In the Dark
A mysterious force is tearing the universe apart!
- By Ed Regis
- Air & Space magazine, March 2005
WHEN SCIENTISTS RECEIVE QUACK LETTERS, they arrive bearing certain characteristic trademarks. They tend to be written in longhand, with pencil, the pages covered with smudges, stains, and inscrutable cabalistic symbols, as if the writer were originally from the planet Weebo.
The author invariably claims to have spotted a hitherto-unnoticed flaw in Einstein’s theory of relativity. For the greater good of humanity, he humbly offers to correct it, proposing a melange of previously unknown forces, particles, energy fields, and, commonly enough, flocks of hidden dimensions, which have somehow escaped the attention of generations of scientists.
Today’s cosmologists might use computers instead of pencils, but otherwise their latest theories bear a suspicious resemblance to the World Classics of Crackpottery. Consider, for example, a briefing held last May at NASA headquarters in Washington, D.C., broadcast live to NASA centers around the country and Webcast over the Internet. The stage of the James E. Webb Auditorium, tucked away in a glassy, modern building a few blocks from the National Mall, is ablaze with light and crawling with television cameras, monitors, microphones, and crew members, as if this were the 400,000th “Oprah” show. Instead, it’s the latest episode of what might be called the Dark Matter/Dark Energy Follies, a series in which a bunch of astrophysicists repeatedly confess that they no longer fathom the universe it is their sworn duty to understand and explain.
“You would think by now scientists would know what the universe is made of,” says Andy Fabian of Britain’s University of Cambridge. “But we don’t.”
“This is the most profound problem in all of science,” says Michael Turner of the National Science Foundation. The most probable solution, he says with a grin, “is almost too bizarre to be true.”
The problem confronting them, however, is simple enough: The expansion of the universe, discovered by Edwin Hubble in 1929, is not slowing down, as astronomers had long thought, but rather speeding up.
The acceleration of the universe’s expansion is a minor catastrophe for astrophysics because for the last half-century or so, theorists had been supposing that the mutual gravitational attraction exerted by all the matter in the universe would be sufficient to decelerate, and perhaps halt or even reverse, the expansion. The most recent observations, however, indicate that just the opposite is happening. The cosmos is flinging itself apart, almost as if gravity no longer exists, or has changed direction, or has been overpowered by some sort of nouveau anti-gravitational, repulsive force.
As if this were a bad horror movie, the force in question has been named “dark energy,” a term coined by Michael Turner. Approximately 75 percent of the universe appears to be made of the stuff. “It’s the most important thing out there,” says Andy Fabian. It is, he thinks, a form of anti-gravity: “It’s like throwing an apple into the air and having it accelerate upward.”
“Only really weird things have repulsive gravity,” says Turner.
To address the problem, astrophysicists have rushed in with a succession of “really weird things” that dark energy could be made of. They’ve proposed exotic new particles such as axions, accelerons, and, jokingly, “bigons.” They’ve proposed strange new force fields and mysterious forms of energy such as quintessence, k-essence, phantom energy, and negative kinetic energy—whatever that is. And they’ve proposed various scenarios for the end of the universe at large: a Big Crunch (a grand cosmic collapse) if dark energy weakens, or a Big Rip (where the cosmos is out of here) if it strengthens.
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.
“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?
When many astronomers heard about invisible forces catapulting the universe hither and yon, the very first thing that popped into their heads was Albert Einstein’s so-called cosmological constant. In 1917, when Einstein published a paper on cosmology, the astronomical evidence of the era still suggested a static universe—Hubble’s landmark observations of receding galaxies were still a dozen years away. However, Einstein realized that according to both the Newtonian law of universal gravitation and his own theory of general relativity, the universe couldn’t be eternally static. Plainly, something had to be done to make his own theory agree with the empirical evidence of the day. And so to make his general relativity equations describe the universe as fixed and unmoving, Einstein introduced a “cosmological term,” a sort of mathematical fine-tuning of his theory that, lo and behold, yielded a static universe after all.
Today we would regard Einstein’s cosmological constant as a fudge factor, a tweak, or, less charitably, a kluge. It was the very image of a gimmick cooked up for no other reason than to “solve” an otherwise intractable problem—and indeed when Einstein learned of Edwin Hubble’s discovery that the universe was expanding, he repudiated the cosmological constant, calling it the “biggest blunder” of his life and saying that it was “theoretically unsatisfactory anyway.”
But Einstein had regarded the cosmological term as representing an inherent property of empty space—an unknown something that pushed against the pull of gravity—and it was exactly this sort of anti-gravitational force that later astronomers needed in order to explain the newly found acceleration of the universe. And so when Adam Riess and his 21 co-authors published their 1998 findings in the Astronomical Journal, the paper’s title was “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant.”
The modern version of the physical force represented by Einstein’s cosmological constant is a phenomenon called “vacuum energy.” Supposedly, according to the esoteric rules of quantum mechanics, a vacuum is not merely empty nothingness; rather it’s just barely something—or at least it can be something, some of the time. Riess, now at the Space Telescope Science Institute in Baltimore, explains: “The uncertainty principle says that the vacuum can borrow energy from nothing, if it has it for a very short amount of time.” This energy—dark energy—exists in the form of virtual particles, which live on borrowed time and borrowed energy. Where do these virtual particles come from and how do they create anti-gravity? “That’s the $64,000 question,” Riess says. “I mean, really, that is the thing we don’t understand. We think it’s a property of the vacuum that has to do with quantum mechanics, that even a vacuum still has energy in it.
“But it’s not the property of being empty that makes the vacuum have anti-gravity,” he adds. “It’s actually the existence of virtual particles that we weren’t really aware of that’s causing the anti-gravity. Apparently, if this is all correct, the vacuum still does have energy in it, in the form of these virtual particles.”
Whatever. Vacuum energy is such a strange and unintuitive phenomenon that it has an equally strange and unintuitive consequence: The bigger the vacuum, the more dark energy there is. And so it stands to reason that since the universe is expanding, there should have been a time in the distant past when the vacuum—and hence the amount of dark energy—were smaller than they are now. And in that case, the relative strength of dark matter would have been proportionately greater than it is now, which means that at some point in the past the universe should have been slowing down after all.
In 2001, Riess found that the Hubble Space Telescope had made repeated images of an extremely distant Type 1a supernova, SN 1997ff, an object that was more than 10 billion years old. It turned out that the object appeared brighter than it would have been if the universe had been expanding at the same rate throughout its history. In other words, the universe had been slowing down way back then, 10 billion years ago. And then it had speeded up.
Later images made by Riess and his colleagues enabled them to determine that the transition occurred some five or six billion years ago. This was the Big Jerk. (Sorry—that’s what they call it.) That was when the universe had expanded to a point at which its dark matter had become dilute enough, and its attractive force had therefore become weak enough, for the anti-gravitational push of dark energy to rise up and overpower it. Says Riess: “As the universe moved through time, it slowly removed its foot from the brake pedal until the point when the accelerator became stronger than the brake and started jerking the car forward.”
It’s an open question where the universe is headed, but the three alternatives are biggies: the Big Lonely, the Big Crunch, and the Big Rip. If the repulsive force of dark energy remains constant, the universe will continue to expand at its present rate. This will make our immediate celestial neighborhood a solitary place, a consequence that the dark energizers refer to as the Big Lonely. If dark energy gets weaker, standard attractive gravity will take over and the universe will collapse in on itself—the Big Crunch.
But if dark energy gets stronger, the current acceleration of the cosmic expansion will speed up even more. The expansion will feed on itself so that the repulsive force of dark energy will get stronger still, with the physical universe finally rending itself apart in a fabulous bacchanal of disintegration—the Big Rip. Riess describes how it will occur: “Large gravitationally bound systems rip apart, and then progressively smaller bound systems rip apart. A cluster of galaxies rips apart first, then galaxies themselves rip apart, and then solar systems rip apart, then planets rip apart, then nuclei rip apart. It’s smaller and more tightly bound systems that will rip apart as there becomes more dark energy in them than binding energy from the ordinary gravity.”
Finally, the material universe will be gone. Where to? Don’t ask.
All of this was so insane, even to astrophysicists, that there just had to be alternatives to dark energy. One of them proved to be that staple of crackpottery, the claim that Einstein’s general relativity theory is wrong, that we don’t really understand gravity. “Perhaps the most radical idea is that there is no dark energy after all, but rather that Einstein’s theory of gravity must be modified,” wrote Michael Turner and Andy Riess in “From Slowdown to Speedup,” in the February 2004 Scientific American.
“Maybe the laws themselves need to be changed,” wrote Georgi Dvali, the NYU physicist, in the same issue. They’d changed before, when Newtonian laws were replaced by Einstein’s, so why not now? Dvali is a proponent of superstring theory, a complex mathematical effort to present a unified account of nature. One of the cardinal assumptions of string theory is that nature has more dimensions than the ones we’re familiar with. “The theory adds six or seven dimensions to the usual three,” wrote Dvali. “The extra dimensions are exactly like the three dimensions that we see around us.”
The existence of extra dimensions provided the perfect opportunity for Dvali to advance his “leakage scenario,” which is his explanation of why the universe’s expansion is accelerating. Dvali thinks that normal attractive gravity is leaking out of our universe’s three dimensions and into those other ones, causing the universe to accelerate its expansion. His theory has a strange sort of logic to it. Who needs dark energy if you have six or seven extra dimensions as escape routes for gravitons, the particles that, according to quantum field theory, are the carriers of gravitational force? “Real gravitons that leak away are simply lost forever,” wrote Dvali. For those of us stuck back at home, “it looks as though they have disappeared into thin air.”
Riess, for one, doesn’t find the idea so crazy. He compares Dvali’s leakage scenario to shining light down a fiber optic cable and getting less light out at the other end than you’d put in. If you lived inside the cable and the cable was your whole universe, then it would appear as if some of the light had simply vanished. “But what’s really happening is that light is leaking out a little bit from the cable,” Riess says, “and you’re missing that.”
Unconventional as it is, Dvali’s theory has the prime advantage of making a scientific prediction that could one day be tested. “I have calculated that graviton leakage would cause the moon’s orbit to precess slowly,” wrote Dvali. “Every time the moon completed one orbit, its closest approach to Earth would shift by about a trillionth of a degree, or about half a millimeter.”
Other experiments to detect, measure, and understand the nature of dark energy are in the works. Saul Perlmutter of the Lawrence Berkeley National Laboratory in California has proposed building a satellite observatory called the Supernova Acceleration Probe to trace the history of the universe through the past few billion years. In 2007, the European Space Agency plans to launch its Planck spacecraft, an observatory that will make yet finer measurements of the cosmic microwave background pattern that constituted one line of evidence for dark matter. And the U.S. Department of Energy and NASA have proposed the sinister-sounding Joint Dark Energy Mission, a space-based telescope dedicated to observing Type 1a supernovas.
Whether any of these observatories will reveal the truth about dark energy remains to be seen. Meanwhile, astrophysicists can only bemoan the predicament of being in the worst scientific fix since the quantum mechanics revolution of the 1920s.
“Dark energy is perhaps the biggest mystery in physics,” says Chandra X-ray project leader Steve Allen at the NASA dark energy briefing.
“We don’t understand our cosmic destiny,” Michael Turner says. “This generation won’t solve the problem; the next generation will.”
For the time being, anyway, the world’s top astrophysicists appear to be in over their heads.
“We’re in a situation where we’re going to need a new idea,” says Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Massachusetts. “We’re in trouble. It might be our ideas are not wild enough.” As if!