Cosmologists have put themselves in the shoes of their future counterparts by pondering the consequences of dark energy, an enigmatic force discovered in 1998 that seems to be pulling galaxies apart at a steadily increasing clip. Eventually, this accelerating expansion of space will yank galaxies away from each other faster than light can travel between them, leaving our galaxy and its immediate neighbors isolated in a vast darkness.
In the process, all current evidence for the big bang would either vanish or become so diluted as to be imperceptible, says cosmologist Lawrence Krauss of Case Western Reserve University, who has studied the idea with Vanderbilt University physicist Robert Scherrer. "It will lead them to the wrong conclusion about what the universe is doing. The universe will look static, and that's vastly wrong, because the universe is expanding so fast they can't see it."
Take the expansion itself, first identified by Edwin Hubble in 1929 from the red tinge of distant supernovae. As space puffs up like a balloon, it stretches the wavelength of light from the bluer side of the spectrum toward the red. Krauss says that in 100 billion years or so the light from receding galaxies will have been stretched like taffy to a wavelength longer than our galaxy (100,000 light-years) and, therefore, be impossible for telescopes to detect.
The cosmic microwave background radiation, a residue of matter's first flash of light following the big bang, will suffer a similar fate. As its wavelength expands, Krauss says, the radiation will start to be soaked up by interstellar dust, potentially becoming indistinguishable from other light sources.
Kiss good-bye, too, any knowledge of the relative abundance of the hydrogen isotope deuterium among all matter in the cosmos, which cosmologists believe was first set by the big bang when the universe was only about a second old. Researchers calculate the amount of deuterium in the heavens by measuring the light from distant quasars shining through interstellar gas, Krauss says. But dark energy will scatter both the quasars and gas beyond our observational horizon.
For their work, Krauss and Scherrer recently won a prize in an annual essay contest sponsored by the Gravity Research Foundation. The paper is slated to appear in the October issue of General Relativity and Gravitation.
So what will would-be cosmologists of the far future be able to tell? Krauss says the motions of stars or planets will probably lead them to general relativity, the modern theory of gravity, and nuclear physics will tell them that those stars have existed for billions of years. They will not realize, however, that their visible universe is only one galaxy cluster in a vast but dilute sea of galaxies.
Although we may know a lot more than they will, there is no guarantee we have a complete picture either, Krauss says. We only discovered dark energy, he says, because we live in a "special" time during which the mysterious influence is neither too weak nor too strong to observe. "This is about the only time in the history of the universe when you could detect it, and that's really weird," he says. "Maybe there are important things we can't observe that will have dramatic consequences for the future of the universe."