Hubble-Lemaitre Law and Cosmic Expansion
The Hubble-Lemaitre law states that distant galaxies recede with velocities proportional to their distances, the founding observational evidence that the universe is expanding.
Definition
The Hubble-Lemaitre law is the linear proportionality between the recession velocity of a distant galaxy and its distance, with the proportionality constant the Hubble parameter; the recession reflects the uniform expansion of space described by a growing cosmic scale factor.
Scope
This topic covers the empirical velocity-distance relation, the interpretation of galaxy redshifts as a stretching of space rather than ordinary Doppler motion, the definition and meaning of the Hubble parameter and its variation with cosmic time, and the conceptual distinction between cosmological and peculiar velocities.
Core questions
- What does the linear velocity-distance relation imply about the history of the universe?
- How does cosmological redshift differ from an ordinary Doppler shift?
- Why is the Hubble parameter not truly constant over cosmic time?
Key concepts
- Hubble parameter
- Recession velocity
- Cosmological redshift
- Hubble flow
- Peculiar velocity
- Scale factor
Key theories
- Velocity-distance proportionality
- Recession velocity scales linearly with distance, a signature of uniform expansion in which every observer sees the same Hubble flow, consistent with a homogeneous and isotropic universe.
- Cosmological redshift
- The wavelengths of light from distant sources are stretched in proportion to the growth of the scale factor between emission and observation, so redshift directly measures how much the universe has expanded.
Mechanisms
As the cosmic scale factor increases, the proper distance between comoving objects grows in proportion to it, producing a recession velocity equal to the Hubble parameter times the distance and stretching the wavelength of light in transit by the same factor.
Clinical relevance
The Hubble-Lemaitre law turned cosmology into an observational science: it provides the redshift-distance relation used to map the universe in three dimensions and to convert a galaxy's redshift into a look-back time, anchoring the interpretation of essentially all extragalactic data.
History
Slipher measured large galaxy redshifts in the 1910s; Lemaitre derived and predicted the velocity-distance relation in 1927 from an expanding-universe model, and Hubble published the observational relation in 1929, with the law renamed Hubble-Lemaitre by the International Astronomical Union in 2018 to recognize Lemaitre's prior contribution.
Debates
- Priority and naming
- Lemaitre's 1927 paper anticipated the velocity-distance relation and even estimated the expansion rate before Hubble's 1929 publication, leading to a long debate over credit that the IAU partly resolved by adopting the dual name Hubble-Lemaitre law.
Key figures
- Edwin Hubble
- Georges Lemaitre
- Vesto Slipher
- Milton Humason
Related topics
Seminal works
- hubble1929
- lemaitre1927
Frequently asked questions
- Is cosmological redshift the same as a Doppler shift?
- Not exactly: while small redshifts can be approximated as Doppler shifts, the cosmological redshift arises from the expansion of space stretching light in transit, and it is most precisely interpreted as a direct measure of how much the universe has grown since the light was emitted.
- Does the Hubble-Lemaitre law mean we are at the center of the universe?
- No: in a uniformly expanding universe every observer sees all other galaxies receding according to the same law, so the apparent recession from us is not evidence of a special central location.