Transcriber: Maria K. Reviewer: Queenie Lee
Hi.
Let's talk about science and don't think otherwise.
Today I am going to talk about
a kind of new scenario for studying the universe,
which is basically about string cosmology: string gas cosmology.
So, in this way, I am trying to combine cosmology,
which is the study of the universe as a whole,
its birth, if any, and its evolution.
And from the other side, I want to combine this with the string theory,
which is the most fundamental theory in physics,
in which the role of a particle is replaced
by a role of a concentration of energies in the form of strings.
The strings could be closed or open.
Since I belong to one of the oldest civilizations on this planet -
I am from Iran -
I was looking whether there is a translation or something
about the string cosmology, in Persian.
And guess what, I found one.
If someone is working on a string cosmology,
it means he talks a lot,
but he doesn't know necessarily what he is talking about.
Basically, he could be a philosopher, in a way, or a liar.
Anyways, we will see which one we are.
Okay, so the motivation here for this scenario,
as I mentioned to you, to combine a string theory with cosmology,
was to see whether we can find an alternative way
to describe the very early universe.
The early universe usually is described by one of the most successful framework
in cosmology and particle physics, which is called inflation.
But inflation is not as fundamental as string theory.
So we were hoping that we would be able to study the early universe
in the context of string theory,
and moreover, whether in this context
we would be able to describe all these structures in the universe,
big structures like galaxies and other stuff in this framework.
Τhis was our motivation.
But this motivation is based on some assumptions.
The first assumption is that we're going to assume that the universe is flat.
Okay?
Like many people, who still believe that the earth is flat.
But this time we are going to assume that the universe is flat.
It's not like sphere. It's flat.
Also, we are going to assume that it has a particular shape,
a topology, which is like a doughnut, okay?
So it's just like a torus.
And it's a higher dimensional torus.
We're going to also assume
that there are three really large spatial dimensions,
although if you relax that assumption,
we will later on see that the only way that we can get correct result
would be that we restrict our spatial dimensions to number three.
Also, we're going to assume,
because we're going to assume that the universe is flat,
we're going to assume that there are so much disorderedness in the universe.
Namely entropy.
So we are going to transit from particle physics,
as you can see,
to strings.
Strings provide more degrees of freedom.
One of the degrees of freedom is winding.
So strings can be wound around the space.
That provides more freedom.
This freedom is associated with a number, which is called a winding quantum number.
Ιt's a new number in addition to what you get
for ordinary particles in physics.
The equation you see on the screen
is not for scaring you to believe that I am not a lawyer.
It depends on some equations.
So if you do this transition, many interesting things happen.
First of all, because the strings have limited and finite lengths,
when they vibrate, the vibrations are very limited.
The limitation of the vibration of strings caused to be an upper limit
for the temperature of the strings that they can achieve.
So as you pump energy to a particle,
the temperature of the particle can go up higher and higher,
ultimately to infinity,
this is not the case for strings.
Strings have upper limits of thermal energy or temperature.
That means there is a maximum temperature for strings,
unlike for particles.
If you do that you, will see there are some strange things happening here.
There is sort of duality between smallness and bigness.
So there is no difference between small and large.
The physics for small entities is same as the physics for large entities.
There is some sort of symmetry, new symmetry in nature,
appreciated by these windings of the strings,
and this new symmetry is called T-duality for some historical reason.
If you add these two features into cosmology,
and we know our universe is expanding,
you will see some strange things happening.
There is a dual to our universe.
So there is [the] expanding part to the universe,
and there is another dual part to the universe which is contracting.
These two universes, which are dual to each other,
one is small, the other large,
are connected through a self-dual point,
self-dual zone, which mimics the big bang.
So in this scenario, there is no initial big bang,
but the self-dual point that connects the two universes.
So for each and all of you here,
there is a dual to you in the dual universe.
Our universe is expanding, the other universe is contracting.
That's a very strange scenario.
As I mentioned to you, in this scenario,
we can have a universe previous to the big bang,
or the so-called big bang
because there is no big bang any more in this scenario,
and the one which is after the big bang.
And all the equations remain finite, no infinity.
All the Einstein equations -
which usually you see, will blow up at the initial big bang -
here would be finite.
So there is no initial singularity in this model.
Now I want to see how I can get the structure formations here,
the galaxies.
The galaxies, all of us, are made of the fluctuations in the vacuum.
Believe it or not, this is the case.
So, I'm going to study the fluctuations of these strings, closed strings,
in little Cosmoland, which I am going to call it Kayhaanistan;
and then, I'm going to study just little of this, little Cosmoland,
which I'm going to call it Kayhaanak;
and then, I'm going to look for the vibrations of these strings.
The thermal vibrations of these strings.
Remember I told you there is an upper temperature
and maximum temperature for strings.
So, in the standard cosmology,
at the beginning, the temperature is infinite.
In this model, the temperature is not infinite, it's finite.
This temperature is called Hagedorn temperature.
So at the beginning, instead of inflationary era,
we have an era which is called Hagedorn era.
These closed strings are vibrating, and as you can see in the graph behind me,
the temperature is maximum and remains constant.
To the right of the graph, you see the expanding universe,
and to the left, you see the contracting universe.
Remember the duality I was talking about?
You can see it even in the temperature.
And then, after this Hagedorn phase,
the universe finds itself in the radiation-dominated universe
and the rest of the story will be the same as the standard cosmology.
So you can see the difference
between string cosmology and inflationary cosmology
is in the inflationary cosmology,
the radius of the universe goes through a huge expansion.
As some people call it,
the fabric of space-time goes under superluminal expansion.
But here, instead of expansion,
the universe started to be really, really huge -
in the context of string theory, huge means one millimeter -
and then collapses to close to the Planck length.
So, the fluctuations generated by strings
will exit the radius of the observable universe
and then re-enter the universe at some later time.
How we produce gravitational waves?
Remember I told you strings are winding around the space.
So now if the pressure of this winding changes by a little bit,
there would be some - as if you have inflating tube,
and this inflation in the tube circulates around the tube,
and that produces the gravitational waves as you saw in that animation.
So, in this scenario, we were able to show
that we can produce both the fluctuations in energy,
which produce galaxies and all of us,
and the second thing, which hasn't been tested yet,
it produces gravitational waves, which is a little bit different
than the way you produce gravitational waves
in inflationary models.
So our prediction is different
than the one which is predicted by the cosmic inflation.
In this scenario, we find out,
because strings provide more degrees of freedom,
for instance, the winding number,
we see some duality between the small and large,
and with that, we see that the temperature of the universe
would be finite and there is a dual to our universe.
There is a quasi-static universe at the beginning
instead of superluminal expansion, which is called the Hagedorn phase,
and then after that,
there would be a phase transition to the radiation-dominated universe.
It is the only theory, which provides you why we are living in three dimensions,
not three spatial dimensions, not more than three,
because it wouldn't match the observation.
And finally, what this scenario lacks
is the following:
The assumption I made at the beginning: Why the universe should start to be flat?
How can we solve the flatness problem?
And why there are some features
that inflation can give a solution like entropy,
we cannot find that solution in this scenario yet.
So maybe you can find out a solution to this problem
and that would be helpful to me.
Please contact me afterwards.
Thank you for your attention.
(Applause)