Physics question: if the observable universe stretches out to the Big Bang, and the Big Bang is extremely dense, wouldn’t the expectation be that gravity would be pulling everything in the universe away from us, rather than potentially causing it to collapse?
As far as I can tell, the Friedmann equations all start with the assumption that the energy density of the universe is homogeneous. But shouldn’t that be incorrect? Doesn’t the energy density increase as you approach the edge of the observable universe?
I’ve stumbled into a pit that is deeper than my YouTube ladder.
First thoughts: interesting question -- a variant of the "why is sky black" paradox that still appears to work in finite universe.
What direction would you expect the pull to go in? Everything appears to be pulled (after taking into account space expansion) away from us? I'm not sure whether that would actually be possible to be the experience of every observer, and I expect that this is a way to figure this puzzle out.
Anyway, I should be sleeping, so will think about this in the coming day(s).
@robryk @isomer I guess my confusion is firstly around what need is there for dark energy to explain the accelerating expansion of the universe? And secondly, are the Friedmann equations actually still relevant, given the Big Bang was discovered long after he made them? Is it just that this effect is negligible? But it doesn’t really feel like it could be.
@robryk @isomer my expectation would be that once you get far enough away (so that the local gravity isn’t a factor), everything will accelerate away from us. And the things that are closest would be moving away the fastest as they have had more time to accelerate, and things further away would be moving slower, but accelerating faster. Which, as I understand it, is basically what’s observed?