According to Magueijo, the variable speed of light (VSL) theory emerged as a solution to a longstanding inconsistency in cosmology known as "the horizon problem" which arises when the speed of light is considered to be a constant.If light has an invariable speed limit, then that means that since the Big Bang it could only have traveled approximately 13.7 billion light years, because approximately 13.7 billion years have elapsed since the Big Bang. The distance that light is able to travel since the Big Bang creates the 'horizon' of the visible universe—this is about 47 billion light years (although light has only been traveling for 13.7 billion years, this number takes into account the expansion of space that is occurring as light is traveling).
"The whole of physics is predicated on the constancy of the speed of light."
According to Afshordi, galaxies and other structures in the universe were only possible due to fluctuations in the early universe's density. These density fluctuations are recorded in the cosmic microwave background as a "spectral index," which might be imagined as the "color" of the early universe. The neutral baseline of the spectral index is a value of 1, which would be a universe with the same magnitude of gravitational fluctuations on all scales. Above this value the universe is "blue" (representing shorter wavelength fluctuations) and below this value and the universe is "red"(representing longer wavelength fluctuations).Although the inflationary model of the universe also would have a "red" spectral index, it is unable to calculate a precise value of the index and as a result the exact gravity fluctuations in the early universe. In their new paper, Magueijo and Afshordi pegged the spectral index at a value of 0.96478, just slightly red, which is two orders of magnitude more precise than current measurements of the spectral index (about 0.968).Now that they've used the variable light speed theory to put a hard number on the spectral index, all that remains to be seen is whether increasingly sensitive experiments probing the CMB and distribution of galaxies will verify or overturn their theory. Both Magueijo and Afshordi expect these results to be available at some point in the decade. But Marsh and other physicists aren't so sure.
"Varying speed of light is going back to the foundations of physics and saying perhaps there are things beyond relativity."