- Angular diameter distance
The angular diameter distance is a distance measure used in
astronomy. The angular diameter distance to an object is defined in terms of the object's actual size, , and the angular size of the object as viewed from earth.
The angular diameter distance depends on the assumed cosmology of the universe. The angular diameter distance to an object at
redshift, , is expressed in terms of the comoving distance, as:
Where is defined as:
Where is the curvature density and is the value of the Hubble parameter today.
In the currently favoured geometric model of our Universe, the "angular diameter distance" of an object is a good approximation to the "real distance", i.e. the proper distance when the light left the object. However, beyond a certain
redshiftit gets "smaller" with increasing redshift. In other words an object "behind" another of the same size, beyond a certain redshift, appears larger on the sky, and would therefore have a "smaller" "angular diameter distance".
[http://icosmos.co.uk/ iCosmos: Cosmology Calculator (With Graph Generation )]
Angular size redshift relation
Distance measures (cosmology)
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