کروند ِ مرپل ِ کیهانی karvand-e marpal-e keyhâni
*Fr.: facteur d'échelle cosmologique*
A quantity, denoted *a*(*t*), which describes
how the distances between any two galaxies change with time.
The physical distance *d*(t) between two points in the Universe can be
expressed as *d*(t) = *R*(*t*).*x*, where
*R*(*t*) is the → *scale factor* and *x* the
→ *comoving distance* between the points. The cosmic scale factor
is related to the → *redshift*, *z*, by:
1 + *z* = *R*(*t*_{0})/*R*(*t*_{1}),
where *t*_{0} is the present time and *t*_{1}
is the time at emission of the radiation. The quantity (1 + *z*) gives the factor
by which the → *Universe*
has expanded in size between *t*_{1} and
*t*_{0}. It is also related to the → *Hubble parameter*
by *H*(*t*) = *R*^{.}(*t*)/*R*(*t*), where
*R*^{.}(*t*) is the time → *derivative*
of the scale factor. In an → *expanding Universe*
the scale factor increases with time. See also the → *Friedmann equation*. → *cosmic*; → *scale*;
→ *factor*. |