Why does a cooling universe suggest that its expanding?

Question

This was one of my GCSE Physics questions, and i wasn't sure of the answer. I would be extremely grateful if someone can provide the answer. Thank you

Answer 1

Taking in to consideration that there is a fixed amount of energy in the universe, as it expands, that same energy has to be distributed in a larger space. Therefore given a fixed volume, the amount of energy becomes less and less because the energy has to be distributed elsewhere in the universe due to the expansion. It is the inverse of the principle behind your average diesel engine, as air is compressed in the combustion chamber, it heats up, and eventually ignites the fuel. If you expand gas on the other hand, it cools, just like in your air conditioner. The matter and energy of the universe follow that same principle..

Answer 2

Answers which suggest the reason is that an expanding gas cools is technically not the correct answer. A cosmic background photon that passes earth today will have exactly the same energy 10 billion years from now *in earth's reference frame", regardless of subsequent expansion.

The correct explanation is that in a homogeneous and isotropic expanding universe, the recessional velocity of any two galaxies is proportional to their separation. Meanwhile, the distance to the original atoms that emitted the cosmic background radiation photons in our part of space at any given time increase with the age of the universe (since they travel at the speed of light). These two fact imply that the recessional velocity of the source atoms increases as the universe ages. That means, over time, the photons passing through our part of space at any given time become increasingly Doppler shifted towards the red.

Answer 3

I would imagine that they are either alluding to a diminishing energy density due to an increase in overall volume of the universe as it expands or to the fact that any radiation will be redshifted by the expansion and will thus "cool" over time, this is the case for the Cosmic Microwave Background which has cooled from very high temperatures to only 3K today.

Answer 4

You're putting the cart before the horse; an expanding universe suggests that it is cooling.

Expansion of space is the only reasonable explanation of the red shift. If the red shift is proportional to the radial component of relative velocity, then galaxies seem to be moving away from us at velocities proportional to their distance.

Cooling because of the expansion seems to be a good guess, for all the reasons listed above. Still, it is just a guess.

Answer 5

For the reasons already listed the answer is yes.
If it didn't cool while it expands, it would indicate that creation of new energy and particles must be occurring.
The creationists would like that theory.
Yet, there is the issue of dark matter and dark energy.
They are so NOT understood that future discoveries may change the laws of physics as we know them.

Answer 6

Because a gas cools as it expands. It's how a refrigerator works, and why a bicycle pump gets hot (compression).

Answer 7

Because the spaces, and time, between galaxies is increasing, thereby diminishing the temperature.

Answer 8

it slows down and blows up