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Q & A: Temperature of the Universe

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Most recent answer: 07/20/2010
How cold is the universe and does it change?
- charlie
Itís pretty cold:   2.725 degrees Kelvin above absolute zero according to the latest measurements obtained by looking at the cosmic microwave background (CMB). 
(See ).   Yes, the temperature does change with time.   As the universe expands, and it does according to Hubble, the temperature drops.  In the "big bang" era 13.7 billion years ago the temperature was enormous, high enough to produce all the particles and anti-particles that we see now. 


The temperature Lee describes is the temperature of the radiation filling the vast parts of space far from stars. Of course some pieces of the universe, the various types of stars and things near them (like us), are much, much hotter. / Mike W.

(published on 10/22/2007)

Follow-Up #1: cosmology and energy conservation

Where was all the heat lost after Big Bang, as the Universe expanded? Something must have acted as heat sink for heat released from the Big Bang? Pl. explain
- Indrajit Kuri
New Delhi, India
Nice deep question.

Let's simplify by considering only the background radiation, without worrying about stars etc. Two things have happened to reduce the density of that background radiation, so that its temperature has dropped.
1. Since the universe has expanded, the photons are more dilute. By itself, however, that would not change the net photon energy.
2. The wavelengths of the photons have stretched out along with the expansion, reducing the energy per photon. This effect does seem, at first glance, to violate conservation of energy.

The resolution is somewhat over my head, but I believe it runs something like this. There's a negative gravitational potential energy density associated with the density of energy. As the energy density drops, this term becomes less negative, leaving the total energy conserved. However, I believe that there is some dispute as to whether this treatment is useful and whether the idea of global energy conservation ought to be preserved in cosmology.

Here's what looks like a good discussion:

Mike W.

(published on 07/20/2010)

Follow-up on this answer.