Hubble's law

Hubble's law states that a galaxy’s recession velocity is proportional to its distance from an observer. It is an observational relation that reveals the universe’s expansion.

Hubble constant: The present-day proportionality between cosmic expansion rate and distance in Hubble's law. It is the slope relating recession velocity to distance.
Edwin Hubble: An American astronomer whose observations established that galaxies exist beyond the Milky Way and linked distance with redshift. His 1929 paper presented the observational relation now bearing his name.
Hubble–Lemaître law: The modern name for the proportional relation between galaxy recession velocity and distance. This name recognizes the theoretical and observational contributions behind the relation.
Doppler effect: The change in an observed wave’s frequency caused by relative motion between source and observer. Cosmological redshift reflects expanding space, not merely ordinary Doppler motion through space.
Cosmological redshift: The wavelength stretching of light as the universe expands during its journey to an observer. Redshift provides the main observable used to estimate distant galaxies’ recession velocities.
Georges Lemaître: A Belgian physicist and Catholic priest who derived an expanding-universe model and estimated a velocity–distance relation. His 1927 work connected relativistic expansion with galaxy recession before Hubble’s paper.
Hubble time: The reciprocal of the Hubble constant, providing a characteristic timescale for cosmic expansion. The law’s slope sets a simple timescale often compared with the universe’s age.
Scale factor: A dimensionless measure of how the universe’s size changes with cosmic time. It describes expansion dynamically, whereas Hubble’s law gives a velocity–distance relation at an epoch.
Cosmic distance ladder: A sequence of methods that calibrates astronomical distances from nearby stars to remote galaxies. Distance measurements supply the other half of the law’s velocity–distance relation.
Vesto Slipher: An American astronomer who measured the radial velocities of spiral nebulae using spectroscopy. His redshift measurements provided much of the velocity evidence later paired with distances.

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