Let’s talk about the dark night.
Yes this post is about Batman.
Our common assumption is that the night sky is supposed to be dark with only few dots of light.
But then, aren’t there supposed to billions upon billions of stars in the night sky emitting light. Yes, they are very far away, but, there is nothing stopping (like air or glass) the light from reaching us. So, shouldn’t all those stars make the night sky (very) bright and not dark ?
This is actually called Olbers’ Paradox.
Let’s look at the problem in another way. We can divide the universe into a series of concentric shells, being 5 light years thick. Thus, a certain number of stars will be in the shell 1,00,000 to 1,00,005 light years away. If the universe is homogeneous at a large scale (i.e., static), then there would be four times as many stars in a second shell between 2,00,000 to 2,00,005 light years away.
But, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell (intensity is inversely proportional to the square of distance). Thus the total light received from the second shell is the same as the total light received from the first shell.
Thus, the argument is that if the universe were static and filled infinitely with stars, the night sky should be much brighter than it is now.
I think you guessed the loop hole here. I said if the universe were static, which it clearly isn’t.
The Big Bang explains this paradox by saying that the universe started at a point, and expanded from that point. Thus, it is not static.
We know that the expansion is accelerating. So, two things happen.
One is that, those stars in the night sky are moving away from us and the distance between them and us increases. This increase the time for to see them and eventually it takes millions of years for the light from those stars to reach us.
Second, which is the more important reason, is that these starts get redshifted away. Redshifting is when the wavelength of an object moving away from us goes towards the red side of the spectrum and eventually, it goes into the infra red, which we cannot see. It is like we listen to a honking truck passing by at great speed. As it moves away from us, the the sound becomes softer and softer and eventually it is inaudible.
So, because of these reasons, we never get to experience the real night sky light. But, it may be a good thing, as otherwise our eyes would be blinded by the light !
Image via Wikimedia Commons