vivian maxine » 26 Nov 2016, 14:55 wrote:https://www.sciencedaily.com/releases/2016/11/161125084229.htm?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+sciencedaily%2Ftop_news%2Ftop_science+%28ScienceDaily%3A+Top+Science+News%29

A theory that challenges Einstein's physics that the speed of light is constant could be tested. Was the speed of light much higher in the early universe? Is the speed of light variable? Scientists have numbers that they think make the variable theory testable.

I have read the actual paper:

The critical geometry of a thermal Big Bang by Afshordi & Magueijo - tough math and plenty of trade jargon! (A pre-print was available since March). This seems to be what the quoted ScienceDaily article is based on. My first impression is that the article is a piece of pretty poor journalism.

The authors' mathematical 'discovery' does not go about a variable speed of light - it is more about

the speed of sound in the very early universe!

^{[1]} The speed of sound in the early "particle soup" approached the speed of light due to the extreme density, but so did the "speed of gravity" (speed of the graviton, if you like). In the paper Afshordi & Magueijo claims that there is a critical (mathematical) solution where the speed of sound exceeds the speed of gravity (and consequently, the speed light is exceeding the constant c). This smooths out the density over an area larger than the gravitational horizon, with no need for inflation to do that.

They claim that this 'critical solution' gives the same acoustic peaks in the CMB as are observed by Planck. This is no different from Inflation, but they also say the solution predicts that there can be no primordial gravitational waves, as predicted by Inflation, created at the BB. So, should future observations detect such GW's, it will rule out this theory - hence it is falsifiable, which makes it a "good theory". If nothing turns up, the theory will stay on the table, but it cannot quite 'proof' it to be correct.

There are other theories as alternatives to Inflation that also predict no primordial GW's, so this particular one is not stirring up too much interest. Also, readers must not be carried away and assume that the speed of light has changed since the time of the CMB. All observations support a locally constant speed of light. We could obviously not yet (and may perhaps never) directly observe earlier than that.

BTW, Dave_O, your statement that the speed of light is only equals c locally is correct - as in locally everywhere. But if we try to measure it over any curved spacetime, it is not c, because you cannot set up an inertial frame that spans over curvature. The LCDM model says that the farthest galaxy that we have observed (redshift ~ 12), recedes from us at 2.2 lyrs per yr, effectively 2.2c. But, its speed relative to other comoving cosmological objects (loosely, through its local space) is actually zero.

=J

[1] There have been many variable speed of light (VSL) theories since the late 1980's, but none could give any observational consequences. Afshordi & Magueijo assume that theories predicting VSL during the inflation period is true and use that in this paper. I think both of them are authors of VSL papers in the 1990's, early 2000's.