Comoving distance

The comoving distance (line of sight) DC between two nearby objects in the Universe is the distance between them which remains constant with epoch if the two objects are moving with the Hubble flow. It can be thought of as the distance with the expansion of the Universe factored out, i.e. the comoving distance between fixed objects remains constant, and is useful in cosmological calculations.

The comoving distance between two events at the same redshift or distance but separated on the sky by some small angle ΔΘ is DM ΔΘ and the comoving distance (transverse) DM is simply related to the line-of-sight comoving distance DC by the product of the Hubble distance DH with trigonometric functions sinh (for a closed universe) and sin (for an open universe) to account for the curvature of spacetime. For a flat universe DM = DC.

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