Cosmic time, or cosmological time, is the time coordinate used in the Big Bang models of physical cosmology. This concept of time avoids some issues related to relativity by being defined within a solution to the equations of general relativity widely used in cosmology.

Albert Einstein's theory of special relativity showed that simultaneity is not absolute. An observer at rest may believe that two events separated in space (say, two lightning strikes 10 meters apart) occurred at the same time, while another observer in (relative) motion claims that one occurred after the other. This coupling of space and time, Minkowski spacetime, complicates scientific time comparisons: neither observer is an obvious candidate for the time reference.

Einstein's theory of general relativity in an isotropic, homogeneous expanding universe provides a way to define a unique time reference. All coordinate points in such a universe are equivalent. Hermann Weyl postulated that "galaxies" in such a universe define geodesics, generalizations of straight lines in spacetime. Each galaxy represents an area of co-moving masses and gets its own local clock. All of these clocks synchronized at the single point in the past where the geodesics intersect. Hypersurfaces perpendicular to the geodesics become surfaces of constant cosmic time.

Cosmic time provides a universal time only as long as the assumptions used to define it hold. There are solutions to general relativity that do not support cosmic time. However, the standard cosmological theory based on the concepts required for cosmic time has been very successful.

Cosmic time ${\displaystyle t}$ is a measure of time by a physical clock with zero peculiar velocity in the absence of matter over-/under-densities (to prevent time dilation due to relativistic effects or confusions caused by expansion of the universe). Unlike other measures of time such as temperature, redshift, particle horizon, or Hubble horizon, the cosmic time (similar and complementary to the co-moving coordinates) is blind to the expansion of the universe.

Cosmic time is the standard time coordinate for specifying the Friedmann–Lemaître–Robertson–Walker solutions of Einstein field equations of general relativity. Such time coordinate may be defined for a homogeneous, expanding universe 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). The clocks measuring cosmic time should move along the Hubble flow.

The ${\displaystyle t=0}$ doesn't necessarily have to correspond to a physical event (such as the cosmological singularity) but rather it refers to the point at which the scale factor would vanish for a standard cosmological model such as ΛCDM. For technical purposes, concepts such as the average temperature of the universe (in units of eV) or the particle horizon are used when the early universe is the objective of a study since understanding the interaction among particles is more relevant than their time coordinate or age.

Cosmic time relies on physical concepts like mass that may not be valid for times before approximately 10⁻¹¹ seconds.