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You probably know this familiar story: In the beginning, the universe was packed tightly
together into a point of infinite density.
It then exploded into the universe we see today.
This is commonly thought of as the "Big Bang."
Astrophysicists will tell you that this image we have of the Big Bang model is incorrect.
First there was no explosion like a grenade where all kinds of shrapnel goes flying away.
There was no substance like stars or galaxies or even atoms that went flying, the entire
universe itself expanded and became bigger.
Atoms only coalesced a few hundred thousand years later as temperatures cooled. And larger
structures took much longer to form.
The big bang was not when the universe had zero size or infinite density.
It is just a moment in time when the universe was very hot and very dense.
And the big bang did not happen at some special point in space, but everywhere in the known
universe at the same time.
And contrary to popular belief, the big bang model is not a theory of how the universe
began.
Science doesn’t currently tell us how or why the universe came to exist or what caused
it to start expanding.
The big bang model is about events that happened in the early universe, which led to the universe
we observe today.
That history is well understood and has been verified by evidence that we can observe even today,
13.8 billion years later.
But, up until the 1980s this model failed to explain some other observed properties
of the universe, which remained puzzles, like why it is so homogenous, why its geometry
appears flat, and why there are no magnetic monopoles.
Along came the theory of cosmic inflation proposed by Alan Guth and others, which solved
many of these puzzles, and is today considered by most physicists to be a standard part of
early Big Bang cosmology.
What is this theory of Inflation?
How does it solve some of the most nagging cosmological puzzles?
And what caused it to happen?
That’s coming up right now…
Today, the evidence for the Big Bang model is so strong that virtually no scientist seriously
disputes that it’s an accurate description of the history of our observable universe.
But some observations leading up to the 1970s had found some mysteries that, the model in
its original form, could not account for.
In order to develop into the universe that we see today, the early universe had to have
some very specific properties, some of which seemed implausible with the early big bang
model.
Specifically, the questions left unanswered in the model were: Why was the early universe
so uniform?
Why is the universe so close to being geometrically flat?
Why do we not see any magnetic monopoles, which could theoretically exist?
In 1981, American physicist, Alan Guth wrote a paper in which he proposed the idea of cosmic
inflation which could solve these apparent mysteries.
This is not the kind of inflation that makes your money worth less.
It means a sudden expansion.
The simplest version of the theory of inflation says that the universe expanded exponentially
fast, faster than the speed of light, near its earliest history – from what is believed
to be the beginning of our universe, from about 10^-36 seconds after the beginning to
10^-32 seconds.
During this time, it expanded a factor of at least 10^78, going from being very small
to being exponentially large in comparison.
You might be thinking, “I thought things couldn’t move faster than the speed of light?”
Einstein’s theory of special relativity shows that this speed limit applies to things
moving within space, but not the expansion of space itself.
So this is permissible.
Note that not all physicists agree that the beginning, that is the point when time first
started, t=0, is the same as the beginning of the Big Bang.
Some physicists believe that the big bang happened after inflation around 10^-12 seconds.
Also note that some descriptions of inflation say something like the universe started out
smaller than an atom and then expanded to the size of a grapefruit.
You should keep in mind that these analogies can be misleading because they imply that
the universe has an edge.
It doesn’t.
There is only the universe and nothing else.
There is no outside.
While we are at it, let’s dispel some other common misunderstandings about the beginning
of the universe.
The idea that the universe started from a singularity, that is a point of infinite density
and heat, is not correct.
It is purely due to mathematical extrapolation.
It is like getting a zero in the denominator of a mathematical equation.
It is undefined and represents the limit of our knowledge.
A singularity is probably not a physical thing, and is really not what the Big Bang theory
says.
It is part of the standard model of cosmology.
Basically, what the theory says is that the universe today is bigger than it was yesterday,
and is much bigger than it was a billion years ago.
So as you extrapolate back, the universe gets smaller and smaller, denser and denser, and
hotter and hotter.
So as you keep going you get to a very small volume of space which is very dense and very
hot.
At some point in this extrapolation, our equations stop working because the volume becomes zero.
But there were probably some other laws of physics that applied at this stage, perhaps
some kind of quantum gravity, that is gravity at infinitesimally small scales.
We don’t have such a theory yet.
This is something that physicists are feverishly working on.
Along the same lines, the universe today is expanding, but galaxies aren’t actually moving at that
expansion rate. Only the space between galaxies is becoming larger and only on very large
scales.
Gravity also plays a role so that on smaller scales gravity still holds stars together
within a galaxy, and certain nearby galaxies are still attracted to each other, like the
Andromeda galaxy is to our Milky Way galaxy.
While the whole universe may come from a tiny volume of space, cosmic inflation would have
caused all points within that space to expand.
And this expansion or inflation happened everywhere in space.
There is no center of the universe or location.
Every point moved away from every other point.
Expansion faster than the speed of light during inflation also means that these points, while
they may have been causally connected initially, would have moved apart after inflation, such
that they became causally disconnected.
This is because causality is limited to the speed of light.
No information can travel faster than this speed.
As a consequence of this, there are certain parts of the universe that we will never
be able to see or detect in any way, because any light or gravity coming from it, will
never reach us.
Let’s now see how Inflation could explain the three problems with the Big Bang model
– the observed homogeneity, the flatness issue, and the no observed magnetic monopoles
issue.
When we look out into space, we see that the universe on large scales is pretty much uniform.
The universe is extremely homogenous and isotropic which is just physics jargon for saying, it
appears roughly the same anywhere your look.
This can also be seen in the cosmic microwave background, or CMB, where the tiny differences
you see on this image represent temperature fluctuations of at most only 0.0001 Kelvin,
or less than one ten thousand of one degree.
To see how inflation explains why the early universe was so uniform, let’s imagine that
before inflation, the universe was completely random.
Maybe in one place the density was very high and a fraction of a millimeter off to the
side the density was very low.
You can imagine it like the surface of deflated balloon.
There may be tiny imperfections like different thicknesses and wrinkles randomly distributed
on it.
Now imagine that this balloon is suddenly inflated to a very large size.
A tiny fraction of a second later, the wrinkles on the balloon’s surface get smoothed out.
Any density differences of the balloons skin become diluted.
This is analogous to what happened during inflation as a tiny volume increased by at
least 10^78X.
Now let’s look the flatness problem.
Imagine living like an ant on the surface of a very small balloon.
It would be a 2 dimensional world.
But if the balloon was tiny enough, it might be obvious to you that this surface was curved
and that you were living in a closed or curved universe.
However, if that ball expanded to the size of the Earth, it would appear flat to you,
even though it is still a sphere on much larger scales.
Now scale that up to human size and the ball being much larger than even the observable
universe.
To you, it would appear to be flat as far as you could see, even though it might have
been obviously curved to start with.
Inflation stretches any initial curvature of the 3-dimensional universe to near flatness.
We don’t know if the universe is 100% flat, but if there is curvature, it is so small
that we can’t measure it with our current technology.
Note that when I say curvature, it would be an overall curvature of the universe in four
dimensions.
This is not easy to visualize, so we have to visualize it as the 3D curvature of a 2D
balloon’s surface.
A closed curvature for the universe would mean that two parallel lines would eventually
converge in our 3D universe, just like two parallel lines on the 2D surface of a balloon
converge.
How does inflation solve the monopole problem, the fact that we can observe no magnetic monopoles
in the universe.
Well, monopoles can only theoretically form at very high temperatures, the kinds of temperatures
that were only present during the big bang.
But once they formed they should be stable enough to survive.
The idea is that Inflation would have quickly cooled the universe because of rapid expansion,
and only monopoles created prior to inflation would then continue to exist in the universe.
But during inflation, the density of monopoles would drop exponentially, so their abundance
would drop to undetectable levels because they would exist in such low densities.
Imagine that just before inflation there were a thousand monopoles and they were all packed
into a cubic meter of space.
A fraction of a second later, those monopoles were spread out in a region 10^78 cubic meters
across.
This would make them so rare that we may never detect it.
Now as the CMB attests, the universe is not completely smooth.
We can see that there were small imperfections, small temperature differences.
This is called the anisotropy of the cosmic microwave background.
You might say well, doesn’t this ruin the idea of inflation if the universe isn’t
completely uniform?
No, these small anisotropies are exactly what we would expect.
Inflation only makes the large scales like the CMB look uniform.
But the small anisotropies can actually explain the origin of the observed structure in the
universe.
Prior to inflation, the universe we see today was microscopic.
The quantum fluctuation within the density of matter on these microscopic scales could
have expanded to astronomical scales during Inflation.
And this is what we see across the universe as higher density regions condensed into stars,
galaxies and galaxy clusters.
Now the big question you may be asking is how did inflation start?
What was responsible for inflation?
This is not well understood.
It is thought that there may have been a scalar inflation field during the time of the big
bang.
What is a scalar field?
As an analogy, imagine a room with a fireplace in it.
Every point in that room will have a temperature associated with it.
So if you were to measure the temperature anywhere in that room, it will have some kind
of distribution.
We would be measuring just a magnitude of the temperature.
This is akin to a scalar field.
It has only a magnitude.
Now imagine that same room with a giant magnet in the middle of it.
Every point in that room will have a magnetic field which will not only have a magnitude,
but also a direction.
In other words, if you had a charged particle or small magnet somewhere in that room, that
object would experience a force, that has not only a magnitude, but also a different direction
depending on its location in the room.
This is like a vector field.
It has a magnitude AND a direction.
Magnetic fields and gravitational fields are vector fields like this.
The Higgs field and Inflation field are scalar fields analogous to the temperature in the
room.
From a theoretical point of view, assuming that an inflation field existed, we can show
this field with the following diagram.
In the very early, very hot universe, the inflation field would have had some value
at the point a.
This would have been the lowest energy density that the field could be in, at the high temperatures
and energies of the universe at that time.
The theoretical inflatons, or particles of the inflation field, would
have remained at this point.
However, point A would not have been the true vacuum, but a false vacuum, because as temperatures
cooled, the field formed a lower energy potential at point c.
Since A was not the lowest point in terms of energy density, it is called a false vacuum.
The natural place for the field to be as the universe cooled, and got to lower temperatures
and energies was actually C, the lowest energy density of the field.
All natural systems tend towards their lowest energy state.
So when the inflation field is stuck at A, it will tend towards C.
But before it can get there it has to overcome a small barrier, B.
This can be accomplished with the help of quantum tunneling.
This can allow the field to overcome the barrier at B, and drop down to its lowest energy state
at C.
When the energy difference between point A and point C becomes very large, larger than
any other energy in the universe, inflation starts.
When the field reaches the lowest minimum energy density in the potential at point C,
Inflation comes to a stop.
This is a very short process because as I said earlier, inflation lasts only from about
10^-36 seconds to about 10^-32 seconds after the beginning.
So the scalar field causes the almost-instantaneous, exponential expansion that we call cosmic
inflation.
As the field reaches its lowest potential, inflation ends, and it decays into other fields
and particles.
After this event, the universe continues to expand, but in the slower way that the original
big bang model described.
So inflation theory solves several problems in one fell swoop.
In my next video, I will cover an even more fascinating aspect of inflation, and that
is the theory of "Eternal Inflation."
This theory, if correct, would mean that our universe is but one of a near endless myriad
of universes, most of which are unimaginably bigger than ours.
This could also actually explain why our universe is like the way it is.
So stay tuned for that.
I’d like to give a big shout out to professor Gary Felder of Smith College, who inspired
this video.
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And if you have any questions or comments for me or other viewers, please leave them
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I’ll see you in the next video my friend.