So armed with general relativity,
theorists were able to explore cosmology.
The first to do so was De Sitter in 1917.
He produced a universe
that contained only vacuum energy.
But it was realized after a few years,
that this predicted Hubble's law,
and in fact,
this prediction was very influential.
A number of astronomers before Hubble,
leading up to Hubble's paper,
which was the most comprehensive
of these analyses,
in 1929, discovered, or
tested, this prediction.
So, although Hubble's Law is often
presented as an unexpected surprise,
in fact the theoretical context led people
to look for a result of this form.
It's interesting to speculate how long
its discovery might have taken otherwise.
But of course, the most general universe
contains more than just vacuum.
And this was solved by
the Soviet physicist,
Friedmann, in 1922 to 24.
Friedmann reached a number
of remarkable conclusions,
the most important of which,
was what we today call the Big Bang.
What this means is that
you can make a plot.
This is time at the size of the universe.
I don't need to say exactly
what I mean by that.
Just pick some piece of the universe,
plot how big it is versus time.
Here it is.
It's getting bigger now, we know that.
Solving the equations
given him by Einstein,
Friedmann showed that in the past this
would've emerged from a singularity.
And the time between the singularity and
today is about one
over Hubble's constant,
which today is 14 billion years.
So, Einstein's dynamics
have given us this strange
conclusion that the universe
was only a finite time.
The other remarkable conclusion
of Friedmann's work was
that the matter content of
the universe affected its curvature.
Think about the Earth.
This is what would be called
a closed surface.
By which I mean that it's finite.
You can walk around it forever.
You never come to a boundary.
But you come back to your starting point.
So the universe can be closed and
have what's called positive curvature.
Three dimensional space
can be curved in exactly the same sense.
But what Friedmann also showed was that
you could have negative curvature.
Now, I can't draw you
a picture of what that means.
But, curvature means that
straightforward geometry doesn't apply.
For example, we know that the sum of these
three angles
adds up to 180 degrees.
That's not true in curved space.
But the negative curved
 universe,
is what's called an open universe.
And it would be infinite.
So unlike a closed
universe which is finite,
the universe with negative
curvature would go on forever,
and it's the density of the universe
that turns one of these into another.
There's a critical density
which is minute,
it's about one atom per cubic metre.
It's a better vacuum
than we can make anywhere on Earth.
But that's enough material to turn
an open universe into one that
closes back in on itself.
And, every now and then,
you might see the symbol omega,
which is the density
divided by this critical value.
And so
we would say that omega equals 1,
tells us to join the universe
at the boundary between open and
closed, which is flat.
And strangely enough for modern
observations, this is where we seem to be.
One of the ways that we learn about the
early stages of the expanding universe is
the fact that it was hot.
So anybody who owns a bike appreciates this.
As you pump up your tires,
you compress the air, it becomes hot.
So the temperature of material in
the expanding universe is actually
proportional just to one over
the size of the universe.
The smaller it is,
the higher the temperature.
This means at early times,
the temperatures can be really extreme.
So, when the universe is
about one minute old,
the temperature is about
a billion degrees.
This means that nuclear
reactions can happen.
So atomic, so nuclei can be assembled.
So, the higher temperatures,
they couldn't survive, so
you have individual protons and neutrons.
But as the universe cools
below this threshold,
these can come together to
make a deuterium nucleus, and
two deuterium nuclei can come
together to make helium.
Now what we see in
the universe today is that all
the stars contain roughly 25%,
by mass, of helium.
When this was first discovered early in
the 20th century, it was unexplained, but
it was then realized that this
was an inevitable prediction of
nuclear reactions in the early universe.
Furthermore, by looking at the relic
abundance of deuterium, you can measure
the density of all ordinary material that
participates in nuclear reactions today.
And the answer is, it's something like
5% of the critical density.
Remember omega equals one was
a universe that was flat.
So ordinary atomic material,
we can be sure was being
synthesized at the time when
the universe was about one minute old.
And we know today it's far
short of closing the universe.
Now, a more direct way
of probing the early
hot universe is the fact
that we can see it.
If we look far enough away,
we can see directly back to a time when
the universe had that temperature.
So, there's radiation left over
in the universe that
comes from great distances.
That's from a,
a shell known as the last
scattering shell, and
that's because at great distances,
corresponding to early times,
as we look at it,
material is ionized so
that light can't propagate freely.
Temperatures thousands of kelvin.
It's just like the surface of the sun.
But eventually, the universe cools
to the point where atoms form.
That is, say for example with hydrogen, you have a proton
and an electron come together to make
a single atom of hydrogen.
That doesn't scatter light so
effectively and then the radiation
can propagate to see us.
So over here, it's say, 3,000 kelvin, but
it's at great distances and
the expansion of the universe red
shifts it by the time it reaches us,
it's a mere 2.7 kelvin.
So radiation of such a low temperature
is characterized by radio waves,
as a wavelength of something
like one millimetre.
This is the so-called CMB,
stands for cosmic microwave background,
and this was found in 1960, well 1964,
published in 1965 by Penzias and
Wilson who received a Nobel Prize for
this work,
even though it was a complete accident.
And it's a strange irony that, elsewhere
in the world, groups who understood
this cosmological transition had
predicted the existence of radiation and
were preparing to search for it.
In any case, it is there
and we can see back, therefore, to this
era, where the time is something like
400,000 years after the big bang.
So, we can get this close to the initial
singularity with direct observations,
and that's extremely powerful.
