At the very beginning, our Universe went through
a period of exponential, faster-than-the-speed-of-light
stretching of space known as inflation.
The stretching was caused by a repulsive form
of gravity and the Universe doubled in size
at least 100 times, going from the from the
size of a billionth of a proton to the size
of a grapefruit.
Inflation ended when the repulsive energy
decayed into ordinary matter and energy,
at which point ordinary Big Bang expansion took over.
The theory of inflation explains two properties
of our observable universe;
(i) the uniformity of space and
(ii) the apparent flatness of the universe.
The observable universe, as the name indicates
is the universe that we can observe from earth;
that is light from the contents of the observable
universe has had time to reach our instruments
on earth.
Assuming that the universe is the same in
all directions, the distance to the edge of
the observable universe is the same in every
direction, so the shape of our observable
universe is a spherical ball with the observer
at its centre.
At the largest of scales the observable universe
displays an extraordinary level of uniformity.
There are of course distinct structures in
space, such as galaxies and stars, however
from a distance far enough away, all of space
seems to look the same from all directions,
similar to how sand in a desert will appear
uniformly distributed if observed from a plane
flying high up enough in the sky.
The theory of inflation allowed the contents
of the baby Universe sufficient time to interact
and come to a uniform density of energy and
temperature before the rapid expansion occurred.
It is that uniformity which is the present
uniformity of space we observe today.
Without inflation the Big Bang model would
not have given the baby Universe enough time
to reach uniformity in this way.
Spacetime incorporates the three dimensions
of space (up and down, left and right
and back and forth) and the fourth dimension of time.
General relativity describes how mass and
energy can curve spacetime, and accordingly
the mass and energy of the Universe can also
curve space time.
There are three possible curvatures the Universe
could have:
1). positive curvature
2). negative curvature or
3). no curvature at all, i.e. be flat
The critical density of the Universe
is the mass and energy contained per unit
volume of space needed for a universe to be flat.
That means the gravity produced by the mass
and energy in the universe is such that it
perfectly balances out the stretching of the Universe.
When the Universe has critical density it
continues to stretch at a decreasing rate
forever until infinite time has passed whereupon
the stretching will finally halt.
But as we all know, infinity has no end.
The density parameter, symbolised by the letter
Omega in the greek alphabet is used
to determine what curvature the universe has.
It is the average mass-energy density of the
Universe divided by the critical density.
If the density parameter > 1, there is positive curvature.
If the density parameter < 1 there is negative curvature.
If the density parameter is exactly 1, i.e the average mass-energy density
of the Universe is the same as the critical density, then the Universe is flat
For the Universe to have positive curvature
there is enough mass-energy density in the
Universe to produce gravity that will warp
spacetime enough to eventually overcome the
stretching of space so that the Universe will
collapse back in on itself to form a closed Universe.
For the Universe to have a negative curvature
there is not enough mass-energy density to
produce enough gravity that can halt the stretching
of space, so the Universe will be an open,
saddle-shaped, Universe which continues to
stretch forever at a decreasing rate.
And we have already covered, that in a flat
Universe there is just enough mass-energy
density to produce gravity that perfectly
balances out the stretching of the Universe.
When describing the Universe as flat, it is
not meant that it's flat in the same sense
as a 2D shape is flat, but rather that the
geometry of the universe is such that parallel
lines will never cross, the angles in a triangle
will always add up to 180 degrees, and the
corners of cubes will always make right angles.
In a closed Universe the angles of a triangle
would be more than 180 degrees, in the open
saddle-shaped Universe, the angles of a triangle
would add up to less than 180 degrees and
in the flat Universe, as mentioned, the angles
of a triangle would add up to 180 degrees.
We can calculate the density parameter of
our Universe using two methods and both methods
indicate that it is very close to 1, therefore
suggesting that our Universe is very close
to being flat.
The first method involves counting up all
the mass-energy in the universe, taking its
average density then dividing that average
by the critical energy density.
The second method involves drawing a massive
imaginary triangle on the cosmic microwave
background radiation and seeing if the angles
of the triangle add up to 180 degrees.
The cosmic microwave background radiation
also known as the CMB is the oldest light
in our Universe that has travelled to our
instruments. As the photons of light travelled
across the universe their wavelengths stretched
transforming the photons into microwave radiation.
When the imaginary triangle was drawn on the
CMB it turned out that the sum of its angles
did indeed add up to 180 degrees indicating
that our universe is indeed flat.
Before we get too excited and state definitively
that our Universe is flat we have to remember
that the exponential stretching of space brought
on by inflation drives the Universe toward
flatness, smoothing out the fabric of the
Universe.
We can only measure the observable universe
which may be part of a much larger Universe
which might not be flat.
The analogy that is often used is how an ant
sitting on the surface of a large inflating
balloon would assume that the surface it is
sitting on is flat because that's how it
would always appear to him from his tiny perspective.
We may be that ant and the Universe is so
massive that whatever we can measure
will appear flat to us.
And that in a nutshell is cosmic inflation.
As always I hope you enjoyed this video and
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