No one has detected a single particle of dark matter, yet it has become a crucial component of modern astronomical theories. Quite literally, without dark matter, much of cosmology simply falls apart.
The conviction that there must be an extra component of unseen matter in the Universe was founded in the 1930s, when Swiss astronomer Fritz Zwicky was studying the motion of galaxies around one another in the Coma cluster, 320 million light years away. He found that the galaxies were moving so fast that they should burst from the cluster: gravity was simply not strong enough to hold them in. Yet there were many such clusters throughout the Universe and none looked as if they were flying apart, so something was clearly keeping the galaxies together. Zwicky reasoned that there had to be more matter hidden somewhere in the galaxies, providing the extra gravity. He thought that this “missing mass” must be hiding in titanic clouds of cold hydrogen and helium that had yet to produce any stars. But, try as he might, he could not detect these clouds.
Skip forward 40 years and technology had become sophisticated enough for astronomers to measure not just the speed at which a galaxy was moving through space, but also how fast it was spinning. They could even split the galactic rotation into sections and calculate how the orbital speed of stars varied with distance from the galaxy’s center. The astronomers were expecting to see stars on the edges of the galaxy orbiting more slowly than stars near the center. This is the pattern displayed by the Solar System’s planets, as discovered by Johannes Kepler (see Why Do the Planets Stay in Orbit?). It is a direct consequence of the fact that gravity decreases with distance. But it turned out not to be the pattern followed in most galaxies; instead, no matter how far stars are located from the galactic center, they all orbit with the same speed. When this was discovered in the 1970s, it presented astronomers with a problem, similar to that of Zwicky’s galaxy clusters, because the outer stars were moving too fast for the galaxy’s gravity to hold on to them. Since nowhere do we see galaxies in the process of spontaneous disintegration, the only solution seemed to be that there must be more matter concealed somewhere within or around the galaxies.
GALAXY ROTATION CURVES
For the stars to all move at the same speed, the amount of this matter would have to increase with distance in a way that would compensate for the expected drop in the gravitational force. This would require a sphere of matter, called the halo, to surround each galaxy; but while circumstantial evidence for this extra matter was too great to be ignored, the searches for it were finding nothing close to the proportions needed. This discrepancy threatened to plunge cosmology into crisis, until particle physics offered a way out.
In the effort to understand and unify the forces of nature, theoretical physicists were contemplating the need for new particles to carry energy. Theoretical predictions of particles had worked well for them in the past; for example, both antiparticles and neutrinos had been predicted to exist before they were observed. Antimatter emerged from calculations made by Paul Dirac in 1928 and was observed four years later (see How Did the Universe Form?). Neutrinos were “invented” by particle physicist Wolfgang Pauli in 1930 to account for missing energy in certain reactions involving the weak nuclear force. He called his invention a “desperate remedy,” but explained that it was necessary either to make up a new particle or to accept that energy could vanish from the Universe. It took 26 years for the experimenters to build an experiment that detected a neutrino.
In light of these successful predictions, particle physicists began talking about whole new swathes of particles in their attempt to unify the fundamental forces. These particles were so unlike normal matter that if they did exist, they would generate gravity but otherwise interact with normal matter hardly at all. Astronomers latched on to this idea immediately, realizing that these theoretical particles might be just what they needed to provide their “missing mass.” Indeed, when they made their calculations they found that the new matter could outweigh normal matter by anything between ten to a hundred times, making them perfect for providing the extra pull of gravity to hold each galaxy together.
“We use the phrase dark matter in the same way the early mapmakers used terra incognita. We don’t know what it is.”
MICHAEL CRISLER CONTEMPORARY COSMOLOGIST
The difficulty was going to be detecting these particles; they were predicted to interact so weakly with normal particles, that there could be giant clouds of them surrounding each galaxy that were completely invisible. Astronomers coined the term “dark matter” to describe this material and gradually fed it into every cosmological problem that needed more mass. At the same time, physicists were trying to deduce the exact nature of this dark matter. They have offered a multitude of possibilities, and currently it is thought that there may be a pantheon of dark matter particles just as there is a zoo of normal matter particles.
An early candidate was called the axion, put forward in 1977 to modify the behavior of the strong force and allow matter to be produced at a slightly higher rate than antimatter (see How Did the Universe Form?). Researchers named the particle after a brand of detergent because they wanted to use it to “clean up” the matter-antimatter problem. In 2005, experimenters thought that they had detected the axion but subsequent investigations disproved this conclusion; the search continues.
In the 1970s, the concept of supersymmetry was propounded by particle physicists. The known fundamental particles can be divided into two categories: fermions and bosons. Fermions are usually associated with matter, because they cannot occupy the same physical space as one another and this resistance leads to matter as we know it. Quarks and electrons are fermions, as are neutrinos. Quarks combine to make the nuclear particles of atoms, which the electrons orbit around. Bosons, on the other hand, are associated with forces, or energy. They can crowd as closely to each other as they like, even occupying the same physical space. Examples include photons, which carry electromagnetic energy, W and Z particles, which carry the weak nuclear force, and the renowned Higgs boson, thought to be responsible for endowing particles with mass. Supersymmetry suggests that every particle has a “superpartner,” thus doubling the expected number of particles in nature. Each fermion has an associated boson, and each boson has an associated fermion. The candidates for dark matter are the bosons’ superpartners, which are fermions and so form matter, with mass; collectively these superpartner fermions are called “neutralinos.”
THE PARTICLES OF NATURE: SUPERSYMMETRY DOUBLES THE NUMBER OF NATURAL PARTICLES
The Large Hadron Collider in Switzerland may be generating large quantities of neutralinos as it smashes particle beams together, but its instruments cannot detect them. This is because neutralinos are so weakly interacting that they will pass through the walls of the cathedral-sized detectors like ghosts, leaving no trace. This does not mean that they cannot be inferred, however. The more energy that is spirited away by the neutralinos, the bigger will be the deficit when physicists add up the energy of all the particles detected and then compare it to the energy that was pumped into the collision. Any difference between the two could mean that supersymmetric particles are being created but then stealing away from the experiment.
If any energy deficit is seen, it is likely to be a long task to identify which of the many possible species of neutralino is involved. Astronomers cannot help because their computer models deal with clouds of dark matter rather than individual particles. The smallest cloud they can usually deal with is more massive than the Sun, and it is impossible to extract individual particle properties from such large-scale simulations. The only way to know what a neutralino is for sure is to do the almost impossible and capture one.
Although dark matter is expected to interact with normal matter only very weakly, it is supposed to exist in such vast quantities that this would counterbalance its poor interaction capability. At any time there is likely to be so much dark matter passing through a detector that catching a particle is at least feasible. Currently, around a half dozen dark matter experiments are running around the world. Each of them is designed to have the necessary sensitivity to discover a dark matter particle, if dark matter exists. Some aim to detect the heat generated when an individual dark matter particle lodges inside the instrument, others look for the dim flash of light created when a dark matter particle ricochets off an atom inside the detector. If any one of the dark matter detectors succeeds, physicists will be able to calculate the properties of the detected particle but, with an expected maximum rate of just a few detections per year, it will take a long time to build up firm results and conclude whether there is more than one type of dark matter.
If neutralinos are detected, this does not automatically mean that they will be the dark matter sought by astronomers; they may not exist in adequate numbers or be sufficiently stable to exist for long enough to provide the gravity needed. When it comes to checking the abundance and stability of dark matter, astronomers will need to join the action. Just because supersymmetric particles interact only very reluctantly with normal matter, does not mean that they do not interact with one another. According to models of dark matter, a pair of identical particles will annihilate one another when they collide, releasing a pair of gamma rays. Astronomers expect that such annihilation signals could be coming from the center of our Galaxy, where the dark matter is expected to be dense, or from neighboring dwarf galaxies. They are already searching for these signals with satellites.
Once astronomers have determined the relative proportions of axions, neutralinos and any other types of dark matter, they will have to take neutrinos (see How Did the Universe Form?) into account because recent experiments have shown that these weakly interacting particles, once thought to be massless, in fact carry a small mass. These would therefore contribute their share of the gravitational pull wherever they are found.
Many astronomers are coming to rely on the existence of dark matter, but others are growing skeptical. This is because dark matter has an Achilles heel in the form of a hidden assumption: its existence relies on Newton’s law of gravity being accurate and applicable to gravitational fields no matter how weak they might be. Scientists already know that Newton’s law fails in strong gravitational fields, where Einstein’s General Theory of Relativity needs to be applied instead. It is possible that the same is true at the other end of the scale, where gravity is extremely weak.
In 1981, as others were beginning to embrace dark matter, Israeli physicist Mordehai Milgrom proposed a change to Newton’s law of gravity to explain rotating galaxies without the need for new particles. Remembering that gravity produces acceleration in objects (see Was Einstein Right?), Milgrom’s suggestion was that Newtonian gravity changes below a certain acceleration value, so that instead of the force dropping as an inverse square law (double the distance, quarter the acceleration), it begins to drop less sharply, as a simple inverse law (double the distance, halve the acceleration). The critical acceleration at which this happens is minuscule—no more than that produced by the gravitational field of a single sheet of paper—but it has a dramatic effect at the outer edge of galaxies.
Milgrom showed that by making weak gravitational fields pull a little harder than expected, theoretical models could successfully show the stars moving with uniform speed all the way out to the extremities of a galaxy. In the vast majority of cases, this produced better agreement with observations than the dark matter models. He called his idea “Modified Newtonian Dynamics” (MOND) and, although it currently remains a minority view, if the dark matter detection experiments fail to produce any results, and neutralinos are not produced at the Large Hadron Collider, then perhaps more and more astronomers will contemplate what once seemed impossible: that Newton’s theory of gravity needs an update.
The drawback that some see with MOND is that it has no theoretical underpinning. This makes many astronomers reluctant to take it seriously, but there are historical precedents for this kind of “discovery first, understanding later.” Kepler’s laws of planetary motion had no theoretical underpinning when he first proposed them early in the 17th century. He simply scrutinized the data and found an equation that reproduced them. Only in 1687 did Newton’s Theory of Universal Gravitation provide an understanding of why Kepler’s laws worked. Throughout the history of astronomy, laws have been deduced from observations before theories could explain them.
Yet MOND is not flawless; the hypothesis struggles to reproduce the motion of clusters of galaxies, requiring more matter than can be seen to make things work. So are we back to needing exotic dark matter? Possibly not, as astronomers think that there may be a substantial amount of normal matter hiding in galaxy clusters, in the form of warm gas. If they can detect this, using telescopes sensitive to the ultraviolet light it is believed to emit, it would solve MOND’s difficulties with galaxy clusters.
In their quest to prove the existence of dark matter, astronomers have recently been using a technique called “weak gravitational lensing.” This idea comes from general relativity’s concept of curved space (see Was Einstein Right?). They find a nearby galaxy cluster that would create a large curvature in the fabric of space, and then look through the cluster at the light from more distant objects. As this light passes through the cluster, it would be deflected by the curvature and this would distort the appearance of those distant objects, rather like the celestial equivalent of a hall of mirrors. By charting these distortions, astronomers hope to map out the curvature of space around the galaxy cluster. If there is more curvature than can be explained by the distribution of the galaxies, then one solution is to invoke dark matter, and use the curvature to construct a map of the dark material. Another explanation, however, is to say that the extra curvature shows where the MOND correction is at work, producing more curving than expected.
A particular battleground for the competing theories is the Bullet Cluster, where two galaxy clusters are colliding. Observations show that the gas between the galaxies has been stripped away, and yet weak lensing indicates that a strong curvature still persists around the galaxies. This can be interpreted as either the gigantic halos of dark matter needed to hold both clusters together, or as regions where MOND needs to be applied. Other galaxy cluster collisions show similar behavior, and likewise do not help astronomers decide between the dark matter and MOND theories. But very recently something has been found that may break the stalemate, an object that perplexes everyone: a most peculiar galaxy cluster collision known as Abell 520.
In this particular cosmic smash-up, weak lensing results imply that somehow the dark matter has been separated from the galaxies. This is completely contrary to what dark matter theory says should happen, and the only way to explain the dark matter behaving like this is to propose that it is interacting with itself, producing a force that only dark matter feels. Such a hypothesis would solve a problem with the Bullet Cluster collision, as well: computer models cannot substantiate the speed at which the clusters are crashing together, but a previously unanticipated dark matter force could provide the extra kick needed.
At present, the picture is more confusing than it has ever been. MOND does not work in all situations: it needs about twice the amount of matter that we see in the Universe; and dark matter needs a new, exclusive force to make its figures tally. So the bottom line is that no one knows what the dark matter is, or whether it even exists. It may be an illusion brought about by our misunderstanding of gravity. In the next few years, the welter of new experiments—the Large Hadron Collider, the dark matter detection devices, and the satellites poised to receive dark matter annihilation signals—promise new information that will surely reveal dark matter to us or disprove it once and for all.