(Related: "Earliest Galaxies in the Universe Spied by Astronomers" [September 15, 2006].)
Redshift is astronomy's version of the Doppler effect: Light from more distant galaxies appears red compared to closer ones, because the wavelengths are getting longer as the galaxies move away from observers on Earth.
Based on observations of redshift, Slipher showed that nebulae are really faraway galaxies that are fleeing from the Milky Way at speeds near 600 miles (1,000 kilometers) an hour (related images: galaxy hunters).
But far into the future such visual evidence of the universe's expansion will disappear.
Other galaxies, Krauss and Scherrer write, will have moved too far away from Earth for even the most powerful telescopes to spot.
And redshift won't be the only characteristic of the modern universe that is likely to disappear, the authors say.
One of the key indicators of the big bang—the cosmic explosion that kicked off the expansion of the universe—is a type of energy called cosmic microwave background energy, which is still observable.
(Related feature: "Beyond the Big Bang" in National Geographic magazine [May 2005].)
But within a hundred billion years, that evidence will be moving so quickly that it will be invisible to current technologies such as spectrometry.
Furthermore, finding trace elements created by the big bang, like helium and deuterium, will become exceedingly difficult if not impossible.
That means cosmologists in the far future may be able to determine the age of objects in their galaxy but will have little idea how those objects came to be.
"Thus, while physicists of the future will be able to infer that their island universe has not been eternal, it is unlikely that they will be able to infer that the beginning involved a big bang," the authors write.
Scott Dodelson of Fermi National Accelerator Laboratory and the University of Chicago, said that the essay is valuable because not many people have thought about what future observers will see around them.
But he points out that the author's scenario rests on an assumption that dark energy—a mysterious, hypothetical force that helps fuel the universe's expansion—will remain constant.
(Related: "Mysterious Dark Energy Has Existed For Most of Time, Scientists Say" [November 17, 2006].)
"This is not necessarily so," Dodelson said, "and a different type of dark energy could conceivably lead to a much different future than the one described in the paper."
Krauss and Scherrer note the same caveat in their essay.
In general, Krauss and Scherrer write, we live in a "very special time" in the evolution of the universe.
We live in "the time at which we can observationally verify that we live in a very special time in the evolution of the universe!" they write.
In fact, they say, we should consider ourselves fortunate for reasons beyond our ability to see the rest of the cosmos: We're lucky that we have a home galaxy at all.
Fast-forward even farther into the future—many trillions of years, Krauss says—and the Milky Way and its neighbors will have collapsed to form a gaping black hole.
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