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Q & A: which way is cosmically down?

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Most recent answer: 12/03/2015
Q:
My question concerns what appears, at least to me, to be a contradiction between relativity's description of the fabric of space time and the seemingly three dimensional appearance of the universe with regard to the positions of the heavenly bodies (stars, galaxies, planets, etc).For example, if the sun, as all bodies of mass in space, can be said to essentially be sitting and, in turn, pressing down upon the fabric of space time and as a result creating a gravity well around which moving planets travel, how can it be that there are cosmic bodies positioned in every direction relative to the sun? How can the sun press down on space time if stars and galaxies are positioned "below" the sun (where the space time fabric it should be pressing down on to create the gravity well should be located) for a seemingly infinite distance. Am I missing something? Clearly the universe isn't believe to be a flat 2D plane where all bodies sit nice and leveled. It seems to go out in every direction. Thus, my question is again, how can mass based bodies press down on the space time plane if the universe does not appear to have a ground. Is it more like filling a balloon with marbles and a very very viscous fluid where the "plane is throughout the open sphere? Please let me know. I hope i articulated this question properly. Thanks. -tom
- Tom Cicillini (age 38)
Long beach, New York
A:

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.

Mike W.


(published on 12/03/2015)

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