Ch.01 Before the UniverseAlternatives to Inflation

Light moved much faster in the early universe. That kept distant regions in touch.

Varying Speed of Light

1999João Magueijo, Andreas Albrecht, John Moffat, Niayesh AfshordiSpeculative3 primary sources, 3 established Reviewed May 14, 2026

Light traveled vastly faster in the early universe, allowing distant regions to come into causal contact without inflation.

Visualisation · Alternatives to Inflation

inflation: A brief burst of accelerating expansion in the very early universe, smoothing out and stretching whatever existed before.
cosmic microwave background: The faint, uniform glow of light released about 380,000 years after the Big Bang, when the universe first became transparent.
inflaton field: The hypothetical scalar field whose slow decay drove cosmic inflation. The fuel, when it burns, space stretches.
cosmological constant: A constant energy density of empty space. In general relativity, it drives the accelerated expansion of the universe. Often written as Lambda.
scalar field: A field with one value at each point in space, like a temperature map. The inflaton is one example.

§1 · The claim, 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.

§2 · Why it might be true

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 1993. 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.

§2.5 · Evidence

Resolves the horizon problem with a single physical mechanism
Makes falsifiable predictions about CMB polarization
Mathematically simpler than inflation in some formulations

§3 · What you'd need to test it

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

§4 · Where it breaks

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

Go deeper

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.

▸§5 · Who built it, and when(3 sources, 3 established)

Variants in this family

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