Let us now turn to the study of the early universe.
First let me review a few of the key ideas.
Obviously as already you were made to realize, as you turn clock backwards and
go to a smaller universe, it was hotter and denser,
in a fairly predictable way, since we know how matter behaves under high
densities and temperatures. Although of course, some new interest in
Physics can happen such as inflation. At any given time particles in universe
have certain temperature, meaning certain energy and that means that particles of
different duress mass energies will dominate at different times.
Essentially universe acts as the ultimate accelerator, reaching energies of
particles that will probably never be generated in the lab, and thus, providing
a very interesting new window into particle physics itself.
So as we go deeper in the past, the energies increase,
and we get into regimes for different interactions and different particles
become more dominant than others. However, the price of that is that the
perigo, the higher the energy the less we actually know.
We currently probe physics out to the energies of order of 100 teraelectron
volts of rest mass. And we do have a fairly reliable theories
what goes on up to the there or thereabouts.
But beyond that things to get to be increasingly more speculative.
And in any case, at some point this has to break down.
Currently the best guess is that it breaks down around the Plack time, which
is more less a guess. We will discuss Plack units later, but at
any rate. 10 ^-43 seconds is certainly the boundary
of what we can even guess. Here is just a schematic outline what
history of the universe might look like. There is a very early period of
inflationary expansion. Which is by now I think, fairly well
established but is by no means proven. And then we go through the early
universe, a hot soup of quarks and other particles until the atoms form, there are
combine micro backgrounds released and that's where the astronomy as we know it
really begins. So let's go quickly through thermal
history of the universe. What are the key moments?
At some point quantum gravity has to be important and dominate the physics of the
universe. We do not know when that is, we do not have a quantum theory of gravity.
And Planck time, 10^-43 seconds is about as good guess as any.
The inflation, if it happened, happened when the universe was around 10^-33
seconds old. And shortly thereafter, we, believe there
was a grand unification or rather, splitting of the electroweak interaction
from the strong interaction. They split from gravity sometime earlier,
probably prior to the inflation. When the universe was around a
microsecond old, baryogenesis happens. Quarks create things like protons and
neutrons, particles that we know about. Between about 1 millisecond after the Big
Bang and about 3 minutes is the time of the cosmic nucleus synthesis, where the
light elm stabilize like [UNKNOWN] to helium 3, helium 4, lithium.
Little bit of beryllium, maybe even some boron were formed and frozen.
At that time, universe is still very much dominated by radiation.
That changes around the time of the, hun, around 100,000 years, age.
After which, matter becomes the dominant component of the dynamics of the universe
and first structures begin to form. The seeds of the first structures begin
to form from the dark matter itself. And when the universe was 380,000years
old, and we know this number fairly well thanks to the procedure in cosmology
measurements from cosmic microwave background.
The gas becomes neutral. Electrons and protons recombine become
atoms of hydrogen and helium and universe continues to expand.
At that time there are no more sources of light.
There are no stars in the galaxies. They, these being to, we think that they
begin to form when the universe is few hundred million years old and that point
they can reionize the neutral gas of the universe and that is pretty much
completed by about age of 1 gigayear and that's where right now the frontier of
our observational cosmology is. Physicists love to plot time history of
the universe on a log axis because that gives lot more space at very short times
in which they're interested, and all of the known astronomy happens just over
there in the corner at the end Well, here is one from Mike Turner, and it's a handy
little chart because shows you the temperature, meaning the energy of the
particles, the density, as well as the time when it happened, which few of the
key moments noted, just as I outlined a moment ago. In tabular format,
here is a quick outline, again, of what are the key times and particles that
dominate or, or processes that dominate at that time.
Not to be to boring, but here is another one.
All of them contain more or less the same information.
I'm offering all of you, all of them to you because you may find one or the other
more useful for, or, more clear than the others.
Well, what is the empirical evidence for all this? The first and foremost is the
Cosmic Microwave Background. Our direct probe of the early hot dense
universe. So that reaches to the times when
universe is a few hundred thousand years old.
Or actually, it's a little in excess of 1000.
And it's certainly based on a very well understood atomic physics.
Next comes the cosmic nucleosynthesis, and that's based on well understood
nuclear physics. The results of that are abundances of
light elements that we can test observationally today.
So, both experiments, in theory, seem to be fairly reliable,
pushing in towards the big bang, all the way down to maybe, 1 millisecond.
Beyond that, things get to be increasingly more speculative.
We think that the a-symmetry between matter and anti-matter in the universe
was set around the time when the universe was about a micro second old.
This is based on a reasonably reliable physics of the standard model of particle
physics. Nevertheless it's not necessarily firmly
established, but and you, you can think of the actual observation that there is a
lot more matter than anti-matter in the universe as an evidence of may have
happened when the universe was a micro-second old.
Then we reach the inflationary year, when universe was 10^-32 seconds old.
Inflation makes, very testable predictions, and so far seems to be doing
fairly well. A lot of people believe that something
like inflation must have happened, which would be really remarkable giving us some
insight into physics of the universe when it was only 10^-32 seconds old.
But the upshot is that cosmological observations.
Observations of the largest things we can think of, can constrain particle physics.
Micro-physics, at the smallest scales, we can also think of.
Well, let's first talk about the cosmic microwave background.
In some sense, its existence is trivial prediction of big bang theory.
If the universe was expanding from hot and dense state, then there has to be a
thermal radiation relic from it. Initially, universe is all plasma,
everything is completely ionized, there are no atoms.
And when it gets cold enough, for atoms to form,
this is where this thermal radiation gets released.
So Alpher, Herman, Gamow and others made a prediction in 1940s and actually
estimated that the temperature of the cosmic microwave background would be
around 5 degrees kelvin. It was only 1960s that cosmologists began
to try to detect this in a serious fashion.
And there were upper limits until the actual discovery by Penzias and Wilson.
However, there was an even earlier not quite discovery, as early as in 1941.
And here is how There are molecules in space and their clouds such as sianogen,
sianogen happens to have some energy transitions that are highly sensitive to
temperature. And probe the regime around a few Kelvin.
So Walter Adams, obtained observations, and McKellar had a theory inferring from
these observations of these subtle little lines of sianogen that that gas was in a
thermal bath of couple degrees Kelvin. They didn't make anything out of it at
the time. You could think of other ways to heat up molecular clouds, like with
starlight. And that stayed forgotten, certainly
Gamow didn't know about it. Until Fred Hoyal, of all people, person
who was against the Big Bang dug up this evidence and pointed it out that this
could be in fact Consequence of this thermal background from the early years.
But then again, it was a little too indirect and subtle for most people to
believe. It took really direct discovery by
Penzias and Wilson to make the solid case for the existence
of cosmic micro background. Clearly this was a real milestone of
cosmology. So as we know, the universe is filled
with thermal radiation, temperature is 2.7 degrees Kelvin today, and is
essentially a perfect black body. we have looked for deviations from the
black body spectrum, and none were found. You could imagine such things happening,
if some energetic processes in the early universe dumped extra energy into it and
changed things away from the equilibrium thermal black body radiation but
Evidently, this did not happen. So the good news is that physics is well
understood. We know exactly how this went.
The bad news is there were no surprises. There was no new physics to be found.
The key discoveries there came from the COBE satellite, Cosmic Background
Explorer. COBE was launched to probe.
Cosmic microwave background in ways that would not be reachable from the ground,
at least, not at the time. And here are pictures of cosmic micro
sky. This is, the top one has the contrast
knob turned up to 1,000. And you can see the dipole radiation,
which was already known due to the motion of the milky way relative to the cosmic
micro background. Subtracting that And turning the contrast
knob to 100,000, you see a motley sky, and a big plane of galaxy with thermal
emission from dust and synchrotron electrons and what not.
And subtracting that galactic emission leaves just motley sky of fluctuations
that were primordial in nature. Cosmologists long looked for those,
but this was the first time they were actually really seen.
And it was one of the reasons why John Mather and George Smoot got Nobel Prize
for some of these discoveries. So let's estimate when, exactly, the
recombination of primordial plasma into the atoms happens.
Hydrogen becomes ionized if it absorbs photons with energies of 13.6 electron
volts or more. And this is 3 * voltsman constant * the
equivalent tempera, temperature of the gas.
So that immediately implies temperature of around 50,000 degree kelvin.
However, turns out, since there are many more photons than there are protons or
electrons by, by a factor of billion. Even a tail of high energy photon,
photons from a cooler gas or cooler temperature would suffice.
And we can estimate that using the Boltzmann equation for equilibrium shown
here. And it happens that given this over
abundance of photons over protons and electrons the temperature could be as low
as 2500 degrees and already one can get recombination.
Since the observed temperature of the micro background is now about 2.7
degrees, this immediately tells you that this
happened around redshift of 1100. Well, obviously it doesn't happen all at
once. There is a transition period as shown
here. What, what is plotted here is the
fraction of ionized hydrogen in the universe.
Which begins at unity. It's all completely ionized.
Then, slowly, the gas recombines,
the electrons are bound into the atoms, and the process ends by about regid of
about 1,100. So when we look at the cosmic micro
background, what we're looking at is a photosphere of the hot, ionized universe,
inside out. When we look at the star we see a
photosphere of ionized ice from the outside in.
So this is the inverted version thereof. You may recall when we talked about
models that dominate, dominated by matter versus radiation.
That because of the density, and it's density of photons changes in a steeper
way, with expansion than it does of the matter.
The two components must dominate with different times.
The energy density of the radiation decreases as the fourth power of the
expansion factor, or for the matter, it's only the third power simply the illusion
evolving. And, so given today's densities of matter
and microbacron radiation, which is well determined through Stefan Boltzmann
formula, we can find at what point did these two cross.
And the answer is around redshift of 5,000.
So this was before the recombination, but well after cosmic nucleosynthesis was
complete. Next, we will talk about cosmic
nucleosynthesis.