De Sitter universe

A de Sitter universe has no ordinary matter content but with a positive cosmological constant (

It is common to describe a patch of this solution as an expanding universe of the FLRW form where the scale factor is given by[1] where the constant

, the scale factor, describes the expansion of physical spatial distances.

), thus satisfying the perfect cosmological principle that assumes isotropy and homogeneity throughout space and time.

There are ways to cast de Sitter space with static coordinates (see de Sitter space), so unlike other FLRW models, de Sitter space can be thought of as a static solution to Einstein's equations even though the geodesics followed by observers necessarily diverge as expected from the expansion of physical spatial dimensions.

Before then, it was assumed that the Big Bang implied only an acceptance of the weaker cosmological principle, which holds that isotropy and homogeneity apply spatially but not temporally.

[2] The exponential expansion of the scale factor means that the physical distance between any two non-accelerating observers will eventually be growing faster than the speed of light.

If our universe is approaching a de Sitter universe then eventually we will not be able to observe any galaxies other than our own Milky Way (and any others in the gravitationally bound Local Group, assuming they were to somehow survive to that time without merging).

[3] The Benchmark Model is a model consisting of a universe made of three components – radiation, ordinary matter, and dark energy – that fit current data about the history of the universe.

These components make different contributions to the expansion of the universe as time elapses.

Specifically, when the universe is radiation dominated, the expansion factor scales as