We'll now talk about Olbers' Paradox

which is the question

Why is the sky dark at night? Have you ever wondered why is the sky dark at night?

So, you probably never stopped to think about why is the sky dark at night

and if you did you probably think

let me guess

Well the stars are so so far away that

by the time the light gets here so it's a little light that it doesn't amount to anything.

Right? But if you really take a little bit more time to think about it

as Olbers did,

you realize that even though there is very little light coming from the

stars that are very far away,

if the universe is infinite. Right?

Let's think for a minute that the universe is infinite.

There's so many stars out there

that the sky should be as bright as the sun

and you can't convince yourself without doing the math, really,

but it's really not that not that hard to even understand the math. You just think about

we're here on Earth

at the center of an infinite universe,

a universe filled with stars

and there are fewer stars close to us

and then there are stars further away from us.

So even though it's true

that

you know, those stars here that are close will appear to us brighter,

we'll get more light from them

There's so many more further from us

that it turns out that

the amount, the light, the amount of light that we receive from each shell

around us is the same.

We can do this by remembering that

the luminousity,

the brightness decreases is a function of one over R square,

if you like math and that the volume,

the volume will increase as a function of one over R cubed

so you have the same amount of light

that comes from the inner shell that comes from an outside shell so universe should

Ooh? What happened to my...?

Hold on.

There. The universe should be as bright

the surface of a star

because, let me go back and

show you that again. See if I can show you that again.

Hang in there. It just goes a little bit too fast, so let me show you.

So what this is gonna show is

different stars populating this piece of sky. Alright?

The first one is one really

close to you so it is really bright

and then they are fainter, but then there are more of them.

So this is what happens.

The sky becomes

as bright as the surface of the Sun.

Alright? So. Well.

So Olbers realized that

and wondering why is the sky,

the sky dark?

So, there are a few options for why the sky is dark at night

First of all, maybe the universe is not infinite. Maybe the universe is finite.

Or are the universe is too young

and we haven't been able to receive the light from most of the

stars in the universe yet.

Or the universe is expanding, which we know it is.

So the lights get red-shifted

so instead of receiving, you know, light

like the light you see on the surface of the Sun. You see redder and redder and redder light.

Or there's too much dust absorbing the light.

Alright? So...

The simple fact that the sky is dark at night

tells you a lot. Right?

In the case of our universe, we don't know whether the universe is finite

or not.

But our universe is very young.

So... young enough that we cannot see

enough

stars to fill it up. Alright.

And on top of that the universe is expanding, so the light that we receive from the shells

that further away from us

are red-shifted. You know, so are redder and redder and redder.

So, I think it's pretty cool that the pure fact that the sky is dark at night

tell us, tells us that our universe

can not be infinitely old and infinite.

Or the sky would be bright at

night.

That's it. I hope it helped