What is the temperature of space?

This is another name for the temperature of the cosmic background radiation which has a temperature of 2.7279 K which was determined by the NASA COBE satellite in 1992. Satellites such as COBE and WMAP have accurately measured this temperature to one part in 100,000 and the temperature is very constant across the sky. There are, however, irregularities in this temperature at a level of a few millionths of a degree. The image below, from the NASA WMAP satellite, shows what these irregularities look like across the entire sky:

The blue colors are areas where the temperature of the cosmic background radiation is slightly cooler than 2.7279 K; the red colors are where it is slightly warmer - by millionths of a degree. These blotches were imprinted on the cosmic background 'fireball' radiation by interaction of this fireball light with vast clouds of hydrogen gas and especially dark matter. Their gravitational wells caused changes in the temperature of this light in accordance with general relativity. Today, even the smallest of these blotches is larger than a cluster of galaxies such as the Virgo or Coma Clusters.

No object in the universe can, for long, have a temperature lower than this unless it is done artificially in some small laboratory using a specially shielded cryostat. This is, however, not the temperature of space itself, but only the primordial fireball radiation left over from the Big Bang which contributes about 400 photons per cubic centimeter of space...everywhere. Space, itself, has no attribute that we know of other than its dimensions (three large ones!) and its shape (flat, closed, or topologically contorted in some way).

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