Tired light

Tired light is a class of hypotheses in which photons lose energy as they travel through space, producing cosmological redshift without cosmic expansion. No tired-light model has matched the full range of observations explained by expanding-universe cosmology.

Cosmological redshift: The increase in observed wavelength of light from distant astronomical objects, usually attributed to cosmic expansion. Tired-light hypotheses aim to explain this redshift without expanding space.
Metric expansion of space: The increase in distances between unbound regions of the universe as its spacetime geometry evolves. It explains cosmological redshift through changing scale rather than photon energy loss in static space.
Fritz Zwicky: A Swiss-American astronomer known for work on supernovae, galaxy clusters, dark matter, and cosmology. In 1929, he proposed that intergalactic photon scattering could produce redshift without expansion.
Tolman test: A comparison of galaxy surface brightness with cosmological predictions for different models of the universe. Observed surface-brightness evolution is inconsistent with the simple static tired-light prediction.
Cosmological principle: The assumption that the universe is statistically homogeneous and isotropic on sufficiently large scales. Any alternative cosmology must explain observations interpreted using this large-scale symmetry.
Photon: The quantum of the electromagnetic field that carries light and other electromagnetic radiation. The hypotheses posit that photons lose energy during intergalactic travel.
Hubble–Lemaître law: The empirical relation that a galaxy’s recession velocity is approximately proportional to its distance at low redshift. Both frameworks can reproduce a distance–redshift relation, but interpret it differently.
Willem de Sitter: A Dutch mathematician and physicist whose solutions to Einstein’s equations shaped early relativistic cosmology. His 1917 cosmological model helped establish the context in which redshift and cosmic expansion were debated.
Supernova time dilation: The observed stretching of distant supernova light curves by a factor that tracks cosmological redshift. Photon-energy loss alone does not stretch observed event durations as expansion does.
Ether theories: Physical theories that posit a medium or preferred frame underlying the propagation of light and other phenomena. Some proposed tired-light mechanisms invoke a medium, raising questions about detectable effects and consistency with relativity.

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Geoffrey BurbidgeCompared with: Burbidge considered non-expansion explanations of redshift amid disputes over cosmological evidence.