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• Hello. Welcome back to the Cosmic Classroom

• 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

Hello. Welcome back to the Cosmic Classroom

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