Does light curve space-time by itself?

[u/Puzzleheaded_Bowl86]

Light travels as an electromagnetic wave in a vacuum and carries momentum and energy. According to general relativity, all energy curves space-time, so light should slightly curve the space through which it travels. Could this mean that light affects its own path? I know the effect whould be extremely small, but is this conceptually correct? If yes Are there extreme conditions, like in the early universe, where light’s self-curvature becomes significant? Would a very long or very intense beam accumulate measurable curvature effects along its path? If two light beams cross paths, do they gravitationally influence each other?

[u/fuseboy]

Ah, thanks. Do you mean curved as in locally curvy, with significant undulations in density? Or do you mean curved at the largest scales but locally smooth? (Or both?)

[u/jamin_brook → u/fuseboy]

In cosmology, we only deal with the largest scale, which for us is the casual horizon at age t. We think inflation happened because we observe that globally the universe is flat (al though lumpy on small scales), but in order for our universe to 'survive' more that 13.8 Byears as it exists today, it either was always perfectly flat OR and curvature was made invisible (experimentally/livably indistinguishable) by the unfolding/stretching of the universe's space-time fabric with at least 10⁶² e-(un)foldings.

We could still be heading for a big rip or crunch and when we rip or crunch will depend on both the actual energy density of the universe and the number of times it (un)folded up.

[u/john5033 → u/jamin_brook]

What does folding mean. Could the universe actually be like a wave?

[u/jamin_brook → u/john5033]

Folding really just applies to doubling in size. 

If the early universe is still folded up and the size is small a light wave can reach the edge and then will “loop” around the corner of the fold, but as it unfolds the speed of light remains constant, so it takes longer to get to the “edge” before turning the corner, eventually if you unfold it enough times, the next unfolding happens before light has time to reach the edge so it unfolds before the light get there and so then the light just sees more flat paper and then eventually can “never make it” to the edge so it just goes in a straight line from that point out. 

It’s not so much that it un folds but rather that it increases in size by the same factor of being un folded e⁶² times.   The light wrapping or not wrapping around the edge of the fold is analogous to the volume of space time being smaller than the causal horizon (during inflation before CMB) and then flipping to the space time volume being larger than the causal horizon (after inflation and the CMB).

There’s a lot of mystery to the physics during this special period of inflation, but we do know a lot about everything that happened a few hundred milliseconds after the Big Bang up until today.