Before “dark energy” and its anti-gravity effect was discovered, scientists had two models for the end of the universe:
Closed Universe: Eventually the combined gravitational pull of all matter in the universe will halt and reverse the universe’s expansion, causing it to collapse into a “Big Bust” to be reborn in another Big Bang, or something.
Open Universe: There’s not enough matter to “close” the universe, so it will keep expanding and expanding forever, until every particle is separated from every other particle and the universe ends in a “heat death.”
As I recall, there hadn’t been enough matter observed in the universe to “close” it, but most scientists were optimistic there would be enough.
Now that cosmologists and physicists are pretty sure dark energy is real, does this mean the universe can’t be closed? Will dark energy keep driving an expansion, no matter how much matter is out there?
I daresay that no one is really sure whether dark energy is real, given that no generally accepted, testable, or indeed, explicable observations about the nature of the energy exists. “Dark energy” is a placeholder for whatever causative agent is responsible for the phenomenon of accelerated expansion of the universe. Some hypotheses about what would cause a long-range, gravitationally repulsive force do exist, but all are based upon one or more premises that are, at this time, untestable or nonrecreatable.
If a sufficient quantity of dark energy (whatever it turns out to be) exists and continues to behave in the way we’ve seen the universe behave from our observations, then yes, we’d expect the expansion to continue ad infinum, until the differential velocities between galaxies, stars, and possibly individual particles are so great that they are no longer able to “see” one another, resulting in quantum thermodynamic heat-death (i.e. inability to recover any information). This is, conceptually, not very different from being trapped at the event horizon of an expanding black hole, leading some to hypothesize that the universe is actually an infalling black hole being stretched by gravitational differential. Kind of a neat idea, but I wouldn’t go taking any bets on it one way or the other, or indeed, on our likelyhood to actually come to an agreement on the correct theory in our lifetimes.
As for the universe being “closed”; as far as we know, the universe is closed, i.e. there is no interaction with anything “outside” of it (if indeed there is anything outside). The various “brane” theories suggest that gravity and other forces, and indeed, fundamental particles are the result of interferences between branes, but this is can be said to be highly speculative in the same way that a red-hot poker in the eye is described as mildly irritating, and obviously beyond our ability to verify.
Mmmmm, no. Closed universe only means that you end up with no more matter/energy than you started off with. How that matter/energy is arranged has nothing to do with the fact that the universe is closed.
Dark energy is currently the leading theory to explain why the universe’s expansion appears to be speeding up. The second law of thermodynamics dictates that entropy will always increase in a closed system, so no matter how much matter and energy is present, while there may be pockets of organization for a finite period, overall the universe will become less ordered, more random, so the two ideas match up just fine.
Even if there were enough mass in the universe to counter the effect of dark energy and cause a “Big Crunch” at the end of time, the second law of thermodynamics would still be in effect, and the mass, as it contracted, would be less ordered and more random on its way in than it was on the way out. (I think. To me, this raises the interesting idea of “reverse entropy”, which I had thought was supposed to be time running backwards. But then, I’m no physicist.)
Cosmology isn’t my specialty, but here’s a stab at it. Any real cosmologist who comes along should feel free to smack me down.
There’s a slightly out-dated premise you’re assuming in this question. “Closed” and “open” don’t really refer to the eventual fate of the Universe, but rather to the “shape of the Universe” at a given time. A “closed” Universe is shaped like a three-sphere, which (among other things) implies that the Universe is finite. An “open” universe", on the other hand, goes on forever. (There’s a third possibility, as well, called a “flat” universe — this one goes on forever as well. This is the case that most cosmologists think corresponds to our Universe.)
Now, for conventional matter, a closed Universe will eventually recollapse, and an open or flat Universe will eventually cool off and end in “heat death” as you describe. But once you add in the possibility of dark energy, it’s no longer true that closed Universes will always recollapse and open Universes will always expand forever. In particular, dark energy (as we understand it) allows you to have closed Universes which expand forever.
So to answer your question: the Universe could be closed, open, or flat and still expand forever. When we look at the experimental data, we see that the Universe is darn close to being flat, and so it’ll expand forever. What’s more, dark energy is unique in that its energy density is constant as the Universe expands, while the energy density of normal matter decreases as the Universe expands; so the more the Universe expands, the more important dark energy becomes. Currently, about 70% of the energy content of the Universe is due to dark energy; I believe that when the Universe is twice as old as it is now, it’ll be over 99% dark energy.
The acceleration of the Universe (whatever its cause) is certainly consistent with a universe with spherical geometry. Current evidence seems to indicate that the Universe is very close to flat, but it is not generally believed to be exactly flat. Rather, it’s generally suspected that it’s a little bit on one side or the other, but we can’t tell which. So it’s possible that the Universe does have spherical geometry.
It’s even possible that the Universe will eventually recollapse. If the acceleration continues like it’s going right now, it won’t. But as Stranger on a Train points out, we know almost nothing about whatever it is that’s causing the acceleration. It’s quite plausible that the mechanism behind the acceleration might at some future time turn off or even reverse, which could lead to a Big Crunch. After all, we had a similar (but much stronger) vacuum energy (if that’s what it is) during the inflationary era, and that eventually turned off. So we don’t know.