Updated: October 21, 2020
The Problem #
People used to believe that the Universe was infinite (in age and size) and static. If this were true
Why is the night sky not as bright as day?
That is to say, if the universe is infinite as are the stars, and the universe is not undergoing any sort of evolution, we should expect every point of the night sky to be filled with the light from any one of its inifnite stars. But we don’t observe this. Why not?
The Assumptions #
In order for the sky to be as bright as the sky in the day time a couple points would need to be true:
- The universe is infinite in size
- The universe is infinite in age
- The universe is static (does not undergo evolution)
- We have an unobstructed view to every star in the universe
- The density and luminosity of the stars would need to be the same
The Resolution #
We know (from Einstein, et al) that the universe is not static. If the universe (as a medium) expands/contracts then the light within it will experience shifts into the red (under expansion) or blue (under contraction) ends of the spectrum and, thus, would be outside of the human-visible spectrum.
The universe is also known to have a gensis point and a massive size. With these points in mind it becomes clear that it’s likely that the light from any number of stars has not reached our part of the universe yet.
We also know that not every star has the same brightness/luminosity as its sibling. So, even disregarding the points above, we wouldn’t expect a uniform brightness in the night sky.