Which way is Cosmically Down?
Most recent answer: 12/03/2015
- Tom Cicillini (age 38)
Long beach, New York
I think I get what you're asking. We say that ordinary clumping stuff (galaxies, ...) tends to pull the universe together via gravity. That would make the expansion slow down, if there were no other effects counteracting it. yet what direction is each galaxy pulled? Aren't they typically surrounded about evenly on all sides, so that on average the gravitational pull on each should be about zero? Which way is "down"?
That's a pretty deep question. The way we usually think of it is that all that gravity is tugging space together, tending to reduce distances. There's no particular direction in the space to that tug.
Here's a maybe useful analogy. Think of a 2-D space, a balloon, that's been blown up. We can picture it in a bigger space, our usual 3-D one. There's tension in the rubber that pulls the balloon together. But there's no net direction in the 2-D balloon space that any little patch of the rubber is pulled. Even without our 3-D picture, however, a little 2-D balloon creature could notice the effect of the pull because it would make 2-D distances get smaller. That's kind of our situation in 3-D, except that there's another effect, attributed to dark energy, that's stronger and makes the distances grow more quickly.
(published on 12/03/2015)