Name: オルバースのパラドックス (Olbers' Paradox)
Uploaded: 2021-01-14T04:46:35.000Z
Duration: 4 min 48 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

Hello. Welcome back to the Cosmic Classroom

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

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

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

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

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

and then there are stars further away from us.

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

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

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

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

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

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.

different stars populating  this piece of sky. Alright?

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

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.

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

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

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.

The simple fact that the sky is dark at night

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

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

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