In our current best model, the Universe is infinite and ever expanding. In this model, it is thought that every line of sight eventually ends at the photosphere (outermost visible layer) of a star. If so, it may be argued that the night sky should be brilliant - as bright as a typical star. The fact that the night is dark is known as Olbers' Paradox.
Here are some explanations:
1) Light dilutes in strength as distance^2.
2) The dust between the stars blocks the light.
3) The expanding Universe "reddens" the starlight to longer wavelengths, since space expands as the light waves pass through it.
Why are all of these wrong or incomplete? E.g., for number 3, why then is the night sky not brilliant at long wavelengths?
What is the most complete explanation, and what poet found the answer?
(In reply to
Eppur si muove by broll)
Yes, what broll says about the Cosmological Principle is completely accurate. Yes, it is an assumption, that, on the large scale (say, a scale greater than the distance between super clusters, or say, between "Great Attractors") the Universe is the same, no matter where you are, and the same physical laws obtain.
There are two upsides to this assumption: 1) it leads us to believe we actually know something about the Universe, and 2) it actually helps solve this problem. (Answer now posted.)
In detail:
1) We currently believe the Universe is infinite and expanding forever, which is based on its observable density (even adding in dark mater and dark energy density), and is also based on its increasing expansion velocity with distance. But, in an infinite Universe, we can only ever study an infinitesimal part, you know, like zero. That is a rather small sample of something from which to draw a lot of conclusions. So, without the assumption of uniformity, we become like the proverbial blind men with the elephant - extrapolating say, from the tail. So, only by assuming the Universe is similar everywhere can use our findings to make a complete model. And the model we come up also provides the causes of uniformity, caused especially well in the early on "inflationary" period.
2) By assuming uniformity we can also assume the current expansion of space is everywhere the same. Space itself is expanding, much like a raison cake in a cosmic sized oven, and expanding uniformly, each hour, where each cubic 1 cm expands to a cubic 2 cm, and so on. In this analogy, sufficiently distant raisins are hurtling away from each other a speeds greater than the speed of light, all due to all the growing size of the centimeters cubes between them. Point 2 thus yields the answer to this problem, which, BTW, is known as Olber's Paradox. (See solution).
Edited on September 25, 2018, 10:24 am
Cosmological Principle
