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In the Dark

A mysterious force is tearing the universe apart!

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Still, some theorists find all this dark energy conjecturing a bit too much. Georgi Dvali, a physicist at New York University, does not think that dark energy actually exists. In “Out of the Darkness,” an article in the February 2004 issue of Scientific American, he wrote: “Researchers commonly attribute the acceleration to some mysterious entity called dark energy, but there is little physics to back up those fine words.”

Not that Dvali’s own solution is any less quirky. The reason that the universe is flinging itself apart, he thinks, is that gravity is leaking out of the cosmos, radiating away, slipping off furtively to somewhere else. Like where? Why, into other dimensions. “The extra dimensions not only sap the strength of gravity,” he wrote, “but also force cosmic expansion to accelerate without any need to stipulate the existence of dark energy.”

Other dimensions? Well, why not? After all, they’ve already successfully explained the disappearance of so many things. The other dimension, as we know, is where Jimmy Hoffa ended up, along with Judge Crater, D.B. Cooper (who parachuted out of a Northwest Orient 727 with $200,000), the missing Florida ballots, all that lost airline luggage, and Elvis.

Truly, these are heady days for astrophysicists.

The first clue that the expansion rate of the cosmos was increasing appeared in 1998, when two separate groups of observational astronomers, one working with the Supernova Cosmology Project and the other with the High-z Supernova Search Team, were canvassing the universe for Type 1a supernovas to measure the rate by which cosmic expansion was, as they assumed, slowing down. Type 1a supernovas are stellar explosions of a known magnitude, so they are regarded as “standard candles,” celestial bodies whose distance can be gauged by their brightness—the farther the object, the fainter it appears.

The expected slowdown was thought to be a simple function of the pull of gravity. The universe, after all, is full of matter—and not only luminous matter, such as stars. A large part of the universe’s mass is thought to be dark matter, a substance whose existence was first postulated in the 1930s by astronomer Fritz Zwicky of Pasadena’s California Institute of Technology to explain his observations of galaxies huddling together in large clusters. From what he could tell, the clusters didn’t seem to have enough visible matter in them to produce the gravity needed to hold them together. Therefore some unseen mass must be exerting the required gravitational effect.

In the years since, estimating the amount of “missing” mass in a galaxy or galactic cluster became a fairly routine business among astronomers, who at one point were saying that up to 90 percent of the universe consisted of the unlit stuff. The fact that dark matter was an inferred rather than a directly observed phenomenon didn’t bother astronomers in the least. Gravity itself is not observed directly, either; its existence is revealed only by its effects.

Astrophysicists had a field day imagining just what dark matter consisted of. In astronomy, “dark” means merely “does not radiate light,” a fairly broad category that includes dead or dim stars, unseen planets, black holes, and miscellaneous flying chunks of matter (all of them collectively known as MACHOs—massive compact halo objects), as well as elementary particles such as neutrinos or more outlandish fare such as photinos and gravitinos (collectively known as WIMPs—weakly interacting massive particles).

Soon theorists had postulated both cold dark matter (composed of slow-moving particles that remained within galaxies) and hot dark matter (particles that had achieved escape velocity and streamed out of galaxies like invisible solar flares). And physicists suggested even wilder theories: Dave Criswell of the California Space Institute proposed that the missing mass was at least partially composed of solar systems enclosed by light-impervious casings built by extraterrestrials.

Anyway, with all that dark matter filling the universe like so much invisible turkey stuffing, what could the cosmos do but, sooner or later, yield to its pull and slow its headlong rate of expansion? And so when in 1998 Adam Riess, a young postdoc at the University of California at Berkeley, and his colleagues in the High-z Supernova Search Team pointed the Hubble Space Telescope toward selected Type 1a supernovas, they had every expectation of finding evidence that the universe’s rate of expansion was decreasing. The supernovas in question, however, were fainter than anticipated. Either they were farther away than they were supposed to be or their light was being dimmed by interstellar dust. In the latter case, however, the dust would impart a reddish tint to the starlight, but the light from the supernovas was not red at all. The conclusion seemed inescapable: Counter to all expectation, the expansion of the universe was accelerating.

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