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The '''expanding universe''' is the standard interpretation of the observed [[Redshift|redshift]] of distant galaxies: that the galaxies are not moving apart through a fixed space, but that the space between them is itself increasing in scale, so that light in transit is stretched in proportion to the growth of the scale factor while it travels.

The picture assembled quickly. Vesto Slipher measured large radial velocities for spiral nebulae between 1912 and 1917; Alexander Friedmann found expanding and contracting solutions of Einstein's field equations in 1922; Georges Lemaître connected the solutions to the observations in 1927 and proposed a "primeval atom" origin; and [[Edwin Hubble]] published the linear redshift–distance relation in 1929 (see [[Hubble Constant]]). Extrapolated backwards, an expanding universe implies a hot dense beginning — the [[Big Bang]] — and with it a finite age, currently quoted as about 13.8 billion years.

Four observations are usually presented as the evidential core: the redshift–distance relation itself; the [[Cosmic Microwave Background]] and its near-perfect blackbody spectrum; the abundances of the light elements, in particular the roughly one-quarter helium fraction by mass, which big-bang nucleosynthesis calculates from a single parameter; and the apparent time dilation of distant Type Ia supernova light curves, which stretch in proportion to (1 + ''z'') as expansion predicts. To these is usually added the Tolman surface-brightness test, in which the surface brightness of galaxies should dim with redshift far more steeply in an expanding universe than in a static one.

Genuine open problems remain inside the model — the [[Hubble Constant#The Hubble tension|Hubble tension]], the unexplained magnitude of the [[Cosmological Constant|cosmological constant]], and the surprising maturity of the earliest galaxies observed by JWST, which has forced revisions to models of early structure formation though not to the expansion framework itself.

==On this wiki==

Opposition to the expanding universe is one of the oldest and largest threads in the literature catalogued here. It is not a single alternative but several, united by the claim that redshift is a property of light's propagation rather than of recession. Related material is under [[:Category:Big Bang]], [[:Category:Redshift]] and [[:Category:Cosmology]].

'''Redshift without recession.''' The main body of work is collected at [[Tired Light]]. [[Lyndon E Ashmore|Lyndon Ashmore]]'s "new tired light" attributes redshift to photon recoil off free electrons in intergalactic plasma (''[[An Explanation of Redshifts In a Static Universe]]'') and confronts the supernova objection directly in ''[[Supernovae Ia Light Curves Show a Static Universe]]''. [[Ari Brynjolfsson]] develops [[Plasma Redshift Cosmology|plasma redshift]], including ''[[Magnitude-Redshift Relation for SNe Ia, Time Dilation, and Plasma Redshift]]''. [[Toivo Jaakkola]]'s ''[[On Reviving Tired Light]]'' and ''[[The Cosmological Principle: Theoretical and Empirical Foundations]]'' set out the case at the level of principle. A dissenting voice from within the same community is [[Roger A Rydin]]'s ''[[A Case Against Tired Light and the Big Bang]]'', which rejects both.

'''Intrinsic redshift.''' [[Halton C Arp|Halton Arp]] argued from observed physical associations between high- and low-redshift objects that redshift is not a reliable distance indicator at all, and that a component of it is intrinsic to the object — the argument of ''[[Seeing Red: Redshifts, Cosmology and Academic Science]]''. If sustained, this breaks the distance ladder on which the expansion inference depends.

'''Static and non-expanding models.''' [[Grote Reber]] and [[Paul Marmet]] argue for a static universe in ''[[Cosmic Matter and the Nonexpanding Universe]]''; Marmet presses the observational case in ''[[Big Bang Cosmology Meets an Astronomical Death]]''. [[Paul Wesley]] wrote ''[[Rigorous Evidence for a Nonexpanding Universe]]'', [[Rati Ram Sharma]] ''[[Nature of the Non-Expanding Universe]]'', and [[James Carter]] ''[[Explaining the Red Shift in a Non-Expanding Universe]]''. [[Robert S Fritzius]] revives an early observational challenge in ''[[Adriaan van Maanen's Challenge to the Expanding Universe]]''. [[Robert L Stilmar]] offers a rotational alternative in ''[[The Universe is Rotating, Not Flying Apart]]''.

'''Alternative dynamics.''' [[Tuomo Suntola]]'s Dynamic Universe (''[[Observations Support Spherically Closed Dynamic Space Without Dark Energy]]'') and [[C Johan Masreliez]]'s ''[[Scale Expanding Cosmos Theory I – An introduction]]'' retain a changing universe but not the standard expansion; [[Bob Ticer]]'s ''[[The Recycling Universe]]'' proposes continuous renewal. [[Glenn Borchardt]]'s [[Infinite Universe Theory]] rejects a finite, originating universe on philosophical as well as physical grounds. General critiques are gathered in [[William C Mitchell]]'s ''[[Bye Bye Big Bang, Hello Reality]]'' and under [[Non-standard cosmology]].

The strongest counter-argument the standard model makes to all of these is that no alternative has yet reproduced the whole set — blackbody background, light-element abundances, supernova time dilation and surface-brightness dimming — from one mechanism. That is the burden the works above are attempting to carry.

==See also==

* [[Big Bang]]
* [[Redshift]]
* [[Tired Light]]
* [[Hubble Constant]]
* [[Cosmological Constant]]
* [[Non-standard cosmology]]

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

Expanding Universe - Revision history

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