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.
