[MUSIC PLAYING]
So I am completely
delighted to be
able to tell you about
our magnificent universe--
my favourite thing.
And I want to tell
you about our quest
to try and understand
this bigger
thing that we are a part of.
And the reason that I
find it so compelling
is that it's our home.
We are here on earth.
But we're part of
something much bigger.
And the stuff that--
it's out there-- is
part of our home, too.
And it's also our story.
If we think about how we
on earth came to be here,
we came to be here
through this long process,
this bigger process
that's embedded
in the story of our universe.
So those two things bring
it into our home here.
And so I'm a
professional astronomer.
I try and ask questions
about the universe.
Those are the questions
that I find most compelling.
It's what's drawn me
to study the universe.
And the kind of
questions I have,
and the questions that I talk
about also in my new book,
are these.
I want to know what is space?
And what is our place in
it, our place here on earth?
I'd like to know if
there's life elsewhere,
which is such an exciting
question that we're
making huge progress on.
I'd like to know if the
universe had a beginning,
and what that even means.
And I'd like to know
what, if anything,
lies beyond our
laws of physics--
the things we
understand already.
And so as an astronomer, we
have to kind of pick the thing
that we're going to spend our
time particularly researching.
And my area that I particularly
research is cosmology.
And cosmology is the
area of astronomy
in which we answer questions
about the whole universe,
on average.
We don't worry so much
about the details.
And we ask questions
like how old is it?
And what is it made
of on the whole?
And is it growing?
And what's going to happen
to it in the future?
And this is my telescope--
my team's telescope-- the
Atacama Cosmology Telescope.
It's one of the
highest in the world.
It's in the northern deserts of
Chile, in the Atacama Desert.
And it's 5,000
metres up in the air.
And we use it to stare as
far out into space and back
in time as we possibly can.
And I use it to
try and understand
the origins of the universe.
But we're going to start a
little closer than the most
distant parts of the universe
and a bit closer to home.
Because I want to
start by taking you out
through our home, our
universe-- from the place
that we're most familiar
with out to the far
reaches, out to the far edges.
So our home in the
universe, the thing
that we are most familiar with,
the thing that we know we're
a part of is the solar system.
That's the thing that
we learn about first.
And we know we've got
our eight planets--
used to be nine--
eight planets and a
whole bunch of rocks
orbiting around our wonderful
sun, the source of our heat
and light.
And one thing I find astonishing
about our solar system
is actually how big
and very empty it is.
If you were to imagine
fitting the solar system
into this hall, into this
room, and you imagine Neptune,
for example-- there's
stuff beyond Neptune.
But let's imagine Neptune
circling around this room,
around the edge.
Then the sun in the
middle of this room
would just be the size of
a peppercorn, just a couple
of millimetres across.
And the earth, which
is 100 times smaller,
would just be a speck of dust.
And the rest of
that whole space,
the thing that we
think of as kind of jam
packed full of planets, is
actually completely empty.
These things are tiny.
And the solar system
itself is pretty big,
although we're
going to go bigger.
Out to Neptune is about
three billion miles.
But these numbers start to
become a little bit intangible.
And what we like
as astronomers--
to make things manageable.
Space is really big.
And it can kind of overwhelm you
if you don't come up with ways
of trying to handle them.
And so one thing
we use a lot of is
using a measurement of
distance that is how far light
can travel in a given time.
So light is the fastest
thing we know of.
It travels at this incredible
pace, at 200,000 miles
a second.
And we think of how long
light takes to reach places.
And so it takes about
five hours from light
to get from Neptune into us.
And that's compared,
for example, the moon,
our closest neighbour, light
takes about a second to reach
us from there.
So Neptune really
is pretty far away.
It's kind of phenomenal
that we can send out probes
through the solar system to
go and visit these places.
And we've known, actually,
how big the solar system
is since the 1760s,
when there were
these marvellous expeditions.
I would say the first big
international astronomy
collaboration took place
when teams of astronomers
from around the world went on
these phenomenal expeditions
to different
locations around Earth
to go and observe the transit
of Venus across the sun.
And by observing it
from different locations
on the Earth, they were able
to judge how far away Venus
is by watching the
different path of the planet
across the sun's face.
And they got the measurement
really remarkably well,
about 100 million miles
from us to the sun.
And that's set the scale
of the solar system.
Now I said we can go
and send probes out
into the solar system.
But that's as far,
really, as we can go.
There are wonderful ideas
about going out to the stars.
But really, they're
really far away.
It's realistically--
we are going
to have to only
sit here on Earth
and look if we want
to go out further.
So the nearest stars to us,
our very nearest neighbour,
is Proxima Centauri.
And it's light takes
four years to reach us--
four whole years since the light
set off and reaching our eyes.
Most of the stars, the beautiful
stars that you see above us,
are tens of years, hundreds
of years, thousands of years
that light takes to reach us.
And if you think about
the Orion constellation,
the beautiful stars
in Orion's belt,
the light from those set off
a thousand years and have been
travelling for 1,000 years.
And just now you
see them in the sky.
And so we have this sort of
cocoon of stars around us.
And are this incredible
distance away.
And now we've known about
how far those are away
since the 1800s.
And the way we kind of
figure out where they are
uses this neat method of
parallax, which we're going
to actually all try right now.
So let's say I wanted to
measure the length of my arm,
but I couldn't be bothered
to measure it actually
with a ruler.
So there's a different
way you can do it.
You can use a thing
called parallax.
Which is you hold out
your arm in front of you--
and let's all do that now.
Yeah, good.
And I want to measure the
distance to my finger.
Now close one eye and
see where your finger
is compared to the wall behind,
or the people behind you.
And now close the other eye.
You should see it move.
Yeah?
Good.
Now bring your arm
closer to your face
as if you had a
really short arm.
And now do it again.
Move further.
Good.
OK, that's parallax.
So here, the farther it moves,
the closer it is to you.
And if I know the distance
between my two eyes--
a few centimetres-- and I know
the angle that my finger moves,
then I can use simple right
angle triangle trigonometry
that many people have learned
in school to figure out
the length of my arm,
maybe 60 centimetres.
And that's all very well--
not so useful for the length of
my arm, but really useful for
stars.
Because I can't get to a star.
But what I can do is
I can do parallax.
So for stars, the
length of my arm
now becomes the distance
to a distant star.
And my two eyes now become
the earth six months apart.
So what you do is
you sit on earth
and you look at a star against a
backdrop of more distant stars.
And then you wait six
months for the earth
to go around the sun--
halfway around.
And you look at it again
from the other eye.
And you see what angle
it moves through.
And the bigger an
angle it moves through,
the closer it is to you.
And so now the distance
between your two eyes
becomes twice the distance
of the Earth to the sun,
about 200 million miles.
And so it's a really
pretty ginormous triangle.
We love triangles in astronomy.
We love enormous triangles.
So this gave the first
measurement to the stars,
again, back in the 1800s.
And this told us how
far away they are.
Now the stars around us are
actually just part of something
bigger.
And we're going to keep going
out until we get to the edges.
Our little group
of stars around us
are just part of a much
vaster thing, which
is the wonderful galaxy that we
live in, the Milky Way galaxy.
And that's a collection
of about 100 billion stars
that are all gathered together
by the force of gravity
of those stars
pulling them together
into this wonderful
swirling desk.
And sometimes, when we're
lucky here on earth,
we actually get to see it.
And this is an
image up here of how
we would see the disc
of the Milky Way galaxy
in the night sky.
Now from London, you
don't get to see that.
And I've only seen
it a few times.
And it's phenomenal.
So I urge you, if
you have a chance
to go to a really
dark sky, please go.
Because it is incredible.
And so what is this?
We're living in this big
disc, this disc of a galaxy.
And why do we see that strip
of light out across the sky?
This is where this mysterious
sauce pan lid comes in handy.
So imagine-- what we think we
have is we have this disc--
a swirling disc of stars.
And we are living in it.
And we think we're living in it
about half way from the middle
to the edge in our little
group of local stars.
And because we're kind
of embedded in the disc,
if I'm in the disc like
this, and I look straight
through the thick disc of stars,
then I'll see a band of light
exactly where that disc is.
But if I look out either
side, I see very little.
There aren't very many
stars in those directions.
Or conversely, if I looked
across that direction,
I'd see a band of light, too.
So this band of light that you
can see in the sky that we call
the Milky Way-- and it's called
the Milky Way because it looks
like Milky Way, right--
is just us being inside
the disc of stars
and seeing a bright light
where you see that edge band.
But how do we know--
and it's big
enough-- so we think
now-- we have pretty
good measurements--
that the size of this disc
of stars that we live in
is about 100,000 light
years from side to side.
So if you set light
off on a journey,
it would take 100,000 years to
get from one side of our galaxy
to the other.
And so let's just remember--
so if our group
of stars around us
were just the ones like tens
of light years away from us,
then we would be like,
again, a little peppercorn
bobbing around the galaxy
as it spins around.
And the whole thing spins
around every 200 million years
or so, this giant
gradually rotating disc.
But then, we might
ask a question,
well how do we know even
the extent of our Milky Way?
Because again, all we get to do
as astronomers is we sit here
on Earth and we look.
And that's all very well.
But inferring where things
are when all you can see
is this kind of two
dimensional surface around you
can get quite challenging.
And the key to this
came from this fantastic
astronomer, Henrietta Swan
Leavitt who I would say
is not nearly famous enough.
Because she did
this awesome thing.
She was an astronomer working
at the turn of the last century
in the states.
She was working at Harvard
College Observatory.
And she started working
there in the late 1890s.
And she was part of this amazing
group of women astronomers
known as the Harvard Computers
who were hired by another
astronomer, Edward Pickering,
to analyse photographic images
of stars that were taken
by their male colleagues--
because as a woman at that
time, you weren't allowed to use
a telescope--
god forbid.
And he gathered this group of
women to study plates after
plates-- photograph plates of
images of stars to classify
them, to measure
their brightnesses,
to measure their colours.
And it was really hard work.
And they weren't really
allowed to do what they wanted.
They were just kind
of set to work.
They weren't paid very much.
But they turned out to be
really quite good at it.
And she discovered
this amazing property,
which is there were
these stars that varied
their brightness with time.
So most stars--
kind of a constant,
it's just twinkling in the sky.
And their brightness
is constant.
But some of them vary.
And there's this
particular class
of stars called
cepheid variables which
actually pulsate over time.
They grow and shrink.
And as they grow and shrink,
they change their brightness.
And she discovered this
pattern about these stars which
was that the longer
they took to pulsate,
the brighter they
were, intrinsically.
And so a star that took
weeks or months to pulsate
was much brighter than
one that took just days.
And this actually proved
to be the key to unlocking
the whole scale of our galaxy,
and actually our universe,
as well.
Because there's this key thing.
If there's this relationship
between the time
a star takes to pulse and how
intrinsically bright it is,
then you can figure out how
far away it is from you.
Because the problem,
usually, in astronomy
is that if all stars were the
same brightness as each other,
intrinsically, if everything
was like a 100 watt light bulb,
then you could figure
out where things
were in space just by saying,
how bright did they look?
You know, a standard
100 watt light bulb,
if it's closer to you,
it looks brighter.
If it's further away,
it looks dimmer.
But if you don't know how
bright it was to begin with,
you can't tell where it is.
But these things,
these cepheid stars,
if you could only measure their
timing of how quickly they
pulsed, which is a relatively
easy measurement to do,
you then know intrinsically
how bright they are.
And then you measure
how bright they seem.
And you can figure
out where they are.
And so Swan Leavitt made
this discovery in 1908.
And then really quickly
astronomers took this law--
it's now called Leavitt's law--
and figured out how
big they thought--
how big the Milky Way galaxy is.
This is an artist's impression
of what we think it looks like.
We'll never be able to
take our own picture of it,
because we can never
get outside it.
It's just much too big.
There's no way we
could ever climb out.
So we think it's these swirling
arms of stars swirling in.
And the darker bits
are where we think
we have sort of debris--
we call it dust,
cosmic dust grains that's
intermixed with the stars,
and also gas swirling
in these spiral arms
to a more central region
of brighter light.
So that's our Milky Way galaxy.
Now until 100 years
ago, astronomers
thought that that was it.
That was our entire universe.
That was the edge,
nothing beyond it.
And actually, it was this
measurement, this discovery
of Leavitt of this
relationship, that
allowed this guy, Edwin Hubble,
to figure out that there
was actually something beyond.
Because 100 years ago, there
were these-- astronomers
had seen that in the sky they
had the stars of the Milky Way,
but there were also
these smudges of light
that no one really
knew what they were.
They were called nebulae--
clouds, basically, of light.
And they weren't identified.
And then Edwin Hubble--
Edwin Hubble-- it's hard
to find a picture of him
without a pipe.
He was he was working
at the Mount Wilson
observatory in California doing
incredibly hard-- back then,
telescopes were much harder
work than they are now,
because you would have to
track stars kind of all night
from the telescope, following
them with your telescope.
Now it's all automated.
And the telescope kind of
follows something for you.
But back then, you had to kind
of painstakingly track a star.
Anyway, he looked at
these smudges of light.
And he realised that there were
some of these magic cepheid
variable stars in those
smudges of light pulsating.
And he used the fact
that he knew then
if he could measure how quickly
they pulsated to figure out
how bright they were--
he realised that they were
far, far, far too dim.
They couldn't possibly
be in our galaxy.
They had to be
much further away.
So if, for example--
if, as we'd figured
out, that the galaxy
is 100,000 light
years across, these
were millions of
light years away--
much, much further.
And so this was this huge
opening up of our horizons
that he demonstrated that there
are indeed these other galaxies
far beyond our own.
And we can now--
he wouldn't have
been able to take these
pictures at the time.
But now we have these
magnificent images
of actual galaxies that
are different from ours.
So this is a real picture
of a galaxy far, far away.
And again, you can see
this beautiful spiral disc.
It would actually be
the shape of a disc.
We're seeing it like that--
with most of the stars in the
middle and these arms of stars
spiralling into the centre.
And so, if we now look out--
again, these are maybe millions
of light years away from us,
or then even further.
They're what we
step out, and we see
the kind of building
blocks of the universe.
And these things tend to
clump together a little bit.
So if we have a
bunch of galaxies,
they tend to like to
group together kind
of in small groupings like
cosmic towns or cities
with hundreds, thousands
of galaxies together.
So this is a nice example
of a galaxy cluster
taken by the Hubble
Space Telescope, named
after Edwin Hubble.
And here, what you're
seeing is a grouping
together of galaxies where every
one of those spots of light
is an entire galaxy,
each of those
with perhaps 100 billion stars.
And this is one of the largest
objects in the universe,
a galaxy cluster.
And I think it's extraordinary.
And so you might have hundreds
to thousands of galaxies
all joined together.
And so these-- we are
now we now on scales
of hundreds of millions
of light years,
these giant objects
in our universe.
And then if we look out further
still, we find more of these.
We find our universe full
of galaxies grouped together
into these clusters of galaxies.
And in fact, there's
an even one level
still up which is, maybe
unimaginatively, called
a supercluster which is where,
again, the local gravity
kind of draws some
of these big clusters
together into a grouping.
And so we live in
one of those, too.
We live in-- in terms--
we don't actually
live in a big galaxy cluster.
We live in a small
group, the local group
which is only three big galaxies
and some little ones, as well.
We live in a relatively small--
we're in a small town.
But we're kind of--
we're pulled together with other
clusters of galaxies around us.
And we thought for
a while we were
part of the Virgo supercluster.
But then quite recently
some astronomers
said, actually,
there's more that's
been kind of pulled
gravitationally
by gravity towards us, which
is the larger supercluster
called Lanae Archaea.
We're not quite sure
which we're part of.
It might be Virgo,
might be Lanae Archaea.
It's quite hard to
tell, because gravity
is kind of pulling stuff in.
But other things, which
I'll come to later,
are pushing stuff away.
But then we've
stepped out, really,
to the-- we can step
out now to the edges
of the observable universe.
So if we look out any
direction in the sky, beyond--
if we try and look
in a gap in the sky.
Don't look at a star.
We just see galaxies
as far as we can see.
So this is just a snapshot
from the Hubble Space Telescope
of a tiny piece of sky where
every one of those spots
of light is a galaxy.
And so we can see out to
this incredible distance.
But it is actually
a finite distance.
So there is this thing that we
call the observable universe.
And it's a funny
concept, because the idea
is that there is actually
only a finite part of space
that we can actually access.
And that's because we actually
think that the universe has
a finite age.
It hasn't been around forever,
and we'll get to that.
But the fact that it has
a finite age, which now we
think is about 14
billion years old,
means that there's an
edge to what we can see.
We can only see
things now on Earth
that light has had time to
travel from since the beginning
of time.
So anything that's so far away
that in 14 billion years light
couldn't have reached us,
it's beyond our horizon.
It's beyond-- and we
call it a cosmic horizon.
It's beyond our horizon.
And it's outside what we
call our observable universe.
So we have this sphere
centred on ourselves,
with us at the
middle of it, which
is the observable universe.
And that doesn't mean that we're
at the middle of the universe,
right?
That's very important.
We're just at the middle
of the part we can see.
And the distance to that
edge, the size of that--
you might have thought
that the size of that,
the distance to the
edge of that would
be 14 billion light years.
It turns out to be actually
further-- about three times
further.
Because space has actually
been growing during that
time, which we'll come back to.
It's nearly 50
billion light years
to the edge of the
observable universe.
And there's this kind
of remarkable aspect
of doing astronomy and
looking out that far
which is that it's kind
of like this time machine
aspect of astronomy--
which is that the further
you look out into space,
the further you
see back in time.
So we sit here at the middle
of our observable universe.
And we look out
through increasing--
the further we look out, the
further back we're seeing.
So the part of the universe
we can see nearby us,
we're seeing quite recently--
as it was quite recently.
But the more distant
bits, we're seeing as they
were quite a long time ago.
And the most
distant parts, we're
seeing as they were
billions of years ago.
So it's a bit confusing,
but very useful.
Because we can't see--
we kind of think that
everywhere in the universe
has evolved kind of the same.
So if we could see very
far away right now,
we think that it's doing the
same as we are, more or less.
But we see it as
it was in the past.
And it's frustrating,
because we can't
see how the whole
universe looks like today.
We just can't do that.
But it's really
useful, because we
get to see how different
parts of it were in the past.
And so by seeing how different
parts of it were in the past,
we can kind of
piece together how
we have come to be here now.
Because we can see
things as they were.
It's a bit like if
you were to see--
if you were to be an alien
coming from a different planet.
And you wanted to understand
how humans have grown.
Then if you give them a room
full of eight-year-olds,
they might be able to figure
out what humans look like.
But they wouldn't
have a good sense
of how they were to evolve.
But if you gave them a room
full of a whole diverse range
of people--
if you gave them some
babies, and some kids,
and some grown-ups,
and some older people,
then you would actually be able
to reconstruct much better how
humans evolve and grow.
So actually, this
idea that we can
get to look at
distant parts of space
as they were a long time
ago is really useful for us.
We can kind of build a picture.
OK, so we have-- this is kind
of the tour of the things
that we can account for
most easily in the universe.
But soon we should be driven
to ask a question of-- this
is not the universe.
Well, it is in the universe.
We should ask the question,
which is, is the stuff
that we're seeing--
these galaxies-- is
that all there is?
Because if we look
at this picture--
this is an image of
the United States--
what you see there-- and if
you were astronauts flying high
in the sky looking down
on Earth at night--
what you would see of the
Earth is the bright lights.
You would see where there's
people living, where
there are cities and towns.
What you wouldn't
see is the darkness
of the land underneath, where
there's no significant amounts
of light.
And so the question
should surely come,
well, I'm looking
out into space.
And I'm only seeing
the bright lights.
What else is there?
And it turns out that
there's quite a lot.
And it's been 50 years since
actually we've had some idea
that there's more to the
universe than meets the eye.
And a big part of
this discovery came
from this great
astronomer, Vera Rubin,
who is pictured here looking
at an observation of using
this thing called a spectrograph
to look at galaxies.
She's an amazing astronomer.
She sadly died a
couple of years ago.
But she made this
great discovery
about the contents
of our universe.
So she is kind of an
interesting character.
She was a great
of both astronomy,
but also a promoter
of women in astronomy.
She had a pretty hard
time starting out,
because she-- well,
a challenging time.
She did her degree in
physics or astronomy
and wanted to go and
do graduate work.
And so she applied to do a
PhD at my current university,
Princeton University,
in the late 1940s.
But she wasn't accepted,
because she was a woman.
And they didn't take
women at the time.
So, undeterred,
she went to Cornell
and successfully studied there.
But then she moved to follow
her husband to Washington,
to Georgetown.
And she had to complete
her studies while looking
after their young children.
So she would go to
lectures at night
after looking after her
young kids during the day.
But nonetheless, that was fine.
And she then-- her big project
that she wanted to work on
was to look at the
motion of galaxies,
to understand how they move.
And she realised that to look at
them in great detail she would
need to be able to use a big
enough telescope to observe
the fine details within
distant galaxies.
To do that, you need a telescope
with a really big mirror.
The bigger mirror you have, the
higher resolution you can see.
And at the time, the
best one available
was at the Carnegie
Observatory--
sorry, the Palomar
Observatory in California.
And so in 1965, she
applied to use it.
But they said, no, only
men can use the telescope.
But she persisted.
And one of the reasons she was
given was there no women's--
there were no facilities,
no bathrooms for women.
She's like, this is ridiculous.
And so she cut out a
triangle of paper--
a skirt-- and pasted it on one
of the men's bathroom doors.
[LAUGHTER]
Like-- her bathroom.
So she was the first
woman to ever use
this magnificent telescope,
this five metre telescope.
And she worked with a
colleague, Kent Ford,
to build this spectrograph.
What does that mean?
It means they were looking
at light from galaxies,
but they were breaking it up
into the rainbow of different
colours from red to purple to
look at the light in different
wavelengths.
They wanted to see how fast
galaxies were spinning,
and in particular, how fast
stars were moving around
in the galaxy.
And you can do that--
so let's imagine, again,
this is my galaxy.
And it's spinning around.
And I want to see--
the stars on one side,
if you see it edge on,
will be moving towards you,
and the stars the other side
moving away.
And if you want to know how
fast it's spinning around,
then you can go and
use the Doppler effect,
which you've
probably come across
with a siren from a
police car, for example.
If something's moving away from
you, the signal coming from it,
be it sound or light, has
its wavelength lengthened
as it moves away from you.
So the signal, the pulse
rate, or the distance
between the peaks of a
signal, the wavelength,
increases as
something moves away.
So for sound, that
lowers the pitch.
And that's what you hear when
a police car goes past you.
You hear it lower pitch.
And with light, that
lengthens the wavelength.
And it shifts the colour
more towards the red end
of the rainbow, which has a
longer wavelength and the blue
or purple.
And conversely, if something
is moving towards you,
it squeezes the
wavelength shorter
and moves it more to
the blue or purple end.
So with a spectrograph, you
can break the light up from
a galaxy into its
different colours.
And you can figure out how
fast it's moving around.
Now why is that interesting?
Well, the speed at which
something moves around
is just connected
to how much gravity
there is in this thing,
how much mass there is.
So for example, the reason
why we orbit the sun
is because it's got mass.
And if we made it more
massive, we'd orbit it quicker.
So the heavier something is,
the faster something orbits.
So she went to look
at all these galaxies.
And she measured very carefully
how fast the stars in them
were rotating.
And what she found was
incredibly surprising--
she and Kent Ford.
And she found that they
were rotating much too
fast, and particularly the ones
at the edge of the galaxy--
about here.
They were really
going much too fast.
And the only way that you could
make sense of it going that
fast was if there was actually
some extra mass in the galaxy
that you couldn't see.
And it looked as if, actually,
the galaxy was actually
many times bigger than
the visible disc of light
was showing up, and with
maybe 10 times as much
mass as you could
see from the stars.
And so the picture of a
galaxy turned into something
that looks a bit like this,
where in the middle of there
is a tiny little blue disc
of the visible stuff--
that beautiful swirling disc
of stars that we maybe thought
was the whole galaxy.
And what Rubin's work
showed was that actually it
seems to be surrounded
by an enormous sphere
or halo of completely invisible
matter whose effect is
to make it all
spin around faster,
but it's completely invisible.
And Rubin actually realised that
this idea had been brought up
30 years before by this
astronomer called Fritz Zwicky.
He was a notoriously
combative but very clever
astronomer working
in California.
And he had looked at
this cluster of galaxies.
And he'd also seen this
behaviour of galaxies inside
the cluster moving
too fast around.
And he'd actually
written a paper about it
in German, bringing up
the idea that maybe there
was invisible matter.
And he called it, in German,
[SPEAKING GERMAN],, which
translates to dark matter.
And so Rubin realised,
made the connection,
that this is what
Zwicky had seen.
And now she and Kent Ford
were seeing it in all
of the galaxies they
were looking at-- many,
many galaxies--
same thing.
And this became established
that there is invisible matter
surrounding all our galaxies.
It's not just
surrounding our galaxies.
It's in them.
It's in here.
It's in this room right now.
And it seems that there's
about five times as much of it
as there is the stuff
that we're made of.
And we have no idea what it is.
It's quite worrying.
We kind of hope--
and I will say that this
happened 50 years ago now.
But the understanding
of it-- of where it is
and how it's behaving-- has
just advanced significantly
since then.
And we're now just
even more convinced
that it's there,
and perhaps even
less sure about what it is.
We had hoped that it might be
a particle, a particle that
gravity acts on but it doesn't
really interact with stuff much
at all.
So a particle of it would
fly through your body
without stopping.
And again, probably
some of it is right now.
Don't worry, though.
But we hoped that
it was something
that we called a weakly
interacting particle, which
means it doesn't interact much.
But it interacts just
a bit, just enough
that we could find
it in a detector.
And there were great hopes
that we could actually
create some of these
particles at the Large Hadron
Collider in CERN.
But we haven't.
And it's possible
that we still might.
But it hasn't happened yet.
So some of our most favourite
ideas about what it could be
have not shown up to be true.
So now it's one of our
outstanding mysteries.
It's there, but we
don't know what it is.
So maybe one of you
can figure it out.
So we've kind of-- this is
the set, I'm doing a sort of--
there's the visible
parts of the universe.
And this is a big
invisible part.
But there's another invisible
part that's very exciting,
which is planets.
Until 30 years
ago, we didn't know
if there were any planets
around the stars in the sky.
Astronomers thought
they probably were.
It would be kind of weird if
we were the only solar system
to have planets.
But they're really hard to find.
Because they're very dim.
They don't give out much
light-- no visible light,
just a bit of kind of
warm infrared light.
And they're very tiny.
So they're mostly
invisible to us.
But we've recently,
with better telescopes
and better techniques,
have managed to find them.
So in the 1990s, the
very first planet
was found orbiting an entirely
different star to ours--
just the first one.
And now today, we've
found thousands.
We've now-- and
many of those have
come from this wonderful
satellite called the Kepler
satellite.
And what I'm showing you
here is a still shot--
and I encourage you afterwards
to go and look it up online--
of a selection of the
solar systems that
have been found
around other stars,
other stars out in the sky.
Each one of those circles
represents a whole solar system
around a different
star in the sky.
And each of the dots on
the circles is a planet.
And they found them in
this really neat way,
which is that they were
using transits of planets.
If a planet transits
in front of-- passes
in front of its
host star, then it
will slightly dim the light
of the star while it passes.
And then the star will go back
to normal again once it's gone.
And so Kepler managed to look
for this very subtle dimming
of light and has found
thousands of planets this way.
And using what
they've seen so far,
they assessed that
probably a good fraction
of the stars in the sky have
their own solar systems.
And the most extraordinary
diversity have been found.
You know, there are planets
that orbit their sun
in hours or days--
orbit their star in hours or
days compared to our 365 days.
There are systems that orbit
two stars at once, for example,
or multiple stars.
And one of the most exciting
places that's going to be
the subject of enormous future
study is this place called
TRAPPIST-1, which this is just
an artist's impression of-- we
haven't taken these images yet--
which is found by
the Kepler satellite.
It's a solar system with seven
rocky planets only 40 light
years from Earth.
It's still quite far.
But in terms of observing
stuff, that's not that far.
It's one of our nearest stars.
And the planets-- again, these
are just an artist's impression
of what they look like.
We have not got
these images yet.
But they're there.
And some of them are thought
to have water on them.
And it's an ideal target
for future observations
with telescopes coming
up in the next 10 years.
This is incredible.
And there are many coming.
There are many coming
that are designed
to be able to study
these in more detail
and look for potential
signatures of life.
So this is a kind of whole
invisible part of this bigger
home we live in, which has
all of these rich planets that
are going around the stars.
And just because
they're tiny doesn't
mean they're not, of
course, fascinating.
We on Earth are much
tinier than our sun.
But I think we'd argue
that we are probably
more fascinating than our sun--
maybe.
We're a bit biassed.
But to me, that
wealth of what we
could find out quite soon,
in the next 10 years or so,
about what's out there in
terms of different planets
is really exciting.
So those are the things
that are in our universe.
But then we can come to thinking
about the story that brought us
here.
And I'm going to focus on the
big story, which is are things
changing in a big way at all?
Because if we look out beyond--
again, out into this bigger
universe full of galaxies,
has it always been there?
And 100 years ago, this
familiar guy on the left,
Albert Einstein, was
absolutely convinced
that the universe was
unchanging, that it is as it
is, that nothing was changed--
as it is now is as
it always has been,
that there could be no
beginning, for example,
and no end to the universe.
But he clashed with this guy
on the right, George Lemaitre,
who was an astronomer,
and also a priest
in Belgium, who argued
against him and said,
actually, I think the
universe could be changing.
And let's go and find out how.
And actually, this
guy on the right,
George Lemaitre, the
reason he thought
that maybe the universe on
a whole could be changing
and could have a
beginning actually
arose from studying carefully
Einstein's new theory
of gravity, the theory of
general relativity, which
explained how space
should behave.
And the idea here is if
you sprinkle galaxies
throughout space, they shouldn't
just stay there, stay still.
If you just sprinkle them
and just plunk them down,
then the gravity of
all those galaxies
should actually tend to want to
pull things all back together
again and actually
shrink the universe down.
Or, if it's already
growing through some means
that we don't know yet,
then it should still
be moving outwards.
And I want to demonstrate
just quickly what
we mean we talk about an
expanding or a shrinking
universe.
And I'm going to use
my universe, here.
This is a universe, by the way.
This is a one
dimensional universe.
We're going to imagine
now that you're
an ant living in a
one dimensional space,
which is this piece of elastic.
And I want you to imagine that
you are living in one of these
blue stickers, which is
your one dimensional-- well,
it's two dimensional-- think
of it as only living along this
line--
that that's a galaxy.
Each of these blue stickers
are a galaxy or a marker
in this ant's space.
And I'm going to model what
I mean by a universe that
grows in this space by simply
stretching the elastic,
stretching it apart.
In a shrinking universe, I
would bring it back down again--
grow it again, like this,
shrink it back down.
Now what happens when I stretch
a piece of elastic is it
grows everywhere.
It doesn't grow from
one central place.
The whole thing expands,
or the whole thing shrinks.
And so when we talk about
an expanding space--
is space growing?
We're talking about
something like this,
where all of the
galaxies in the universe
sprinkled through it are
moving apart from each other.
And a shrinking
space would be one
where they were all
moving together like this.
And that's very different from
the idea of maybe something
bursting out from
a point in space.
We think a space that grows is
a space that grows everywhere.
Now this is just a one
dimensional universe.
We think that we might be
living in a universe that's
like this in three dimensions,
a bit like a stretchy elastic
in three dimensions.
And an analogy for that is a
bread dough full of raisins.
Imagine you take a
small bread dough,
fill it with raisins and
yeast, and let it rise.
The raisins will all move
apart from each other
in that dough universe.
And that's kind of
what we think of when
we think of an
expanding universe,
is everything moving apart
from everything else.
So this guy George Lemaitre,
who was brilliant--
but he made one error,
which was that he wrote
about his predictions and
his interesting findings
in a really obscure Belgian
journal in French that none
of the relevant people read.
So he wrote this fantastic
paper where he said,
I think the universe is growing.
And here's how we could go and
figure out if it's growing.
And oh, I have actually
gone to figure it out.
And, oh, I think the
universe is growing.
And it started in a big bang.
It was huge news--
really important results.
No one read it.
And it wasn't until
like four years later
that it got translated
into English
and was read by the
wider community.
But he made this prediction
that others did, too, which
is that in such a universe--
OK, let's imagine.
Let's do this all together.
Imagine I live in this--
I'm back in this ant, and
I'm in my stretchy universe.
My universe is growing.
And I want to know if
I, as an ant living
in one of those blue
spot galaxies, how
I can tell whether the
universe is growing or not.
And as an aside, I'll say
that the reason that these
are blue spots and not
drawn on with blue pen
is that we don't think,
in an expanding universe,
that a galaxy
itself will expand.
We think the gravity
in that is too strong
compared to how we
now-- how fast we now do
think space is growing.
When we think about expanding
space, anything inside a galaxy
is not growing.
So even if space is
growing, this room
is not growing right now.
So if you were an ant
in one of these galaxies
and you want to know if
your space is growing,
you can look out at
galaxies around you.
And they all should
appear to move away
from you if space is growing.
If you picked any one of
these galaxies and looked out,
you'd see space.
You'd see everything
around you moving away.
And actually, you'd see
something even more specific
than that, which is that the
galaxies very close to you
wouldn't move very far in the
time I'd take to stretch it.
But the galaxies very far
from you move further.
And how that can be seen
is that distant galaxies
should be expected to be
moving away faster from you
than ones close by.
So this very specific prediction
of an expanding universe,
which I'm showing
here-- let's talk
through it, which is if we
have along here the distance
of a galaxy from the Milky Way
and the speed that it appears
to be moving away from us, then
if the universe is growing,
all the galaxies in it
should approximately
lie along this line where
the nearer galaxies should
be moving away from you slower.
And the further ones should be
moving away from you faster.
And if the universe
is not growing at all,
then there should be no pattern.
They shouldn't follow a pattern.
And so both Lemaitre, and
then, with better data,
Edwin Hubble, in
the late 1920s went
to look at all these
galaxies that they just
discovered were actually
separate galaxies
beyond their own.
And they measured how
fast they appeared
to be moving away from us.
And they found that
it was, in fact,
true that almost all of the
galaxies around us in the sky
are, in fact,
moving away from us.
And they followed this trend
that the ones further away
from us are moving
away from us faster.
Again, that doesn't
mean that we are
at the middle of the
universe with everything
moving around us.
If you hopped over
to another galaxy,
you would see the
same thing happen.
Everywhere you
jumped to in space,
you would see galaxies
moving away from you.
And so this was big news.
This said that the
universe is growing.
And actually, by measuring
how fast things are going,
you can do something
even better.
You can work out
when in the past
that growth should have started.
And when the growth started,
when-- imagine in your heads
you can wind back.
Now, when I shrink down my
elastic, I can't get it to--
let's say I'm out here.
If I wind back time in my head,
I shrink back down the elastic.
It stops there, because my
elastic has a finite length.
But imagine, if you
will, that the elastic
could keep shrinking down,
and down, and down, and down,
and further until
all those spots were
on top of each other.
That would be what we
call the big bang--
the beginning of
time, when everything
that's now stretched
apart was right back
on top of each other.
And so actually, just by looking
at how galaxies are moving away
from us, how fast, you could
imagine even just doing this
with a simple, you know, if you
know the speed that something
is moving and how
far it is away,
I'm pretty sure we can all
figure out what time it set off
from me, right?
That's an evaluation
that we can do.
And so, by looking at the
galaxies all around us,
we were able to
make-- astronomers
could make the first estimates
of how old the universe is.
Now Edwin Hubble
and George Lemaitre
both showed that the
universe was growing,
and it had this pattern.
But they actually
got the distances
wrong by about
eight times wrong.
And so they actually
estimated an age
of the universe that was
any two billion years.
And after that, there
was this subsequent kind
of years of
understanding exactly
how to use these pulsating
stars in an accurate way.
Until in 2001 this
astronomer, Wendy Friedman,
American astronomer,
led a team that
used the Hubble Space Telescope
to measure these pulsating
stars even better
and managed to get
a really good estimate of how
fast the universe is growing.
And it was her
team's measurement
that gave us one of our
best current day estimates
of the age of the
universe, which
is about 14 billion years.
There's another way
you can do it, too,
which is actually
what I do, which
is I look at distant light
from the big bang itself.
And we can use that to infer
the age of the universe
in a slightly different way.
And it gives us even a slightly
more precise measurement.
But the measurements
agree, broadly.
And they point to this time in
the deep past, the big bang,
when everything started.
So when we look around-- so
this big picture of our story
is that the universe is growing.
Galaxies are moving
apart from each other.
There was a beginning,
some big bang.
Something quite strange
is happening now
that 20 years ago, we
went out to measure--
we thought, right, the
universe is growing.
Some initial energy set
off this initial expansion.
But, you know, the stuff
throughout space filling it,
and the gravity of
all the galaxies
still in space should tend to
slow down the growth of space,
and should, eventually,
either just slow, and slow,
and slow it down, you know,
getting slower and slower
forever, or maybe
slow down it enough
to turn around, to
stop the expansion,
and to reverse and
start shrinking again.
And this was genuinely
an interesting question
20 years ago.
It's still interesting, but it
seems to be no longer the right
question.
Because astronomers went to
measure very distant galaxies
and looked at how fast they
were moving away from us.
And they did this clever
trick of comparing
how fast the universe
is growing in the past
to how fast it's growing now.
And they thought
they'd see that it
was growing faster in the past
and not as fast now-- slowing
down.
But they saw the exact reverse.
They saw that the universe
is growing faster today
than it was in the past.
It's speeding up.
The galaxies are flying
apart from each other faster
and faster.
And this is as strange as
if I threw a ball in the air
and instead of coming back down,
or in the most crazy example,
I throw it so hard that
it just kind of coasts off
up into space slowly--
unlikely-- instead,
this is as weird
as if I threw the
ball in the air
and it sped up
away from my hand,
away from gravity's
pull of the Earth.
And again, this is--
along with dark matter--
it's one of our big mysteries
right now in astronomy
and in cosmology,
which is that there
appears to be something
in the universe that is
making the whole of space
grow faster and faster.
And again, we don't
know what it is.
We call it dark energy.
And that really means we
don't know what it is.
It might be the energy
of empty space itself.
It might be that as
space grows and grows
that every box of empty
space has its own energy,
and that as space gets
bigger and bigger,
this becomes more dominant.
And it can make space
expand faster and faster.
But it could be something else.
This is, again, one of the big
things we want to find out.
Now, just as I finish,
I just want to--
I've had time to talk
about the big story.
There's also our local
story, which is--
you know, our sun has
not been around forever.
And we think that it was born
about five billion years ago
in a place a bit like
this-- not this place.
This is the Eagle Nebula.
This beautiful image of a
stellar nursery, a place
where stars are born--
and so within the galaxies
that I talked about,
these galaxies have these
spiralling discs of stars.
But they also have these
clouds of gas and debris
from older stars.
And this is an example of
them, these beautiful clouds
of dust and gas.
And inside those are where
new stars are forming,
where balls of gas are
condensing into new stars
and drawing around
debris discs of rocks
and other things that will
eventually form into planets.
And so we think
that our sun itself
was born about five billion
years ago with some planets
that coalesced around it.
And it was born from the
remnants of older stars.
Older stars are where our
ingredients were created.
Everything we're made of was
created in the core of a star.
Not everything-- the
only thing that's
not created in the core of a
star is hydrogen and helium.
At the beginning
of the universe,
that's all there
was, nothing else.
And so everything-- all the
oxygen, nitrogen, carbon--
everything that's
in our bodies--
was made in the furnace of
older stars that then exploded
and sent out their material
out into space into some
of these stellar nurseries
where new stars were born
with planets around them that
were made of the kind of things
we're made of.
So we really are made of stars.
And, I have to skip past these.
And so there's this story of--
there's this cycle of
the stars that create us.
And there's the bigger story of
how the whole of the universe
is growing.
And so we've really found
out so much and so quickly
about our magnificent universe
that's happened really
in the last decades--
our knowledge has
advanced so much.
But there are still tonnes of
fascinating and interesting
questions.
And what is true is that
today's astronomers-- and I--
won't get to answer them all.
But happily, someone else will--
maybe some people in this room.
Thanks.
[APPLAUSE]
