In cosmology (or to be more speci c, cosmography, the measurement of the Universe) there are many ways to specify the distance between two points, because in the expanding Universe, the distances between comoving objects are constantly changing, and Earth-bound observers look back in time as they look out in distance.

In standard cosmology, comoving distance and proper distance (or physical distance) are two closely related distance measures used by cosmologists to define distances between objects. Comoving distance factors out the expansion of the universe , giving a distance that does not change in time except due to local factors, such as the motion of a ...

We define a proper distance, as the distance between two events, A and B, in a reference frame for which they occur simultaneously (tA = tB). Once again, we start with the Robertson-Walker metric, and set dθ=dφ=0 and dt=0, so that

It forms the observational evidence from which astronomers infer the distances of most distant galaxies, by using some cosmological model to convert from redshift to distance. The look-back time to an object is the time taken for its light to reach us.

Distance measures are used in physical cosmology to give a natural notion of the distance between two objects or events in the universe.

2021年3月13日 · if we could construct a cosmological-sized ruler between the galaxy and the Earth then we would measure a quantity known as the proper distance to the galaxy. This is the separation between the Earth and the galaxy at a given time.

2021年12月1日 · if we could construct a cosmological-sized ruler between the galaxy and the Earth then we would measure a quantity known as the proper distanceto the galaxy. The proper distance is the most widely used measure in cosmology and when cosmologists used the term “distance” without further qualification they usually mean the proper distance.

This module provides a set of routines to calculate a number of cosmological quantities based on distance and time. Some are a bit complex - e.g. the volume element at a given redshift - while some, such as the conversion between flux and luminosity, are more mundane.

Three distances based on observational methods are the angular diameter distance ra, proper motion distance rm, and luminosity distance rl. rl = (1 + z) rm = (1 + z)2 ra. This is closely related to the second law of thermodynamics (see Linder 1988) and so …

All cosmological tests essentially consist of comparing some measure of (relative) distance (or look-back time) to redshift. Absolute distance scaling is given by the H0.