Hubble–Lemaître law

Hubble–Lemaître law is a fundamental principle in physical cosmology describing the observation that galaxies are receding from Earth at speeds proportional to their distance. This linear relationship, expressed as velocity equals the Hubble constant multiplied by distance, provides direct evidence for the expansion of the universe. The law's discovery, based on measurements of galactic redshifts and distances, revolutionized our understanding of cosmology and underpins the Big Bang model of the universe's origin.

Historical background and discovery

The groundwork for the law was laid by earlier theoretical work, notably Albert Einstein's general relativity and the Friedmann equations derived by Alexander Friedmann. In 1927, Georges Lemaître, a Belgian priest and physicist, published a paper deriving the theoretical expansion of the universe from Friedmann's solutions and used observational data from Vesto Slipher on nebulae redshifts to estimate an expansion rate. Lemaître's work, published in the relatively obscure journal Annales de la Société Scientifique de Bruxelles, initially received little attention. Independently, Edwin Hubble, working at the Mount Wilson Observatory with the Hooker telescope, conducted a systematic program to measure distances to spiral nebulae using Cepheid variable stars as standard candles, a technique pioneered by Henrietta Swan Leavitt. Hubble collaborated with Milton Humason to obtain spectroscopic redshift measurements. In 1929, Hubble published a seminal paper in the Proceedings of the National Academy of Sciences presenting a clear linear relationship between recessional velocity and distance, which became widely known as Hubble's law. The International Astronomical Union voted in 2018 to rename it the Hubble–Lemaître law in recognition of Lemaître's prior theoretical and empirical contribution.

Physical interpretation and significance

The law is expressed by the equation \(v = H_0 D\), where \(v\) is the galaxy's recessional velocity, \(D\) is its proper distance, and \(H_0\) is the Hubble constant. The observed redshift of light from distant galaxies, interpreted as a Doppler effect due to this recession, is not motion through space but rather the stretching of wavelengths caused by the expansion of spacetime itself, as described by the metric expansion of space in general relativity. This interpretation, championed by Lemaître and later George Gamow, signifies that the universe is dynamic and evolving, overturning the previous dominant model of a static universe held by figures like Einstein. The reciprocal of the Hubble constant, the Hubble time, provides an estimate for the age of the universe, linking directly to the Big Bang theory. The law's discovery shifted cosmology from speculative philosophy into a quantitative, observational science.

Observational evidence and measurements

Initial evidence came from Hubble's 1929 plot combining distances to galaxies like M31 in the Andromeda constellation and the Virgo Cluster, measured via Cepheids, with redshift data. Later, major projects like the Hubble Space Telescope's Hubble Key Project, led by Wendy Freedman, used Cepheids in the Virgo Cluster and Fornax Cluster to calibrate other standard candles like Type Ia supernovae and the Tully–Fisher relation. Modern measurements employ diverse techniques including cosmic microwave background studies from the Planck mission, baryon acoustic oscillations from surveys like the Sloan Digital Sky Survey, and gravitational lensing from Hubble Space Telescope and the James Webb Space Telescope. These efforts have refined the value of the Hubble constant, though a persistent tension exists between values derived from the early universe (Planck) and the late universe (SH0ES project), known as the Hubble tension.

Relation to cosmology and the expanding universe

The law is the primary observational pillar for the expanding universe and the Big Bang cosmology model. It implies that the universe was denser and hotter in the past, leading to predictions like the existence of the cosmic microwave background, discovered by Arno Penzias and Robert Wilson. The law's form is consistent with solutions to the Friedmann equations, which describe the evolution of the universe within the framework of general relativity. The scale of expansion is governed by the scale factor, and the law's linearity holds only for distances where the velocity is much less than the speed of light. For more distant objects, cosmological redshift and effects of dark energy, associated with the cosmological constant originally proposed by Einstein, become significant, as revealed by observations of Type Ia supernovae by the Supernova Cosmology Project and the High-Z Supernova Search Team.

Alternative explanations and theoretical context

Before the expansion interpretation gained universal acceptance, alternative explanations for the redshift were proposed, such as tired light hypotheses, where light gradually loses energy over vast distances. These models, advocated by figures like Fritz Zwicky, have been largely discarded as they fail to explain observed phenomena like the time dilation of supernova light curves. The theoretical context for the law is firmly rooted in general relativity and the cosmological principle, which assumes homogeneity and isotropy on large scales. The law's simple linear form is a first-order approximation; a more complete description involves the Friedmann equations and parameters like the density parameter for matter, radiation, and dark energy. Competing cosmological models, such as the now-defunct steady-state theory championed by Fred Hoyle, Thomas Gold, and Hermann Bondi, attempted to explain the redshift-distance relation without an initial Big Bang but were ultimately contradicted by the evidence of the cosmic microwave background and quasar distributions.

Category:Physical cosmology Category:Astrophysics Category:Scientific laws