Expert: Jayendra Upadhye - 8/18/2009

Question
Hai Sir,
It's me again, Steffi.
The temperature in the universe is assumed to be 2.726K. Does that mean that the temperature in the universe is constant? I would like to know is it not possible for the temperature to differ in different parts of the universe?
Waiting for your answer....

Answer
Hi,
No No!
I think you are a bit mixed up here.
The temperature of the universe is NOT constant.
15 billion years ago it was astronomically high!
Then when matter and energy decoupled, it was comparatively lower.
NOW its where you say it is.
This is the temperature of the Cosmic Microwave Background radiation. The word "BACKGROUND" is important.
It does not preclude the fact that localised hotspots may exist in the universe, represented by stellar systems. There is a regular inverse square temperature gradient as one progresses away from the star's surface, all the way to its heliopause, where its radiation pressure matches that of the galactic tenuous gas and terminates in a bow shock! (our sun has one, and so have all the stars as they move in the galactic plane). The solar wind is a regular hydrodynamic flow till that far away!
The general understanding is that if the hotspots were removed, (as in intergalactic space), far away from any stars, space would still be awash with microwave energy representing a temperature of 2.726 deg K.

In other words, nothing and nowhere in the universe, is anything cooler than THAT!

It means also, that 7 billion years ago or 10 billion years ago, the intergalactic medium was boiling hot! It also means that in THAT epoch, planets as far away as pluto, or even bodies in the kuiper belt WOULD NOT BE FROZEN, but would be at a pleasant 100s deg C!! All the way to the heliopause and beyond into the intergalactic space, nothing could have been cooler than 100s of  deg C! As space itself was that hot!

Thus one ends up saying, Space is cooling and has been cooling ever since the bang, though it allows localised hotspots such as stellar interiors with 100s of millions of deg K, it has a BASLINE temperature below which nothing can be cooler, and which is called the temperature of the CMB.
It will also continue cooling far into the future, but at slower and slower rates.

regards
Jayen