So I'd like to summarize a few of
the elements of modern cosmology.
And let's begin by saying that this
is an incredibly recent story.
No more than 100 years ago,
our knowledge about the universe was
little advanced from the Stone Age.
Whereas today,
we have a feeling that almost,
observational study of the universe
is converging in some sense.
How is this possible?
Well, it's possible by
the technology of telescopes.
Telescopes have shown us, different
lines of sight into the universe.
And we know the universe
consists primarily of
the building blocks of galaxies.
Just like the Milky Way,
each one, 10 billion stars.
Throughout this universe of galaxies,
we're able to see how things
have changed with time,
because the beauty is that
light travels at a finite time.
So, the further away we're looking,
the further back in time we're seeing.
So we're able to study
the universe as it was then and
as it is in different directions.
So we learned the universe of galaxies is relatively uniform, so
we can be confident that by
studying part of the universe,
we're learning something that's
statistically representative of the whole.
And we're able to learn
about the entire history of
the development of this as well,
given a large enough telescope.
Now, the most important thing to
know about the universe of galaxies
is that it's expanding.
If that's a set of galaxies
at some time, little bit later,
the whole
distribution is more diffuse.
Everything's moving apart, and
this expansion obeys Hubble's Law,
which we can write the velocity, V,
is a constant H times a distance, D.
This dates from 1929.
How do we know that
the galaxies are in motion?
The answer is the doppler shift that
we can measure through spectroscopy,
if I have a galaxy emitting radiation.
Now, if I move it away,
that radiation is stretched out.
It becomes radiation of a longer
wavelength, a redder color.
So we would say that the wavelength
is replaced by the wavelength
times the correction,
which is roughly the recession of
velocity times the speed of light.
So if we observe in detail, spectroscopic
observations of the galaxies,
we can measure this velocity.
Many people assume, that this is
what was first done by Hubble.
But actually,
the astronomer that deserves credit for
beginning this work was V M Slipher,
who worked in Arizona.
His first observations were made
just over a century ago in 1912,
and really, for the next decade,
he had the field to himself.
So he was the great pioneer
who first revealed that all
galaxies were moving away from us.
Grasping what it means to say
the universe is expanding isn't easy.
First of all,
the distribution of galaxies in
this cartoon might go on forever.
So there's no way of ever escaping and
observing the expansion from the outside.
It's not as if material is flowing out
to fill a pre-existing empty space.
Secondly, there's no centre.
That is,
although it seems to us that all galaxies
are moving away,
from the point of view of this one
it would be as if
the Milky Way was moving away.
So, uniform expansion makes
the expansion democratic.
Everyone sees material moving
off in all directions.
Finally, you should not think of
this, although it's a common analogy.
People will say that
galaxies aren't moving apart,
it's just the space between
them getting bigger.
The space here is not getting bigger.
The universe as a whole gets larger.
But that's only on scales where
space-time is curved, and
that's not an important
phenomenon here in this room.
So really, you should just imagine
the galaxies are moving in flat space, and
that's a good description.
Now, to follow the evolution
of an expanding universe,
we've got to deal with gravity.
In many ways,
we still use Newton's treatment.
From the 17th Century, that is,
the inverse square law of attraction
between two particles.
Halve the distance between them,
the force goes up by a factor of four.
But this is replaced by general
relativity in 1915 by Einstein,
where he took the view that you
should think of this force as
arising from a curvature of space and
time.
Einstein addressed a question which
really, Newton was unable to answer,
which is what happens in the case
of a uniform mass distribution.
Newton thought that if you had
a lot of particles of mass,
that the whole thing would be static.
Because the gravitational
attraction in this direction
would balance, and so
there'd be no net motion for any particle.
Einstein showed that even within
Newton's gravity, that was wrong.
You need to modify
the law of gravity itself.
The change that Einstein made to the law
of gravity was to introduce the thing he
called the cosmological constant.
We use different names
today, for the same thing.
Although it's still very much
a feature of modern cosmology.
The most common one
would be dark energy,
or sometimes vacuum energy.
What this means is that you
give empty space itself some weight.
A sphere of absolutely nothing
of volume V, contains a mass,
which is the volume times
a density of the vacuum.
We'll discuss a little bit more
later on how that's possible.
But if we just accept it for
the moment, Einstein proved,
a further strange consequence,
which is that
such a system would
exhibit anti-gravity properties.
That is, the effective mass inside
the empty space would tend,
rather than attracting material around it,
to push it away.
So you could obtain a static
universe by balancing this repulsion
with the inward attraction
due to the matter inside.
Whereas today we believe
that it was nearly right,
but it's just that in the present
universe this is out of balance,
and the anti-gravity from the vacuum,
in fact, dominates.
So the expansion of
the universe is accelerating.
