How do we know that the 3 degree K measured by COBE is the temperature thru Universe the same?

Answer 1

Temperature measures the level of thermal energy in a body of some sort of a substance. If there is no substance, there is no temperature. This is what 0DegK defines. Temperature of nothing, empty space, perfect vacuum. So in “empty space” without any traces of energy temperature would be 0DegK. Of course empty space does not exist. There is always some virtual particles popping in and out of existence, space debris and when COBE stick out its measuring probe into empty space the space is not empty anymore. Basically COBE’s temperature probe measures its own temperature when all (almost all) of its energy radiated away into empty space. So because of the not quite empty space the temperature is not quite zero. This is valid statement for the empty space in the entire universe according to definition of Kelvin scale.