Expert: Courtney Seligman - 8/29/2011

Question
Okay, so I have had a question that has been bugging me about all energy in the universe being constant. According to heat transfer physics and Olbers' Paradox, if the universe was not expanding, all of the heat given off by stars would infinitely increase the heat of the universe, so we determined that the universe must be expanding. However, due to a general physics law, all energy in the universe is constant. My question is this, if space is expanding and the energy in the universe is constant, how come all of the energy in the universe isn't being constantly stretched infinity thin?

Answer
As it happens, the solution to Olbers' Paradox isn't that the Universe is expanding, but simply that it is too young and too thinly populated with stars for the light from infinitely distant stars to have had time to reach us, and fill up the space between the stars we can see. Of course, if the Universe weren't expanding, eventually the light of distant stars would fill up the spaces between the stars we can see, but by then the stars we can see now would all be cold, dead objects giving off so little radiation that it really wouldn't make much of a difference. So in the current era the expansion of the Universe is only a minor player in the solution to Olbers' Paradox, and it is only in the infinitely distant future that the expansion becomes really important.

However, that doesn't answer the part of your question about the energy being stretched infinitely thin. The fact is, it is being stretched -- that's why light from distant objects is redshifted -- so although space is filled with the light from the Big Bang (that is, the so-called Cosmic Background Radiation), that light is filling a larger and larger volume of space with less and less energetic photons. The result is that the energy per unit volume of space is going down just as fast as the volume is going up, so the total energy does remain constant. (This ignores the question of Dark Energy, which may or may not exist, but in either case would not be counted in the conservation of energy laws which you are referring to.)

That's a pretty brief answer, but your question was brief and to the point, so hopefully the answer is adequate for your purpose. If not, just let me know and I'll do what I can to please.