This lecture we're going to go back all the way to the beginning, or
at least as close to the beginning as we can possibly get.
We're going to ask the question, how far back in time does our physics extend to
the very beginnings of the Universe and the big bang.
We're going to explore the Universe in its earliest phases in a lecture I call
The First Three Minutes.
The Universe we see today is cold, dark, and low density.
We see hundreds of billions of galaxies out to billions of light years away from
us in all directions.
But it didn't always look this way.
13.8 billion years ago, the Universe was small,dense, and hot enough to be opaque.
When the Universe was 380,000 years old it suddenly become
transparent to light producing the cosmic background radiation that we see today.
This red and blue map that you see here in the middle of the screen,
is the best map to date made of small fluctuations in temperature seen in
the cosmic microwave background.
We're seeing the Universe as it was when it was only 380,000 years old.
How far back however can we go in cosmic history?
How far back does our knowledge pf state of the art physics press and
how close can we get to the very beginnings of everything at
the very start of the big bang.
To understand what governs the physics as we go back into the hotter denser
phases of the Universe, we need to introduce the concept of binding energy.
Binding energy is the energy that is needed to break apart or
unbind different forms of matter.
It often helps to express binding energy as an equivalent binding temperature
at which different forms of matter become unbound and melt.
Many of us find it easier to think in terms of temperature
rather than energy expressed in some unit like joule or electron volts
because we have at least an everyday human scale notion of hot and cold.
So too we can use temperature, as a stand in for energy, because temperature and
energy are just different ways of expressing the same thing.
Here are four different states of matter, the relevant size scales and
the typical binding temperatures.
Atoms consist of electrons bound to atomic nuclei.
The typical size scale is 10 to the -10 meters and they become unbound.
The electrons are stripped away from the nuclei
when the temperature exceeds the binding temperature of about 1000 degrees Kelvin.
For example, when we talked about the cosmic microwave background the Universe
became transparent when the temperature dropped below about 3,000 degrees Kelvin.
And electrons could then combine onto hydrogen nuclei and
theUniverse suddenly became transparent.
This number for
the binding temperature that I'm going to use is a round number temperature.
Atomic nuclei consist of protons and neutrons.
They have size scales four orders of magnitude smaller than atoms,
at 10 to the -14 meters.
And the binding temperature of an atomic nucleus is 10 to the 10 degrees Kelvin,
10 billion degrees Kelvin.
Protons and neutrons can actually break apart into quarks.
Protons and neutrons are made each of three different types of quarks.
The typical size scale of a proton and neutron is 10 to the 5 is 15 meters and
have a binding temperature of about 10 to the 11 degrees Kelvin,
100 billion degrees Kelvin.
Finally the most fundamental form of matter of all, quarks.
The individual particles of which all heavy particles are made up, have a size
scale three orders of magnitude smaller still and 10 to the minus 18 meters and
have binding temperatures of 10 to the 13 degrees Kelvin.
When the temperature exceeds 10 to the 13 degrees Kelvin, quarks and antiquarks come
into equilibrium with photons, and we no longer have stable quarks existing.
The quarks melt into a sea of mass energy, as a way of thinking about it.
For example, atoms ionize into ions and electrons at about 3000 degrees Kelvin.
Nuclei in the cores of massive stars begin to melt when the temperature exceeds
about 10 billion degrees Kelvin.
If you remember back to Unit 2 when we talked about the evolution of massive
stars, it was when the core of the star reached 10 billion degrees that the nuclei
began to melt and hastened the star to the collapse that led to a supernova.
The idea of binding energy is derived from each of the four fundamental
forces of nature.
We're used to a couple of these forces on every day scales.
The gravitational force is the weakest force of nature.
It acts to bind masses together at very long range.
Planets are bound to stars.
Stars are bounded to galaxies.
And galaxies are bound to other galaxies within clusters and
super clusters throughout the Universe.
Gravity is probably the one force we have the most every day encounter with
if you jump up, gravity pulls you back down.
The electromagnetic force is the other force we have everyday experience of.
It is about 10 to the 36 times stronger then gravity And
it acts to bind atoms and molecules together.
Our experience of the electromagnetic force, in many ways,
is much more personal.
We are held together by the electromagnetic force.
The weak nuclear force is about 10 to the 25 times stronger than gravity.
And the strong nuclear force is about 10 to the 38 times stronger than gravity,
are the two forces that work on very small scales, on nuclear scales.
The weak nuclear force is the force fundamentally behind many forms
of radioactivity in nuclei the neutron, being unstable, breaking into protons and
electrons, is a manifestation of the action of the weak nuclear force.
The strong nuclear force is the force that binds protons and
neutrons together into atomic nuclei.
The range of this force, both the weak and
the strong nuclear force, are examples of what are called short range forces.
They do not occur on scales of human beings or on the scales of rooms.
They occur only on the scales of the atomic nucleus.
They have ranges of activity between 10 to the minus 15 and
10 to the minus 18 meters.
On those scales the weak and
strong nuclear force dominate over all other forces on the problem.
When I go up to atomic scales, the electromagnetic force is stong.
And it continues to stay strong, even to the scale of people and large objects.
Lightening is a good example of the electromagnetic force in action.
Finally, the gravitational force works on the very largest scales of all.
Scales extending all the way up to the Universe, itself, on large scales.
At very high energies, our current models of physics show that the physical
behaviors of the four fundamental forces begin to merge or unify together.
All of the important milestones in the evolution of the hot
dense early universe are going to occur at the times
of the unification of the various fundamental forces among each other.
This diagram that I'm going to show,
which looks like a tree, shows the division of all the forces of nature.
In the present day, here we are at the extreme right of the diagram.
The temperature of the Universe is about three degrees Kelvin,
three degrees above absolute zero.
And we live in a universe in which there is gravity, the electromagnetic force, and
the strong and weak nuclear force.
But this was not always the case.
As we go into the past,
as we go to the left in this diagram, The temperature will increase and
at various temperatures we will see that some of these forces will begin to
merge together into a smaller number of fundamental forces or interactions.
Many of the major milestones of the Universe will occur when these changes
in the interaction among the four fundamental forces occur.
This is part of what allows us to push our ideas of physics further and
further back in time.
What's the story of the evolution of the Universe,
from as far back as our physics works?
Let's go. The very first place we can talk about
in the beginning of the Universe is not time T equals zero.
But a time that occurred 10 to the minus 43 seconds afterwards
we enter was called the Planck Epoch.
The Planck Epoch was when all the four fundamental forces of nature
were unified into a single superforce.
There is no gravity,
weak force, electromagnetic force or strong force as separate entities
each working with their separate interactions on their separate scales.
All the forces are unified into one.
Everything in the Universe is packed together.
The temperature exceeds 10 to the 32 degrees Kelvin and
all of the forces act as one superforce.
We can't say too much about what the Universe is like at this scale,
because we do not yet have a good quantum theory of gravity.
We do not know how to incorporate gravity with the strong electromagnetic and
weak forces.
All three of which have reasonable good descriptions in the quantum theory.
As a consequence we don't really know in detail what goes on.
But what we can say is that spacetime as we understand it today
would have emerged from whatever the Big Bang was at the end Planck Epoch.
Before the Planck Epoch with gravity combined in and
unified with the weak, strong, and electromagnetic forces,
our notions of spacetime today would have been very, very different.
How different?
We really won't know until we can crack the puzzle
of the quantum description of gravity.
But this epoch doesn't last very long.
By the time the 10 to the -43 second Planck era ends, when the temperature
reaches about 10 to the 32 degrees Kelvin, gravity separates from the superforce.
And now there are two forces at work in the Universe, gravity and
a combination of the strong, weak, and
electromagnetic force called the Grand Unified theory or GUT force.
The strong electroweak, the strong and electroweak forces are all still unified.
In the Universe in this GUT Epoch is a hot, dense soup of photons,
quarks, and antiquarks all in equilibrium.
A quark and an antiquark collide, annihilate together, and
divide up into two photons, carrying away their energy.
Two photons of that same energy colliding will suddenly pop a quark-antiquark pair
out of the vacuum.
Before those quarks and any quarks could do anything interesting, like form
some other particles, they will quickly annihilate and turn back into photons.
The entire universe is a soup of matter, coming in and
out of existence with gravity.
Spacetime as we understand it, is now fully formed, at least we think it is,
[LAUGH] you never tell with this stuff it's really early in the Universe.
But matter, as we understand it, has not yet emerged.
We're still a hot, dense soup of energy and matter, changing back and
forth in form.
This changes at 10 to the minus 35 seconds, at a temperature of 10 to
the 27 degrees Kelvin when we enter the Inflationary Epoch.
The Inflationary Epoch begins when the strong force separated from the GUT force.
At this point, there are now three forces in the Universe,
gravity, the strong nuclear force and the unified electromagnetic and
weak force that we we call the electroweak force.
At this phase, we end up with three forces and
there's a rapid separation of the electromagnetic and strong force triggers
a rapid exponential growth inside scale of the Universe.
This rapid exponential growth is so fast it is called inflation.
In the Inflationary Epoch,
the Universe grew exponentially by a factor of almost 10 to the 26,
between to the minus 36 and 10 to the minus 32 seconds from the Big Bang.
It started out very small and very compact, but at 10 to the minus 36
seconds, it suddenly blew up enormously in scale and
began the general slow expansion that now see today.
Before the Inflationary Epoch we only have a best speculation on what the physics
was, but after the inflation area epoch, the Universe is more and more beginning
to resemble very hot, very dense version of the type of universe we see today.
Not only in terms of content, but in particular in terms of scale.
It's what we would expect if we took the current day expansion in the scale of
the universe and ran it backwards in time we would end up at a very early time, 10
to the minus 45 seconds, with the Universe being a lot larger then it actually was.
So our real knowledge of physics really starts to begin
at the end of the Inflationary Epoch.
What is inflation do for us?
One of the things that inflation does is it helps explain why the Universe is so
nearly perfectly geometrically flat.
As we mentioned in previous lectures,
there is a lot of observational evidence to show that we live in a flat
infinite universe with omega naught approximately equal to one.
Why is it we are almost exactly flat?
Because the range of possibilities from high density to low density is enormous.
Why do we pick and zero in on exactly flat?
The answer, surprisingly, may be that the Universe may have had no choice but
to be flat because of the action of inflation.
Here's why. Let's view the Universe
in a very simplified form as a steadily expanding sphere.
It really isn't but this is a way to look at the problem of inflation geometrically.
When the sphere is small, and
we look at it on the scale of the sphere, we can see the curvature of the sphere.
It's obviously, well, spherical.
But now blow it up by a factor of three.
Now on the local scale of our square the curvature is a little bit less.
Blow it up by another factor of three to a factor of nine and
the curvature becomes flatter.
And by the time we get to a factor of three times three times three, or 27,
now the grid lines are looking pretty flat.
The curvature is not as obvious.
But example of this every day?
Walk outside and look around.
We're standing on the surface of a sphere, nearly 13,000 kilometers in diameter.
And yet, the Earth doesn't look round to us, the Earth looks flat.
Why?
Because the scale on which we are able to see is much smaller
than the radius of curvature of the Universe or the curvature of the Earth.
So to it is with the Universe.
When we look at the Universe today, we see it as nearly flat.
It's radius of curvature must be at least 40 billion parsecs.
Perhaps even more, it is to all intents and purposes, geometrically flat.
So while the Universe may have started out in a shape more like the sphere down here,
what inflation does is it so
rapidly expands the size scale of the universe that it effectively flattens out.
it would be very hard to have a flat universe from the beginning,
Ab initio, in a classic way without invoking this exponential inflation.
This is going to be a very important fact to us,
because this inflation is very important for
understanding not only why the universe is flat but why the Universe has structure.
Structures that are important because those structures gave birth to stars,
planets, and ultimately to us.
And that's what we'll pick up in the next lecture.