The universe seems neither benign nor hostile, merely indifferent. ~Carl Sagan
Another day, another theory of how the universe will end.
As you no doubt know, the ultimate fate of the universe, somewhere out a google years from now, depends on the amount of mass it has. According to the prevailing theory, if it has enough, it will expand up to a point and then contract, ultimately crushing down to a primordial particle (or black hole) in a "big crunch." Alternatively, if there isn't enough mass, the universe will expand forever, become more diffuse and disorganized, due to entropy, ending as a sort of fuzzy haze of elementary particles.
The latter outcome has never been very popular, probably because it seems so dismal. The idea of a sort of cyclical "bang-crunch-bang" universe appears to be more comforting to people, probably because there is a guaranteed succession of universes.
The desire to know which outcome is probable has been one of the driving reasons for astronomers to try to find out just how much stuff is in the universe. I've talked about this before, so I'm not going into detail here. Suffice it to say, the search for matter has led scientists to determine that there's more out there than we can see. Part of it is something called dark matter; most of it is called dark energy. What the two have in common is that we don't know what either is.
Dark energy is the current darling of the skywatching community. It appears to be causing the expansion of the universe to be increasing rather than slowing, as one would intuitively expect. An accelerating expansion leads to some potentially strange outcomes.
One of the strangest is coming out of Case Western Reserve University (just so you know, your extremely humble author graduated at a very humble level from Case Institute of Technology, class of 1970; Case Tech is now part of CWRU). Lawrence Kraus and Robert Scherrer (out of Vanderbilt, which apparently doesn't have as good a publicity department) have proposed that ultimately radiation will, well, radiate away, leaving some very old matter scattered about an immense universe.
Fundamentally, matter won't decay fast enough to replace the dwindling radiation energy. There will still be such energy, but matter will be the dominant stuff of the future universe. But bits of matter will be so widely scattered as to be invisible to one another.
All this depressing outcome is due to the accelerating force of dark energy, which, according to Kraus and Scherrer, will begin to "accelerate the most distant galaxies and stars beyond the speed of light." What that means is that some parts of the universe will be beyond observable range of others. The long range outcome of this is a cold, scattered universe where an observer perched on one of the remaining bits of matter won't be able to see any others.
In fact, the scientists say that some objects that could be seen when the universe was about 7 billion years old aren't visible any more. In around 10 trillion years, only the local cluster of galaxies will be available for any astronomers still around in that distant epoch to observe. Eventually, all matter will have followed the expansion of space until all the stuff is out of sight of all the other stuff.
In an attempt to be properly apocalyptic about all of this, Mr. Krauss says, "The future is bad. A universe with dark energy is the worst of all possible universes for the long-term future of life."
And you though global warming was bad.
Of course, all of those doom-and-gloom depends on understanding how dark energy works in the long run. This is the same dark energy that we haven't directly detected and can't actually describe in physical terms. To show how fuzzy the concept is, in the article, cited above, I mention a study (near the end) where a study of distant supernovae seemed to indicate that dark energy had been at work for 9 billion years or more. The only fly in that ointment is that the study disagrees with the principle that dark energy was once an attractive force that later flipped into a repulsive force, which is fundamental to the dark energy theorists.
I also recorded that another physicist has offered a theory that doesn't require dark energy to account for the apparent repulsion of the universe, because there is no actual repulsion. It's an illusion created by the effect of local mass concentrations (like clusters of galaxies) on the immense voids between concentrations. The universe is expanding, yes, but local clusters are contracting as well, having less effect on the void areas, creating an illusion of accelerated expansion.
Dark matter is faring better, as at least one Chandra observation seems to indirectly indicate the presence of some unseen dark matter in a galactic collision (as I reported here). However, like dark energy, no dark matter has been directly observed. And, so far, the properties of dark energy and dark matter are purely conjectural. So, while Krauss and Scherrer may have an interesting model, that model rests on a lot of assumptions which may well be overturned in the next few years. That is, of course, the essence of scientific progress.
In other words, we're still very much in the dark, but the light is out there somewhere.