Neutrinos are generally expected to travel a tiny bit slower than the speed of light. Nobody really expects them to travel at the speed of light even if we are currently unable to measure the tiny difference.
I was a physics student in 1987, when neutrinos were captured from a far-away supernova. I rushed into my professor's office and asked breathlessly: "Does the timing of the observations give us a limit on the mass of the neutrino?"
He calmly replied: Sorry, kid. Somebody already submitted a paper on that.
That raises an interesting thought: Does interstellar space have a non-unity refractive index? If there's any kind of matter out there, then I suppose the answer has to be yes. But I wonder if it's quantifiable.
The term 'speed of light' is somewhat confusing. Just to clarify: there's the universal maximum speed, c; this is the speed with which light travels in the vacuum; in a medium light happens to slow down, while neutrinos do not, the result of which is their moving not slower, but even "faster than light."
the speed of light is based on two specific values of the medium in which the photons are travelling. The permeability and the permittivity, one affects the magnetic the other affects the electric.
Hence by the change of these two values, the speed of light (magnitude) changes. Hence, if we are able to create media in which these values are appropriately specified, we can create media in which the specific speed of light is less than c0 or greater than c0.
It, therefore, may be possible to create a medium in which light will pass through faster than a vacuum.
He calmly replied: Sorry, kid. Somebody already submitted a paper on that.