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martyb writes:

A report from researchers at Imperial College London suggests that, contrary to Einstein's theories, the speed of light in a vacuum may not be constant. The proposed new theory provides a prediction that could be used to test its validity.

Scientists behind a theory that the speed of light is variable - and not constant as Einstein suggested - have made a prediction that could be tested.

[...] The assumption that the speed of light is constant, and always has been, underpins many theories in physics, such as Einstein's theory of general relativity. In particular, it plays a role in models of what happened in the very early universe, seconds after the Big Bang.

But some researchers have suggested that the speed of light could have been much higher in this early universe. Now, one of this theory's originators, Professor João Magueijo from Imperial College London, working with Dr Niayesh Afshordi at the Perimeter Institute in Canada, has made a prediction that could be used to test the theory's validity.

[Continues...]

Professor Magueijo said: "The theory, which we first proposed in the late-1990s, has now reached a maturity point – it has produced a testable prediction. If observations in the near future do find this number to be accurate, it could lead to a modification of Einstein's theory of gravity.

"The idea that the speed of light could be variable was radical when first proposed, but with a numerical prediction, it becomes something physicists can actually test. If true, it would mean that the laws of nature were not always the same as they are today."

The testability of the varying speed of light theory sets it apart from the more mainstream rival theory: inflation. Inflation says that the early universe went through an extremely rapid expansion phase, much faster than the current rate of expansion of the universe.

'Critical geometry of a thermal big bang' by Niayesh Afshordi and João Magueijo is published in Physical Review D.

Article text (excluding photos or graphics) available under an Attribution-NonCommercial-ShareAlike Creative Commons license.

The above-referenced journal article is paywalled, but arXiv.org has a preprint available.

It is well-worth reading if for no other reason than they posit the actual existence of a warp factor! Its Introduction raises some interesting shortcomings of the current theory:

1. Introduction. In spite of its mathematical simplicity and observational triumphs, the Big Bang model of the Universe remains an unfinished work of art. Many of its late-time successes can be traced to the initial conditions postulated for its early stages, and these are put in by hand, without justification, other than to retrofit the data. The main culprit for this shortcoming is the so-called horizon problem: the cosmological structures we observe today span scales that lay outside the ever-shrinking "horizons" of physical contact that plagued the early universe. This precludes a causal explanation for their initial conditions.

Several extensions of the Big Bang model have been proposed with the aim of opening up its horizons. An early bout of accelerated expansion [1–3], a contracting phase followed by a bounce [4], a loitering early stage [5], and a varying speed of light (VSL) [6, 7] have all been considered. None of these proposals evades the criticism that retrofitting the data is still used to select in detail the primordial fluctuations that the model should produce. Once primordial causal contact is established, work can start on concrete physical mechanisms for spoiling perfect homogeneity (e.g. vacuum quantum fluctuations or thermal fluctuations). Typically it is found that one can produce a wide range of initial conditions including, but not circumscribed to those explaining the observations.

Are there any cosmologists/astrophysicists in the house who can weigh in? Years ago when I was in college, I took several astronomy courses, so I understand enough of the material to get the general idea, but it is well beyond my background to follow the details.

Specifically, if the spectral index is found to match their prediction, does that mean that the speed of light did, or did not, vary? And, if it DID vary, what impact would that have on our current understanding of the universe?

Original Submission