Comoving distance is the distance between two points measured along a path defined at the present cosmological time. For objects moving with the Hubble flow, it is deemed to remain constant in time. The comoving distance from an observer to a distant object (e.g. galaxy) can be computed by the following formula (derived using the Friedmann–Lemaître–Robertson–Walker metric): χ χ = ∫ ∫ t e t c d t ′ a ( t ′ ) where a (t ′) is the scale factor, t e is the time of emission of the photons detected by the observer, t is the present time, and c is the speed of light in vacuum.