One of the big mysteries of modern cosmology is the fact that the Universe is so uniform on large scales. Observations tell us our Universe is topologically flat, and the cosmic microwave background we see in all directions has only the smallest temperature fluctuations. But if the cosmos began with a hot and dense big bang, then we wouldn’t expect such high uniformity. As the Universe expanded, distant parts of it would have moved out of reach from each other before there was time for their temperatures to even out. One would expect the cosmic background to have large hot and cold regions. The most common idea to explain this uniformity is early cosmic inflation. That is, soon after the big bang, the Universe expanded at an immense rate. The Universe we can currently observe originated from an extremely small region, and early inflation made everything even out. The inflation model has a lot going for it, but proving inflation is difficult, so some theorists have looked for alternative models that might be easier to prove. One recent idea looks at a speed of light that changes over time.
The idea that light may have had a different speed in the past isn’t new. Despite the assertions of some young Earth creationists, we know the speed of light has remained constant for at least 7 billion years. The well-tested theories of special and general relativity also confirm a constant speed of light. But perhaps things were very different in the earliest moments of the cosmos. This new work looks at alternative approach to gravity where the speed of gravity and the speed of light don’t have to be the same. In general relativity, if the speed of light changed significantly, so would the speed of gravity, and this would lead to effects we don’t observe. In this new model, the speed of light could have been much faster than gravity early on, and this would allow the cosmic microwave background to even out. As the Universe expanded and cooled, a phase transition would shift the speed of light to that of gravity, just as we observe now.
Normally this kind of thing can be discarded as just another handwaving idea, but the model makes two key predictions. The first is that there shouldn’t be any primordial gravitational waves. Inflation models predict primordial gravitational fluctuations, so if they are observed this new model is ruled out. But it might be the case that primordial gravitational waves are simply too faint to be observed, which would leave inflation in theoretical limbo. But this new model also predicts that the cosmic background should have temperature fluctuations of a particular scale (known as the scalar spectral index ns). According to the model, ns should be about 0.96478. Current observations find ns = 0.9667 ± 0.0040. So the predictions of this model actually agree with observation.
That seems promising, but inflation can’t be ruled out yet. This current model only explains the uniformity of the cosmic background. Inflation also explains things like topological flatness and a few other subtle cosmological issues this new model doesn’t address. The key is that this new model is testable, and that makes it a worthy challenger to inflation.
Paper: Niayesh Afshordi and Joao Magueijo. The critical geometry of a thermal big bang. arXiv:1603.03312 [gr-qc]