Cosmic time (also known as
time since the big bang) is the
time coordinate commonly used in the
Big Bang models of
physical cosmology. It is defined for
homogeneous,
expanding universes as follows: Choose a time coordinate so that the universe has the same
density everywhere at each moment in time (the fact that this is possible means that the universe is, by definition, homogeneous). Measure the passage of time using
clocks moving with the
Hubble flow. Choose the big bang singularity as the origin of the time coordinate.