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{{Infobox theory
| name = Hubble's law
| type = Observed redshift–distance relation and its disputed interpretation
| author = [[Edwin Hubble]] (1929); interpretation disputed — see researchers listed below
| keywords = Hubble's law, Hubble constant, redshift-distance relation, expanding universe, tired light, [[Big Bang]]
| year = 1929
}}

'''Hubble's law''' is the observed relation that the [[Redshift|redshift]] of a galaxy is proportional to its distance. Discovered by [[Edwin Hubble]] in 1929, it is universally presented as the observational foundation of the expanding universe and hence of the [[Big Bang]].

The relation itself is a well-established empirical regularity. What is disputed — and disputed at length in the literature catalogued on this wiki — is the '''interpretation''' invariably attached to it: that redshift measures recession velocity, and therefore that the universe is expanding. That step is an assumption added to the observation, not a consequence of it. Notably, '''Hubble himself never accepted it.'''

==The observation==

Hubble found that the spectral lines of more distant galaxies are shifted further toward the red, and that the shift increases in direct proportion to distance. Written as ''v'' = ''H''₀''d'', with ''H''₀ the Hubble constant, the relation is read in standard cosmology as a recession velocity increasing with distance — the signature of uniform expansion, extrapolated backwards to an initial singularity.

==Hubble's own doubts==

The man whose name the law carries was consistently sceptical of the expansion reading, and grew more so over time. In correspondence with Willem de Sitter in 1931 he deliberately used the phrase '''"apparent velocities"''' to keep the empirical relation distinct from its interpretation, adding that "the interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority." Writing with Richard Tolman, he preferred the same guarded language, noting that if redshift were not due to recession its explanation would probably require "some quite new physical principles."

His clearest statement came in 1936:

<blockquote>"If the redshifts are not primarily due to velocity shifts … [then] the velocity–distance relation is linear; the distribution of nebulae is uniform; there is no evidence of expansion, no trace of curvature, no restriction of the time scale. … The unexpected and truly remarkable features are introduced by the additional assumption that redshifts measure recession."</blockquote>

The point is precise and remains unanswered: '''without''' the recession assumption the observations describe a uniform, unremarkable universe; the paradoxes of modern cosmology enter only '''with''' it. In his later years Hubble inclined toward an infinite, static universe in which redshift arises from an unrecognised principle of nature.

==A linear relation does not require expansion==

A linear redshift–distance relation is exactly what any mechanism producing a constant fractional energy loss per unit path length would give. If light loses a fixed proportion of its energy per distance travelled — as proposed in tired-light, plasma-redshift and photon-scattering models — the observed proportionality follows automatically, with the Hubble constant measuring the '''rate of energy loss''' rather than a rate of expansion. Hubble's law is therefore consistent with a non-expanding universe, and does not by itself select between the two accounts. See [[Redshift]] and [[Tired Light]].

==The Hubble constant and its tension==

Determining ''H''₀ has proved persistently troublesome. Values derived from the cosmic microwave background under standard assumptions (about 67 km/s/Mpc) disagree significantly with those measured from the local distance ladder of Cepheids and supernovae (about 73 km/s/Mpc). This '''Hubble tension''' has resisted resolution and is acknowledged within mainstream cosmology as a serious problem.

Critics regard it as symptomatic: a "constant" that takes different values depending on the method used, within a model that already requires dark matter and dark energy to fit the data, suggests a difficulty in the framework rather than in the measurements. Work catalogued here has also reported '''spatial variation''' in the constant and large deviations from the law itself.

==Criticisms from researchers on this wiki==

Hubble's law and the constant are examined critically by numerous researchers catalogued here:

* ''[[Hubble's Law and Dark Energy are False]]'' — challenges both the law's standard interpretation and the dark energy inferred from it.
* ''[[Evidence for Large Deviations from Hubble's Law in Normal Spiral Galaxies]]'' — reports departures from the linear relation in well-measured systems.
* ''[[Spatial Fluctuation of the Hubble "Constant"]]'' — argues the constant is not constant across the sky.
* ''[[Modified Hubble Law, Time-Varying Hubble Parameter and the Problem of Dark Energy]]'' — reconsiders the law with a varying parameter, bearing directly on the dark-energy problem.
* ''[[Galilean Interpretation of the Hubble Constant]]'' and ''[[On Interpretations of Hubble's Law and the Bending of Light]]'' — offer non-expansionary readings of the relation.
* ''[[Recoil Interaction Between Photons and The Electrons In The Plasma Of Intergalactic Space Leading To The Hubble Constant And CMB]]'' — derives the constant from photon–electron interaction in intergalactic plasma rather than expansion.
* ''[[Theoretical Calculation of the Hubble Constant and Relation to CMB and CIB]]'' and ''[[Planck's Constant From Hubble's Constant: Cosmological Origin of Terrestrial ZPF & Zitterbewegung]]'' — derive the constant from other physics.
* ''[[About Hubble's Law and the Big Bang (And the Hypothesis of Core Vortices as Sources and Engines of Galaxies)]]'' and ''[[Baseless and Irrationality of Modern Cosmology]]'' — question the inferential chain from the relation to the Big Bang.
* Non-expanding cosmologies that reproduce the relation include ''[[A Non-Expanding, Non-Relativistic Universe]]'', ''[[Theory of Non-expanding Universe]]'' and ''[[Nature of the Non-Expanding Universe]]''.

The common position is that Hubble's law is a genuine observation carrying an unexamined premise, and that the premise — not the data — is what produces an expanding universe. Further material is indexed under [[:Category:Cosmology]].

==See also==

* [[Redshift]]
* [[Intrinsic redshift]]
* [[Big Bang]]
* [[Non-standard cosmology]]
* [[Tired Light]]
* [[:Category:Cosmology|Category: Cosmology]]

[[Category:Cosmology|Hubble's law]]
[[Category:Theory & Models|Hubble's law]]

[[Category:Redshift]]

[[Category:Big Bang]]