According to SR, mass cannot move at the speed of light. The $24,000 question is why is there a wall at that particular speed? I am not questioning the velocity of this wall, but rather I am asking why is there a wall in the first place?
One way to answer this question, is to first look at mass with respect to gravity/GR. The properties of mass and matter, subject to gravity, is not fully defined by GR, since GR does not take into account phases changes of matter due to the induced pressure. GR fixates of space-time but not all observed details that are connected to mass, that happen concurrently. Something important is left out, with the excluded offering a way to address the question at hand.
If we start with a cloud of matter/mass, acted upon by gravity, space-time will contract as mass density increases. GR does a good job here. What also happens is the increasing mass density (of the material substance) will lead to higher and higher pressures, causing phases changes within the matter, as the interaction of forces change due to gravitational pressure.
With respect to space, the higher mass density is contracting physical distance between the matter, while space-time contraction is contracting space within space-time. These follow the same general direction. Time does the opposite. The higher mass density causes matter to interact at higher energy and higher frequencies, thereby speeding up time with respect to matter. Gamma based interactions in the core of a star uses the fastest time scales. Space-time does the opposite with time slowing down within space-time as mass density increases. One should notice there are two time attributes, generated by mass/gravity with each going in the opposite direction; mass times speeds up and space-time shows a slower time.
Although space goes in the same direction for space-time and mass/pressure/phases, time goes in the opposite direction. Say these two opposing time directions were to cancel, since one speeds up , as the the other slows down, the net effect is zeroed time in space-time. With SR we add energy and form what Einstein called relativistic mass which appears to have the same time canceling effect on local space-time. The net result is all one will have is space, at the speed of light, but without net time to change mass velocity.
In case of GR/gravity/mass pressure, there is an internal countering time clock, where the local space-time reference does not see mass behaving by the rules of space-time but rather it goes by rules defined by mass pressure. These two times, both generated by mass, cancel in a black hole. The time remains zero, until it loses some pressure to add net time. An expansion will be endothermic, causing no core energy to escape. This added energy at lower pressure adds pressure but this hits the wall of no time.
