Nice, added to the reading list.
Absolutely not. Things with mass cannot reach the speed of light, for a start. The ##c^2## in ## E=mc^2## is a conversion factor between energy units and mass units - that's all. In a less artificial unit system than SI, the conversion factor is 1, and the equation is just ##E=m##.
Well, first off, c IS the speed of light, as far as we know. More importantly, there is a "universal maximum speed" and THAT is the constant "c". Experimentally, and theoretically, there is no reason to believe that light travels slower than c, but it is not impossible. If light were found to travel slower than c, that would have no effect on c or equations using it, it would just mean we'd have to come up w/ a new symbol for the speed of light.
It is true. But the constant ##c## appears all over the place as a conversion factor, and would continue to appear in the maths even if we were to discover that photons had a (tiny) mass and hence that light did not travel at ##c##.
Actually, the "Big Bang Theory" starts at the end of the (presumed but not 100% confirmed) inflation in the first tiny portion of a second after the creation event (whatever THAT was) and moves forward from there. What went before inflation is unknown. Some models say that spacetime was created as part of the creation event but that's a model. Nature doesn't care about our models and we don't know what was really going on.
Whereby till now we couldn't confirm that what seems to act like matter is really matter.
In the terms of dark matter, it basically means that it isn't detectable by light (or any other frequency of the electromagnetic spectrum). The common possible candidates are MACHOs (MAssive Compact Halo Objects), which could be objects like Black holes, etc, or WIMPs (Weakly Interacting Massive Particles) , These would be particles that have mass, but just don't interact electromagnetically, so they do not emit, absorb or scatter light(other than by gravitational lensing) The neutrino is such a particle, though there are reasons why the neutrino's we are familiar with don't make up dark matter.( though a hypothetical type of neutrino, the "sterile" neutrino has been proposed as a candidate).
I was thinking of the methodology we use to place experimental restrictions on the photon mass. As far as I understand, you use Proca's equation to derive some effect (e.g. a non-zero E-field inside a charged sphere) the strength of which depends on photon mass, and then go and measure that effect. So far the results are consistent with zero mass to ever tighter limits, but if it's ever found to be non-zero then light doesn't propagate at the invariant speed, ##c##. But that wouldn't change the role or value of ##c## in relativity.
Light travel slower than c in a medium with an index of refraction that's less than 1
As Ibix pointed out, I'm using standard of light traveling in a vacuum. Light traveling in a medium is irrelevant to this discussion.
Why did you say that it is not impossible for light to travel slower than c?
See post #20. If photons turn out to have mass then they do not move at ##c## even in vacuum. We are currently unable to detect any mass, and therefore we model light as traveling at ##c##.
Again, see post #20. You can describe a massive photon and the diffrent effects this would have on measurements as you let the mass vary. Our actual measurements put an upper bound of 10-54kg or something like that, but the upper bound is not and never will be zero (barring a complete revolution in physics). So a massive photon is a possibility, albeit fairly remote.
A massive photon just seems contradict the entirety of modern physics. Could we have a stationary photon? What about a massive anti-photon? What about black holes, the equivalence principle? There's a lot of disturbing consequences.
So a massless photon is its own antiparticle, but if photons DID have mass they would have a separate antiparticle? Interesting. I never thought about that.
Peter has answered your specific points. I think the general point is that you are confusing how we normally model light (massless, always travels at ##c##) with reality, which is whatever it is. Reality isn't measurably different from our model (it wouldn't be a useful model if it was), but that doesn't mean the model is exactly correct.
