(dramatic music) - Thank you, I'm pleased to be part of this post-COVID effort. I think we have to keep up the good work and have these face-to-face
meetings wherever we can. So ladies and gentlemen, good evening and to those who are joining us online, good morning, good afternoon or if you're in Australia good eye, 'cause I think it's probably
in the middle of the night. Anyway a very warm welcome to everyone. The first time I came to
the Royal Institution, I was sitting up there somewhere. It was many decades ago. It was during the time when
George Porter was the director so that dates me. And the subject of the
lecture was the arrow of time why the past is different from the future. And the reason I was interested in that was because I'd done my PhD on that very topic at
University College London and so I wanted to hear, I think it was actually George
himself who gave the lecture I wanted hear what he had to say. And I came across this
quotation from Michael Faraday which I think, and of
course his ghost stalks the corridors of this building. I think it was addressed to me because no matter what you look at if you look at it closely enough, you're involved in the entire universe. And indeed I had found that
the source of times arrow can be traced back to the
origin of the universe. So these are very appropriate words. Well, these days as you
heard I live in Arizona but it's a delight to
be back here in London away from the unrelenting
blue sky and warm sunshine. Blue sky does have one
advantage in Arizona. It's good for astronomy. And in fact in some ways, Arizona is the world capital of astronomy. There are many observatories. But it's also the place where the subject of cosmology began. Now cosmology is the study
of the universe as a whole as opposed to astronomy,
astronomers study bits and pieces, cosmologists study the big
picture, the whole thing. And we can trace the origins
of cosmology back to Arizona and in particular back to an observatory that was built at the
turn of the 20th century, end of the 19th century by rich
businessman Percival Lowell in Flagstaff which is a three-hour drive north of Phoenix where I live and work. And the reason that Lowell
built this observatory was not to study the grand
architecture of the universe. It was to look for martians
that at that particular time there was a lot of speculation that Mars was inhabited
by intelligent beings who dug canals and the
search for the canals on Mars seemed to be a high priority among those who believed all this and Lowell himself produced
these elaborate maps of the martian canals and it turns out that they are entirely a fiction of his imagination. He was convinced that there
were intelligent beings there but then when NASA launched
two spacecraft to Mars called viking, this is what they found. A freeze-dried desert with
withering ultraviolet radiation, no sign of any life, and
no sign of any canals. And so that was a sort of waste adventure but the observatory itself
had other projects lesser, I mean, on smaller telescopes. And one of these was a little
known astronomer called Vesto Slipher who decided that
he was going to investigate what at that time was one of the great unsolved
problems of the universe which is how it's organized. Now astronomers were
familiar with the fact that there were fuzzy patches of light, they were called nebulae
stellar, called nebulae but what were they? Were these simply clouds of
gas within the milky way? Or were they like other
milky ways much further out? And how was this going to be resolved? Well, Slipher, and I should
point out that in those days an astronomer's lot was not a happy one because Flagstaff's
very cold in the winter, gets lots of snow, that's the
best time to look at the stars and you can imagine, and you
can't heat an observatory because the air currents
mess everything up and so it's freezing cold work and it's not like today when
it's all hooked up to computers and they do all the work
and you sit somewhere warm and just download all the images. In those days it all had to
be done with pens taking toil, photographic slides literally exposing the plates and developing them and basically tweaking the
instruments as you went along and so this was an astonishing
and painstaking endeavor and so basically what he was looking at were things like this fuzzy. I mean it's not the projector,
it's deliberately defocused. Because that's what you saw
with the early telescope. So what he was able to do, by analyzing the quality of
the light using a spectroscope, he found that the fainter of these nebulae were redder than the less faint ones. And there seemed to be a
systematic relationship. And he had an explanation
for why they might be red. It's called the doppler shift. If an object is rushing away from us, then the light from it is stretched out and it shifted towards the
red end of the spectrum. And that was known, so
it's known as the redshift. So he discovered the faint and therefore farther away objects he surmised were rushing
away from us faster. And this work came to the
attention of Edwin Hubble, a pipe smoking lawyer turned astronomer who had a much better telescope, the 100-inch Mount Wilson telescope and he was able to look more
closely at these nebulae and to see that they were
made up of individual stars and not only that, he was able
to use some of those stars to measure the distances to them. And settled the matter that many of these nebulae are
in fact other milky ways other galaxies as we would now call them millions of light years away. But he took Slipher's
results and added to them and produced this rather
dubious looking graph but it's sort of trying
to be a straight line based upon these observations. And what this suggested to Hubble is that if there is that linear relationship, it suggests that the whole
universe is expanding getting bigger all the time in that type of linear relationship. And he published those results in all places the New York Times. This was in 1924. That was the first the world heard of it. And you you might have thought that a declaration that the
universe is expanding would be a media
sensation around the world but it really wasn't. Although Hubble became quite famous. But the thing is nobody quite
knew what to make of all this. Today, we think the expanding universe is part of what I think everybody knows, every school child knows that
the universe is expanding but what does it mean? Expanding into what? Coming from where? What is its trajectory?
How do we think about it? And the first thing that is pretty obvious is if the universe is
getting bigger every day must have been smaller
yesterday than it is today. And then if you run the
great cosmic movie backward and you go back some billions of years because Hubble could measure roughly how fast it was expanding and it was sort of
billion year time scale, if you go back billions of years then surely everything
was scrunched together that it must have sort of
come into existence somehow and what could you say about that? Well, in the 1920s nobody
really wanted to be drawn, well, almost nobody wanted to be drawn. But one person who did
draw the obvious conclusion was a Belgian priest. The abe George Lemaitre who in 1927, sort of
said the obvious thing that if everything is coming
out of this tiny volume, then he had this term cosmic egg, he imagined that in the
beginning, not literally an egg, but that's why I'm depicting it that way, but that it exploded and gave rise to the universe we now see. It wasn't very well received, Einstein said your mathematics is correct but your physics is abominable. And at the time almost
all astronomers said, well, this was a speculation too far. So the idea that the expanding
universe implies on origin at some finite moment in the
past, few billions of years ago really took a back seat to
other issues in astronomy such as what makes the stars shine. And if we fast forward
to the post-war years, then an arch skeptic of this idea of this sort
of abrupt cosmic origin was the astronomer Fred Hoyle, the Cambridge astronomer Fred Hoyle. Now Fred with a couple
of others had developed another way of thinking
about the universe. Fred, of course, realized it was expanding but did that mean it had an origin? No, not necessarily, he said supposing every time
the universe doubles in size then new matter equivalent
to what was there before trickles in to the universe and fills up the gaps left
as the universe expands. And I should just say as a corollary, 'cause people often say well, you know, what does
expanding universe mean? The best way of thinking
about it is it's the expansion of space itself. So every day the universe has somewhere in
the book I've worked it out how many more gallons or liters or whatever is your favorite unit of space appear in the universe every day. So the reason that the
galaxies are retreating from us it's not because they're all blasted from some common center over there. It's because the whole universe is, the space is getting bigger and
bigger and bigger everywhere and so therefore the galaxies are conveyed apart from each other. There's no center and no edge. That's you have to think of it that way. And Fred had this idea that there was no beginning
to the universe and no end but it was in a steady state. That over billions of years, it would look more or less
the same on a large scale. And so this is a way of depicting that with the new matter coming in and new galaxies forming and so on. And that was taken very
seriously in the 1950s. The steady state theory is a rival of the cosmic origin theory and it was Fred, in a
radio BBC radio interview that coined the term big bang. And it was a term of derision. He said, well, some astronomers think that the universe
began with a big bang. You know, the self-evidently
upset. And the name stuck. And that's what we call it
today, the big bang theory. Now the nail in the coffin
of the steady state theory came a little bit later in 1964 with this extraordinary contraption. This was a radio antenna built
at Bell Labs in New Jersey and it was built with the intention of communicating by satellite. I'm probably the oldest person here, I can well remember Telstra, the first communication
satellite in the early 60s that was able to certainly
send telephone signals. I'm just trying to remember whether that was when we
got the first TV signals from the United States. It was very memorable. Anyway, so this was the pinnacle
technology in those days. I mean, look at it. And what they found was that
there was an annoying hiss in this instrument and there
was some a lot of effort to try and figure out what it was. Could it be pigeons
nesting inside the antenna? So they cleared the
pigeons out. Wasn't that. But they eventually realized that this hiss was coming from outer space and it is in fact the fading
afterglow of the big bang. This is the big bang's smoking gun. This radiation, because the universe was very
compressed at the beginning, it was also very hot and this
radiation has nowhere to go it fills the whole universe but as the universe
expands, it cools down. Remember I told you space is stretching. So what that means is that light waves traveling through space
get stretched as they go and that's how you think of the redshift. And this radiation has been stretched by a factor of about a thousand from the time that it
was last in equilibrium with the matter when the universe
was a sort of fiery plasma at about 380,000 years
after the beginning. That sounds like a long time. But it's a tiny fraction of
what we now know is a 13.8 billion year old universe. And so this radiation
is coming untrammeled almost undisturbed from
that very early time. And today we can observe
it with satellites and if you do that this
is a textbook spectrum. So this is the heat radiation, the spectrum of the radiation that is the energy distributed
across different wavelengths. And this is, when I was at
school you saw this curve 'cause this was deduced
in the 19th century that if you had a perfect black body or a system which was in perfect
thermodynamic equilibrium, it should have a radiation
spectrum that looks like this. And this is the the real thing. It's the best example we have. You can't get this good in the lab. So it was pretty obvious
I think to everybody in the late 60s that the universe did have
a beginning with a big bang and Fred surrounded himself
with a heroic band of die-hard steady-staters, states persons and here's a picture from
1970 with Fred Hoyle, this was the Institute of
Theoretical Astronomy in Cambridge with Fred Hoyle sitting
in the front row there still tenaciously clinging to those ideas and you might also
recognize in the front row a very young looking Stephen Hawking and a very young looking Martin Reece. And if you look very
carefully at the back, you'll see a very young
looking Paul Davies and so the reason I'm belaboring this story about Fred Hoyle is 'cause he gave me my first job and there I am on the
job standing at the back about to embark on my career in cosmology. And what a charmed career that has been because I've lived through
the golden age of cosmology which really began not
with Slipher or Hubble. It really began with the
cosmic background radiation the CMB, we often call it, the
cosmic microwave background which carries so much information etched in the details of that radiation. It's information about the first split second of the universe and it was obviously at
that time going to dominate cosmology for the coming decades. So cosmology in about 1970 was
really a backwater of science a speculative backwater. It's now precision science. And a great deal of that comes from investigating
that heat radiation. Now already at that time of course there was a lot of discussion if the big bang was real, everybody wanted to know
what caused the big bang and what happened before it. And people still ask those questions. If I go to dinner parties and I usually try to keep
quiet about what I do but if cosmology comes
into the conversation people always pounce on me. Aha, you scientists may be clever but what happened before the big bang? You know, that's got you stumped. So let me just address that question because it's an important question. I'm not trying it aside at all
and we don't know the answer but let me take you through the concepts. Now what does it mean anyway, what happened before the big bang? So a simple version of the
nature of space and time might be something like this. Time stretching back for all eternity forever and ever into the past. And then some particular moment
C for creation if you like the universe comes into being, okay? So there's nothing down here and then boom there's a
universe physical existence. Is that the case? Why would that happen? Obvious problems with it. What caused C? I was just saying that. What caused the big bang? Doesn't have to be a big
bang. Whatever it is. What caused it? Seems to be only two sorts of answers a natural process or a
supernatural process. If it's a physical process
or something like God to make that happen. But both of these have their problems because if it was a physical process, what type of physical
process needs all of eternity and then it suddenly happens. If it can happen with a finite probability why didn't it happen an infinite time ago? So that's a very strange
thing to think of it as a physical process. But it's equally strange in theology because you can ask, or people did ask back in the early days of the narrative of a
universe having an origin or creation. What was god doing before
creating the universe if the time goes on for infinity. And Augustine in the fifth century, he was a christian theologian came up with a clever answer to this. He said that the world was
made with time and not in time. In other words that before what we would now call the big bang, there was nothing at all and I don't mean empty space
sitting there for all eternity. I mean no space, no time,
no matter, no thing. So literally nothing. And that was Augustine's view. And so the idea being that time itself just sort of switched on
at this particular event. And that's pretty much the way it was when I was in Cambridge at that time. The standard view of the
universe was there was a big bang and it was the origin of space and time and so the question about what
happened before the big bang or what caused the big bang was simply dismissed as meaningless. And Stephen Hawking I think
later expressed it quite well by saying, asking what
comes before the big bang is like asking what lies
north of the north pole. The answer is nothing, not because there's some
mysterious land of nothing there but because there ain't no such place as north of the north pole. And in the same way in this
simple picture of a big bang there's no such time
as before the big bang. So this is my pathetic
attempt to depict this. So here we see the great eruption, the bringing into being of the universe. Very misleading 'cause we
don't think it has an edge but anyway and here we see
time, the clock itself, coming into being. But the idea that time would
just sort of switch on, you know, there wasn't
time and then, it appeared it does appear a little bit magical. So although you can't talk about a cause, you can still ask for an
explanation why would that happen, how could that happen. And there's one branch of physics that gives a clue to how
we might explain that and that is quantum
mechanics, quantum physics. And in the 1980s, Jim
Hartle and Stephen Hawking worked on a theory that had the universe
with a quantum origin. Now let me explain to you in simple terms why that could work. So imagine that you have an excited atom and as most of you probably know, the atom can then decay or
de-excite and emit a photon. That simple process and that's described by quantum mechanics. So this is a process where
in the beginning is no photon and then there is a photon. And according to a best
understanding of quantum mechanics, you can't predict exactly
when that will happen. That the process is
intrinsically uncertain and indeterministic. And so if you say, oh the photon appeared at
four o'clock on a Thursday, but why didn't it appear
two o'clock on a Wednesday? There is no answer to that. So in other words quantum mechanics is a
description of the world the physicists favor that says that there are genuinely
spontaneous processes down at the atomic level. So the reasoning is, if the universe was once so
squashed to the size of an atom then quantum effects
would have been important and we could talk about
the spontaneous or abrupt coming into being of a
universe from nothing by a quantum process without
having to worry about a cause. And so that was the drift of it. It was called the no boundary proposal because I don't want to get
into too many technical details but you'll notice that
what is depicted here is space in the horizontal
plane and time going vertically and this is the universe, we're only showing two space dimensions and then it's a bowl shaped thing. And so if you think about this being time, what this really says is that down here there's only space and up
here there's space and time. And at the time itself, it
doesn't switch on abruptly. It sort of emerges out of space. And again this is my
attempt to show you this. So imagine that the origin
the universe is like that. So time doesn't just suddenly switch on. It emerges. It takes a
duration to emerge if you like. I mean, that's an abusive language. And what I just showed you is speeded up about that number of times. So it would have been, for most purposes, we could
regard it as instantaneous. So was this credible, was it credible that the
origin of the universe could be explained as a quantum event? Well, I think there is
some evidence in its favor and I'm going to now pick
up the main narrative which is the exploring
this cosmic background microwave background radiation because Penzias and Wilson who discovered the horn-shaped antenna couldn't really map the
whole thing properly. But the first satellite to do it called cosmic background
explorer or COBE many years later was able to produce a
heat map of the whole sky. And here it is. It was published in the early 90s and became instantly famous. And what you're seeing there
in those color coded blobs is slight variations in temperature. This is the whole sky
sort of mapped out there. So these blobs are big in in size but very tiny and temperature variation few parts in a million. But the overwhelming story is not the variation it's the smoothness that the temperature over there and the temperature over there in the sky are almost exactly the same to a very high degree of precision. Once you factor out that the earth is sort of
ploughing through all this stuff. And so those variations
were very important. Now today, there's a much
better satellite called Planck and that's what we see now. So very detailed data
mining of that radiation. Now I had a small part to play in all this because in the 1970s, I was a lecturer at
King's College in London, not far from here and I
had a student, Tim Bunch and we needed to get him a PhD and so we were at that time,
there was a group of us we were interested in quantum effects in the expanding universe. So getting close to what
I've just been talking about. And there's a particular, the equations are really difficult but there's a particular
one where they're easy and that's where the
universe doubles in size in a fixed time so it's
exponential expansion. And then you can solve the equations. I mean it was still a lot of
work for Tim but he got his PhD he solved all that. And what we were interested
in was the quantum vacuum that even empty space we
know in quantum mechanics is not totally empty but
it still has activity with particles that come
into existence fleetingly and then disappear again. And I'll come back to this a little later. But just know for now, that even if you remove
all atoms and all photons and all particles from a box there's still things
happening in that box. And there is a certain amount of energy that attaches to what's happening there. And this being quantum
mechanics is uncertainty and indeterminism and fluctuations. So any given quantity fluctuates. And so we'd worked out the
properties of that quantum vacuum never thinking that anybody would ever be remotely interested. And it was only two or three years later that suddenly a new theory
of the big bang appeared and it was called the inflationary theory and in a nutshell, what that theory says, is that during the first split
second after the big bang, the universe left in size
by an enormous factor as if it had taken a sudden deep breath and that that was a
fundamentally quantum process and when it stopped and continued expanding
at a more sedate rate then the quantum fluctuations that Tim and I had worked out were imprinted on the universe. So sort of writ large in the sky and that's actually what
you're seeing there. We think these are the fossils
of quantum fluctuations from the first split
second after the big bang. So very strongly suggesting
a quantum origin. But like everything else in
cosmology, it may be superseded, it remains the best explanation
quite some decades later and it's more than just,
you know, looks right. Statistically, it does come out right and people have done these analyses, I'm not going to go into
the technicalities here but what you see with the curves is the theoretical prediction based on this vacuum state
that Tim and I worked out and then the red dots are
the observational points. And you'll see that it
does come out very well. And that's good because without
these quantum fluctuations that gave you those slight variations, we literally wouldn't be here. Now why is that? Because the hot spots and the cold spots represent variations not just
in temperature but in density. And in cosmology, there's only
really one force that matters it's gravitation. And gravitation is a pulling force. So if you have an over dense
region of the universe, it'll drag in material
from the surroundings and enhance that density contrast. So the few parts per million
back at the beginning grew and grew and grew until they turned into
something like this. A universe with galaxies
and clusters of galaxies and within the galaxy's stars and planets and on these one, intelligent beings. So this growth of the
large-scale structure that led to the universe we now see and permits the existence
of life-bearing planets flows from the quantum activity
in that first split second, I keep saying split second, I'm actually talking about
something a little bit, just a little bit longer than a trillion trillion
trillionth of a second. So that's the degree of confidence that we can express in our theories that it's possible to
actually model the universe at that ridiculously early time. So now it does look like a
case of cosmic perfection. I'm describing to your universe that was born in almost
complete uniformity, beautiful if you like but with just the right
degree of variation to lead to interesting things like people. And there are some people
that think that's it, that there must be a sort of principle of primordial simplicity
or primordial perfection or something like that because we don't just
live in any old universe. If the big bang was any old
bang, it would be a total mess. It was a highly highly orchestrated bang and in addition to the uniformity of it, the bigness of the bang
is really very special because if the big bang had been smaller, the universe would have collapsed back on itself a long time ago and if it had been bigger, the parts would have spread out so much that there will be none of
this large-scale structure. And so it turns out and this
was known quite some decades that the rate of expansion matches the gravitating
power of the universe almost precisely. And that mystery is explained
by the inflation theory. So what we seem to have is something that is very well set up. If you were gonna make a universe, this is a pretty good one to make. But, do you remember this picture? Well, down at the left-hand end here, we see that there's some discrepancies. Now that left-hand end represents the largest angular scales. So what we're talking about is that the small-scale
variations match very well. But when you get to that
one over there over there, it's not so good. And that said some people to wonder whether the universe is
actually a botched job, a nice try a perfection but
there's some flaws in it. Blemishes in an otherwise
perfect universe. And what's eating the universe, you'll see how that relates to this but let me just show this picture this is the same heat map of the sky. And there are a number of weird things but a couple I just mentioned one is that there's a slight lopsidedness between the hemispheres
with the temperature. The other is that there's
a curious cold patch in the constellation of Eridanus which is in the southern hemisphere we can't see it from here. And it's colder than it should be as that if it was just
a random fluctuation in the background heat radiation. So what was that? Well, I'm going to suggest
one idea in a moment. But in order to tackle
problems of that sort and to get back to this big question, what happened before the big bang, then we need to sort of
enlarge the discussion a bit and the point being that if the big bang, what if the big bang were not the ultimate origin
of all physical things? Not the beginning of space
and time as I've told you? Supposing there was
something there before it not the universe we know and love but nothing anyway, then we would have a very
different set of issues to address. And if the big bang was a natural event and most scientists prefer
to think of it as that and not a supernatural event, then it's very odd natural
event that happens only once. So you might expect there to
be bangs going off all the time scattered throughout space and time. And so maybe our universe is or the universe, what we call the universe nothing of the sort it's just our universe it's just a microcosm in this much larger a
more elaborate system which has come to be
known as the multiverse. The multiverse of many bubbles. And so the picture, I like to say the picture that is favored among the type
of people I have coffee with is that our universe is
just one bubble among many, is this just the realm of science fiction? Well, some people think
so but it's not quite that because it's one thing to just say, well, there are many big bangs
many bubbles and away you go. There has been some effort to discuss universe generating mechanisms, that is that there are laws
of physics in this multiverse and these laws can give
rise to bubble universes and that each universe, each bubble would have a beginning like a big bang and they would evolve
over a period of time and then maybe have an end as well. And so our universe would
have this life cycle and then the inevitable question is, where are these other bubbles? And is there some way of accessing them or knowing what they're doing. Now this question about
the origin of our universe and the origin of our bubble can't be disconnected from a deeper issue as one of the great unsolved
problems of the universe which is where the laws
of physics come from. It's all very well saying
the laws of quantum mechanics permit a universe to pop
into being from nothing which I said earlier but did those laws of quantum physics somehow exist before the
universe or outside the universe? How do we think about that? So again when I was back in
those early days in Cambridge the view was that the laws of physics were somehow imprinted on the universe from the get-go like the maker's mark. It's just like a hallmark
bump, there are your laws. Away you go. And then in the multiverse version, each one comes with its own set of laws and they'd be different. That's the hypothesis. And that they might well
be randomly different just sort of spread around differently so the laws that apply in our universe will be different in one
of these other bubbles. And the reason that that
appeals to many cosmologists is because one of the oddities, one of the unexplained
mysteries, cosmic mysteries, about our universe is that it is extraordinarily
well suited for life for the emergence of life. And let me explain that. Supposing you didn't like this universe and you wanted to play God
and change a few things and imagine you have a
machine in front of you like a designer machine
with 30-something knobs and you twiddle this one and
all electrons get a bit heavier and you twiddle that one and the weak nuclear force
gets a bit weaker and so on, well, we can't afford to
do the real experiment but you can do the thought experiment, you can do the calculation and it's been known for a long time it's like actually Fred Hoyle who started this whole line of reasoning that there are some things that look like if you twiddle
the knob by only a tiny bit, it would be lethal. There will be no life. That there could be no
possibility of life. And that looks suspiciously
like a universe that's sort of rigged in favor of life, scientists don't like that idea and so if there are many many universes with randomly distributed laws and yes we're winners in a cosmic lottery that here and there just by chance the cosmic cookie would crumble
in the appropriate manner and life would become possible. And here we are in ours and it's no surprise that
we we live in a universe that can support life, obviously, we couldn't
live in a sterile universe. And so that's the sort of
favorite idea of this multiverse why it's so appealing. But to get back to that, scar
that blemish in Eridanus, it's been suggested by Laura
Masini Howton a cosmologist that maybe this was a
collision with another bubble. Another bubble universe. And I'm gonna talk a little
bit more about this in a moment it's one of the things that can happen in this multiverse model. If the big bang wasn't
the ultimate beginning, there's something outside of our universe or before our universe we have to take into account
those sorts of possibilities. So that brings me quite naturally to talk about the end of the universe. There are many many things
in this book, I should say. I'm only giving you a subset. I'm trying to tie it
together into a narrative. Let me say something about
the end of the universe. So again this sort of
trip down memory lane, when I was a student
you had three choices. Either the universe would go on expanding and then it could expand so fast that basically everything would lose touch with everything else. Or it would somehow reach a maximum size and collapse back on
itself to a big crunch like the big bang in reverse. And so this is the sort
of big crunch idea. And then there would be this dividing line this curve c which was sort
of exactly on the border, you couldn't really tell what the ultimate fate
of the universe would be. And the observations really did
suggest that we were sitting on this dividing line very frustrating, couldn't say what it is but the inflation theory
explained that very nicely that that's exactly where
we should be sitting. And so that predicted that the universe would go on expanding but at a decelerating rate and eventually become cold dark and empty and really really boring. And that would go on for all of eternity. And all this was transformed
in the late 1990s when astronomers discovered
to their consternation that the rate of expansion of the universe is actually speeding up. That it looks more like this with this curve going off and up and up. So the reason it curves
downwards like this in the early stages is because
gravity is a pulling force so it acts like a break
as the universe expands. But now if it is going up and up and up, that looks like something
like anti-gravity, that looks like something very different. And that leads me to talk about the dark forces of the cosmos. If you look at the cosmic pie, normal matter, that's the stuff that you,
me and the stars are made of makes up only about 4%
of what's out there. About five times as much as that is made up of some other form of matter, we don't know what it is
it's called dark matter, partly because we don't know what it is but partly because it doesn't shine. And this dark matter could be, the favorite idea is
that these are particles, perhaps very heavy particles may be much heavier than the proton but interacting so weakly that they just pass straight through us. And if this is true, even
as we're sitting here, these ghostly dark matter particles are just traversing
our bodies continuously and we don't feel a thing. It's not a creepy
sensation though, isn't it? To think it's going through us but we know this is true of neutrinos. Neutrinos are incredibly
penetrating particles that we can detect just about. They're very weakly interacting. They go straight through the earth. You see and they go straight
through you as well. Just very occasionally you
can stop one and study it. That's how we know they exist. So the plan is that the dark matter might be like neutrinos much heavier and that would make up that. We know it's there, this dark matter, because it exerts a gravitating effect and part of the clustering
that I showed you, the clumping together
is due to the existence of the pull of that dark matter. So you can sort of measure.
You can see it's pulling. So just for example the milky way galaxy the stars on the periphery are orbiting around much too fast for the amount of material we can see. The visible matter. It's
got to be dark stuff. Otherwise it would unravel
like an exploding flywheel. And so we know the dark matter's there. But fully 75% of the
stuff that is out there is not even that. It's called dark energy and this causes confusion,
dark matter, dark energy. Dark is a word that astronomers like and now I've noticed 'cause many years ago biologists said, oh the physicists are so clever, they get it, there's a mystery and they call it dark
energy dark matter and so on and we don't understand DNA
and we call it junk DNA. Well, I've now noticed
they're starting to talk about dark DNA. So they've learned to trickle to. Now, this dark energy is what explains the accelerating
expansion of the universe. And this is an idea that
goes right back to Einstein who gave us our best
understanding of gravitation space and time. And he did that back in 1915 with his so-called general
theory of relativity. It was a total triumph
of the human intellect. A wonderful theory and
right up to this day, there is no known contradiction with the predictions of that theory to the extent they can be measured. The most recent triumph, won't have escaped your attention I'm sure was the discovery
of gravitational waves predicted by this theory, predicted by Einstein over 100 years ago and just recently discovered, opening up a whole new
window on the universe, a gravitational astronomy doing with gravitational waves what optical and radio telescopes do with electromagnetic waves. And so the general theory
of relativity is it when it comes to describing the universe and in 1917, Einstein introduced the idea of something we would
now call dark energy. And he did this for all the wrong reasons he thought the universe was static and if it was static why
didn't it all fall down into a big heap? What was keeping all the
galaxies or the stars out there if everything was pulling
on everything else? Newton worried about that as well. And that's a whole different story, I don't have time to get into but Einstein wanted to fix it up by inventing an anti-gravity
force, something like this. So normal gravity diminishes with distance like the famous inverse square law and Einstein said in addition
there's this other thing that gets bigger with distance. And that you could balance the
two, the anti-gravity force you wouldn't notice much at short range, it's not very strong but
over cosmic distances, it becomes big enough to rival
the weight of the universe and then you could hold
the whole thing static. Well, it was much his chagrin later on when he went to the
United States met Hubble, he discovered the universe
is actually expanding and he realized that this was a blunder, the worst blunder of his life, he said because had he not
introduced this extra term in his prized field equations, then he would surely have concluded the universe must be expanding
'cause it's not collapsing. Hasn't collapsed. But he missed the trick. But in spite of the fact
Einstein introduced it for the wrong reason, turns
out he was right all along. There is an anti-gravity force. Whether it's just this
term in his equation or something more subtle
or some type of field, we don't know, it does make a difference but in case you're getting
excited about anti-gravity, it's not much help for this
type of thing I'm afraid because the total amount
of force on a human being tends to the minus 26 grams. So we don't notice it locally. Now, so what then is this lambda term, I've showed in the graph, what is this antigravity? How do we think about it? Well, it is nothing more. It's very simple to think about it's nothing more than
the weight of empty space. Just that. How much does space itself weigh? And you might think, well,
why should space buy anything? 'Cause there's nothing there. But as I've explained
to you that's not true. There is something there there's
all this quantum activity going on. So how do you weigh
space? How do you do that? You obviously can't put a
box of space on a balance like I'm showing here. The only way you can actually weigh space 'cause it's everywhere is in cosmology from
the universe as a whole. And so astronomers can measure how fast the universe is expanding, how fast it's picking up in speed and that gives you a number
for the weight of space. Now you might be thinking,
well, hang on hang on. If space has weight, shouldn't
that contribute to gravity? Shouldn't that be a pulling
force like everything else? And the answer is yes, that's true but in addition to having energy and e equals mt squared so
weight, energy, mass, weight all using them interchangeably. In addition to that there is pressure in Einstein's theory of gravitation. Pressure is also a source of gravitation. We don't normally think of that. We think of the mass of the earth keeping our feet on the ground. The pressure of the earth, we think doesn't contribute to that. But it does a tiny amount.
But it certainly does. And where you have a lot of pressure, then you have an enormous
gravitational effect. And in this particular way of introducing this anti-gravity or dark energy, the pressure is negative. And negative means anti-gravity. And it outweighs the
positive effect of the energy by a factor of three. So the net effect is to
give you a repulsive force. And so the next slide just shows very, in a pathetic way really the
idea of this quantum frolic. You have to very carefully with particles that sort of flit in to and
out of existence like that. And then you can say well,
let's do a calculation to see how much energy
or weight is contained in that quantum vacuum. And I was talking earlier about this was just the sort of stuff Tim Bunch and I were
doing back in the 70s. And it's easy enough. It takes two minutes to sit
down and do a calculation how much does all this vacuum, quantum vacuum stuff end up weighing. And when you do that, what you find first of all what the astronomers measure is to just to give you a number in a cube, 100,000 kilometers across
the total weight of space is about one gram and
when you go to the theory, what you find is the density is about 10 to the 90 grams
per cubic centimeter. That's what the theory gives you whereas expressed in those units it's more like 10 to the minus 30. so this is a bit of a discrepancy Stephen Hawking said it's the
greatest failure of theory known to science. So if you try to deduce from
theoretical foundations, what this dark energy is,
that's as far as you could get. So we're sort of stuck on that and that's one of the great
unsolved problems of cosmology for any young people
listening to this lecture that are thinking of
getting into the subject. There's plenty still to be done and that's one of the
outstanding questions. Now to pick up the story
at the end of the universe, I'm just trying to think when
is the end of the lecture getting rather close to it. Let me just say that if the
dark energy remains constant then the universe will just
go on expanding more and more and more until it becomes
dark cold and empty but rather faster than it
did in the old picture. But there's another alarming possibility if the dark energy
increases in its intensity then the rate of expansion can go literally through the roof. It can just escalate
until it becomes infinite. And at that point space-time
would cease to exist because it would be
expanding infinitely fast. And that goes by the name of the big rip and what it means is that the
universe would end its days not in a crunch where everything collapses
and space-time comes to an end in a high dense phase,
this would be the opposite. It would be ripped apart. Of course whether it's really is the end, it's like what happened
before the big bang. We don't know because
we're not confident enough in the application of our of
quantum physics to gravitation to be able to answer what
happens after the big rip or after the big crunch
or before the big bang. There are many possibilities. But if the big rip is gonna happen, it's not gonna happen anytime soon. We're talking billions and
billions of years in the future but meanwhile, there are other
things that can be happening to accelerate the end of the universe and to pick up the the
title of the lecture, what's eating the universe, I did already mention the possibility of the cold spot in Eridanus being something that bumped into us, is it possible our universe
could be swallowed wholesale by another universe? And is that something
we should fret about? So I've just got a list of
death by devourment here. We know that the universe is being eaten from the inside out by black holes. Black holes follow everything
that comes near them and there are monster black holes at the centers of galaxies. And I'm talking about millions
or billions of solar masses. And over time these are swallowing prodigious amounts of material. So the universe is eating
itself from inside out but it could be being
eaten from outside in. It could be swallowed by another universe. There's another horrible consequence that comes out of the physics
I've been talking about and it's the same thing as
I've just been discussing, this quantum vacuum. So we're pretty confident that empty space has this vacuum energy but is it the lowest
possible energy you can have? Is it conceivable that there is a state, a quantum state with lower energy than the one that we have in our universe? And if that's so, just like everything else
in quantum mechanics, there's always a probability
it can make a transition just like an excited atom, it's a probability it would
de-excite and emit energy. Could the universe de-excite? Could it tunnel in some
way into this lower energy quantum vacuum state? And if so, what would happen
to the energy released? And the calculations suggest, it's been known for some decades, that this energy would be
concentrated in like a shell wall or a bubble wall, it'd be another bubble, it's a bubble that would expand out of very close to the speed of light and and basically devour
the universe that we know. Would swallow everything
up and transform it not into nothing but into this
lower energy quantum state. We can only guess that. And so that could happen at any time. You know, any time this
bubble could arrive and would annihilate us faster
than the speed of thought. There's another really weird idea that goes back to the work
of Ed Whitton in Princeton which is not that the
universe would be transformed by a bubble or swallowed
by another universe but it would be swallowed
by bubbles of nothing. Nothing at all. And I've been at pains to point out that there's a big difference
between empty space and literally nothing. And we can imagine that just
like in the simple picture before the big bang there was nothing, in the multiverse there was something but in the simple picture
there was nothing. Maybe there are bubbles of nothing that can grow so it's a
bit like a Swiss cheese where the cheese is space and the holes are bubbles of nothing. And then it's as if the
holes grow and grow and grow until there's no cheese left. That's another possibility
that's discussed. And it's not there, I mean
there are many other ways that the universe in its
present condition could fail. Space-time itself, many
physicists think that space-time is not a primitive entity, it's not the one of these ultimate things that goes in to making a universe. It might itself be an emergent feature, it might be something that comes out of some
sort of pre-space-time or pre-geometry, something like that and it maybe is being held
in a state of equilibrium but if something destabilizes it, space-time itself could collapse. I think I have we started
like a minute late I just want to raise but
not address in detail 'cause we can deal with
it in question time, raise the other big question that I've been very much associated with. Are we alone in the universe? Is there any life out
there, any intelligent life? I mentioned at the beginning
about canals of Mars, we don't take that seriously but we must be open to the possibility that there's not just life
but maybe intelligent life elsewhere in the universe. I'm actually personally
very skeptical of it but I think it's a really
important question to ask. And there's an entire field called SETI search for extraterrestrial intelligence started by Frank Drake
the astronomer in 1960 addressing this question,
is anybody out there? And the reason I think that we
find that very hard to answer and I'm out of step with
my colleagues on this one, Frank wrote down this equation, it's called the Drake equation. With all the terms you need to know to estimate the number of
communicating civilizations in the milky way at this time and if you look at that equation, the first one is the
rate of star formation and then the next one is the
fraction of stars with planets and then the next term
is the number of planets that are earth's like. And those things when
frank wrote this down, those things weren't known very well. But we now know them rather precisely. So that is fine but then we hit a term where
we don't know what to say because f sub l is the
fraction of earth-like planets on which life emerges. And when I was a student,
the prevailing view was that number would be exceedingly small that life is so complex that so specific that it would require a dream
run of chemical reactions for it to happen. And the chances of that
dream run happening anywhere else in the
universe were infinitesimal. Francis Crick summed it up, he said, life seems almost a miracle so many are the conditions
necessary for it to get going. So this is the big problem,
how did life begin, was it a bizarre fluke, is it
somehow built into the nature, is the universe rigged in favor of life, is it built in to the nature
of chemistry and physics? We don't know. Darwin himself said,
it's a wonderful quote, it's mere rubbish thinking
at the present time the origin of life, one might as well think
of the origin of matter and I always fond of
saying well we physicists have now explained the origin of matter but we haven't explained
the origin of life. And if you don't know what
turned non-life into life, you can't work out the betting odds. You can't work out the
probability it's going to happen. So we're absolutely stuck. The universe might be teeming with life uh but it might be that we
are the only planet with life. Which is very sad. I'd love to believe
there's life out there. But we don't know. I'm gonna bring this to a conclusion. I've got a lovely quote
here by Alfred Wallace. He was Darwin's rival came up
with the theory of relativity and he wrote a book about, called "Man's Place in
the Universe" in 1904 and he was also of the
view the improbabilities of the independent development of man sort of cranes and politically
incorrect terminology I apologize, his words not mine even in one other world and now shown to be so great
is to approach very closely be actually impossible. Having run out of time, I'm going to just skip over Jacques Mano and Christian de Duve's notion on this and just say that I regard the
biggest of the big questions as the fact that the
universe is comprehensible, the fact that we can even
come to understand it uh by applying this wonderful
thing called science and mathematics that the human intellect is
able to make sense of the world through the scientific endeavor. Uh And this is I think really important, I said that some people
like the multiverse idea because it explains why our universe looks so friendly for life
because that's what we observe but we're more than observers. uh Einstein being a classic example that we don't just see
the world around us. We've come to comprehend
it and Einstein felt that that was the most incomprehensible thing about the universe that
we can comprehend it. And I agree with that. And I began with a quote
from Michael Faraday so I'm gonna leave with a
quote from Michael Faraday, it's rather nice, he says
lectures which really teach will never be popular
lectures which are popular will never really teach. Well I hope this evening I've given you something
which is a bit of both. I hope it's been an enjoyable experience but I hope you've
actually learned something and I'm very happy to move
on and take questions. So thank you. (audience applause)
