> Sokolsky has calculated that at 3×1020 eV, even a single proton could travel no farther than 10 megaparsecs

I wonder how fast proton looses energy (due to interactions with photons of background radiation).

How much energy it would need to travel to us from edge of observable universe?

Particle have (very short) wavelegths associated with them. It might be interesting to observe larger amount of such high energy photons coming from same point in the sky especially if there is a black hole near their path that could bend it slightly. Maybe we could get some diffraction patterns. ... yeah I'm probably insane.