Angular diameter distance is a concept used in cosmology and astrophysics to relate the observed angular size of an astronomical object to its actual physical size and its distance from the observer. It is particularly useful in understanding how we perceive the size of distant objects in the universe, such as galaxies or stars.
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The angular diameter distance is defined by for a small transverse physical size subtending angle . In a spatially flat Friedmann-Lemaître-Robertson-Walker metric,where is the comoving radial distance.