What is the temperature of a perfect vacuum?

Dave - That is a very good question. It is often not very well defined. For something to have a temperature, it's got to have something in it to measure the temperature of. A perfect vacuum with no atoms, you can't measure the temperature of the atoms. So what's left in the vacuum? Quite often, there's light going through the vacuum, there's photons. I don't know of anywhere in the universe with not many photons there. So, you could try and measure the temperature of the photons. The problem is quite often that photons don't have a well-defined temperature. They're not interacting with each other or averaging out enough. So, sometimes there'd be lots and lots of very high energy photons. There might be a be load of x-ray photons and not very many microwave photons. So, you can do it on the photons, but often, it's not very well defined.