Variable Light Speed: Could Tiny Photon Mass Explain Cosmic Anomalies?

By Michael Kappel – April 23, 2025

Modern cosmology is built on the assumption that light in a vacuum always travels at an invariant speed. This tenet, stemming from Einstein’s 1905 postulate that the speed of light c is constant in all inertial frames :[1], underpins the standard Big Bang model of the universe. But what if this assumption is ever so slightly wrong? Imagine that photons – the particles of light – possess an extremely tiny but nonzero mass. Such “massive” photons would no longer be obligated to race at exactly c; instead, their speed could vary minutely with energy :[2]. Over the unimaginable distances of cosmic travel, even a tiny deviation might accumulate into significant effects.

Over the past few years, astronomers have reported surprises that don’t fit neatly into the textbook picture of a universe expanding uniformly since the Big Bang. The new James Webb Space Telescope (JWST) has unveiled galaxies that appear far too massive and evolved for their estimated age :[3], and some distant objects even look larger or brighter than expected under standard assumptions. These findings have led scientists to question whether our understanding of cosmic history might be incomplete. In this article, we’ll explore a radical alternative idea: that the anomalies could be hinting at new physics of light itself. Specifically, this hypothesis might help explain:

“Impossible” early galaxies: observations of galaxies that seem too large, massive, or mature for their high redshifts (e.g., those found in JWST data).
Distant objects’ unexpected size: far-away galaxies and structures appearing larger (and/or brighter) than the standard expanding-universe model predicts.
Rethinking redshift: the idea that the redshift of light might result from photons gradually losing energy due to a tiny mass (a concept akin to “tired light”), rather than from the expansion of space itself.
Light slowing over vast distances: the possibility that the speed of light is not absolutely constant over cosmological scales – a massive photon could slow down, altering the travel time from distant corners of the universe.

It’s a far-reaching proposal that challenges decades of accepted wisdom. However, history shows that big paradigm shifts can sometimes arise from addressing just such outlier evidence. Let’s delve into the mysteries and see how a variable light speed due to photon mass could offer a unifying explanation for these cosmic conundrums.

The Mystery of “Impossible” Galaxies

When JWST began sending back images of the deep cosmos in 2022–2023, astronomers were astonished. In particular, they identified several galaxy candidates at high redshifts (meaning we see them as they were very early in the universe) that appeared way too massive and developed for their age :[4]. These galaxies – colloquially dubbed “universe breakers” – seem to have the mass and brightness of modern Milky Way-sized galaxies, yet exist just 500–700 million years after the Big Bang :[5] :[6]. According to our best cosmological models, there simply shouldn’t have been enough time or material in the infant universe to form such large, complex galaxies so soon.

:[7] *JWST’s First Deep Field (SMACS 0723) reveals thousands of distant galaxies, including some of the faintest and most distant ever observed :[8]. Such rich detail at high redshift allows astronomers to test cosmological models – and has uncovered surprises that challenge the standard Big Bang paradigm.*

The discovery of these “impossible” galaxies has cosmologists both excited and perplexed. If the observations are confirmed (ongoing studies are working to verify the distances and masses of these objects), they would “[disagree] with about 99 percent of our current models of the early universe,” one researcher noted :[9]. In other words, we’d have to seriously rethink how galaxies formed and grew in the cosmic dawn. Proposed explanations within the standard framework include exotic ideas like a different initial mass function of stars (so that early stars might have formed and burned out differently) or faster-than-expected star formation. Yet, an even more daring possibility is to ask: could our interpretation of their redshift – and thus their inferred age and distance – be misleading us? This is where the notion of a tiny photon mass comes into play as an alternative explanation.

Distant Objects That Appear Too Large

Another puzzling trend from JWST’s observations is that some distant galaxies and structures appear larger or more extended on the sky than conventional cosmology would anticipate. In an expanding universe model (like ΛCDM, Lambda Cold Dark Matter), the relationship between an object’s redshift and its apparent size is non-linear – beyond a certain distance, objects actually start to appear larger in angular size up to a peak, and then smaller again due to the curvature of spacetime. Early JWST data, however, hinted that high-redshift galaxies might be unexpectedly large and bright, without the dimming and shrinking expected from billions of years of expansion. In fact, researchers performing an “angular size vs. redshift” test found potential deviations: remote galaxies in JWST images at redshifts corresponding to less than a billion years old look surprisingly similar to nearby galaxies in terms of brightness and size :[10]. It’s as if the vast gulf of time and stretching of space assumed by the Big Bang model isn’t making as much of a difference as it should.

One way to reconcile this could be to imagine that the universe isn’t expanding in the way we think – or at least, that the light from these distant objects isn’t being stretched purely by cosmic expansion. If photons lose energy during transit (due to having a small mass), their light would redshift (become stretched to longer, redder wavelengths) without requiring the intervening space to expand. In a non-expanding (or slowly expanding) scenario, distant galaxies could indeed look bigger and brighter than expected, because their light hasn’t been diluted by the stretching of space over time. Intriguingly, a recent analysis comparing JWST data to different cosmological models suggests that a static universe model with “tired light” (the idea of photons gradually losing energy) can naturally explain the observed sizes and brightnesses of high-redshift galaxies, better than the standard ΛCDM model that has to add ad-hoc evolution tweaks :[11]. In that static model, the puzzle of well-evolved early galaxies basically disappears – they no longer are “early” in the sense of being only 500 million years old; they could be much older objects whose light simply took billions of years to reach us, losing energy along the way.

Redshift Reinterpreted: Energy Loss vs. Expansion

Central to this discussion is the concept of redshift. In the orthodox view, redshift is a result of the Doppler and cosmological effect of recession: as galaxies rush away in our expanding universe, the light they emit is stretched to longer wavelengths, making it appear redder by the time it arrives here. This is how we gauge distance and look back in time – higher redshift usually means farther away and further back in time :[12]. Edwin Hubble’s discovery of the galactic redshift-distance relationship cemented the idea of an expanding universe, which became the foundation of Big Bang cosmology.

However, the notion of redshift being caused by something other than expansion is not new. In 1929, astronomer Fritz Zwicky proposed the “tired light” hypothesis: light might gradually lose energy as it travels enormous distances, perhaps through interactions with matter or other subtle effects, leading to redshifted wavelengths. In a tired-light scenario, the universe could be static (not expanding significantly), and yet we would still observe the light from distant galaxies shifted to the red. For many decades, this idea was largely set aside because observations (such as the detailed behavior of supernova brightness and time dilation) favored an expanding universe. In particular, one strong argument against tired light was that it didn’t naturally produce the time dilation of distant supernova light curves – whereas in an expanding universe, events appear stretched in time by the same factor as the redshift (a supernova at redshift z=1 takes twice as long to fade out, for example).

Light with a Speed Limit (and a Mass): The Physics of a Non-Zero Photon Mass

If photons did have a non-zero mass, even an extremely tiny one, it would represent a fundamental shift in physics. In Einstein’s relativity and in Maxwell’s electromagnetism, the photon is massless, and that’s why it travels at exactly c (about 299,792 km/s) in a vacuum. A massive photon, by contrast, would obey a modified dispersion relation. In simple terms, lower-energy (redder) photons would travel a bit slower than higher-energy (bluer) ones. The difference would be extraordinarily small for any feasible photon mass – after all, measurements have put an upper limit on the photon rest mass on the order of 10⁻⁵⁰ kg :[13], which corresponds to an energy of around 10⁻¹⁴ eV. This is so tiny that for most practical purposes light behaves exactly as massless. But if this tiny mass exists, there are profound implications: for instance, the “force” of electromagnetism would not reach infinitely far but would have a finite range (though again, so large as to be virtually infinite at human scales), and crucially, the speed of light would subtly vary with color :[14]. We would, in essence, have to refine Einstein’s statement to: nothing can exceed c, but light itself doesn’t always attain c – it goes just a smidgen slower, depending on its energy.

A Possible Solution – and New Questions

Let’s step back and consider what accepting a tiny photon mass and variable light speed could buy us in terms of explaining the cosmological anomalies, and what it would cost us in terms of rewriting physics. On the plus side, this single hypothesis offers a unified explanation for multiple observational surprises:

If light loses energy (and slows down) over long distances, then high redshift doesn’t necessarily mean an object is unbelievably distant in an expanding cosmos – it could mean the light has simply traveled a very long time and lost energy. The universe might be far older or larger than the Big Bang model implies, giving galaxies plenty of time to grow. Those “impossible” galaxies would no longer be so impossible; they could have formed in a vast, steady cosmos and we’re just seeing their light after eons of travel :[15].
The surprisingly large apparent sizes and high brightness of distant galaxies would make sense because there’s no tremendous expansion diminishing their light or shrinking their angular size. In a static or slowly expanding universe, a galaxy’s observed size on the sky is just a simple function of its distance, and JWST’s ability to resolve them is not so unexpected :[16].

However, the costs of this idea are also significant. It would force us to revisit the success stories of the Big Bang model: the cosmic microwave background (CMB) radiation, for example, is brilliantly explained as the afterglow of a hot, dense early universe expanding and cooling. If the universe were static and photons simply lost energy to reach the microwaves, we’d need a different explanation for the uniform glow of the CMB (some have proposed it could be starlight that has thermalized over aeons in a static universe, but this is highly speculative). Big Bang nucleosynthesis, the theory explaining the abundances of light elements like helium and lithium, is another triumph tied to an early hot phase of the universe – a tired-light cosmology would need an alternate account for those as well.

Challenging the Paradigm

Speculating that light might slow down over cosmic distances due to a tiny mass is a bold hypothesis. It directly challenges Einstein’s foundational principle and the standard interpretation of a century’s worth of cosmological data. Yet, science progresses by questioning assumptions, especially when new evidence pokes holes in the old theories. The JWST findings of unexpectedly mature galaxies and other potential discrepancies have certainly poked some holes – or at least raised eyebrows. A 2022 study even posited that we should “challenge [the] assumption” that the tired-light model is ruled out, showing that a static universe with photon energy loss can fit the JWST observations without the need for exotic new galaxy formation physics :[17]. If more data from JWST and other observatories continue to show inconsistencies with the expanding universe model, scientists will be pressed to investigate ideas along these lines more seriously.

Looking Ahead

As JWST and upcoming telescopes (like the Nancy Grace Roman Space Telescope) collect more data on early galaxies, and as we observe more high-redshift supernovae and other phenomena, we’ll gain a clearer picture of whether these anomalies persist. If they do, interest in alternatives like variable light speed might grow. We might also devise new tests: for example, looking at whether the redshift-distance relation for different types of objects remains consistent with expansion or starts to hint at an energy-loss law. Astronomers could also examine whether the surface brightness of galaxies (how their luminosity spreads out in area) matches the $(1+z)^4$ dimming predicted by expansion, or follows a different curve that might favor tired light. Early hints from JWST already suggested less dimming than expected, though this is still being debated.

Article URL: https://www.arcsecs.com/blog/variable-light-speed.

ArcSecs.com: Rethinking Lightspeed