Centauri Dreams‘ recent post on the eventual merging of the Milky Way with the Andromeda galaxy took us to a future some five billion years from now. But it also speculated on something even more distant in time. What happens if the universe’s expansion does not stop accelerating? Eventually the galaxies beyond our own Local Group will exit the visible universe. Astronomers of that era would have no way of knowing those galaxies had ever existed, and would shape their cosmology accordingly.
Meanwhile, our Local Group should still be visible — the merged Andromeda/Milky Way elliptical galaxy and the survivors of the more than thirty galaxies, held together by mutual gravitational attraction, that make up the LG today. These galaxies should remain gravitationally bound despite the effects of the accelerated expansion, according to a paper by Lawrence Krauss (Case Western Reserve) and Richard Scherrer (Vanderbilt) to be published in October.
A starry island in an endless black sea. It’s an odd scenario, to be sure, but one that violates no currently understood laws of physics. The reason the more distant galaxies will seem to disappear is that space will be expanding faster than the speed of light. Einstein, to be sure, told us that nothing could move faster than the speed of light within space, but his theories put no speed limit on the expansion of spacetime itself.
As for those distant galaxies beyond the Local Group, they will still be out there, but their light will be unable to reach us. Unless, of course, our understanding of the universe’s expansion is incomplete (also a reasonable assumption). After all, we don’t understand the dark energy that almost has to exist to explain current observations, nor can we necessarily assume (although it would seem logical to do so) that dark energy will always have the same effects that we observe today.
All we can do is extrapolate from what we know and be aware of the gaps in our knowledge. Those gaps will be profound in the universe of the far future. Listen to Krauss and Scherrer on the question of whether or not physicists of that era will be able to puzzle out the Big Bang:
The answer is no. The inference that the universe must be expanding or contracting is dependent upon the cosmological hypothesis that we live in an isotropic and homogeneous universe. For future observers, this will manifestly not be the case. Outside of our local cluster, the universe will appear to be empty and static. Nothing is inconsistent with the temporary existence of a non-singular isolated self-gravitating object in such a universe, governed by general relativity. Physicists will infer that this system must ultimately collapse into a future singularity, but only as we presently conclude our galaxy must ultimately coalesce into a large black hole. Outside of this region, an empty static universe can prevail.
Which leads to this extraordinary assessment:
Observers when the universe was an order of magnitude younger would not have been able to discern any effects of dark energy on the expansion, and observers when the universe is more than an order of magnitude older will be hard pressed to know that they live in an expanding universe at all, or that the expansion is dominated by dark energy. By the time the longest lived main sequence stars are nearing the end of their lives, for all intents and purposes, the universe will appear static, and all evidence that now forms the basis of our current understanding of cosmology will have disappeared.
How do we know our own era doesn’t suffer similar constraints? Are there events in the early universe of which we are unaware because of the expansion of spacetime, and how does our lack of understanding distort our own view of reality? “It’s very important for all cosmologists to be very humble,” said Fred Adams, co-author (with Greg Laughlin) of The Five Ages of the Universe: Inside the Physics of Eternity, when queried about Krauss and Scherrer’s work in this Discovery News story.
Indeed, cosmology seems to teach humility above all else (The Five Ages of the Universe is a useful primer). The Krauss/Scherrer paper is “The Return of a Static Universe and the End of Cosmology,” slated for the Journal of Relativity and Gravitation and available online.
Here’s a thought that I’ve had for a long time. Observing objects at the extreems of the visible universe reveals them as they were over 10 billion years ago, when the universe was young, relatively a short time after the big bang. If we assume a model of the universe wherein there was the great rapid expansion, followed by a slow decrease in the expansion as gravity slowed it down, we would expect that we would “see” everything rapidly receding from us if we look at it going farther and farther back in time, with objects nearer in time receding from us less rapidly, and objects located in time and space very close to us actually contracting towards us. Well, that is what we see isn’t it? We aren’t seeing these distant galaxies receding from us “today”, we are seeing them retreating from us billions of years ago. If the light was red-shifted at the time it left those distant stars, would it not be seen as red-shifted today? I don’t follow the logic that red-shifted light from distant galaxies as seen today, and the ones farthest away being the most red-shifted, I don’t get that it follows that today’s universe is expanding. What it tell me is that everything was expanding billions of years ago faster than it is now. The farther back in time, the faster it was expanding. What is actually going on with those distant galaxies presently is ALREADY lost to any possibility of knowing since we can never get there. For all we know the expansion of the universe has already stopped and those galaxies may be accelerated towards us. But since the only light we can see from those distant galaxies left them at a time when indeed the universe was rapidly expanding, we have no way of knowing what those distant objects are doing today, or where they are located. Where is my idea wrong?
These are fascinating questions, David, and of course you’re right that what is happening today to the objects whose light we examine at vast distances is unknown. But we can also tie in the redshift findings with other factors like dark energy. Supernovae observations in the last decade resulted in the current view that the universe is not only expanding but accelerating in its expansion. Oddly, the expansion seems to have decelerating up until about five billion years ago — actually, this fit older models well, because it makes sense that gravitational attraction would slow the rate of expansion (I think this is just what you are saying). But current thinking about dark energy gets around this problem and the idea of accelerating expansion now seems firmly entrenched. We have much to learn about how dark energy works, of course, but if it does increasingly come to dominate the universe, then the outcome foreseen by Krauss and Scherrer may well happen.
Hi David
The red-shift isn’t due to the pace of the distant galaxies themselves, but the expansion of the space-time interval that their light has travelled across to us. That means the red-shift we see is a kind of summing up of how much space-time has stretched between us and those distant galaxies.
As for whether they’re still accelerating or not we look to relatively nearby galaxies and what they’re doing because the Universe is either expanding or contracting as a whole – thus what galaxies nearby are doing is related to what galaxies very far away are doing also. You might think that sounds like some faster-than-light influence, but really it’s because the whole is expanding due to lots of little expansions between galaxies.
The finite speed of light – and gravity – means that our “little” region of expanding Universe is all we can currently know. So you’re quite right that the distant objects – ones beyond our current horizon of about 45 billion light years – might be doing something quite different to what we can see. Cosmologists can only work with what they can currently observe and speculating on unseen events beyond our horizon is pretty fruitless. A good cosmological hypothesis has observable effects this side of the horizon – if all the action is where we can’t see it then there’s no way of knowing if the hypothesis is true or false.
That doesn’t mean we can’t do cosmology, but many assumptions – like an infinite Universe which many cosmologists accept – are just assumptions for philosophical reasons and are really just the borders of our current maps. I believe in a finite Universe for several reasons, but the data could go either way.