Varying Speed of Light
Light traveled vastly faster in the early universe, allowing distant regions to come into causal contact without inflation.
Placeholder for a 3D visualisation of Alternatives to Inflation. The interactive scene will land in Phase 3. Standard inflationary cosmology explains the universe's observed flatness, uniformity, and lack of monopoles by positing a brief period of exponential expansion. These theories propose different mechanisms to explain the same observations without invoking inflation.
In one sentence
VSL proposes that the speed of light was much higher in the early universe, solving the horizon problem by allowing distant regions of the universe to exchange information directly.
The claim
The horizon problem is one of the strongest arguments for inflation. Regions of the cosmic microwave background that are too far apart to have ever exchanged light signals nonetheless have nearly identical temperatures. Inflation explains this by saying these regions were in causal contact before being stretched apart by exponential expansion. VSL offers a different solution: if the speed of light was much higher in the very early universe, then those regions could have been in causal contact without requiring inflation.
The idea was independently proposed by Andreas Albrecht and João Magueijo in 1999, and by John Moffat earlier in 1992. The mechanism varies between formulations, but the common feature is that the speed of light c is not a true constant, it depends on the energy density or temperature of the early universe and decreased to its present value as the universe cooled and expanded.
Afshordi has worked on VSL in collaboration with Magueijo. Their version makes specific predictions about the CMB power spectrum that can be tested against observations. The model is unusual in physics because changing 'fundamental' constants is widely viewed as dangerous, but VSL proposes that c was a 'running constant' in the same sense that gauge coupling constants are known to be running.
The family stance
A different physical mechanism in the early universe produced the same observable features that inflation is invoked to explain. The pre-Big Bang state varies by variant, but none of these models require an inflationary phase.
Predictions
- Specific shape of the CMB power spectrum that differs slightly from inflationary predictions
- A different relationship between temperature fluctuations and gravitational waves than inflation predicts
- No requirement for a scalar inflaton field
- A naturally fine-tuned cosmological constant from the same mechanism
Evidence
- Resolves the horizon problem with a single physical mechanism
- Makes falsifiable predictions about CMB polarization
- Mathematically simpler than inflation in some formulations
Counterpoints
- Changing the speed of light requires very careful formulation to avoid violating relativity in unwanted ways
- Inflation is observationally more successful and is the established framework
- Different VSL formulations make different predictions; no single canonical version exists
- Strong constraints from precision tests of the equivalence principle
Variants in this family
▸Go deeperTechnical detail with proper terminology
The Albrecht-Magueijo 1999 paper formulates VSL as a phase transition in the early universe where c transitioned from a much higher value to its present value. The transition can be modeled as a scalar field with specific couplings to the cosmological metric.
In Magueijo and Afshordi's later work, the changing speed of light is connected to the running of fundamental constants in quantum field theory at high energies. The model makes predictions about the running of c that can in principle be tested through precision cosmology.
Critics including Lee Smolin (who has written about VSL favorably elsewhere) and others have argued that the VSL framework is sensitive to which fundamental quantities are taken as constants. In some formulations, what appears as a varying c could equivalently be described as a varying particle mass or varying gauge coupling.
References
- EstablishedAlbrecht, A., Magueijo, J. (1999). 'A time varying speed of light as a solution to cosmological puzzles.' Phys. Rev. D 59, 043516
- EstablishedMoffat, J. (1993). 'Superluminary universe: A possible solution to the initial value problem in cosmology.' Int. J. Mod. Phys. D 2, 351
- Needs verificationAfshordi, N., Magueijo, J. (2016). "Critical geometry of a thermal big bang." Phys. Rev. D 94, 101301
Last reviewed May 14, 2026
Spotted an error? Have a source to add?
Prefer email?
You can also send a prefilled email with the variant URL already filled in.