Is our universe finite or infinite?
We know that about 13.8 billion years ago, the Big Bang gave birth to the universe we
live in.
We know also it's been expanding and cooling ever since.
Actually, we know a lot about it, but something is missing.
For example, we can't really answer the question: is our universe finite or infinite?
We know a bunch of things about its size.
We know it's way bigger than we can imagine.
Also: Is it a flat universe?
Or a curved one?
Follow me in this video to get to know more about the shape and size of our universe!
I'm already excited!
What about you?
Cosmology is the discipline that tries to explain how our universe began, and how it
evolved.
Cosmology also tries to put some bounds to its shape and geometry.
It does so with the help of some basic principles of physics.
For example, cosmologists are very lucky because lights propagate with finite velocity.
From our point of view, we can look back some 46 billion light-years in all directions,
thanks to the speed of light and the expansion of space.
This is of course a huge distance, but...it is not an infinite distance.
It's just all that we can see.
What if we asked ourselves what lies beyond that?
Could the universe be infinite?
That's what cosmologists want to know.
Cosmologists also know very well that as we look to greater distances, we look back in
time.
For example, the nearest galaxy, the Canis Major Dwarf galaxy, is some 25.000 light-years
away.
This means it appears to us as it was 25.000 years ago because the light requires that
much time to journey to our eyes from when it was emitted.
Of course, more distant galaxies appear as they were tens of millions, hundreds of millions
or even billions of years ago.
Again, looking further is looking back in time.
So, here comes a simple and ingenuous question: why not go all the way back to the beginning?
When the light was emitted?
Why not go to 13.8 billion years ago?
Actually, we can't.
At that epoch the universe was so hot that all the atoms were ionized: if we had a powerful
telescope capable of seeing back to 13.8 billion years ago, we wouldn't see anything.
The universe was opaque, at that time.
But there is one type of radiation that we know originated 13.8 billion years ago, and
we can still detect it, even if it is losing energy time by time.
It is called the cosmic microwave background, which is the leftover glow from the Big Bang.
When astronomers looked at the properties of the CMB, they found that the universe was
almost perfectly uniform in the beginning, but some regions were more or less dense than
average, by only 1-part-in-30,000.
That's enough to grow into the stars, galaxies, galaxy clusters, and cosmic voids we see today.
But these early imperfections that we see from this cosmic snapshot encodes an incredible
amount of information about the Universe.
One such piece of info is a startling fact: the curvature of space, as best as we can
tell, is completely flat.
If space were positively curved, like we lived on the surface of a 4D sphere, distant light
rays would converge.
If space were negatively curved, like the surface of a 4D saddle, distant light rays
would diverge.
Instead, distant light rays move in their original direction, with the fluctuations
we have indicating perfect flatness.
But what does the CMB have to do with the size of the universe?
Well, combining both the cosmic microwave background and the large-scale structure of
the Universe we get some constraints that tell us that if the Universe is finite and
loops back in on itself, it needs to be at least 250 times the extent of the part we
observe.
Since we live in a 3D volume, 250 times the radius means (250)3 times the volume or more
than 15 million times as much space.
This is a huge number.
Our universe is very big.
Probably we can't even think about this huge size.
However, a lower bound of the Universe being at least 11 trillion light-years in all directions
is tremendous, but it's still finite.
So it seems we definitely live in a finite universe.
But the reality is always more creative than math.
If one thinks about it, there's a reason to believe our Universe is even bigger than that,
though.
And the reason resides in the so-called cosmic inflation.
What is that?
And why is it the reason to believe our universe could be infinite?
"Before finding out the answer to this question, be sure to like or dislike the video so that
we can continue to improve and make these videos better for you the viewer.
Plus, be sure to subscribe to the channel clicking the bell so that you don't miss ANY
of our weekly videos."
The hot Big Bang marks the beginning of the observable Universe as we know it, but it
doesn't mark the birth of space and time itself.
What was before the Big Bang?
We think the Universe underwent a period of cosmic inflation.
At that epoch, instead of being filled with matter and radiation, the Universe was: filled
with energy inherent to space itself.
It also was expanding at a constant, exponential rate and it was creating new space so quickly
that the smallest physical length scale, the Planck length, a veeeery small length, of
about 10 to the -35 meters, would be stretched to the size of the presently observable Universe
every 10 to the -32 seconds.
Amazing, isn't it?
We know that in our region of the Universe, inflation came to an end.
But there are some open questions we don't know the answer to that has a huge influence
on how big the Universe truly is, and whether it's infinite or not.
For example, how long did inflation go on prior to its end and the resultant Big Bang?
The thing here is that we can only see the observable Universe created by inflation's
end.
Another thing that we know is that inflation must have occurred for at least some ~10 to
the -32 seconds or so.
But it possibly went long for longer.
Cosmologists are trying to understand how much longer.
Seconds?
Years?
Billions of years?
Maybe inflation could even go on for an infinite amount of time, maybe it is still going on
at our epoch.
Also, did inflation have a beginning?
Or is it eternal?
And how did all the space emerge?
Did it emerge from nothingness a finite amount of time ago?
These are really hard questions to answer.
All that we know until now from our best observations is that the Universe is a lot bigger than
the tiny part we can observe.
We believe beyond what we can see there's plenty more Universe.
But how is it?
Well, probably it is governed by the same laws of physics, the same types of cosmic
structures, and so the same chance of complex life.
We also know that, unless inflation went on for an infinite amount of time, the universe
will turn has to be finite in extent.
To date, we don't have enough information to solve this kind of problem.
The only accessible information is the one inside out observable universe, which has
an extension of about 46 billion light-years in all directions.
The only thing we can do is to keep studying and asking questions to ourselves.
Maybe in the next couple of years, we will have a clearer scheme and description for
our home universe.
Meanwhile...remember to keep it curious!
"This video ends here!
Thanks for watching everyone!
What do you think, is our universe finite or infinite?
Let us know in the comment below!
See you next time on the channel!