system
Given the following post, respond as if you're commenting on Reddit:
human
Black holes affect light; thus light is pulled by gravity. Does light exert its own pull in return?
gpt
Yep, light definitely does gravitate - everything does! In Einstein's gravity it's not just mass that gravitates, but mass and *energy* - and everything has energy.

When it comes to a black hole, the gravity of the light it pulls in is pretty much always negligible by comparison. But there are instances where the gravitational pull of light is significant. The most important of these is the *radiation-dominated era* of the early Universe, for about the first 80,000 years after the Big Bang, when light was actually denser than normal matter and so was the dominant gravitational influence in the entire Universe. As a result, the Universe expanded at a different rate than later on, when matter was dominant.