- Scale factor (Universe)
The

**scale factor**, parameter ofFriedmann-Lemaître-Robertson-Walker model, is a function of time which represents the relative expansion of theuniverse . It relates physical coordinates (also called proper coordinates) to comoving coordinates.:$L\; =\; lambda\; ;\; a(t)$

where $L$ is the physical

distance , $lambda$ is the distance in comoving units, and $a(t)$ is the scale factor.The scale factor could, in principle, have units of length or be dimensionless. Most commonly in modern usage, it is chosen to be dimensionless, with the current value equal to one: $a(t\_0)\; =\; 1$, where "t" is counted from the birth of the universe and $t\_0$ is the present

age of the universe : $13.7pm0.2,Gyr$.The evolution of the scale factor is a dynamical question, determined by the equations of

general relativity , which are presented in the case of a locally isotropic, locally homogeneous universe by theFriedmann equations .The

Hubble parameter is defined::$H\; equiv\; \{dot\{a\}(t)\; over\; a(t)\}$

where the dot represents a time derivative.

**External links*** [

*http://www.astro.ucla.edu/~wright/cosmo_03.htm Relation of the scale factor with the cosmological constant and the Hubble constant*]

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