Hello and welcome back.
When you look at the night sky, or a photograph of space, what is it that strikes you more,
what is it you see?
The answer probably will be the stars, but in reality there is another thing that should
hit you in the same way, which is the darkness of space, the darkness of the night sky.
Have you ever wondered why the sky at night is dark?
Why is space dark?
It may seem a silly question but in reality the answer is not so obvious as it seems and
tells us important things about the way it is in the universe.
Let’s try to figure this out.
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Why should it surprise us that the space is dark and that there is so much darkness in
the night sky?
I’ll try explaining it to you: Imagine you are in a forest surrounded by trees , but
also the empty space between trees.
But now imagine that you’re increasing the number of trees, you will see less and less
empty space between a tree and the other.
If the trees become an infinite number you should not see empty space between one tree
and the other, you should be completely surrounded by logs of trees in any direction.
Of course the trees closest to you will seem bigger, while the far away ones may seem smaller,
but the overall result is that all the look should be completely closed by trees and in
all directions.
Well, this example of an infinite forest and endless trees should help us understand what
we should expect if the universe were infinite and contain an infinite number of stars.
Well in this case all the space (and therefore also the night sky) should shine like the
surface of a star.
Let’s try to understand why.
If there is an infinite number of stars and are distributed so uniform throughout the
space, looking in any direction you should see a star.
Clearly the nearest stars shine more and the stars farther away shine less but this fact
is compensated by another fact or by that looking further away in the same space region
fall more and more stars.
The brightness of a light source and then a star decreases with the distance, but the
number of stars grows with the distance.
The same area is filling more and more stars, therefore the decrease of brightness and increasing
the number of stars are perfectly balanced.
The final result should be that the whole night sky should shine like the surface of
a star.
This fact was thought about when it was noticed and began to create some doubts to astronomers.
This included Kepler in 1600 when he had accounted for this problem and then later the German
astronomer Olbers spoke in one of his works and this is the reason why the question that
the night sky is dark today comes often called Olbers paradox.
Why is it a paradox?
Well, because what we’re looking at isn’t a night sky as bright as a star but a dark
sky, so we must actually explain why things are so.
There are several possibilities, one possibility would simply be that there is no infinite
number of stars in the universe.
This is something that astronomers of the past took in and yet it seemed a bit strange
because if you imagine that there be an infinite number of stars in the universe, this not
only would say that there would be a point privileged within the universe or the centre
of this distribution of stars which is a contrary thing to the idea of Nicolaus Copernicus (that
there is a center in the distribution of matter of the universe).
But there would also be another problem or the fact that one finite star distributes
a number it wouldn’t be stable.
Sooner or later this finite number of stars would end up collapsing into the midpoint.
This is the reason why astronomers of the past, before Einstein and before knowing the
theory of general relativity, preferred an infinite universe to an infinite space that
contained an infinite number of stars.
But this thing was in contradiction with the observation that the night sky is dark.
Another possibility is that the universe does not exist always and this is a possibility
that brings us a little closer to the answer that we think today is the right one.
The idea is basically that if the universe has not always existed (so stars have not
always existed also if there is an infinite number), some of these stars actually are
so far away that the light has not yet reached us.
If stars don't always exist but they lit up in a certain era, looking far enough we see
areas of space where still the stars didn’t light.
This as I said it brings us a little closer to the solution of the paradox of Olbers and
the thing curious is that this idea is actually was exposed for the first time for written
in a work not by a astrophysicist, but by a writer, by Edgar Allan Poe, who in one of
his essays of 1848 published shortly before of his death, he wrote that possible solution
to the paradox of dark sky is that the universe and the stars, they haven’t been around
forever.
Well, but we come to the modern explanation of the paradox of Olbers or the fact that
the sky at night is dark.
The explanation that we think is correct is that of the model of the Big Bang, which is
a model that tells us two things: In the meantime the universe does not have always existed
and then that the universe expands.
Both explain the reason why we do not look at a sky bright but instead we see wide dark
space regions.
Since the universe doesn’t exist forever and looking far into space we look even back
in time because the light takes a finite time to reach us, when we look sufficiently far
in space we see regions of the universe in which the stars were not still lit.
And this is a first piece of the explanation of the paradox of Olbers.
But the Big Bang model also says something else, it tells us that in the past the universe
was denser and hotter.
There was even an era in which the whole universe shone like the surface of a star because the
universe was full of this hot gas incandescent that filled all the space.
So you could actually come in mind that the explanation does not work altogether, because
looking far into the space you are true at some point not we’ll see more stars, but
we should see this wall of fire caused precisely by incandescent gas that pervades everything
the universe.
But here comes the second piece of the explanation or the fact that in the model of the Big Bang
the universe is expanding.
And as we saw in a previous video the expansion of the universe causes a redshift, a cosmological
redshift, or a shift towards the red of electromagnetic and light radiation visible.
And this means that you are true in the past when the universe shone like the surface of
a star, but this wall of fire has suffered the phenomenon of red shift for us who observe
it today and then came out of the visible region of the electromagnetic spectrum.
In other words we cannot see with the eyes, we can receive it in the form of low electromagnetic
radiation frequency, high wavelength, and therefore in radio waves, which we actually
do.
We receive this signal called cosmic radiation but we can’t see it with our eyes.
So actually when we look at the night sky and we see these big dark regions of space,
in a sense we have before our eyes a test of the nature of the universe and of the correctness
of the Big Bang model.
Looking into those dark regions we are not seeing stars but we are seeing an ancient
light that comes from the origin of the universe but we can not see more with our eyes.
Of this ancient light of radiation cosmic background, we will talk in a next video then
continue to follow the channel and see you at next time!