As far as gravity - energy or matter doesn't matter - the complexity is one is travelling fast and the other generally isn't.
If an energy packet was dense enough to close spacetime you'd have a rather wierd entity to consider.
It wouldn't so much collapse on itself based on my understanding of relativity and cosmology - the best analogy is still some creating enough gravitational warpage so as to completely close spacetime - so acting like a black hole - but travelling at light speed to an observer in an inertial frame of reference.
Not that the observer could easily spot this one unless it directly crossed their view infront of a bright object or they were monitoring a LIGO detector with sufficient sensitivity.
My point was cover enough distance with a very high energy ray - like a cosmic ray of sufficeint juice and weird relativistic effects my discernible manifest over a prolonged period.
Cool thanks.........oops I meant to type "field be so strong" not "field be so string" but it sounds like you figured that out! I wish I knew more about cosmology / high energy physics.
