I'm quite novice about all this, only have read some vulgarization books and sites, but I wonder: could all the observable universe be falling outwards on some non-observable, more compact kind of barrier - I'll call it "edge"? Could this be a part of the explanation of cosmological accelerated expansion?

Like say we would be in a kind of bubble, or egg-shape thing? This thing could be inflating, and the closer matter would be from the edges, the quicker it would fall?

Good question "why would we be at the center of it?"... I know already there could be a kind of "flow" of matter at large scales which is already kind of violation of universe isotropy. Could we remove this issue with some kind gravity propagation itself?

Any sense to anybody it makes?

  • 1
    $\begingroup$ Your question does make sense (except the last paragraph), but a very good reason for this scenario not to be true is given by yourself: It would require that we were located exactly in the middle of the Universe (because the expansion is the same in all directions). You would have to come up with a plausible explanation for this. An even better reason is that the gravitational forces inside a hollow sphere vanish, so if we were surrounded by dense matter, we wouldn't be dragged to the edge. See the shell theorem. $\endgroup$
    – pela
    Jan 28 '17 at 21:58
  • $\begingroup$ related question here: astronomy.stackexchange.com/questions/12463/… $\endgroup$
    – userLTK
    Jan 29 '17 at 0:37
  • $\begingroup$ @JamesK, Edit: I removed the part that didn't make sense. Could my question be rehabilitated? $\endgroup$
    – J. Chomel
    Jan 30 '17 at 8:16

It's a pretty clever idea and a solid question that unseen mass might cause gravity outside the observable that tugs on the universe and might be the cause of dark energy as opposed to some unknown energy that seems to permeate all space. It's not a new idea, but it's worth answering.

There's 2, possibly 3 (or 4) pretty big problems to that approach. The first is that Dark Energy expansion is very uniform. For it to be explained by gravity, that gravity would need to be close to uniform around the observable universe, with Earth close to the center. A giant "edge" as you called it, or maybe we're in a low mass bubble surrounded by higher mass. Not impossible, but somewhat improbable that the Earth is in the center of this theoretical gravity field pulling the observable universe apart.

The 2nd problem is time. Gravity travels at the speed of light, so this gravity field would need to have existed prior to the big bang or be very close to the edge of the observable universe. Still not impossible, but it's a difficult theory to work with as it requires a few leaps of faith.

A possible 3rd problem is that measurements show that the universe is very flat. I don't know if a gravity field of the sort you suggest would be flat or not, I suspect not, unless it was enormously large.

Given enough time, maybe they can find a test to detect gravitons (if they exist) or, find dark energy and answer this one with better certainty. The question is asked here on Quora if interested.

I'll add a 4th problem. The velocity of observed young galaxies at the edge of the observable universe is close to the speed of light away from us in all directions. Relative velocity that high is difficult to explain by high mass outside of the observable universe cause that mass would need to be enormous, measurably greater than the mass of the observable universe. All the factors taken into account, it's an improbable explanation

If I've missed anything obvious or broken any laws of physics with the answer above, corrections are always welcome.


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