Cosmic time, or cosmological time, is the time coordinate commonly used in the big bang models of physical cosmology. It is a criterion for defining and measuring time adopted by cosmologists that takes as its reference the local conditions of certain parameters, such as the expansion rate of the chronotope, the local density of galaxies, the temperature of the microwave background radiation, the gravitational redshift, etc., in order to establish a time scale characterized by the progressive (local) evolution of these parameters. An observer will observe identical cosmological time scales if the measurement of cosmological parameters relative to two different points in the Universe yields identical results.
Such time coordinates can be defined for a homogeneous, expanding universe, such that the universe has the same density everywhere at any point in time (the fact that this is possible means that the universe is, by definition, homogeneous). The clocks that measure cosmic time should move along the Hubble stream.
Although general relativity allows the laws of physics to be written in any coordinate system, there may be coordinate systems in which theoretical developments are easier. In the big bang paradigm, the most natural choice is comovent coordinates, that is, those of an observer who is “swept along” by the expansion of the universe and is therefore called a “comovent observer”. A comovent observer (and thus a constant triplet of comovent coordinates) is characterized by the fact that it perceives the universe (and in particular the cosmic background radiation) as isotropic. The background radiation measured by any other observer who is not at rest with respect to the comovent coordinates will be shifted toward blue in some regions of the sky and toward red in others, depending on the direction and intensity of the observer’s velocity.
The time measured by a comovent observer is called “comovent time” or “cosmological time”; the zero instant is obviously that of the Big Bang, and the value of cosmological time coincides with the age of the universe.