Combined with the cosmological principle, the fact that the Universe is homogeneous at all points and all times, then the finite speed of light means that observation so distant galaxies are equivalent to lookback time.
Lookback Time - The length of time that has elapsed since the light we are now receiving from a distant object was emitted
Luminosity - The rate of total radiant energy output of a body ...
Lookback Time: The time required for light to travel from an emitting object to the receiver. Hence when we look at a distant object we are "looking back" in time.
A trio of telescopes - Spitzer, the Hubble Space Telescope and the Japanese-built Subaru Telescope - have observed a galaxy that existed at a lookback time (the time it has taken for its light to reach us) of 12.
Nevertheless, the overall trend of increasing blue galaxy fraction with increased lookback time is an important clue to how galaxies form.
the spherical surface centred upon us, here on Earth, which is at a distance corresponding to the lookback time to the era at which this radiation was last scattered by matter).
QSOs, some BL Lac objects, and the most powerful radio galaxies (at their redshift distances, which is an issue we've dealt with already) are luminous enough to be detected at large lookback times (beyond 80% of the Hubble time these days) and with ...
 See also: Redshift, Light, Distance, Density, Background
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