### comoving units

(units of measure that incorporate the scale factor)

**Comoving units** are units of measurement
that divide out the scale factor.
**Comoving distance** is basic, and from it,
any measure based on distances, areas, or volumes
can be expressed in comoving units.

Two distant objects observed in the past,
e.g., galaxy clusters, can be further
apart today, purely due to Hubble flow.
*Comoving distance* expresses the distance
between them as a distance times
the scale factor at that time,
to factor out that effect. It is used, when appropriate,
to compare phenomena at different cosmological
times, and also to allow for future refinements
of the Hubble constant that would affect
determinations made from distant observations.

**Comoving coordinates** are coordinates using
*comoving distance* and **comoving space density**
is density (e.g., of mass per unit volume) based
on the same. Comoving distance is also called
**coordinate distance**.

A similar kind of coordinates can be used in
describing black holes, with coordinates moving
inward. **Coordinate speed** is the rate at which
the coordinates shift.

(*cosmology*)
http://en.wikipedia.org/wiki/Comoving_and_proper_distances

**Referenced by:**

star formation rate (SFR)

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