A warmer universe

[claymore]

with all the trillions of stars in the universe putting out so much energy ,why is n`t the universe warmer than than it is, what happens to all this energy ,does it just disappear, does the law of conservation of energy have any bearing on this?

[beso]

Most of the Universe is next to empty and very cold. It is an awefully long way between stars. Despite their incredible temperature and the amount of radiation they produce they don't heat up the vast void much at all.

Look at a star. It is very hot but does it burn your eyes? No, it is so far away the intensity of the radiation is vey small. However if you calculated the surface area of a sphere whose radius was the distance to that star and multiplied that miniscule intensity you would find it is still quite a lot of energy. Just extremely diffused.

[Tom Braider]

Yes, it's all about conservation of energy. If the universe is a closed system then it always contained, and always will contain, the same amount of energy. This energy can exist in various forms but only matter can have a temperature - the spaces between stars, in so far as they are truly "empty space", are neither hot nor cold. In terms of the question, the universe, in so far as it can be said to have a temperature at all, is as warm as it is, period. By definition, that "temperature" includes the radiated energy from stars.

Entropy is the tendency of the energy content of a system to "level out", so that eventually the "temperature" of the universe will be the same everywhere within it. This means that areas that are now colder than the average will become warmer, but the "temperature" of the universe as a whole will be just the same as it has always been.

[wildwood]

I wonder if this has any bearing on the continuance of the Universe's expansion.

[Rev. Green]

Yes, the expansion of the Universe is vital. If the Universe were static then every line of sight would end on a star and the whole sky would be as bright and as hot as the sun.

[jake-the-peg]

Oh fine question Claymore!

One of the best! In fact it's a varient of Olber's paradox "why is the sky dark at night?"

As hinted at by the last two answers it's a piece of evidence for the expansion of the Universe

http://en.wikipedia.org/wiki/Olbers'_paradox