اپست ِ هممیاو apest-e ham-miyâv
*Fr.: distance comobile*
1) A distance in → *comoving coordinates*
between two points in space at a given cosmological time. In other words,
the distance between two nearby objects in the Universe which
remains constant with epoch if the two objects are moving with the
→ *Hubble flow*.
More specifically, it is the → *proper distance*
divided by the ratio of the → *scale factor*
of the Universe between then, *a*(*t*)_{em},
and now, *a*(*t*)_{obs}:
*D*_{C} = *D*_{proper} .
[*a*(*t*)_{obs}/*a*(*t*)_{em}].
In terms of → *redshift* (*z*),
it is the proper distance multiplied by (1 + *z*).
At the present epoch, i.e. *a* = *a*(*t*_{obs}) = 1,
*D*_{C} = *D*_{proper}.
If the objects have no peculiar velocity their comoving distance at
any time is the same as their distance today.
The comoving distance of the → *cosmic horizon*
is about 48 × 10^{9}→ *light-year*s.
2) *Transverse comoving distance*: In a non-flat Universe,
the comoving distance
between two events at the same → *redshift* but separated on
the sky by some angle. It is expressed by trigonometric functions of
→ *curvature*, → *comoving distance*,
and the → *Hubble distance* accounting for
the curvature of space.
In a flat universe (Ω_{k})
it is the same as the → *comoving distance*.
3) *Line-of-sight comoving distance*:
The total line-of-sight comoving distance from us to a distant
object computed by integrating the infinitesimal comoving distance
contributions between nearby events along the radial ray from
the time *t*_{emit}, when the light from the object was emitted,
to the time *t*_{obs}, when the object is observed. → *comoving*; → *distance*. |