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abstraction > mathematical concept > number > quantity > time > lookback time

lookback time
subject			fact

lookback time	has definition Phenomenon that, owing to the finite velocity of light, the more distant an object being observed, the older is the information received from it. A galaxy one billion light-years away, for instance, is seen as it looked one billion years ago.
	has definition The time in the past at which the light we now receive from a distant object was emitted. Galaxies of a certain type (redshift and luminosity) can be seen only at a certain distance.
	is an instance of time
time	has unit time unit
number	has value