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2.6. Redshift and temperature

The redshift measures the expansion of the Universe via the stretching of light

Equation 17 (17)

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

Equation 18 (18)

The expansion preserves the thermal form, in the absence of interactions. In its earliest stages the Universe may have been arbitrarily hot and dense.