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Dark Matter in Galaxies Like Our Own

In the 1970s, astronomer Vera Rubin of the Carnegie Institution of Washington was measuring the rotation speeds inside spiral galaxies similar to the Milky Way, then considered rote and unglamorous work. But her efforts yielded a major discovery: Objects at the galaxies’ far edges were moving at startlingly high velocities. The only way these outer regions could remain intact was if they were bound together by the gravitational embrace of a much larger halo of invisible material, five to 10 times as massive as the visible galaxy.

To many of Rubin’s colleagues, the existence of so much dark matter seemed absurd. Some researchers suggested the dark halo consisted of ordinary material that simply didn’t emit light. For instance, a ball of gas with less than 8 percent of the sun’s mass would not be squeezed tightly enough by its internal gravity to ignite the nuclear reactions that cause stars to shine. It would form a brown dwarf—a near-invisible object almost as hefty as a star. Systematic searches for these objects turned up surprisingly few, however, and computer simulations indicate that interstellar clouds such as the Orion nebula convert just a small fraction of their mass into brown dwarfs. The first stars were probably more massive than today’s, making it even less likely that brown dwarfs make up a significant portion of the invisible universe. On the other hand, early massive stars could have collapsed into black holes when they exhausted their nuclear fuel. Black holes emit nothing at all.

In the late 1980s, Bodhan Paczynski of Princeton University and several other astronomers realized there was a way to detect unseen compact bodies that might be lurking in the halo of our galaxy. If one of the objects happened to pass directly in front of a bright star, the dark interloper’s gravity would temporarily bend and amplify the light. To improve the odds of spotting such a chance alignment, three groups of researchers used specifically modified telescopes and computers to monitor millions of stars at a time. The rarity of these events—only 15 meaningful ones, seen in the direction of our satellite galaxies, have been recorded—confirmed that brown dwarfs and black holes are far too scarce to make up a significant fraction of the dark portion of our galaxy.

For more clues to the nature of dark matter, astronomers have looked out beyond our neighboring galaxies, into deep stretches of space where the influence of the unseen material shows up in other, more dramatic ways.

Farther Out

Dark Matter in Galaxy Clusters

Some of the most dramatic evidence of dark matter shows up in images of large clusters of galaxies. The gravitational pull of matter in the cluster bends and twists the light from more distant galaxies, producing a plethora of strange optical effects ranging from distorted arcs to multiple images of the same background object. This process is called gravitational lensing. The intensity of the distortion indicates the strength of the overall gravitational field, and hence the total mass, of the lensing cluster.

The mass of galaxy clusters inferred from lensing studies is about 10 times as great as that of all their stars combined with the hot X-ray-emitting gas that fills the space between the galaxies. The excess mass must be dark matter. (That conclusion assumes gravity behaves the same on cosmic scales as it does on Earth—a logical but untested article of faith. See “Nailing Down Gravity,” Discover, October 2003.) In clusters, too, dark matter seems to form a massive halo around the bright parts. Earlier this year, Jean-Paul Kneib of Caltech and his colleagues analyzed gravitational distortions produced by a galaxy grouping named CL0024+1654. The resulting map, pictured below, exposes the location of dark matter in and around that cluster.

So what makes up all this dark material holding together clusters of galaxies? Burned-out or failed stars cannot account for nearly enough mass. In fact, there are strong reasons to think that much of the dark stuff is not made of atoms at all. The Big Bang theory makes detailed predictions about the total number of ordinary atoms and about the relative abundance of deuterium (heavy hydrogen) and helium in the universe. These predictions match the observed cosmic composition if the modern universe has an average density of 0.2 hydrogen atoms per cubic meter—far less than the amount of matter seen around galaxies and galaxy clusters.

As a result, astronomers are convinced that most dark matter must consist of particles that do not influence nuclear reactions. That rules out all atoms but allows many other types of elementary particles. The most plausible of these dark matter candidates are neutrinos. These ghostly particles are so unreactive that trillions of them pass through you each second without disturbing any of the atoms in your body. They then continue right on through Earth, with absolutely no effect.

Theoretical calculations indicate that there should be as many as 100 million neutrinos for every atom in the universe. Because neutrinos are so abundant, they could be the dominant dark matter even if each weighed only a tiny fraction as much as an atom. Until recently, physicists thought neutrinos carried no mass at all, but studies completed in 1998 at the Super-Kamiokande detector in Japan indicate otherwise. The inferred mass is so slight, however, that neutrinos cannot account for all dark matter. At most, they could match the mass of the stars. Adding up all the dark forms of ordinary matter (gas clouds, brown dwarfs, black holes, and so on) still leaves 95 percent of the mass in the universe unaccounted for.

Clues about the nature of the missing remainder emerge from elaborate computer simulations showing how large-scale cosmic structures—clusters and superclusters of galaxies—evolved out of a slight lumpiness in the early universe. Over time, gravity amplified the lumpiness by causing the denser regions to collapse and fragment. The manner in which that happened depends on the properties of the predominant form of matter. Ordinary atoms are easily agitated by radiation, so they would take too long to settle into the observed structures. The simulations reveal the likely nature of the matter that seeded the formation of today’s galaxy clusters. It should be dark—that is, it shouldn’t get stirred up by radiation—and it should be slow moving relative to the speed of light, or “cold” in astronomical parlance. (Neutrinos, in contrast, are fast moving, or “hot.”) Literal-minded cosmologists have named this stuff cold dark matter.

The latest theories that attempt to construct a unified model of physics list a number of potential particles that seem to match these properties. The exotic particles, the lightest of which would be at least 100 times as heavy as a hydrogen atom, are referred to as weakly interacting massive particles, or WIMPs. They are purely hypothetical, but if they exist it should be possible to detect them. If there are enough WIMPs to make up all the dark matter in our galaxy, then there should be several thousand per cubic meter in the region around Earth. They would be moving at about the same speed as the average star in our galaxy, 135 miles per second. Most of these electrically neutral particles would, like neutrinos, go straight through Earth. On rare occasions, however, one might interact with an atom in the material they pass through.

Several international collaborations, including the DAMA (DArk MAtter) project at the Gran Sasso laboratory in Italy and the UK Dark Matter Collaboration headquartered at the Boulby mine in England, have designed experiments to detect the minuscule recoil when a WIMP hits a slab of ultrapure silicon or some equivalent material. These detectors must be cooled to an extremely low temperature and placed deep underground to eliminate other kinds of particle impacts that could drown out the dark matter signal. So far, the only claimed detection of a dark matter particle—made three years ago by a team at the University of Rome—has been strongly disputed. Meanwhile, the search goes on at ever-increasing sensitivity.

The Ultimate Scale

Dark Matter and the Shape of the Universe

Despite uncertainties about the nature of dark matter, we at least have a good idea of how much of it is out there. Cosmologists measure the density of the universe against a number known as critical density—the amount of mass needed to cancel out the curvature of space. (Curved space is one of those baffling concepts that emerged from Einstein’s general theory of relativity. In the simplest cosmological model, critical density just means that the inward pull of gravity is exactly strong enough to halt the outward expansion of the universe.) Adding up the inferred gravitational effects in galaxies, galaxy clusters, and large-scale structures implies that the total amount of matter in the universe, including dark matter, comes to about 30 percent of critical density.

Yet even that is apparently not a full tally of the invisible universe. Data from NASA’s Wilkinson Microwave Anisotropy Probe, or WMAP, provide compelling evidence that the cosmos has the full critical density. If so, there is an enormous additional dark component out there. WMAP measured the density of the universe as a whole in a very indirect way, by recording the distribution of the cosmic microwave background, the afterglow of the Big Bang. This pervasive radiation contains slight irregularities where regions of compression or expansion in the early universe caused the microwaves to appear slightly hotter or cooler than average. Cosmologists can calculate the typical size of such primordial irregularities. Comparing that size with the angular dimensions of the features seen in the WMAP images, which are most prominent on a scale of about one degree, reveals the net geometry and makeup of the universe. Last February, Charles Bennett of NASA’s Goddard Space Flight Center announced the result: The overall geometry of the universe is flat, meaning the amount of mass and energy exactly matches the critical density. Further analysis of WMAP data indicates the cosmos consists of 27 percent matter and 73 percent something else.

Whatever the something else is, it does not show up in our studies of galaxies and galaxy clusters. But if the extra component is real, it would have to affect the expansion of the universe. In 1998 two groups—one led by Saul Perlmutter of Lawrence Berkeley National Laboratory, the other by Brian Schmidt of Australian National University—reported that they had succeeded in measuring how the expansion has changed over time. The researchers looked at an unusually bright, uniform type of exploding star, called a Type Ia supernova. By identifying supernovas at varying distances from Earth, measuring how much their light had been stretched, and determining how much the light had dimmed over distance, researchers could probe the rate of expansion at different points in cosmic history. To everyone’s amazement, the studies showed that the expansion is speeding up.

All matter, whether dark or light, produces gravity that should cause the universe to slow down. Cosmic acceleration implies that the enigmatic “73 percent” element must cause a repulsive effect that counteracts gravity over enormous distances. Strange as it may sound, most cosmologists have come to believe that this invisible element consists of energy present throughout seemingly empty space. Such dark energy might arise from the tangle of  fields that fill the vacuum on the subatomic level. The simplest versions of quantum theory actually predict far too much energy, so physicists typically assumed the energy from all these fields somehow canceled out to zero. Now they speculate that (for reasons equally unknown) a residual trace of this quantum energy remains and accounts for the inferred dark energy. Other researchers invoke a hypothetical energy field called quintessence, a play on the ancient Greek term for a heavenly “fifth element,” which would also cause cosmic repulsion but which could decay as the universe expands.

Whichever theory is correct—and both may be wrong—the discovery of dark energy is forcing astronomers to rethink the life history of our universe. Early measurements of cosmic density indicated there is so little matter out there that the expansion could continue to infinity. As more and more dark matter turned up, researchers began to consider that gravity might eventually cause the Big Bang to reverse and initiate another cycle of creation. Now it seems that dark energy, whatever it is, controls the destiny of the universe.

In this new view, we occupy a startlingly peripheral place in the universe. Dark energy accounts for most of its mass, exotic dark matter comes in second place, and ordinary matter—the atoms we are made of—lands in a distant third place, with just 4.4 percent. Even the bulk of ordinary matter is dark. Everything we know, see, and touch is an insignificant part of the whole. It is a sobering thought, but it leads us into a world far more curious and marvelous than we could have ever imagined. For the first time in history, we are able to look past the luminous flecks of foam dotting the surface of the cosmic sea and begin to explore the vast, murky depths.