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The '''Hubble constant''' (''H''0) is the constant of proportionality in Hubble's law, which states that the [[Redshift|redshift]] of a distant galaxy increases in proportion to its distance. It is quoted in kilometres per second per megaparsec, and in standard cosmology it is read as the present-day rate of expansion of the universe.

The redshift–distance relation was published by [[Edwin Hubble]] in 1929, building on radial-velocity measurements made by Vesto Slipher from 1912 onward and on Hubble's own Cepheid distances; Georges Lemaître had derived essentially the same relation from theory two years earlier. Hubble's original slope was about 500 km/s/Mpc, which implied an age for the universe smaller than the known age of the Earth — an embarrassment resolved only when the distance scale was repeatedly revised downward over the following decades. Modern values are near 70 km/s/Mpc. The reciprocal of ''H''0, the Hubble time, is about 14 billion years for that value, and is the origin of the familiar figure for the age of the universe.

==The Hubble tension==

There is a real and unresolved conflict here, and it should not be understated. Measurements that build outward from nearby distance indicators — Cepheid variables calibrating Type Ia supernovae, as in the SH0ES programme — give values around 73 km/s/Mpc. Values inferred instead from the angular power spectrum of the [[Cosmic Microwave Background]], notably by the Planck satellite, give about 67 km/s/Mpc. The quoted uncertainties on both are small enough that the discrepancy is several standard deviations, and a decade of work has failed to make it go away. Independent methods — the tip of the red giant branch, gravitational lensing time delays, and more recently JWST observations of Cepheids — have been marshalled on both sides without settling it.

Within the standard framework the tension is a parameter problem: either one of the measurements has an unrecognised systematic error, or the concordance model needs modification in the early universe. Outside that framework it is read as a symptom of the interpretation itself.

==On this wiki==

The researchers catalogued here mostly deny that ''H''0 measures an expansion rate at all, and instead treat it as a measure of how much energy light loses per unit distance travelled. Related material is under [[:Category:Redshift]] and [[:Category:Cosmology]].

'''H0 as a photon energy-loss rate.''' [[Lyndon E Ashmore|Lyndon Ashmore]]'s "new tired light" derives the Hubble constant directly from the density of free electrons in intergalactic space, in ''[[Recoil Interaction Between Photons and The Electrons In The Plasma Of Intergalactic Space Leading To The Hubble Constant And CMB]]'' and ''[[An Explanation of Redshifts In a Static Universe]]''. On this reading the same number that cosmologists call an expansion rate is a property of the intervening medium, and the quantity is a constant of ''space'', not of ''time''. [[Ari Brynjolfsson]]'s [[Plasma Redshift Cosmology|plasma redshift]] work makes a comparable claim. See [[Tired Light]] for the general class of models.

'''Is it constant?''' [[Jean Pierre Vigier]], [[Toivo Jaakkola]] and [[Jean-Claude Pecker]] argued in ''[[Spatial Fluctuation of the Hubble "Constant"]]'' that the measured value varies from one direction and region of sky to another — which, if true, is fatal to reading it as a global expansion rate but quite natural if it reflects the properties of intervening matter. [[Halton C Arp|Halton Arp]]'s ''[[Seeing Red: Redshifts, Cosmology and Academic Science]]'' makes the more radical claim that a component of redshift is intrinsic to the object and unrelated to distance at all, which would corrupt the distance calibration on which ''H''0 rests.

'''Alternative frameworks that reproduce Hubble's law.''' [[Tuomo Suntola]]'s Dynamic Universe derives the observed magnitude–redshift relation without dark energy in ''[[Observations Support Spherically Closed Dynamic Space Without Dark Energy]]''. [[C Johan Masreliez]]'s ''[[Scale Expanding Cosmos Theory I – An introduction]]'' reinterprets the relation as an effect of changing scale rather than recession. [[Bob Ticer]]'s ''[[The Recycling Universe]]'' and [[Grote Reber]] and [[Paul Marmet]]'s ''[[Cosmic Matter and the Nonexpanding Universe]]'' pursue static-universe readings.

The common thread is not a dispute about the ''measurement'' — the redshift–distance correlation is accepted here — but about whether the constant of proportionality describes motion or describes propagation.

==See also==

* [[Expanding Universe]]
* [[Redshift]]
* [[Tired Light]]
* [[Big Bang]]
* [[Edwin Hubble]]
* [[Cosmological Constant]]

[[Category:Cosmology]]
[[Category:Redshift]]
[[Category:Astronomy]]
[[Category:Big Bang]]

Hubble Constant - Revision history

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