Hubble's law
Hubble's law, also known as the Hubble–Lemaître law, describes the large-scale relationship between a galaxy's distance and its recession rate: the farther away a galaxy is, the faster it generally recedes as the universe expands. In its simple form, the law is written v = H₀d, where H₀ is the present expansion-rate parameter. The relation links astronomical redshift and distance and provides a foundational observational description of cosmic expansion.
Statement of the law
On sufficiently large scales, the recession rate of a galaxy is approximately proportional to its distance. The commonly used linear expression is v = H₀d: v is the recession rate, d is distance, and H₀ (the Hubble constant) is the proportionality factor. H₀ is expressed in units of speed per distance, such as kilometres per second per megaparsec. As an illustration, a value of 70 km/s/Mpc would correspond to about 70 km/s of recession rate for each megaparsec of distance in this simple relation. This example explains the units; it is not a universal prediction for an individual galaxy's measured motion.
Redshift, distance and cosmic expansion
Light from a receding distant galaxy is observed at longer wavelengths than when it was emitted; this shift toward the red end of the spectrum is called redshift. Astronomers measure spectral shifts and estimate distances to galaxies to study how recession rate varies with distance. NASA describes the observed pattern as part of the evidence for an expanding universe: in general, more distant galaxies show greater redshift and recede faster. Individual galaxies also have motions relative to their surroundings, so the simple large-scale relation describes the expansion pattern rather than every galaxy's complete local motion.
The Hubble constant
The value H₀ is the present-day rate parameter in the distance–recession relation. It is not the speed of one particular galaxy, and it should not be confused with the velocity of light. Because it has units of speed divided by distance, H₀ indicates how quickly the recession rate increases with distance in the linear approximation. Astronomers estimate it from observations, and its value is central to measuring the expansion history of the universe.
History and name
The relationship is associated with Edwin Hubble's 1929 study of galaxy distances and redshifts, building on earlier observations of galactic spectra. The International Astronomical Union later recommended the name Hubble–Lemaître law, recognizing Georges Lemaître in the name. Both Hubble's law and Hubble–Lemaître law are used in astronomical writing.
Key facts
- Basic relation
- In its simple linear form, v = H₀d: recession rate equals the Hubble constant multiplied by distance.
- Meaning of H₀
- H₀ is the present expansion-rate parameter and has units of speed per distance.
- Observational connection
- Distant galaxies generally show greater cosmological redshift and recession rate, a pattern associated with the expansion of the universe.
- Alternative name
- The International Astronomical Union recommended the name Hubble–Lemaître law.
Ask about this article
Answers use only “Hubble's law” and its references, with citations.
References
- 1.Hubble’s law
Encyclopaedia Britannica · other · reliability: high · retrieved 21 October 2026
Supports: Definition of the law, proportionality between a galaxy's recession velocity and distance, linear formula, and historical context.
- 2.Hubble Cosmological Redshift
NASA Science · other · reliability: high · retrieved 21 October 2026
Supports: How cosmological redshift arises, how galaxy redshifts relate to distance and recession, and their role as evidence for cosmic expansion.
- 3.IAU members vote to recommend renaming the Hubble law as the Hubble–Lemaître law
International Astronomical Union · other · reliability: high · retrieved 21 October 2026
Supports: The IAU's 2018 vote to recommend the Hubble–Lemaître law name.
References are suggested by the research engine and may be incomplete. Unverified URLs are omitted rather than guessed.
Article history
- Version 121 October 2026 · EditorCreated by Zapia