Lookback time

Lookback time is the time light has traveled from an astronomical object to an observer. Because light travels at a finite speed, distant objects are seen as they were in the past.

Speed of light: The speed at which light and other electromagnetic waves propagate through a vacuum. Finite light speed means every astronomical observation shows an object at an earlier time.
Astronomical unit: The mean Earth–Sun distance, used as a standard unit for distances within planetary systems. It anchors familiar light-travel delays within the Solar System.
Cosmic distance ladder: A sequence of calibrated methods for measuring distances to astronomical objects beyond the reach of direct parallax. Distance estimates help place sources whose light-travel times are then inferred from cosmology.
Galaxy formation and evolution: The processes by which galaxies assemble, change their structure, and develop their stellar populations over cosmic time. Different lookback times reveal galaxies at different stages of their development.
Cosmological redshift: The stretching of light to longer wavelengths as the universe expands during its journey to an observer. Redshift helps infer how long light has traveled through the expanding universe.
Light-year: The distance light travels in vacuum in one year. A light-year converts travel time into a distance, but does not by itself give cosmic lookback time.
Type Ia supernova: A thermonuclear stellar explosion whose predictable peak brightness makes it useful for measuring cosmic distances. Their distances and redshifts trace expansion across a range of lookback times.
Cosmic star-formation history: The changing rate at which the universe formed stars across cosmic time. Observations at varied lookback times reconstruct when stars formed most rapidly.
Scale factor (cosmology): A measure of the relative size of the universe at a given time in cosmological models. Its change over time determines how expansion affects a light signal's journey.
Parsec: A distance unit equal to about 3.26 light-years, defined through stellar parallax. Astronomers often report source distances in parsecs, which must be interpreted cosmologically at great distances.

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