Luminosity distance

Luminosity distance is the distance that makes an object's observed flux equal its intrinsic luminosity divided by four pi times distance squared. In an expanding universe, it accounts for redshift and the arrival rate and energy of photons.

Flux: The rate at which energy crosses a unit area, often measured for astronomical sources. Observed flux is the measured brightness that luminosity distance relates to intrinsic luminosity.
Comoving distance: A distance measure that removes cosmic expansion from the changing separation of objects over time. It is a geometric input to luminosity distance in standard cosmological models.
Type Ia supernova: A thermonuclear explosion of a white dwarf, used after calibration as a standardizable candle. Its brightness-distance relation traces luminosity distance across cosmic history.
Proper distance: The distance between objects measured at a specified cosmic time using the expanding universe's spatial geometry. Unlike luminosity distance, it is a geometric separation rather than a brightness-equivalent distance.
Luminosity: The total energy an object emits per unit time across all wavelengths. It is the intrinsic power paired with observed flux in the defining relation.
Angular-diameter distance: The distance measure relating an object's physical size to its observed angular size. Its relation to luminosity distance clarifies why different observations yield different distances.
Cosmic distance ladder: A sequence of calibrated methods for measuring distances from nearby stars to galaxies and beyond. Luminosity distances connect calibrated source brightness to distances on cosmological scales.
Lookback time: The elapsed time between the emission of observed light and its detection. It measures cosmic history in time, not the flux dilution of a source.
Cosmological redshift: The stretching of light to longer wavelengths as the universe expands during its journey. Redshift alters photon energy and arrival rate, effects included in luminosity distance.
Hubble constant: The present-day rate of cosmic expansion per unit distance. Its value sets the distance scale inferred from redshifts in the nearby universe.

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