3 ms·

Kind of related to something that I've been chewing on: I've read things (outside of the OP) that say the universe is expanding and also speeding up it's expan

Kind of related to something that I've been chewing on: I've read things (outside of the OP) that say the universe is expanding and also speeding up it's expansion, as neighbors exert less of a pull on each other the further spread out things get. However, that gravitational pull is still there -- will it at any point slow down the expansion and retract back inwards?

hairytrog 8y ago
The Big Crunch will probably not come about - https://en.wikipedia.org/wiki/Big_Crunch https://en.wikipedia.org/wiki/Big_Crunch
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pdonis 8y ago
> that gravitational pull is still there -- will it at any point slow down the expansion and retract back inwards? No. If we are talking about the current best model we have of the universe, the dark energy density (which is what drives the acceleration of the expansion) is constant in time, while the density of matter (which is what causes the expansion to decelerate) decreases with time. Up until a few billion years ago, the expansion was decelerating, because the density of matter was high enough to dominate. But since a few billion years ago, the density of matter has been small enough that the dark energy dominates, and that is never expected to change.
- SiempreViernes 8y ago
  Though to be fair, our expectations on what dark energy will do aren’t very well grounded. We already have inflation and dark energy but no good reason to expect either of them to be there at all. Like, the book on dark energy I’ve seen go: ”here are a bunch of pretty good guesses”, thats not a field full of confidence in its expectations (though admittedly the book is 9 years old by now).
  - pdonis 8y ago
    > We already have inflation and dark energy but no good reason to expect either of them to be there at all. I have seen plenty of heuristic arguments for why we should expect inflation to be there, but I'm not really qualified to judge. None of them seem to have convinced enough physicists to be considered standard. For the cosmological constant, I think it depends on what you consider the "natural" condition to be. The simplest way to derive the Einstein Field Equation is to start from the Einstein-Hilbert Lagrangian and vary it with respect to the metric. The justification for using the Einstein-Hilbert Lagrangian is that it contains the only scalar which contains up to second derivatives of the metric and is quadratic in the metric and its derivative, namely, the Ricci scalar. But that statement is not actually true: there's another obvious scalar that meets this requirement, namely a constant. So the most "natural" version of the Einstein Field Equation contains the cosmological constant: it should be there on theoretical grounds, and that means "dark energy" should be there as well. Of course that also raises another problem, which is indeed one we don't have a good answer to at present: why is the cosmological constant so small?
Hendrikto 8y ago
> However, that gravitational pull is still there -- will it at any point slow down the expansion and retract back inwards? No. Even if dark energy stopped right now, almost ever pair of galaxies has long reached escape velocity. Their relative speed will slow over time, making them drift apart slower, but it will never reach 0, or even become negative (= contraction).

hairytrog 8y ago

The Big Crunch will probably not come about - https://en.wikipedia.org/wiki/Big_Crunch https://en.wikipedia.org/wiki/Big_Crunch

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pdonis 8y ago

> that gravitational pull is still there -- will it at any point slow down the expansion and retract back inwards? No. If we are talking about the current best model we have of the universe, the dark energy density (which is what drives the acceleration of the expansion) is constant in time, while the density of matter (which is what causes the expansion to decelerate) decreases with time. Up until a few billion years ago, the expansion was decelerating, because the density of matter was high enough to dominate. But since a few billion years ago, the density of matter has been small enough that the dark energy dominates, and that is never expected to change.

SiempreViernes 8y ago
Though to be fair, our expectations on what dark energy will do aren’t very well grounded. We already have inflation and dark energy but no good reason to expect either of them to be there at all. Like, the book on dark energy I’ve seen go: ”here are a bunch of pretty good guesses”, thats not a field full of confidence in its expectations (though admittedly the book is 9 years old by now).
- pdonis 8y ago
  > We already have inflation and dark energy but no good reason to expect either of them to be there at all. I have seen plenty of heuristic arguments for why we should expect inflation to be there, but I'm not really qualified to judge. None of them seem to have convinced enough physicists to be considered standard. For the cosmological constant, I think it depends on what you consider the "natural" condition to be. The simplest way to derive the Einstein Field Equation is to start from the Einstein-Hilbert Lagrangian and vary it with respect to the metric. The justification for using the Einstein-Hilbert Lagrangian is that it contains the only scalar which contains up to second derivatives of the metric and is quadratic in the metric and its derivative, namely, the Ricci scalar. But that statement is not actually true: there's another obvious scalar that meets this requirement, namely a constant. So the most "natural" version of the Einstein Field Equation contains the cosmological constant: it should be there on theoretical grounds, and that means "dark energy" should be there as well. Of course that also raises another problem, which is indeed one we don't have a good answer to at present: why is the cosmological constant so small?

Hendrikto 8y ago

> However, that gravitational pull is still there -- will it at any point slow down the expansion and retract back inwards? No. Even if dark energy stopped right now, almost ever pair of galaxies has long reached escape velocity. Their relative speed will slow over time, making them drift apart slower, but it will never reach 0, or even become negative (= contraction).