2.6. Redshift and temperature
The redshift measures the expansion of the Universe via the stretching of light
Redshift can be used to describe both time and distance. As a time, it
simply refers to the time at which light would have to be emitted to
have a present redshift z. As a distance, it refers to the
present distance to an object from which light is received with
a redshift z. Note that this distance is not necessarily the time
multiplied by the speed of light, since the Universe is expanding as
the light travels across it.
As the Universe expands, it cools according to the law
The expansion preserves the thermal form, in the absence of
interactions. In its earliest stages the Universe may have been
arbitrarily hot and dense.
