 
Hello I'm Andrew Liddle from
the Institute for Astronomy
at the University of Edinburgh.
In this section
of the MOOC we're
going to consider cosmology,
and in particular the hot Big
Bang, and how that relates to
ideas from fundamental particle
physics.
The discovery of
the Higgs particle
completes the Standard
Model of Particle Physics.
It contains all
of the particles,
the quarks, electrons,
muons, and it also
describes the forces between
these particles, as well.
We have the Higgs
particle, which tells us
how those particles
got their mass.
Standard Model is a
very ambitious theory,
because it's trying to
explain all of physics,
with the exception of gravity.
It should explain the
physics of our everyday life
- atomic physics,
nuclear physics.
If we combine it with
the theory of gravity,
for instance, Einstein's
Theory of General Relativity,
we should then be able
to use it to describe
the entire Universe.
And that's where
cosmology comes in.
The Universe is expanding,
but it's very, very cold.
It's only a couple of
degrees above absolute zero
at present, which we can measure
from the cosmic microwave
background radiation.
But any gas you have
which is expanding,
like the Universe itself,
cools down as it does so.
For the Universe,
the relation tells us
that the temperature
of the Universe
is inversely
proportional to the size.
What that means is that the
young Universe would have been
very much hotter than
the present Universe,
and that affects the kinds
of physical processes
which will be relevant.
 
The first thing
we need to look at
is the relationship between
temperature and time
in the Universe.
The Universe was dominated
by radiation for roughly
its first 100,000 years
or so, and we can readily
compute what the
temperature is, and how
that relates to the
age of the Universe.
And I can write that
relationship in this form here.
One way to interpret
this formula
is that it says that when
the age of the Universe
is one second, then
the temperature is
2 times 10 to the 10 Kelvin.
Now for our purposes,
in fact, temperature
is perhaps not the
most useful quantity.
We have to know what
temperature means.
And temperature is
really just a way
of telling us about the
energies of individual particles
in the gas.
So we can rewrite
this formula in terms
of the energies of
those particles.
And that lets me
write it this form,
where I've used
particle physics units
to say that the energy is in
units of mega electron volts.
For comparison, the energy
of typical photon of light
that you might see is
about one electron volt.
So for particle, this is
quite a significant energy.
It's about the energy that
corresponds to nuclear physics
interactions, such as beta
decay in a radioactive process.
And this is saying that when
the Universe is about one
second old, of the particles are
moving with about this energy.
And that's telling us that
the source of processes
that will be relevant are
nuclear physics processes.
Now, if we go to CERN,
at CERN the beam energy
is measured by an
even larger quantity,
the tera-electron volts.
It's about 10
tera-electron volts.
This is a million mega
electron volts here.
And so this is 10
to the seven MeV.
So I can now ask, at what
stage of the Universe's history
were the typical particle
energies comparable to those
that we can generate at the
CERN large hadron collider?
So what I need to do is
substitute this number
into this formula, and find
the corresponding time here.
And what I'll find, is the
corresponding time is about 10
to the minus 14 seconds.
 
At that era, everything
in the Universe
is happening with
the sorts of energies
that are generated in a
large hadron collider.
And if we can consider the
Universe at even younger
ages, would be energies
which are beyond anything
we're able to create
here on Earth.
First, let's cover the standard
successes of the Hot Big Bang
model.
We have two main pillars
which have convinced people
that the Hot Big Bang model
is a very good description
of our Universe.
One of these is the cosmic
microwave background.
This is radiation
which was left over,
a kind of a relic radiation
from the Hot Big Bang, that
suffuses the entire
Universe, and which
we can detect readily today.
And it's readily explained
as having its origin
in atomic processes,
which took place
around about the age
of 400,000 years or so.
The second main
pillar is the theory
of Nucleosynthesis, which
is why I've highlighted
the time of one second
as being characteristic
of nuclear processes.
It's believed that the very
light elements in the Universe,
particularly
hydrogen and helium,
were formed in the first
instants of the Hot Big Bang.
And we have a theory of
that, cosmic nucleosynthesis,
which is able to predict the
abundances quite accurately,
and in very good agreement
with observations.
All of this happened when the
Universe was just a few minutes
old, starting from
perhaps one second old,
finishing off maybe three
or four minutes later.
And by that time, the
basic chemical properties
of the Universe
were all laid down.
So taking together the
cosmic microwave background
and nucleosynthesis, adding
with that the observed expansion
of the Universe, discovered
by Hubble long ago,
that led to cosmologists
accepting the Hot Big Bang
Model certainly by
the 1970s, and it's
by far the dominant
paradigm just now.
However, from our particle
physics point of view,
perhaps the most interesting
thing about the Hot Big Bang,
is that it contains
quite a few things
that the Standard Model
of particle physics
apparently does not explain.
And what we've just learned,
is that during cosmology we
can have very high energies, if
we go back far enough in time.
And we may be probing
physical processes which
are even more
energetic than those
that we can explore at CERN.
So perhaps cosmology
is the way that we
can learn about these
very energetic processes.
So what we want to do is look
at processes, or phenomena,
which we can't explain readily
within the Standard Model.
And there's quite
a few of these.
The first thing, which has
been known for a long time,
is that the Universe is
comprised entirely of matter.
There's no antimatter
in the Universe
but the Standard Model
doesn't distinguish
between matter and antimatter,
except at a very fine level.
It can't explain why
there's no antimatter
at all in our Universe.
A second mystery is
that the Universe
appears to be full of
so-called dark matter.
We observe its
gravitational force,
it holds galaxies together,
it lets galaxies form
in the first place,
it's responsible
for the visual appearance
of the Universe.
But we don't know its nature.
We don't know even whether
it's a fundamental particle,
or comes in some other form.
The Universe is observed
to be accelerating.
That's a mystery
which has already
been discussed within this MOOC.
Another thing is,
that we don't know
what caused the
irregularities in the density
field of the very
young Universe.
If we look at the cosmic
microwave background,
it's not perfectly smooth,
it has quite significant
variations in temperature that
we can measure accurately.
And these variations
are very important,
because they later
give rise to galaxies
and other structures
in our Universe.
The standard model again
provides no explanation
before these irregularities
might come from.
So collectively, taking
these things together,
we have a lot of
evidence that there
may be new physics beyond
the Standard Model,
applicable at even
higher energy scales.
Although the Standard
Model doesn't
explain these
phenomena, we certainly
have ideas of how
they might have
come about which are
telling us something
about possible new
ideas in physics.
For example, the
matter-antimatter asymmetry
doesn't get explained
in the Standard Model,
but it's believed that the
forces, such as the strong
and the electroweak
interactions,
might become unified at very,
very high energies, called
grand unification.
And that process allows
matter and antimatter
to be distinguished,
and perhaps provides
a way of explaining
where that comes from.
Dark matter is widely believed
to be fundamental particles,
and particle physics
actually offers us
lots of possibilities, lots of
candidates that might explain
why the Universe
contains dark matter.
The most popular by far
are so-called weakly
interacting massive particles.
The acronym is WIMPs,
and these are associated
with a theory called
supersymmetry.
And that's probably everyone's
favourite possible particle
that might be left over
from the early Universe,
and provide the extra
gravitational attraction.
It's not the only
one, though, we
have other possible
particles, and the axion
is a particle which
is been considered
by quite a lot of people
as a reasonable candidate
to be the dark matter
of the Universe.
Why is the Universe
accelerating?
That's probably the
biggest puzzle cosmologists
face at the moment.
One possibility is
that it's caused
by a nonzero energy
of quantum fields,
the so-called cosmological
constant, which
even in the vacuum state can
give an energy of empty space
that drives an expansion.
 
Finally, I turn to the density
irregularities laid down
in the very early
Universe, and there we
have quite a strong theory.
The theory is known as
cosmological inflation, which
tells us that the
Universe underwent
a period of
accelerated expansion
when it was very young.
This has almost become part
of the standard cosmology
now, because there are
well-developed theories that
tell us if we take
quantum physics,
and we take the rapid expansion
of the inflationary era,
it is inevitable that
we will finish up
with some irregularities
in the Universe.
And these can be
exactly of the form
we see in the cosmic
microwave background.
And then later on, they
cause galaxies to form,
they cause stars
to form, eventually
allow us to exist
in the Universe.
 
So let's sum up
what we've learned
about in this
section of the MOOC.
What we've seen is that at
early stages in the Universe's
evolution, particle
energies could be very high
through the relationship
between temperature and energy.
In fact, if we go
to young enough
in the Universe,
early stages, we
may find that particle energies
are even higher than those
we can create on Earth at
accelerators such as CERN.
So the early Universe
is a probe of physics
at very, very high energies.
What we're hoping
for, of course,
is that that physics
will leave behind relics
that we can explore
and investigate today.
The cosmic microwave background
is already one example,
and perhaps dark matter is
another example as well.
Overall, what we've learned is
there are quite a few phenomena
in the Universe which
don't seem to be
explainable within the
framework of the Standard Model.
So perhaps we are
already seeing evidence
that there is physics
beyond the Standard Model,
and perhaps that
cosmology will prove
to be the best way
to learn about it.
 
