(inquisitive music) (audience applauding) - Hey, thank you very much. Thank you, thanks for coming tonight. So yeah, so thanks for the introduction and for inviting me to come speak here. So tonight what I thought I would do is have a look at an
exploration of the universe in which we live. So tonight I'd like to just
zoom out from the earth to give you a perspective of
what the universe looks like on the largest scales in
both space and in time. And then what I would like to
do is put that all together and show you what our
model of the universe is and talk a little bit about how
we go about building a model of the universe itself. Okay, so I guess the
place to start, I think, is our planet, planet Earth, which you'll soon see is
just a mere speck of dust in the cosmos that exists. And in fact, it's not
just a speck of dust, but the little part of it that we inhabit, the few kilometers in the atmosphere above the earth's surface where we live is obviously a very, very special place. Our picture of the earth
is a planet that rotates with pictures of it like this, taken by the Copernicus satellite, it's obviously a very modern
way to look at the earth, and it wasn't always like this. So about 500 years ago, this was the picture of earth that we had. This is a Waldseemüller map, and it's the first map
with all of the continents sort of in the right place. So back then people knew the
world was obviously round, kinda knew where the continents were, where the oceans were,
and that sort of thing. And I would argue that our
understanding of the universe as a whole is roughly similar to our understanding of the
earth was about 500 years ago. So we kind of have a
picture of where things are. We don't know all of the details. So moving on to astronomy, I'll just zoom out from
our position on earth. Obviously the most important
astrophysical body for us is the sun, which obviously creates the energy that powers all of our lives. So the sun is about a million
times the size of the earth. It's pretty massive, it's
also like very dynamic. There's a lot going on in the sun. At its heart is a fusion
reactor surrounded by, essentially, a giant plasma with all sorts of crazy stuff going on. And as you can see on
the bottom right picture where the earth is shown to scale against a closeup image
of the sun's surface, we would drop into the
sun with barely a splash. So we're not the only planet, obviously. We live in a solar system
with other planets. And just a bit of historical
perspective, again, it's very easy for us to see
ourselves in our solar system. It's the third rock from the sun, going round it with the other planets. Again, this is a fairly modern perspective in the history of humans to understand that the earth is not the
center of the universe. And the Copernican revolution that led to the enlightenment, essentially, in the scientific revolution, removing us from the
center of the universe to being just an average
place in it, I guess, was a major part in our
understanding of our positioning in our universe and really the, the first understanding of cosmology. So when we look at the night sky, so this is what the sky
looks like outside of London. If you ever get the chance to go outside, we see stars everywhere. We see the Milky Way galaxy, which I'll come to in just a minute. But the stars, if we actually focus on the
kind of stars that exist, they exist all over the place. The Milky Way is full of them. It's about 300 billion
stars in the Milky Way. And our sun is a fairly typical star. It's called the main sequence star, which means that it's sort of
halfway through its lifecycle. It's about 5 billion years old
and it will live for another 5 billion years or so until
it will swell up and become a red giant star. Like the ones that we see
in this bottom graphic. Stars are not only clustered in galaxies, but within galaxies they can
cluster into things called globular clusters, which is a picture on the top left consisting of many, many thousands of stars in a
sort of fairly dense region. Stars themselves are
living dynamic objects. They're born, from gas that collapses under gravity, which we can see in different
areas of the Milky Way that we see. So that's what we've zoomed into on the right here. So stars themselves are
living objects with a finite lifetime surrounded by a living
solar system in many cases, putting everything together
that we see in the night sky forms the Milky Way galaxy, which we live. So here's a picture that
an amateur astronomer took traveling all over the
world, piecing together many, many different photos of the night. Sky compressed onto this ellipse
to show a whole sphere of where you would see the
Milky Way all over the world. So it's quite spectacular. There's a band of stars across
the middle implying that the Milky Way itself is kind of
a flat disc and as I say, it consists of about 300 billion stars, which is a fairly average
number of stars for, for a Galaxy, Andromeda, our nearest neighbor has about
three times as many stars and it's kind of massive. So it's about... the number
in kilometers is there, it doesn't really make much sense. So we measure it in light
years the size of it, and it's about a hundred
thousand light years across. So that is when we look at
the far side of the galaxy, the light that was emitted
from there has taken about a hundred thousand
years to get here. So just for a bit of perspective, that's from around the time
when Neanderthals were also sharing the planet, the Milky Way itself. As we learn more and more about it, we zoom into the center of it. This is another common feature
of galaxies when we zoom into the center of it, it actually
is in the very center, is a super massive black hole. Now, I'll come onto black
holes a little bit later, but in the very center is a a black hole, which is about 10 to the 6
million times more massive than our sun, compressed into
an area of about a million kilometers across. And what we've zoomed into
here is a picture taken by the Event Horizon telescope, which is a network of radio
telescopes around the earth, which have created the first ever images, direct images of a black hole. And what we see here is a
giant accretion disc around it. So our galaxy then viewed from
the outside is the sort of cartoon on the right. It's a barred spiral galaxy, fairly normal kind of galaxy
with the sun halfway out on a spiral arm, it rotates. So it's a dynamic and
evolving system itself. So it rotates every 200
million years or so. And as we zoom into the center, this is another graphic made
by the European Space Agency. So people spend their time
looking at the stars in the center of the galaxy and then themselves, we can actually see them moving
on our timescale over years. So over several years we can
see the stars at the center of the galaxy moving around
the central black hole that exists there. So there's these two pieces
of evidence for this central black hole in the center of our galaxy, both a direct image of its secretion disc, which is essentially just stars, which are being churned up and
are getting sucked into it. And the dynamics of the stars
moving around it both indicate the same sort of mass of
black hole in the center. And so the simulations
that people can do now, bottom left is showing
simulations of what will happen in the future too. A cloud of gas that's falling
into towards that black hole. So the simulations of what
happened in the coming years in the center. So that's our galaxy,
that's where we live. We're still not anywhere
near cosmological scales yet. So still looking at one galaxy, there are all kinds of different galaxies. They come in many, many
different shapes and sizes. Here's a kind of random selection. So our Milky Way looks kind of similar to the top central galaxy, but they come in different types
and they also come in pairs and triples and colliding galaxies and all sorts of other things. So here's some examples. So top right, bottom left and bottom right
are all examples of galaxies that are in the middle
of giant collisions. And these collisions for
galaxies take many hundreds of millions of years to go on. So in fact, we see galaxies that have
undergone collisions or, or undergoing collisions. So we see that with the galaxies called the mice
in the bottom left picture. The two galaxies are actually
in the process of colliding and we can now do simulations
of these so giant computer simulations of galaxy formation. So on the left we've got galaxy
forming over several billion years of time with flying
through the center. And then on the right is what
the galaxy looks like today in this simulation. And this is a fairly typical
galaxy in a simulation that comes out to be similar
to our Milky Way galaxy. So by studying simulations of galaxies, we can understand a lot
about how galaxies form and how they live. And in fact the simulations are
getting quite sophisticated. So here's an example of one
where instead of looking at the stars in it, we look at just the gas
swirling around the center falling in on itself under
the gravitational force. And then on the bottom right
image is a blow up of the central region where all the
star formation is happening. So with the sort of the violence and the, the 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 process. So understanding galaxies
is a whole area of research. I'm not gonna say much more about that. What I'd like to do is now zoom
away from single galaxies to see how they behave in the neighborhood. So this is our local
neighborhood in the universe. So our local galactic group
is highlighted in red. It's part of a small cluster of galaxies. We happen to be falling
gravitationally falling towards the Virgo cluster in the right
hand side of this picture, forming what's known as the
Virgo super cluster of galaxies, a few million galaxies
which are gravitationally bound together. That's our neck of the
woods of the universe. It's about 50 million light
years to the Virgo cluster. So the light from then is
would be emitted at around the time the dinosaurs went extinct. When we look at the Virgo cluster, and that's really our nearest
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. We're now onto the scale of cosmology. So this is a fly through of a survey. The slow and digital sky survey
sampled a little bit of the sky, I'll say more about that in a second. And flying through all of these galaxies, we see there are just
millions of them everywhere. The scale now we're genuinely
on cosmological scales. We're into like looking at
things over the timescale of billions of years, billions
of light years now. So the light emitted from
these galaxies was emitted billions of years into the past. And for reference, the earth was formed about four and a half billion years ago. The sun 5 billion years ago. Life started about 3 billion
years ago and the dinosaurs went extinct about 0.1 billion years ago. So just to put this into context, how far back in time we're looking at when we look
to these distant galaxies, And we can see just from this, this fly through here that they
come in all different shapes and sizes, different colors. Many are spiraled like
ours, many are just blobs, elliptical blobs, some are
undergoing collisions and so on. So just to give a sense of
perspective on the scale of what we're looking at here. So on a practical level, what we do as cosmologists
is part of what we do anyway, is to try and make maps of the universe. In the same way, 500 years ago, people were trying to
make a map of the earth, a map of our local neighborhood. These days we try and make a
map of our local universe using galaxy surveys. So on the top right is a, is a spin around of kind of
the bits of the universe that we've surveyed so far. So when galaxy surveys happen, generally a telescope points at the sky, watches everything come pasted and finds where the galaxies are records their angular
position and their distance away from us. And then from that one can
make essentially a map. And one thing to notice about
this map on the bottom left, this is a 2-DF survey. This is quite a long time ago now this one is, you can see that
there's some structure in the way galaxies are distributed. So every dot in the bottom
left picture is a galaxy that's been observed. And you can see in the
distribution that there's some sort of structure there. It's not quite random. So as I say, we're still in the process of
mapping the whole universe. This is what we've got to so far. There's a number of satellites
going up or there's a satellite and some
ground-based experiments in the coming years, which are gonna hopefully fill
in a lot of the gaps so that we have a more complete picture, of our universe around us. So that goes back, those surveys go back about 5
billion years into the past. What happens if we try and look further? Well, here's a beautiful picture taken by the Hubble Space Telescope over several years called
the Hubble Deep Field. It's essentially staring
at a very dark patch of the sky with a very long exposure
photograph essentially, which we've got a fly
through simulation of here. Obviously it doesn't see this, but it was able to see
roughly 10 billion years into the past. So looking at galaxies that emitted light 10 billion years ago,
way before our galaxy was even forming. So it seems like there are just galaxies absolutely everywhere. Everywhere we look, there are just galaxies
upon galaxies upon galaxies. And there's thought to be about
one to 2 trillion galaxies in our observable universe, which of course we have only
measured a small fraction of so far. So what might the universe
look like if we could see all of it? Well, here's the result
of a computer simulation, the millennium simulation, which reveals what we think
the large scale distribution of matter will look like of galaxies. So in this image, this is a simulation of
what's called a dark matter simulation where each point
of light roughly is gonna represent where there
will be some galaxies. And where you can see
particularly bright spots in there would be clusters of galaxies
like our Virgo super cluster for example. We can see that they're
scattered about non randomly. It's not a random scattering where the way galaxies are distributed. And it's named a cosmic web
because it looks like a just a large network of galaxies. So it's super clusters are
very bright spots joined by filamentary structures and
very empty regions known as cosmic voids. So this simulation is roughly
2 billion light years across. Roughly speaking, it's not
as we would see the universe, we'll never see the universe
like this because light takes time to travel to us. We can never look at a patch
of it all at once if you like. So it's completely inaccessible
to us just because the light travels to us at a finite speed. So this is what we think
the universe looks like. If we could imagine it being
able to see it all at once. We can imagine it in computer simulations. I'll say a bit more about that later. But for the moment, let's continue our exploration
of the universe and say what happens if we try and look
further than Hubble did, further than the Hubble deep
field was able to measure. Do we just keep seeing more
and more and more galaxies ever into the distant past or
does something else appear? Well, this is what the
universe looks like as far as we will ever be able to see it. And I'll explain why later. So if we look 13 and a half
billion light years away, the universe changes completely
from what we've seen from Hubble where there's lots of galaxies, it becomes completely smooth, there's virtually no
structure of any kind. So galaxies disappear. And in fact, 13 and a half billion light years away, the universe was completely
smooth and just pure radiation. And what we see on this
picture is that radiation, the radiation that comes to us today, it's 2.725 degrees kelvin
with the last decimal place on that fluctuating over the sky. So in every direction we look, we can see this microwave
background radiation of 2.725 degrees Kelvin. The next decimal place is a fluctuation, which is plotted on this sphere. And that temperature fluctuation
we'll see tells us an awful lot of information about the
big bang and actually late time structure as well. The universe then was very, very smooth 13 and a half
billion years ago and 10 billion years ago it had loads and
loads of galaxies in it. And today of course we know
that it has gazillions of galaxies forming this kind
of cosmic web of structure. So let's just put everything in place. So this next slide zooms out
from our location with kind of galaxies put in their right
position from what we've seen. So let's just go through
that zooming out from us with galaxies that have been
observed in the right place in a three-dimensional sense. So in a short second this is
gonna spin around and then we'll see just the very
narrow slices of the universe. We've been able to survey so far. So galaxy surveys work by
targeting patches of the sky where the Milky Way is particularly
dark 'cause we've gotta see far beyond the Milky way
to see these galaxies. And essentially just mapping
out in thin slices where all the galaxies that we can see are. So as we spin round, you can see that there's
relatively few slices that have been done. There's quite a lot to
fill in in this picture. And then as we zoom out
further from our position, we're also zooming backwards
into time here and zooming further out, we get to this
cosmic microwave background, which you can imagine there's
a giant sphere around us in some sense in the sense that
we have put our observations together in this way. And so that's kind of the
current state of what we know about where stuff is in the universe. So there's a little bit
more to it obviously. And we come now to the next
part of what we know about the universe, which is that it's expanding. So this is where modern
cosmology really began about a hundred years ago with Edwin
Hubble and George Lamaitre shown here in the two
pictures at the top left. And what they did was they made, they were just studying galaxies. So they were measuring galaxies positions and their velocities. So measuring their
positions and velocities, a natural thing to do
is make a graph of that, of the position of a
galaxy and its velocity. So galaxy velocity you would
think if it's moving towards us or away from us would be fairly random. But in fact what they found
was actually there's a systematic trend that the
further away a galaxy is from us, the faster it was moving away from us. And so on this plot on the
right where we have distance plotted horizontally and
velocity plotted vertically, we can see the straight line
roughly going through all of these data points is telling us that the further away a galaxy is, the faster it's receding from us. So the implications of
this are quite profound. It means that the universe
is expanding around us and because we assume that
we're not at a special place in the universe, we can assume that the universe
is expanding for all other observers as well. So an analogy that's commonly
used is if you imagine a balloon being blown up
with some dots drawn on it, those dots are moving away from
each other at the same rate. So any dot will see all the
other dots moving away from its particular position. So this very important fact
that the universe is expanding today means that as we go
backwards into the past, all of the pictures that
we saw going backwards into the past, the universe was not only
younger but it was also smaller as well. So when we observe the past, if we look billions of light
years into the past or billions of light years away from
us means billions of years into the past, the
universe was a lot smaller. And if we look all the way
to the cosmic microwave background for reference,
the furthest we can see. So 13 and a half billion years ago, the universe was about 1000th of its size. So it's something that was
a kilometer or let's say something that was one meter
at that time would now have expanded to be a kilometer in size. So that's the last key piece
of information that we need really before we can start
building a model of the universe to understand how it came to
be like that and why there is structure on all of
these different scales. So before I get to how we
go about building a model, I thought I would just run
through a kind of timeline of the universes evolution. So in this cartoon we have
13.7 billion years of evolution with the horizontal direction
being timed from the Big Bang. It went through different
phases, phase called inflation, which I'll mention a bit more later. Then we have this Cosmic
microwave background was formed when the universe was
about 380,000 years old. So very early on in the
university's history this was formed. The universe was very,
very smooth at this time with these tiny fluctuations
in the temperature. Okay, over time those
fluctuations grew under gravity to form first galaxies, the first stars. So that essentially started the
kind of universe that we see today with lots of
stars, planets and so on. And then over 13 billion
years of evolution, more and more stars basically were formed. There's obviously quite a lot
that goes into that model. Lemme just put it into into words. So we start with a big
bang, some sort of big bang, which is when the universe
underwent an inflationary period. During that time, quantum fluctuations in
the inflaton field seeded all of the structure. So the tiny fluctuations
that we observe in the cosmic microwave background are
essentially a blowup of the quantum fluctuations that happened
during the Big Bang. It went through then two kind of phases. It went through a radiation
era for the first few hundred thousand years where
radiation was a dominant gravitational component. The universe then was just a
plasma essentially of protons, electrons and radiation. And it ended with the cosmic
microwave background formation. Now say a bit more about that later. It's just to put things in a
kind of timeline to understand what's coming. It then went through a
matter era where galaxies and things formed. And it seems to be today
transitioning to a dark energy dominated era where the
expansion rate of the universe is actually not slowing down but speeding up. So I'll come to each of
these things in turn, but let's just briefly start
with how we go about finding out this information. So we started from looking at
the universe and saying where everything is. And in the parlance of
young people on Love Island, they might be like, well it is what it is. But we would maybe want to try
and do a bit better than that with scientific method, which is to come up with ideas about why it might be like this. We then test those ideas by
confronting them with the data that we've observed. So 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 is
gonna look like for example, in reality of course
the scientific method is a long running argument with
people with competing theories, competing observations, trying to sort out what
is actually reality. So in fact, in cosmology, there's been a long running
argument about what the actual expansion rate of the universe is today. People haven't been able to
agree on this for for the last 60 years or so, and everybody converges onto
their own separate answers. And so that's kind of
the way we go about it. So you start from observations that we just take for granted,
come up with theories, argue about it and try and
come to some sort of scientific consensus about what's really going on. So how do we go about
modeling the universe? The first thing is to identify
what the most important force acting on cosmological scales is. And the most important force
once we zoom out to galactic scales is gravity. The universe is neutral. Electromagnetism only
plays a very minor role. So the main force that's
going on is gravity and things falling in on themselves under gravity. So how do we go about that? Well, we use Einstein's theory of gravity. So Einstein came up with a
theory of gravity in, excuse me, about 1915, known as general
theory of relativity. The equations for it
are in the bottom corner and it envisages gravity not as a force. So the Newtonian picture of
gravity is that there is a force pushing us down onto the floor right now. There is a force attracting
the earth to the sun, the moon to the earth and
everything in the galaxy to each other. Einstein's theory completely
does away with that picture and reimagines it as space
time itself being curved, a curved manifold in which we
all live and objects falling on that curved manifold on
effectively straight lines. So there's quite a lot to take in. But a good analogy that's
often used is if you imagine you're an ant walking on
the surface of an apple, just because you're an ant, you don't know that apple
is a curved surface. So two ants walking from say
the equator up to the core would find themselves walking
towards each other even though both of them are walking
along straight lines in their minds, whatever minds that they have. But they might interpret that
as being dragged together through a force of nature that
they don't quite understand. And as they move around
the core of the apple, they might interpret that as
a peculiar force doing all sorts of very strange things
when in fact we can see that they're just ants walking
on a curved surface. And so the imagining of space
time that Einstein came up with was to imagine that
space time is curved in a similar way and the
objects like the earth are freely falling on that. On this space time. The key thing for us when
thinking about cosmology is that it allows for a dynamic and
expanding universe and it gives us a framework in which to build a model of the whole cosmos. So what does it look like? Well these are Einstein's field equations, obviously a bit complicated
to try and understand. You need to do a physics degree to, to get the hang of these. Essentially there are equations
that balance the energy density of all the matter in the universe on the right hand side. So take everything, turn it all up, shove it into the equation. On the right hand side, the left hand side represents
a space time geometry that reacts to the matter in it. So as we move around this room, the space time curvature reacts
to our mass and the energy contained within us. Obviously a very tiny amount
for objects tiny like ours, but for objects the size
of the sun, solar system, the galaxy, it becomes quite a significant space time curvature that changes. So general relativity has
very important predictions, which confronting with
observations has led us to believe that it is genuinely
correct or nearly correct theory of nature. So some of its predictions are
that space time bends and it moves under motions of mass. So the conventional picture is
kind of on the bottom right. Imagine a rubber sheet with
some heavy balls on it. That's kind of the same sort of idea. The rubber sheet will change
in reaction to the weight of the balls. The 3D picture is more like
the mesh shown on the left in order to realize this is
a force objects traveling on that curved space time
travel on a curved path, which is actually a straight
path or the shortest possible path in space time. So our earth falling around the
sun in free fall is actually traveling on the shortest
possible path in space time given that the sun is there
curving space time around it. So that's the kind of the
picture for massive objects. A really important prediction
from Einstein's theory, which was later observed by Eddington shown here in the bottom, was that light itself is also
bent by a gravitational field. So this is a big divergence
from Newtonian gravity, which doesn't have this prediction. So for example, in fact the experiment that Eddington
did was that stars that are just behind the sun or just
near the sun's position on the sky, will actually have the light
deflected by the gravitational field of the sun, the space
time curvature around it. And the star will actually
move its position when the sun moves around it. And so this was observed in
1919 or so, verifying Einstein's theory and actually made Einstein a bit of a star at the time. So that's one key prediction
that light is deflected by a gravitational field. And in fact when we go back
to cosmological scales, it gives us a phenomenon known
as gravitational lensing. So gravitational lensing is
kind of the collective process of this by large objects
such as clusters of galaxies. So the top two images here
are images taken by telescope Hubble and the James
Webb's face telescope. And what we can see is in
the center of these images is cluster of galaxies. And then the galaxies behind
that cluster have actually had their light completely distorted
by the sheer gravitational mass of that cluster in the middle. And so on the bottom right is a simulation of a cluster
moving past a distant star field and seeing a
distant galactic galaxy field, sorry, and the shape of those
galaxies getting distorted by the mass of the cluster in front. So this is another important
prediction of Einstein's theory and as we can see in the top right, it creates all sorts of weird
effects of galaxies being stretched out in funny ways. Another prediction of Einstein's
theory that was developed over many years was the
prediction that black holes must exist, if objects are
sufficiently massive and start collapsing in under their own mass. It's been proven that there's been, there is no way to stop that
collapse all the way down to a singular point. So this was predictions by
Roger Penrose and Steven Hawking back in the '60s essentially
asserting that black holes have to exist if Einstein's
theory is correct. So if a sufficiently massive star dies, the pressure can no longer keep it up against its
gravitational mass and it starts collapsing and nothing can
stop it till it gets to a singular point. These objects are
surrounded by what's called an event horizon. The gravitational field of these
objects as you approach the singularity gets so strong
that light can't get away from it basically. So you can go in but you
can never come out again. The point at which point of
no return is the event horizon for that object and that's
known as a black hole. These things come in all
sorts of different sizes from roughly about the mass of our
sun all the way up to one's found in the center of galaxies, which are about a million
to a billion times the mass of our sun. And so they're gobbling up all
sorts of stuff shown in the simulation and observed by
the Event Horizon telescope in those two top circular images. These are actual images made
of black holes that we observe. The top left one is from
our center of our galaxy, the other one is from
the center of Andromeda. So these are quite amazing
observations that have been made just in the last few years. So the final prediction
of Einstein's theory is of gravitational waves. The gravitational field is no
longer a kind of static forces kind of thing, but actually a dynamic manifold
in which things happen when two black holes meet each other
and start to spiral around each other and collide together. Shown in the simulation,
they release huge amounts of gravitational radiation, essentially the space time
responding to those objects moving radiates that information
outwards and contains enormous amounts of energy in doing so, in the top right this
is gonna creepy picture moving about is showing how
the actual wave travels through space time. So if it comes towards you, it would make your head
oscillate in opposing ways. And in fact, gravitational waves have now
been detected the first time a few years ago by the LIGO Observatory. This is a picture of Virgo in Italy, which is part of the network
of gravitational detectors that are being built around the earth. And so what they've been able
to observe is kind of the following situation. So this is a simulation of two
black holes that were thought to being observed by LIGO. They spin around each other,
shown against a star field. We can see the space time
curvature is so strong shaking the stars behind these two
black holes as they spiral in towards each other. They're both about 30
solar masses, so very, very large black holes
and yet they're only a few kilometers across. So a few kilometers across with a mass of 30 or 40 suns each. And as they spiral in together, they emit gravitational radiation, which you can see is a shaking
of the star field behind it. So in this simulation it slowed
down massively 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 indicating that
they had found the first black hole merger and really
solidifying Einstein's theory as a correct theory of gravity. So really impressive, really
impressive achievement. And a gravitational wave
binaries as they're called are being observed all the time now. Okay, so let's get back to cosmology. Let's see how we can go about
using the fact that we now have a gravity theory that we
can trust to build a model of the universe and understand
the large scale dynamics of the universe itself. So a simple question is
the universe is expanding. How fast is it expanding? How long has it been expanding for? Well let's go back to
Einstein's equations. We're balancing the energy
density of matter on one side, dynamics of the space time
curvature on the other side. And what this means is we
need to know what's in the universe, what the proportions of the, the cosmic stuff in the universe matters. So what is in the universe? Well, putting everything together, you can have a kind of pie
chart of stuff in the universe if you like, if you mashed everything up and worked at its energy density. The surprising thing about
this picture is the proportion of heavy elements. So that is everything that we
know in our world makes up the tiniest fraction of a
percent of all the stuff in the universe. So elements heavier than
helium form basically no energy content to the universe. So we're really very an
insignificant part of it. Next comes stars. Stars you would think up, make up most of the mass. In fact they don't. They make up about half a
percent of the mass of stuff 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 yet could in the future. That makes up about 5% of
the energy density of stuff in the universe. And the rest is made up of
things that we don't know what it is. One component is called dark matter, another component is called dark energy, dark matter, we have a fairly good idea of kind of what it's like,
dark energy, less so. And lemme just go through
pieces of evidence for both of these to persuade you that
were not completely crazy. Dark matter, evidence
for dark matter comes in many different forms. The first discovery really was
from the way galaxies rotate. So we saw in the simulations
of galaxies rotating over the course of hundreds of millions of years, we can measure that rotation
quite accurately in galaxies in our neighborhood. The expected way that they rotate, if they just contain the stellar
matter that we see is shown in the yellow curve. But the observed way they rotate is shown in the green curve. And what this suggests is, is that galaxies themselves
don't just consist of what we can see in them, that is the stars, but actually as they live in
a halo of dark matter which is larger than the galaxy itself. So the galaxy that we see
actually extends much further but with material which is completely dark. We also see dark matter
in gravitational lensing. So this is sort of important picture of one of
the first gravitational lensing events by a cluster that was taken. So the arcs that you can see in that image of the lens image, the central part contains a
large cluster of galaxies and by examining the arcs in the bending of the, the galaxies in the background, one can work at the mass of
the cluster and the mass of the cluster from gravitational lensing, which is purely gravitational effect, does not equal the mass of
all of the visible matter in that system. And this is true for clusters
all over the universe that we see. And so this is another piece
of evidence of why we think dark matter exists on
the scale of clusters. And I'll come to another
piece of evidence later, dark energy is much more mysterious. It shows up in the very large
scale dynamics of the universe and it shows up because it
seems to be making the expansion rate of the universe accelerate
rather than decelerate. If you imagine if you
take a bunch of galaxies, send them flying apart, their combined gravitational
attraction you would imagine would slow down that expansion over time. And in fact it seems that the
expansion of those galaxies moving apart is getting faster
over time and not slower. And this has been going
on for the last five or 6 billion years in
the universe's history. So dark energy is very peculiar. It acts in an anti-gravity way. So it doesn't work in the normal
way we think gravity works. And it also has very
strange properties like if you compress it, it
doesn't get any more dense. So it's a very peculiar kind of thing. Again, we have lots of separate
pieces of evidence for it. One of the first was measuring supernova. So supernova are special types
of exploding stars that we can see to very large distances
and we have a good idea of how bright they are so we can work out how far away they are. And continuing Hubble's diagram further allows us to see kind
of the expansion rate of the universe over time. So this key piece of, in this key piece of
information actually led to the Nobel Prize a few years
ago for understanding that the universe's expansion rate
is accelerating over time. And so there's a lot of effort
underway to kind of keep observing these exploding
stars known as supernova. So now that we know
what's in the universe, the composition of the
cosmos, dark energy, dark matter, little bit of all the other
stuff that we actually understand, we can put
all that together in a, into Einstein's equations, solve them and work out the dynamics of the universe over time. So pictured here is what's
known as the relative scale of the universe relative today. So as we go backwards in time, the universe is getting smaller,
going backwards in time. How fast is it getting smaller
and at what point did that contraction reach zero size? So these curves on here show
us different universe models for different amounts of these
different cosmic materials that go into making up a model. And you can see you can
have universe do all sorts of things, collapse upon
themselves, expand forever, the expansion rate
getting faster and faster. Our curve is the one that's kind of green, just the transition
between orange and blue on the background, implying that there's
a big bang singularity about 13 billion years into the past. So this is a very large
scale simplified model of the universe, smoothing
everything out in it. Just what is the expansion dynamics? How old is the universe? Will it expand faster into the future? Which is what we think it is. So if we understand how
the overall dynamics work, can we explain this cosmic
web of galaxies that we see. And can we explain the
cosmic microwave background kind of at the same time 'cause they're sort of
related to each other. This cosmic web is known
as large scale structure in the universe. How does this large
scale structure evolve? So the seeds of structure were
laid down during inflation kind of initial conditions. Big bang set off this quantum
fluctuations which exploded up to classical scales. So that's a little bit
speculative, that area but seems to be the case. So the early universe was
radiation dominated until about 380,000 years after the big
bang that went through a matter dominated era, during the
radiation era, the universe, the stuff in the universe oscillated, you can actually picture it a
little bit like the interior to the sun, mostly hydrogen, little bit of helium with
sound waves propagating through it and gravity acting upon those. Essentially what happened is
over dense regions collapse into galaxies and the material
that led to their collapse is comes from these void like
regions at late times. And so a very simplified
level that's really all that's happening is, slight over
densities at early times just get heavier and heavier and
gravitationally collapse at the expense of the
slightly under dense regions. Obviously there's a bit
more to it to than that. So let's look at the radiation era. The radiation era was relatively simple. The universe was incredibly
smooth, because it's smooth, it's actually really easy to model from a physics point of view. So the physics of what was
going on at this early time, less than 300,000 years
or so after the big Bang, we just have a soup of
photons, mainly radiation, dark matter particles,
protons and electrons. The universe is very hot, it's
above 3000 degrees kelvin, it's much, much smaller than it is today. And because it's above
3000 degrees kelvin, that's the ionization
temperature of hydrogen means that there are no atoms. The protons electrons
are free bouncing around in a big plasma just like in the sun. This also means that the photons
are trapped in that plasma. They can't free stream, they
bounce off charge particles. And so that's kinda what's going on 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
pool of those oscillations even though the dark matter itself
doesn't interact directly with the protons and electrons and
photons electromagnetically. So those are dragged by gravity
and we actually observe the end of the irradiation era in the cosmic microwave background. So what happened then is as
the universe is expanding, it's cooling down all the time. If you take a gas, you expand it, it cools down. So as it cooled down
below 3000 degrees kelvin, the ionization temperature of hydrogen, suddenly all across the universe, protons and electrons combined together to form neutral hydrogen. Neutral hydrogen no
longer scatters photons. So the photons were free
then to free stream, the universe, all of a sudden, almost instantaneously
across the universe went from completely opaque to completely clear. And one way to think about
how we can now see that cross microwave background, those photons able to
free stream 13 and a half billion years, till today,
we can actually observe them is a bit like when we look
at a cloud, inside a cloud, photons are bouncing around
of all the water molecules. Photon gets to the edge of a
cloud and it can free stream towards our eyes. And so we perceive it as a surface. And so a very similar kind of
physical processes going on when the universes temperature
cools below 3000 kelvin. So it releases in the
cosmic microwave background. So what we've got here on the
bottom right is the map of the cosmic microwave background
made by the plank satellite shown in the bottom left. And it's the whole sky but
compressed into an ellipse. So we can see it on one picture. The fluctuations on this map
are one part in 10 to the five. So it's an incredibly smooth surface, much smoother than the
smoothest billiard ball you'd find for example. And when we take a mathematical
transformation of that, we show the graph on the top left. Now when we look at the
map, you can see that the, the red spots representing
slightly warmer regions and blue bits representing
slightly cooler regions are kind of the same
size all over that map. And you can see by eye there's
a kind of characteristic scale in that, map when we
do a mathematical transform of it, see the big peak in
the middle of that transform the function represents those scales. We can see it about a
degree scale on the sky. So about that size on the sky, it's a characteristic scale in that map. And so that characteristic
scale actually tells us an awful lot of information about
how the universe was at those early times. So it tells us, firstly,
the shape of the universe, whether it was curved or
not, seems to be flat. It tells us the age of the universe. It tells us proportions of
dark matter to hydrogen, to helium. So if you vary all of these
in the models that we make of the early universe, it changes the way this cosmic
microwave background looks. And one can do just do a best
fit analysis to see what it kinda looks like. And it also contains
information about what happened in the Big Bang. So there's huge amounts of
information that we've been able to get from this map of the
very early universe in this cosmic microwave background. And so how do we get from
that to this cosmic web? Well one can do some work analytically, typically it has to be
done in simulations. So simulations look something like this. Take a box, factor out
the universe's expansion, give it the initial condition
to the cosmic microwave background, watch everything collapse. And when you watch everything collapse, you can actually see that gravity does all of the work for us. And the simulation just shows us this, it produces a cosmic web just
because that's the way it is. And so we can then take the
results of those simulations and then compare them with the
galaxy surveys that we measure. So here's an example where in blue I think is real universe maps that are made, red are analogous things which are made purely from simulations. And one can try and match the
statistics of those two things and decide which model
fits the observations best. And so by looking at
structure and all of these different scales, one can work out what's
called the power spectrum of density fluctuations. So essentially these functions in code, the statistics in the cosmic
web that we can see by eye. It's difficult to see by eye
the differences that come from all of these changes, but in mathematical
functions we can do it. And so these simulations
become quite sophisticated. The previous one was just
dark matter simulation, just throw away all the actual stuff that we think is important. But modern simulations can
now model the gas in these. So the hydrogen and helium,
star formation rates, watching things explode
as things go on and so on. And they become quite sophisticated
on these small scales. So on the left is a dark matter. On the right is all the gas, which is hydrogen helium making
stars and things exploding. So dark matter is absolutely
crucial for this picture of structure formation. It's the final piece of the puzzle. So it's not just gravitational lensing, not just rotations of galaxies,
it's the whole shebang, the whole 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, 'cause we only
interact with it gravitationally, which means it's very hard to discern what it might be made of. There's obviously a lot
of work underway trying to understand its particle nature, how it interacts with what's
called the standard model of particle physics, which is everything that we
know that we see around us. Dark energy is also an important
part of this model because we measure the expansion rate speeding up, but we really don't know what this is. It's actually contained in
Einstein's field equations through what he called
the cosmological constant. So in his field equation
there are unknown constants, speed of light, which we can measure
gravitational constant G, which obviously we can measure. But there's another one called
the cosmological constant, which is there by the nature
of his field equations. It's mathematically allowed, but we can't measure it
on day-to-day scales. We can only measure it
on cosmological scales. Seems to be there, seems to be non-zero, but it doesn't have a
satisfactory fundamental explanation for it. So a lot of work is underway
to try and determine if the cosmological constant is
constant or if it's evolving or perhaps Einstein's theory is
not the correct theory to be using on the very, very large scales, perhaps a modified theory
of gravity is required. So one way to determine it is
actually in the large scale distribution of galaxies, the characteristic scale that we saw in the cosmic microwave background, those hotspots and cold spots, which looked about one degree across, that characteristic scale is preserved as we evolve further into the
future into the cosmic web. So the cosmic web picks up that
same scale and by comparing that scale at late times and early times, one can understand through
quite a lot of work whether the dark energy itself is evolving or not. Is it a cosmological
constant or is it something even more strange? So just a picture of what dark
energy constraints look like. So this is the sort of stuff you get in scientific papers about. This one tries to
parameterize dark energy, say it's evolving. What might it be evolving, by
parameterize that by saying, giving it some functional
form and then constrain those parameters with observations. And a key goal is to determine
whether it's a cosmological constant or not. And then one produces aerobars on that, et cetera, et cetera. Can say with certain confidence
that it's certainly not doing something else. So another aspect of this model, which seems to be also
peculiar but also seems to be required, is a period of inflation
right at the beginning of the universe. So I've kind of glossed over
this mainly 'cause I don't really understand it very well, but during the Big Bang, the universe seemed to go through
another anti-gravity phase where there was something
driving the expansion of the universe at an exponential rate. So it's known as the inflaton
field and it seems to explain a lot of things about
the universe for touches, why it's flat, it explains the nature of
the perturbations or the fluctuations that we see in the
cosmic microwave background. And oh yeah, also explains why the universe is uniform, which is also something that
you wouldn't just expect for a randomly created universe. So inflation itself might be
a prediction that in fact that there are other universities
and we form part of a larger multiverse of universes as well. So by observing these things
we can maybe try and find answers to those questions. So just to finish off,
I mentioned that the, our map of the universe is not complete, it's a long way from being
complete and there's many surveys underway to try and fill
in our understanding of the cosmic web. So Euclid and the square kilometer
array in South Africa, Britain is both well
involved in, another one, the large synoptic survey telescope will measure millions of supernova, try and fill in the Hubble
diagram out to further and further distances and in more
and more detail to try and understand the expansion dynamics. And in fact, there's new avenues opening
up with what's called the Einstein telescope, which is a super
gravitational wave detector, which is supposed to, if they
get round into building it, detect all gravitational wave
merger events that happen. So there's a huge amount of
stuff going on for the next generation of surveys and
the kinda questions that we need to answer, what is dark matter? These get progressively more complicated. What is dark energy, what is inflation? And what happened in the Big Bang itself? Related to that is probably what's at the heart of black holes. They're probably related questions. Are there other universes? That's a question we might
try and be able to answer. And then of course there's
the bigger questions, like why does anything exist at all? So it's kinda like, I guess that's gonna be harder to explain, but if we just reflect back on
where we were 500 years ago, just trying to make a
map of the earth itself, we've come a very, very long way. So I'll leave it there. Thank you very much. (audience clapping)
