[jprete]

I would really like to hear from an actual physicist on this question since both of you seem correct for one of the reference frames and the only way I see to reconcile the two is with weird singularities like "the particle reaches the edge of the visible universe in infinite time according to the rest observer". (And I don't think that is right either.)

[scientator]

The edge of the visible universe functions for us like a cosmic event horizon. Similar to the event horizon around a black hole. A particle leaving earth at light speed can never reach or go beyond that horizon. Even in infinite time. That's assuming the universe continues to expand. If it starts to contract then, yeah, the horizon is going to crash in on us.

[jprete]

What does this look like from the photon's frame of reference, then? Or is it nonsensical to try to describe things from a frame of reference at C?