 
Hello, I'm John
Peacock, professor
of cosmology at the
University of Edinburgh.
I'm going to tell you some of
the ways in which the Higgs
Field intersects with some of
the big questions of cosmology.
Almost everyone has heard
that the universe is expanding
and that it began in a big bang.
In other words, here we are.
The universe is big.
It's getting bigger with time.
You run that backwards
and you conclude
that about 14 billion years
ago everything diverged.
The density of the
universe was infinite.
And for many years
we were embarrassed
because we'd be asked
what happens here,
the times before the Big Bang.
Or indeed, at the Big
Bang, why does the universe
start off expanding
the first place?
Nobody had any good
answers to offer.
Since about 1980, though, we
have had some potential answers
that connect to the
Higgs Field, and that's
what I want to
tell you about now.
It all comes down to one
very simple but apparently
obviously wrong idea, which
is that the vacuum can
have weight.
I mean, you take
some scales, you
put a box of absolutely
nothing on the top,
and yet the scales go down.
That sounds bonkers, but
actually astronomical data
tell us this is true.
 
So if you're prepared to
believe one impossible thing,
then maybe you could
go for another.
 
That is, the vacuum
not only has weight,
but rather than attracting,
its gravitational effects push.
This has a profound
influence on the expansion
history of the universe.
If we didn't have
any of this stuff,
you'd have a piece of
the expanding universe,
and there's mass inside,
and that attraction--
that mass-- would slow
down the expansion.
And it's just like
gravity here on Earth.
I take my keys, I throw
them up, they go up,
but they slow down at the top.
Why is that?
They have kinetic energy.
The kinetic energy
transfers itself
into gravitational
potential energy.
So this sphere, the
potential energy
changes as the sphere expands
and the kinetic energy
is drained away.
But if I have a sphere
of absolutely nothing,
and make that bigger,
and if it has weight,
it ends up with
more mass inside it.
So rather than becoming
less important with time,
the gravitational binding
effects become more important.
And so the only way to
conserve total energy
is for the expansion
to speed up.
And so the vacuum actually
tries to blow itself apart.
A little bubble of absolutely
nothing just wants to explode.
 
OK, so the idea that the
vacuum can have weight
actually goes back
to Einstein in 1917.
He invented the thing called
the cosmological constant, which
was his name for it.
 
What Einstein wanted
was a static universe.
 
That's because he didn't know
about the evidence that already
existed in 1917 from
Slipher's observations.
The universe was expanding.
So he tried to balance the
kind of attractive nature
of normal gravity
where a sphere of stuff
would tend to fall in
itself with the repulsion
of the empty space
within which it sits.
And he tried to make these
two equal and opposite.
So the whole thing
just sat there,
neither expanding
nor contracting.
He was very close.
The modern picture we have
is that in the universe today
the effect of the vacuum is
about three times as large
in its anti-gravity properties
as the gravitational attraction
of ordinary matter.
And therefore, the
expansion of the universe
is actually speeding up
because of this repulsion.
 
So today, astronomical
observations
tell us how much
acceleration there is
and we can measure the
density of the vacuum.
 
Roughly equivalent
to the density you
would get from one hydrogen
atom in a cubic metre of space.
Almost nothing, but on
a cosmological scale
it's immensely important.
Interesting thing
to bear in mind,
though, is it's only
important today.
Imagine how the
density of material
is going to change with time.
For the vacuum, for a
cosmological constant,
it doesn't change.
The vacuum is the
same at all times,
at least as far as we know.
Ordinary material, though,
reduces in density.
 
So at early times the
vacuum might as well not
be there because the
universe is dominated
by the density of
ordinary material.
There's a crossover
point and it turns out
we live just to the right
of that crossover point,
when the vacuum is about
three times as large
in density as ordinary material.
 
So we can now see what
these different eras
of matter domination
and vacuum domination
are going to do
for the expansion
history of the universe.
At early times it's
going to decelerate.
But at late times it
starts to accelerate.
So let's draw the
history again, the size
of the universe versus time.
 
Our old picture was of some
continuous deceleration
like this.
So we'd be here and the
universe would be decelerating.
But actually, as we've
seen around this time,
the vacuum starts to
become more important.
And so we're picking up
an accelerating expansion.
So we're still left,
though, with the problem
of the need for a singular
origin - the Big Bang -
and the question of
what happened before it.
 
But before we talk about
what might have happened
before the Big Bang, let's
deal with the first nutty
thing, which is surely how can
the vacuum have weight at all?
How can this acceleration and
expansion be taking place?
Well, the explanation
for it traces back
to the uncertainty principle.
 
And this says that on
the subatomic scale
you can't have precise
knowledge of the position
and the motion of particles.
This is around us every day.
Think of an atom.
 
The electron orbiting around
a proton in the hydrogen atom.
Why is it the size that it is?
The answer is that the
uncertainty principle says
that the product of the
uncertainty in its position--
let's say x-- times the
uncertainty in its mass times
its velocity, its
momentum, is some constant,
Planck's constant.
So if we made the atoms much
smaller than they actually
are the uncertainty
in the velocity
would have to be so large
that it would exceed
the speed of light,
and that's impossible.
So this elementary fact
governs our everyday life.
The uncertainty principle
is all around us.
So let's think about what the
uncertainty principle does
for the vacuum.
 
All right, so what
is the vacuum?
We can easily take out all the
atoms and so on out of a box,
but we're still
left with fields.
So what are fields?
Let's take the
electromagnetic field.
When we look at
something with light,
you can think perhaps of a
kind of elastic string, which
is the kind of lines of
force that one sees, say,
from sprinkling iron
filings around a magnet.
Think of that string connecting
you to the light source,
and if you wiggle that
string, the disturbance
will run along it.
And that's what
electromagnetic radiation is.
So our vacuum is full of
all these elastic strings.
And if there's light in the
box these strings are excited.
So in a vacuum we take away all
those excitations, don't we?
No.
The uncertainty principle
says that it's not
possible to know that
that line of force
is completely stationary.
So the energy in one
of these wave modes,
the energy is n for the number
of photons times the energy
- Planck's constant -
times the frequency,
plus what's called
the zero point energy.
A half times the energy
of the oscillation.
So the vacuum says the lines
of force are unexcited,
but you're still left
with a zero point energy.
So the vacuum has to have weight
just from all these zero point
motions.
 
So that's the good news, that
the vacuum should have weight
and we can calculate it.
The bad news is when
we do the calculation
it's a bit embarrassing.
 
The problem with this
is that we add up
wave modes of higher and
higher frequency and the energy
diverges.
 
A better guess is
that new physics
will intervene to save this.
So there'll be some
maximum energy of photon
that it makes sense
to think about.
That had better be higher
than the kind of physics
we've been able to
probe at the moment.
So let's say 10 TeV.
That would be above the energy
reach of the Large Hadron
Collider.
If you do that you
get a better answer,
10 to the 36 kilograms
per cubic metre.
So that's still
grotesquely greater
than the observed
10 to the minus 26.
So there's a big problem
here with this result.
And what it screams
at you is there
have to be other
contributions to the energy
density of the vacuum that
somehow can cancel out
this huge embarrassingly
large number.
 
