The Search for Dark Matter
Failing to spot some 70 percent of the matter in the visible universe may seem like a glaring oversight, but astronomers were aware that telescopes simply could not capture all the objects that must be lurking in space. The first confirmation that much of the matter in the universe is invisible came when they noticed that the outer portions of a significant number of galaxies were rotating inexplicably fast.
Those outer stars seemed to be pulled by far more gravity than could be accounted for by adding up the contributions of the visible stars. The most plausible answer was the galaxies also contained clouds of what they dubbed “dark matter” that could not be seen by conventional means, but which exerts a gravitational tug.
Riess wondered if there might be enough dark matter in the universe to comprise the missing 70 percent predicted by cosmologists. And he thought he could prove it by measuring the rate at which the expansion of the universe was slowing down. If the universe’s expansion was slowing significantly, it would be fair to conclude there was unaccounted gravity pulling it back on itself, with a huge amount of dark matter at the root. Just how much there was would, in turn, dictate whether the cosmos would grow forever or end with a crunch.
The race was on to measure the expansion rate of the universe, involving a long slog of telescope observations and data analysis by both groups. In early 1997, Perlmutter saw the first hints of something unexpected. Measuring brightness, he found that supernovae for a given redshift were much farther from Earth than anyone had ever dreamed. If correct, the implication was shocking: Instead of the gradually slowing cosmic expansion predicted by cosmologists, the universe was speeding up. But before publishing such a claim, Perlmutter would need to check and recheck the finding.
Riess, meanwhile, had created a computer program that spit out calculations on cosmic density, and he was getting the same strange results. Instead of pointing to a universe in which matter comprised some 30 percent — the figure needed for the expected scenario of a universe expanding forever — his program seemed to mock him with a seemingly nonsensical result: negative 30 percent. The value did not appear to correspond to anything physically reasonable at all.
At first Riess thought his program was malfunctioning. But eventually he realized there might be an explanation, something totally unrecognized until now: Perhaps dark matter wasn’t the only “stuff” contributing to the overall density of the universe by exerting either a gravitational push or pull. Maybe something else was lurking as well.
You've Got Mail
This is when Riess emailed the crucial graph to Schmidt in Australia, not daring to tell him that he believed that it showed the universe was not only expanding, but that this growth was ramping up faster and faster with time. With the bizarre results in hand, Riess and Schmidt, too, now stood on a ledge, and they had to intensely examine their results.
The timing could not have been worse for Riess, who not only was getting married within weeks, but was holed up in his lab while his fiancee made last-minute arrangements. “Christmas vacation occurred, and I just stayed at work,” Riess laughs.
By early January 1998, Schmidt and Riess agreed that the result was real and told their team, allowing Riess to take a break to get married and go on his honeymoon.
Finally it was time to present the results to the world. At a talk at the American Astronomical Society meeting later that month, Perlmutter’s team presented data on accelerating expansion that Schmidt’s colleagues immediately realized fit with their own. Both teams were now in accord. The universe was flying apart faster than anyone had thought, propelled by an unknown force unaccounted for by any theory of physics to date.
The term dark energy began to be bandied about to describe the mechanism that drove the expansion. But in truth, it was little more than a label highlighting physicists’ ignorance about what it was, where it came from, and how and why it was acting the way that it apparently did.
In 2000, the observation moved beyond a reasonable doubt, thanks to measurements of microwave radiation that rippled out from the original Big Bang. The measurements taken by the BOOMERanG Experiment (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) and MAXIMA (Millimeter Anisotropy eXperiment IMaging Array) represented “cosmic stretch marks” that were proof positive of expansion at ever-greater rates.
With each astronomical observation over the past decade, evidence that the universe is composed of 30 percent matter — made up from adding both visible and dark matter components together — and 70 percent dark energy has grown stronger. Riess has since hunted down supernovae that exploded more than 7 billion years ago, filling in gaps: The universe first slowed down as the inward pull of matter dominated over the relatively mild outward push of dark energy.
As the cosmos expanded, matter gradually spread out, and its gravitational grip weakened, hitting a balance with dark energy about 5 billion years ago, causing the expansion to coast at a steady rate for a while, neither accelerating nor slowing down. In more recent times, because the universe has continued to grow but no new matter has formed within it, matter has been diluted further, spreading across an ever-growing space. As the density of matter in the universe has steadily dropped, the universe’s expansion has sped up.
Dark Energy's Origin Story
Despite such insights, physicists remain in the dark about dark energy’s origin. In one model, cosmologists propose that dark energy emerges from the fuzzy laws of quantum physics, which govern the subatomic realm. Quantum mechanics is famed for its weirdness because it states that before you look at a particle, it does not have any set properties; instead, it exists in multiple places simultaneously.
This built-in caprice means that you can never say for sure that a particle is not there; even the supposedly empty vacuum is bubbling with particles that pop in and out of existence for fleeting moments. This roiling froth of “virtual” particles adds energy to empty space itself, though so far the theory predicts far more energy than we actually see.
Could quantum effects be creating dark energy? And could some kind of constant resembling Einstein’s cosmological constant predict the phenomenon overall? “Because of this, cosmologists have been flailing around trying to find some explanation for dark energy that sort of looks like a cosmological constant, but isn’t a cosmological constant,” says Schmidt.
An alternative model is “quintessence,” the idea that the cosmos is pervaded by a field that lay dormant for much of its early history, but then kicked into gear, driving expansion only in the recent past. It’s possible to pit the two models against each other because in quintessence, the strength of dark energy varies while the cosmological constant (as the name suggests) is always the same.
Working with NASA on its Wide-Field Infrared Survey Telescope (WFIRST) mission, due to launch sometime after 2020 (see “Mapping the Dark,” page 47), Perlmutter will help choose between the different models by studying groups of supernovae that lie farther out in space than any yet studied, following the universe’s expansion history back in time. By peering into this distant past, he should be able to tell whether dark energy has remained the same or has varied over time as quintessence predicts.
And quintessence is only one possible alternative to the cosmological constant. Another explanation holds that our universe is housed inside a black hole — the superdense stellar corpse left behind in the aftermath of certain supernovae explosions. Calculations from cosmologist Stephon Alexander at Dartmouth College in New Hampshire show that when squashed together by an attractive force, subatomic neutrinos can form a cosmos-spanning superfluid with an anti-gravity effect matching the strength of dark energy.
The catch? Squeezing the neutrinos into a superfluid requires the kind of pressure generated inside superdense cosmic objects, meaning that in this model, our universe would need to be contained within something like a black hole. “It sounds crazy, but I think it is minimalist,” Alexander says.
Riess greets these theories and their many rivals with bemusement. “Over the past decade, there has been greater desperation. It’s understandable because this is a really hard problem.” Perlmutter adds: “There’s been around one paper about dark energy every day by a theorist for the past 12 years.”
But rather than pick a front-runner, Riess plans to search for evidence with an unbiased mind. “Like a baseball umpire, I’ll stay impartial and call the strikes as I see them,” he says.
Beyond choosing among the multitude of possibilities for the origin of dark energy, experiments may also help answer the question that fascinated Perlmutter, Schmidt and Riess in 1994: What is the ultimate fate of the universe? Where it was once thought that cosmic destiny would be governed by the density of matter, it now seems to be at the mercy of dark energy’s whims.
If dark energy continues on its current course unchecked, some versions of the theory suggest it could cleave the cosmos in a Big Rip, tearing apart stars, planets and atoms. If dark energy slows and then flips, eventually pulling in tandem with, rather than against, gravity, the Big Crunch — in which our local universe is crushed down into an infinitely small speck — will be back on the cards, although this seems less likely now.
One upcoming chance to tease out the answer could come from Euclid, a European Space Agency telescope designed to study the dark universe, and poised to launch into orbit around Earth by 2020 (See “Mapping the Dark,” at right). Euclid should begin generating data eight to 10 years from now, but, Perlmutter warns, it may not serve up the answers we anticipate. “If experience has shown me anything, it’s that experiments can lead somewhere completely different than what you thought.”
Schmidt notes the long wait following Isaac Newton’s 17th-century theory of gravity for Einstein’s general relativity. “To explain why the cosmological constant is there, we’ll need another Einstein — and we just don’t know when that brilliant insight will come,” he says. “It could happen tomorrow, but it could take another 150 years.”
Mapping the Dark
A group of new satellites and observatories will help map the dark side, shedding real light on the nature of the universe and its eventual fate.