In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects. Proper distance roughly corresponds to where a distant object would be at a specific moment of cosmological time, measured using a long series of rulers stretched out from our position to the object's position at that time, and which can change over time due to the expansion of the universe. Comoving distance factors out the expansion of the universe, giving a distance that does not change in time due to the expansion of space. Comoving distance and proper distance are defined to be equal at the present time. That is, the two distances only differ at times other than the time at which they are measured: The universe's expansion results in the proper distance changing, while the comoving distance is unchanged by this expansion.
Read more about Comoving Distance: Comoving Coordinates, Comoving Distance and Proper Distance, Proper Distance Vs. Comoving Distance From Small Galaxies To Galaxy Clusters
Famous quotes containing the word distance:
“After climbing a great hill, one only finds that there are many more hills to climb. I have taken a moment here to rest, to steal a view of the glorious vista that surrounds me, to look back on the distance I have come. But I can rest only for a moment, for with freedom comes responsibilities, and I dare not linger, for my long walk is not yet ended.”
—Nelson Mandela (b. 1918)