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.