Doesn’t the evidence of an expanding universe make it unlikely that the universe is in an infinite cycle of expansion and contraction?
This seems to indicate that the “big bang” is a one time event. It also seems to mean that the universe is “diluting” and will eventually turn “cold”. Effectively, life in any shape or form will be relatively impossible at this point becasue enough energy to support life cannot concentrate in one spot.
Is there any reason to believe that the universe may eventually contract again?
The fact that the Universe is expanding now in no way implies that it won’t contract at some later time. By way of analogy, if I throw a ball in the air, it’s going up now, but some time later, it’ll be coming down.
That said, there is currently additional evidence that strongly suggests that the Universe will not ever re-collapse.
To expand on Chronos’ ball analogy, if you throw a ball up in the air, one of three things can happen. You only push the ball up at the beginning of the throw. After that, gravity wants to constantly pull it back down. Case 1: If you throw the ball too lightly, the ball is just going to go up and come back down. Case 2: If you throw the ball really really really hard, it’s going to escape the earth’s gravity and keep going forever. Case 3: You throw it just hard enough that it ends up exactly in between case 1 and case 2, so the ball goes up, just barely escapes the earth’s gravity, and stops. Case 3 is admittedly a razor thin line and is very difficult to hit exactly.
You’ve got the same 3 cases with the universe. The Big Bang pushed the universe out, but now that the Bang is over with, it isn’t pushing any more. But gravity is still trying to pull everything back together. So case 1 is that the universe expands outward until gravity overcomes the momentum from the big bang, and everything starts collapsing back towards each other (this is often called the Big Crunch). Case 2 is that it expands forever, and turns cold, dilute, and pretty much dead eventually. This is sometimes called the Big Freeze (as you can see, these terms play off of the “big bang” name) Case 3 is exactly the balance point between 1 and 2, where the universe expands outward to a certain distance and then stops.
Determining which case is our universe is not trivial. One of the problems is that there is a lot of matter out there that we can’t detect (so called “dark matter”) and there is also energy out there that we can’t detect (“dark energy”). We take a lot of measurements and combine them with our best guesses.
Our most recent measurements (that the rate of expansion seems to be accelerating) and guesses about the way things work have led to the conclusion that the universe will end in a Big Freeze.
Ah, yes, that’s what I was referring to. There’s still the caveat that we have very little understanding of the phenomenon causing the acceleration, so for all we know it might turn off or even reverse at some point in the future. In fact, early in the Universe’s history, a similar phenomenon called inflation did come to an end. So if that happens, then the Universe might still re-collapse.
But that’s an argument from ignorance. The best bet is still that it’ll keep on going more or less as it is now, which would preclude collapse.
What I can’t understand is, if all the galaxies are accelerating away from an initial formation point, how can the Andromeda galaxy be on a collision course with our own?
The expansion only shows up on a large scale. It’s not so much that individual galaxies are moving away from each other, as that clusters and superclusters of galaxies are moving away from each other. The Milky Way and Andromeda galaxies are part of the same cluster, and are very close together, so our mutual gravity is easily more than enough to counteract the cosmological expansion.
There is no “escape” from gravity; it continues to weaken in an inverse-square manner with distance but never ceases completely. A projectile launched at precisely escape velocity will continue moving away forever, approaching zero velocity (relative to the body it was launched from) without ever quite reaching it.
The analogy that makes a lot of sense to me is to compare the expanding universe with a star. Normally, we’d expect a star to collapse under its own gravity, but nuclear fusion provides an outward force. In the case of a star, this provides an equilibrium for most of its lifespan, but only while there is still nuclear fuel available. After that, the star continues its gravity-driven collapse.
A person with no knowledge of fusion might reasonably conclude that “dark energy” (i.e. an energy he doesn’t understand) is causing an expansionary force. Without understanding that energy, the person could not know that the star will collapse after its fuel runs out. In fact, he might predict that the star will expand because stars are losing a small amount of mass all the time. Imagine his surprise when it goes through the red giant phase (matching his prediction) and then suddenly collapses into a ridiculously tiny white dwarf, neutron star or black hole.
From a purely scientific standpoint, we have no evidence that the universe will do anything but continue expanding. However, the evidence also tells us that we just don’t know very much in the first place.
As I understand it, within the general framework of expanding space, the galaxies are moving about in random directions and at various speeds, under the influence of one-another’s gravity, but also under the influence of their own momentum (it was explained to me as being like the random movement of molecules in a gas). For galaxies that are relatively near one another, this random motion can be considerably faster than the speed at which the relevant region of space is expanding, so if the random motions happen to be sending them towards one another (and gravity, I guess, makes it a bit more likely that this will be the case), they may actually be getting closer, even though the general expansion of space is tending to pull them further apart.
Yeah, there’s really no such thing as stopping. An object with mass in space will always be attracted to the objects around it no matter how far away / weak the gravitational pull of those objects may be.
There is no position at which one can be said to have escaped gravity, but there can be a combination of a position and a speed that can be said to have escaped.
dracoi, what little we do understand of dark energy does not suggest that there is some finite resource being expended to maintain it. Which is not to say it might not turn off for some other reason: Like I said, we just don’t know.