Perfect vacuum, 0K or 4K?

[alpha2cen]

In the perfect vacuum, there is no molecules in it, and the vacuum has no surround objects, i.e., too far away from the heat sources. As CMB theory, it is about 4K. Which one is correct 0K or 4K? If a vacuum is 4K, how can we drop the temperature to 0K?

Edited July 18, 2011 by alpha2cen

[Fuzzwood]

We can't.

[J.C.MacSwell]

In the perfect vacuum, there is no molecules in it, and the vacuum has no surround objects, i.e., too far away from the heat sources. As CMB theory, it is about 4K. Which one is correct 0K or 4K? If a vacuum is 4K, how can we drop the temperature to 0K?

 

A "perfect" vacuum would not even have photons in it. Nothing would remain that could define a temperature.

[mathematic]

As a practical matter, there is no way to avoid photons.