How did distant supernovae reveal accelerating cosmic expansion?

A distant supernova can be a clue to how the universe has expanded during the light's journey. Two different measurements are needed. Redshift measures how much the expanding universe has stretched the light's wavelengths. A distance estimate helps astronomers test which expansion history could connect that redshift with the object's observed position in cosmic history.

Type Ia supernovae provide one distance tool because the shapes of their light curves and their colors help astronomers estimate and standardize their intrinsic brightness. Intrinsic brightness describes the light output; apparent brightness describes how much light reaches the observer. At greater distance, a given light output is spread over a larger area, and cosmological effects must also be included. Thus a standardized object that appears fainter can indicate a greater distance. The supernovae are not all identical bulbs: calibration, dust, corrections and uncertainty matter.

Surveys found that distant supernovae were dimmer, and therefore more distant, than expected in the decelerating expansion models being tested at those redshifts. The comparison supported a history including a period of accelerating expansion. This was not simply a claim that farther things look fainter, which was already expected. It was a mismatch between measured brightness and the brightness particular expansion histories predicted. Populations of supernovae and checks against alternative explanations made the inference meaningful.

Other observations provide complementary tests. In the standard cosmological model, dark energy names the component used to account for the acceleration; a cosmological constant is one possibility. Its underlying physical explanation remains unsettled. Establishing how expansion has changed and explaining the ultimate cause are different achievements. The evidence concerns widely separated, unbound regions, not a claim that every object or gravitationally bound system expands along with the universe.