In the Dark
A mysterious force is tearing the universe apart!
- By Ed Regis
- Air & Space magazine, March 2005
(Page 3 of 6)
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.
“The most plausible explanation,” Riess said later, “is that the light from the supernovas, which exploded billions of years ago, traveled a greater distance than theorists had predicted. And this explanation, in turn, led to the conclusion that the expansion of the universe is actually speeding up, not slowing down.” So surprising was this result that, initially, many of the scientists involved were too embarrassed to publish it. What if they were wrong? After repeatedly analyzing their data, however, and seeing no flaw in it, they did, with Riess as the lead author.
Odd as the supernova findings were, they were buttressed within a few years by two other types of observations. The first was a survey of the cosmic microwave background, the distant remnants of the Big Bang. In 2001, data from NASA’s Wilkinson Microwave Anisotropy Probe, an observatory orbiting the sun 900 million miles beyond Earth orbit, discovered variations in the temperature of the background radiation; the variations, through a tortuous chain of astrophysical reasoning, also pointed to cosmic acceleration. And then last year, a third study, involving measurements that NASA’s Earth-orbiting Chandra X-ray Observatory made of hot gas inside galaxy clusters, confirmed the results of the other two.
The Chandra observations also enabled the scientists to estimate what proportion of the universe consists of what type of substance. The conclusion was that 75 percent of the universe is made up of dark energy, 21 percent is dark matter, and only a scant four percent is ordinary matter—the stuff you can see and touch.
Finally there was no longer any doubt: Cosmic expansion was speeding up, and the visible universe of ordinary, everyday, boring reality was being eclipsed by a haze of mystic dark stuff. The question was: What did it all mean?