Why Isn't Night sky Light?

Most recent answer: 12/02/2014

Since light from stars radiates like ripples from a stone in a pond, except in all directions not linear, and considering there are billions of stars in our universe, why is space primarily dark and not illuminated? Thanks
- Mike Mitchell (age 56)
Murrells Inlet, SC U.S.A.

That's a great question with a long history. It goes by the name Olber's paradox. () People started to worry about it around 1610 AD.

In a simple static picture of an infinite universe, a first calculation of the incoming light intensity would give infinity, since on average each distant spherical shell would contribute the same amount. The loss of intensity due to distance would just cancel the increase in the number of stars in the larger shells. When you take into account that you would only see light from the first star in a line of sight, not stars behind it, you get that every direction should be about as bright as an average star surface, more or less the same as the brightness of the Sun. Obviously that's not what we see.

The explanation lies in the Big Bang. Distant stars are traveling away from us. Their light is Doppler shifted to lower frequencies. (To be more accurate, the shift is due to the expansion of the universe and the full general relativistic expression for the frequency shift differs from a simple Doppler shift.) Also, the distant regions would not look the same as things do now because they represent an earlier stage in the history of the universe. At any rate, if you look any direction you do find a uniform glow that started off as looking very much like the surface of the Sun. However, that glow has been shifted to much lower frequencies by the expansion. It now appears as the cosmic microwave background. It is just like the light emitted by an object at around 2.7K, a little colder the liquid helium. When it started off, it was more like light from an object at around 3000K, which is quite visible though reddish colored and not as bright as the Sun's surface.

Mike W.


(published on 12/02/2014)

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