with it gravitationally, meaning it is very difficult to discern what it is. .

There's obviously a lot of work going on to try to understand its particle nature, how it interacts with what's called the standard model of particle physics, which is all we know and see around us, dark energy is a big part of it too. from this. model because we measure the acceleration of the expansion rate, but we don't really know what it is, it is actually contained in Einstein's field equations through what he called the cosmological constant, so in his field equation there are constants unknown, the speed of light, which we can measure. gravitational um constant G that we can obviously measure but there is another one called cosmological constant that is there by the nature of its field equations it is mathematically allowed but we cannot measure it on day to day scales, we can only measure it on cosmological scales it seems to be there, It appears to be non-zero, but it has no satisfactory fundamental explanation, so a lot of work is being done to try to determine whether the cosmological constant is constant or whether it is evolving or perhaps.

Einstein's theory is not the correct theory to use on very large scales, perhaps a modified theory of gravity is required, so one way to determine it is actually on the large scale distribution of galaxies, the characteristic scale that we saw on the cosmic scale. The microwave background, those hot and cold spots that looked like about a degree on that characteristic scale, is preserved as we evolve into the future on the cosmic web, so the cosmic web takes that same scale and compares that scale in times late and early. one can understand through a lot of work whether dark energy itself is evolving or not, is a cosmological constant or is something even stranger, so just a picture of what the limitations of dark energy look like, so this is the kind of thing that Get Scientific Papers About This tries to parameterize dark energy, say it's evolving, what could be evolving?

Parameterize that by saying, uh, give it some functional form and then constrain those parameters with observations and a key goal is to determine if it's cosmological. constant or not and then one produces error bars on that, etc., etc., we can say with some confidence that it is certainly not doing something else, so another aspect of this model that seems to be also peculiar but that also seems to be necessary is a period of inflation. at the beginning of the universe, so I've glossed over this mostly because I don't really understand it very well, but during the Big Bang, the universe seemed to go through another antigravity phase where there was something driving it. the expansion of the universe at an exponential rate, which is why it is known as the infoton field and appears to explain many things about the universe, for example why it is flat, it explains the nature of the perturbations or fluctuations that we see in cosmic microwaves. background and oh yeah, it also explains why the universe is uniform, which is also something you wouldn't expect from a randomly created universe, so inflation itself could be a prediction that there are, in fact, other universities and we are part of them. a larger multiverse of universities as well, so by looking at these things maybe we can try to find answers to those questions hmm, so just to finish I mentioned that our map of the universe is not complete, it is very far from complete and there are many studies underway to try to complete our understanding of the cosmic web, so Euclid and the set of square kilometers in Africa.

Britain is heavily involved in another: the Large Synoptic Survey Telescope will measure millions of supernovae. Try to complete the Hubble diagram. to an increasingly distant season in more and more detail to try to understand the expansion dynamics and, in fact, new avenues are being opened with what is called the Einstein telescope, which is a supergravitational wave detector that is supposed What if they decide to build it. detect all the gravitational wave merger events that happen, so there's a lot of stuff happening for the next generation of surveys and the kind of questions we need to answer what is dark matter, they get progressively more complicated what is it? dark energy what is inflation and what happened in the Big Bang itself related to that is probably what is at the heart of black holes, probably related questions, are there other universities?

That's a question we could try to answer and then of course there's the bigger questions like why something exists, so I guess that's going to be harder to explain, but if you just reflect on where we were 500 years ago, just trying to do a map of the earthsame, we have arrived. a very long road so I'll leave it there thank you very much