When Albert Einstein was starting out on his cosmological quest 100 years ago, the universe was apparently a pretty simple and static place. Common wisdom had it that all creation consisted of an island of stars and nebulae known as the Milky Way surrounded by infinite darkness.
We like to think we are smarter than that now. We know space is sprinkled from now to forever with galaxies rushing away from one another under the impetus of the Big Bang.
Bask in your knowledge while you can. Our successors, whoever and wherever they are, may have no way of finding out about the Big Bang and the expanding universe, according to one of the more depressing scientific papers I have ever read.
If things keep going the way they are, Lawrence Krauss of Case Western Reserve University and Robert J. Scherrer of Vanderbilt University calculate, in 100 billion years the only galaxies left visible in the sky will be the half-dozen or so bound together gravitationally into what is known as the Local Group, which is not expanding and in fact will probably merge into one starry ball.
Unable to see galaxies flying away, those astronomers will not know the universe is expanding and will think they are back in the static island universe of Einstein. As the authors, who are physicists, write in a paper to be published in The Journal of Relativity and Gravitation, "observers in our 'island universe' will be fundamentally incapable of determining the true nature of the universe."
It is hard to count all the ways in which this is sad. Forget the implied mortality of our species and everything it has or has not accomplished. If you are of a certain science fiction age, like me, you might have grown up with a vague notion of the evolution of the universe as a form of growing self-awareness: the universe coming to know itself, getting smarter and smarter, culminating in some grand understanding, commanding the power to engineer galaxies and redesign local space-time.
"There may be fundamentally important things that determine the universe that we can't see," Krauss said in an interview. The proximate culprit here is dark energy, which has been responsible for much of the bad news in physics over the last 10 years. This is the mysterious force, discovered in 1998, that is accelerating the cosmic expansion, causing the galaxies to rush away faster and faster.
The leading candidate to explain that acceleration is a repulsion embedded in space itself, known as the cosmological constant. Einstein postulated the existence of such a force back in 1917 to explain why the universe did not collapse into a black hole, and then dropped it when Edwin Hubble discovered that distant galaxies were flying away -- the universe was expanding.
If this is Einstein's constant at work -- and some astronomers despair of ever being able to say definitively whether it is or is not -- the future is clear and dark. In their paper, Krauss and Scherrer extrapolated forward in time what has become a sort of standard model of the universe, 14 billion years old, and composed of a trace of ordinary matter, a lot of dark matter and Einstein's cosmological constant.
As this universe expands and there is more space, there is more force pushing the galaxies outward faster and faster. As they approach the speed of light, the galaxies will approach a sort of horizon and simply vanish from view, as if they were falling into a black hole, their light shifted to infinitely long wavelengths and dimmed by their great speed. The most distant galaxies disappear first as the horizon slowly shrinks around us like a noose.
A similar cloak of invisibility will befall the afterglow of the Big Bang, an already faint bath of cosmic microwaves, whose wavelengths will be shifted so that they are buried by radio noise in our own galaxy. Another vital clue, the abundance of deuterium, a heavy form of hydrogen manufactured in the Big Bang, in deep space will become unobservable because to be seen it needs to be backlit from distant quasars, and those quasars, of course, will have disappeared.
Eventually, in the far, far future, this runaway dark energy will suck all the energy and life out of the universe. Our future cosmologists will puzzle about why the visible universe seems to consist of six galaxies, Krauss said. "What is the significance of six? Hundreds of papers will be written on that," he said.
Although by then the universe will be mostly dark energy, Krauss said, it will be undetectable unless astronomers want to follow the course of the occasional star that gets thrown out of the galaxy and is caught up in the dark cosmic current. But it would have to be followed for 10 billion years, he said -- an experiment the National Science Foundation would be unlikely to finance.
"This is even weirder," Krauss said. "Five billion years ago dark energy was unobservable; 100 billion years from now it will become invisible again."

NYT News Service