# Thread: Cooling down of the Universe after the Big Bang...

1. I was thinking of this topic while I was studying Thermodynamics. We know the formula:
[Change in internal energy] = [Heat gained] - [Work done by the gas on the container]
Let's use this formula with the Universe.

The Universe is HUGE!! So, I assume that we can take all the matter in the Universe as gas particles in a huge container (assuming the Universe has a boundary). The internal energy of the Universe must remain the same as there are no external forces and effects (Law of Conservation of Energy). And Heat gained or lost will be zero. After the Big Bang the Universe has been continuously expanding, and is also continuously cooling down. I suppose this will be due the work done by the 'gas' on the boundary. Is the cooling down of the Universe related only to the expansion, or is there something else?

Hey! Maybe I'm missing out the fact that there are immense amounts of radiations which carry energy. But still the internal energy is conserved (Law of Conservation of Energy). So the temperature must remain the same.

If we assume that the Universe has no boundary (as is believed now), then the 'gas' cannot do work on any'thing'. So it must not lose any heat. But it did lose a lot of heat! (From millions of degrees to almost 3 kelvin)

There is this thing called Absolute Temperature, which can't be below zero in any case. Can the Universe ever cool down to absolute zero? If it can, then there must be a boundary to the Universe.

Also, answer this (or just post your theories), where did the energy initially come from??

2.

3. You are confusing cause and effect. It is not the hot gas that drives the expansion. The universe is cooling down because of the expansion.

4. when you leave a container with a loose piston, the gas will push the piston and this will lead to some decrease in temperature.

You say that "The universe is cooling down because of the expansion." The Universe must push on something to do work and decrease it temperature, isn't it? If there's no boundary to push on, then how is it possible?

5. Originally Posted by Sudhamsu
You say that "The universe is cooling down because of the expansion." The Universe must push on something to do work and decrease it temperature, isn't it? If there's no boundary to push on, then how is it possible?
Ummm - do not confuse temperature with actual energy.

The total heat energy in the universe will stay the same. The expansion of the universe means that this energy will be spread over a larger area. A consequence of this spreading is that the energy concentration (it's temperature) will be reduced.

No work needs to be done to change energy concentration, as long as the total amount of energy stays the same.

6. Originally Posted by Sudhamsu
when you leave a container with a loose piston, the gas will push the piston and this will lead to some decrease in temperature.

You say that "The universe is cooling down because of the expansion." The Universe must push on something to do work and decrease it temperature, isn't it? If there's no boundary to push on, then how is it possible?
The hot gas is not working on the volume of the universe. Work is done by a yet unknown force (hypoth. dark energy) on the universe with the hot gas being in it. This leads to cooling. It is an adiabatic process. You can do some calculations on this website. Altogether, the reduction of the temperature can also be seen as the result of an increase of the entropy of the universe. The same thing is basically happening, when you pump up the tires of a bicycle. The pump does not expand, because of the hot air. You are expanding the pump which leads to cooling. The air is heated, because you compress it in the pump.

7. thank you...

can any one try to answer the last two questions...

Originally Posted by Sudhamsu
There is this thing called Absolute Temperature, which can't be below zero in any case. Can the Universe ever cool down to absolute zero? If it can, then there must be a boundary to the Universe.

Also, answer this (or just post your theories), where did the energy initially come from??

8. Originally Posted by Sudhamsu
thank you...

can any one try to answer the last two questions...

Originally Posted by Sudhamsu
There is this thing called Absolute Temperature, which can't be below zero in any case. Can the Universe ever cool down to absolute zero? If it can, then there must be a boundary to the Universe.

Also, answer this (or just post your theories), where did the energy initially come from??
Absolute zero is like the speed of light: a known quantity, but one not achievable by matter. We can get extremely close to absolute zero, though, just as we can accelerate leptons to speeds close to the speed of light.

9. If you regard the temperature as the macroscopic equivalence of the average kinetic energy of an ensemble of many particles, a temperature of 0K would mean no movement at all. Everything comes to a halt. I would think that this has some important implications for the quantum mechanics, because the uncertainty principle postulates that it is impossible to measure both the momentum (or speed) and the location of a particle with high accuracy. However, if something reaches 0K, its momentum is 100% defined, so its location must be infinitively uncertain.

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