Five-sixths of the universe is missing. That statement feels strange to write, and I'm sure it feels pretty strange to read as well. Given the vastness of the cosmos--and given how little of it humans have explored--how can we know for sure that anything is out of place? The claim sounds positively arrogant, if not delusional.
Color-coded, composite of the galaxy cluster Abell 520. Green denotes hot gas; orange highlights starlight from galaxies; blue shows the inferred location of dark matter. (Credit: NASA, ESA, CFHT, CXO, M.J. Jee, and A. Mahdavi) And yet scientists have assembled a nearly airtight case that the majority of the matter in the universe consists dark matter, a substance which is both intrinsically invisible and fundamentally different in composition than the familiar atoms that make up stars and planets. In the face of staggering difficulties, researchers like Samuel Ting of MIT are even making progress in figuring out what dark matter is, as evidence by teasing headlines from last week. Time to come to terms, then, with the new reality about our place in the universe. Here are seven key things every informed citizen of the cosmos should know.1 Dark matter is real. The evidence for dark matter goes all the way back to a paper published by visionary Swiss astronomer Fritz Zwicky in 1933--less than a decade after Edwin Hubble definitively proved the existence of other galaxies. Zwicky noticed that galaxies in clusters were moving so quickly that the clusters should be flying apart, and yet the clusters remain intact. He concluded that there must be dunkle Materie (dark matter) scattered through the clusters, providing the extra gravitational pull that holds everything together. At the time, most of Zwicky's colleagues considered the evidence too tentative, and the idea too weird, to believe. In the 1970s, American astronomer Vera Rubin changed their minds with the same kind of observations carried out in much greater detail. She found that galaxies systematically rotate so quickly that they should fly apart unless bound together by dark matter--or unless our understanding of the laws of gravity are wrong. More recently, astrophysicists have run elaborate computer models of how galaxies form. These models beautifully fit the observed structure of the universe, but only if they include dark matter into their equations.
Stars in the outer regions of spiral galaxy M74 move much more quickly than expected if they were held in orbit only by the visible matter. The best explanation is that they are being pulled by a large halo of unseen, dark matter. (Credit: Gemini Observatory/GMOS Team) Two other lines of evidence strongly support dark matter. One comes from observations of gravitational lensing, the bending of light due to gravity. Astronomers can make crude maps of where the matter is in galaxy clusters by observing how they distort the light of more distant galaxies. These maps not only confirm the presence of huge amounts of dark matter, they also show that the dark stuff moves independently from hot gas in the cluster, something that alternate theories of gravity cannot easily explain. Another, completely independent line of evidence comes from studies of the cosmic microwave background, radiation left over from the Big Bang. The distribution of that radiation on the sky is very sensitive to the exact composition of the early universe. The observed pattern allows a very precise measurement of the makeup of the universe, as I described recently, in which dark matter outweighs visible matter by a factor of 5.5 to 1. All three types of observations not only show evidence of dark matter, they also show the same amount of dark matter. That's awfully persuasive. 2. Dark matter can be visible...sometimes. That sounds like a contradiction of all that I've just said, so bear with me. Dark matter seems not to interact with light or any other form of electromagnetic radiation (radio, x-rays, etc), but it may be able to interact with itself. One of the leading theories of dark matter holds that it consists of fundamental particles called WIMPs (weakly interacting massive particles) that can destroy each other if they happen to smack into each other. In the vastness of space, particles don't collide very often but it will inevitably happen occasionally. If two WIMPs annihilate each other, they might create visible radiation in the form of gamma rays; or they might give rise to more familiar types of particles, such as electrons and their antimatter partners, positrons. In fact, two space-based experiments are currently looking for both signals, and both see some intriguing signs of something strange going on in the depths of space. NASA's Fermi Gamma-Ray Space Telescope has picked up an extremely faint but unusual glow of gamma rays having a very specific energy: 130 giga-electron volts (GeV), or about 60 billion times the energy of visible light. That looks a lot like the breakdown of a dark-matter particle, but Christoph Weniger of the Max-Planck Institute for Physics cautions that the evidence right now "is as ambiguous as it can be." Further hints of dark matter come from Samuel Ting and the $2 billion Alpha Magnetic Spectrometer, or AMS, experiment aboard the International Space Station. That's the one that just made headlines last week. (New York Times: "Tantalizing New Clues into the Mysteries of Dark Matter".) AMS is picking up a slight excess of positrons from all directions of the sky, which is again consistent with the presence of dark matter but not yet at all conclusive. Stay tuned for more results; Ting says it will take "several more years" before he has enough data to say for sure.
The Alpha Magnetic Spectrometer experiment (top left) aboard the International Space Station. (Credit: NASA/AMS) 3. Dark matter might show up here on Earth. In theory, we are swimming in dark matter all the time. It should be passing through you right now. Because dark matter is so unreactive, most of the time it keeps going and nobody here is any the wiser for it. But starting in the 1990s, a few hardy (or foolhardy, depending on your perspective) physicists decided to try to sense dark matter particles as they pass. The idea is that on very rare occasions, a dark matter particle might strike an atom of ordinary matter, giving it a kick. That could potentially be detected as a thermal signal: a minuscule dose of heat. Several experiments along these lines have claimed tentative sightings of dark matter signals. The most celebrated results have come from the detector known as DAMA, short simply for DArk MAtter. Beyond a core of true believers, nobody considers these results convincing, however. A new experiment called LUX should clarify the situation. "The sensitivity is significantly better than previous direct detection experiments," promises LUX principle investigator Richard Gaitskell of Brown University. By the time LUX finishes its first full run in 2015 it will be, he hopes, "a very definitive experiment." 4. We might be able to create our own dark matter. That is one of the great goals for the ambitious Large Hadron Collider: making dark matter in the lab so that scientists can study it. The core concept of the LHC is that the mad smashing of particles into other particles will shake loose all kinds of things that do not show up in the calm and quiet of everyday physics. In essence, the huge amounts of energy created at the LHC can be spontaneously transformed into various particles (mass and energy being equivalent--remember your e=mc²?). That is how the physicists at the LHC (probably) found the Higgs Boson. If WIMPs have the kinds of masses that theorists expect, the LHC should create them too. Such dark matter particles will be hard to track down, because of their elusive nature. They tend to fly right out of the detectors, unseen, and so would initially show up as missing energy in the LHC reactions: One more shadow to chase. Still, if the WIMPs really are there, the crafty researchers and enormous computers that sift through data from the LHC should be able to find them when the collider restarts in 2015. 5. Dark matter is a totally different thing than dark energy. In 1998, two competing teams of cosmologists discovered that the expansion of the universe is speeding up. The force behind that cosmic expansion is now known as "dark energy," a term that was coined by Michael Turner at the University of Chicago as a deliberate (if sometimes confusing) counterpoint to dark matter. Both are dark in the sense that they are unseen, and both are dark in the sense that they are mysterious. But dark matter seems to consist of some sort of particles, and it exerts a gravitational pull that tends to bring things together: it glues together galaxies and galaxy clusters, and may have provided the extra attraction that allowed these structures to form in the first place. Dark energy, on the other hand, is even less well understood but it seems to be a form of energy that is embedded into the fabric of space itself, and it exerts a repulsive force--almost like antigravity--over extremely long distances. To add further confusion, dark energy has the equivalent of mass (if you didn't remember your e=mc² before, try remembering it now) and when you total up all that mass, dark energy is the dominant component of the universe.
6. The dark stuff really dominates.
Based on the latest observations from the Planck observatory, the universe consists of 68.3 percent dark energy, 26.8 percent dark matter, and 4.9 percent ordinary matter. A little perspective: More than 95 percent of the universe is dark and fundamentally unobservable, most of the universe does not consist of matter, and most of the matter does not consist of atoms like the ones that make up you and me. Feeling insignificant yet? 7. The dark universe might have a life of its own. A few years back, Savas Dimopoulos of Stanford University postulated that dark matter could form dark atoms that create their own dark chemistry. Neal Weiner at NYU has kicked around the thought problem of how a hypothetical scientist composed of dark matter might be able to find the visible universe (which of course would be invisible to him or her). The answer: It wouldn't be easy. And just recently a group of Harvard University physicists led by JiJi Fan and Lisa Randall have theorized that some dark matter might be able to cool and collapse just the way ordinary hydrogen gas does, leading to the possibility of dark galaxies, perhaps even dark stars and dark planets. Right now nobody knows. Perhaps these ideas are just flights of fantasy, but they are fantasies that are consistent with what we currently understand about how the universe works. In fact, they are supported by some of the best available current observations. Douglas Finkbeiner, an astrophysicist at Harvard, neatly sums up the wonder and uncertainty of this kind of research: "We should all be lying in bed awake wondering what dark matter is." Follow me on Twitter: @coreyspowell
