Hi Peter,
over 6c from Hubbles viewpoint without breaking the 2nd postulate or troubling causality.
thus we can discovery our universe because we can go more far than c, like I said before this limit of c is for me a wall for the check of th space between cosmolgical spheres, thus it is a possible hypothesis above our physicality, but frankly c is sufficient for the transfet of information on Earth, in the future it will be interesting to discover our galaxy but we are youngs dear Peter and thus where are the priorities....
When Eisntein spoke about the restrictive relativity, it was about all the physicality before the walls.Thus how can we accelerate or multiplicate this velocity if the code is so far about the rotation of quantum spheres, because it is there the secret dear Peter, the rotation of spheres.
Thus in my line of reasoning more the velocity of rotation is important less is the mass , thus you can understand the mass of the light and its linearity due to an other sense of rotation than the stable gravity.
Thus for a c more important you must increase its velocity of rotation , there the mass is zero towards in this serie and probably rest zero, thus the gravity must change its main sense and after accelerate the spinals rotations of the quantic architecture.
If we don't change the sense ,thus it is the mass which lost its mass with an acceleration of the rotations.....Of course the temperature is a thermodynamical parameter important because the universal gauge is correlated.
Best Regards
Steve