Although we live in a renaissance era of cosmology, in which theories and observations have advanced to the stage where ideas can be precisely tested, we also live in the dark ages. About 23 percent of the universe consists of dark matter, mysterious stuff that exerts gravitational forces but doesn’t interact with light. Ordinary matter makes up just 4 percent. (Another 73 percent is dark energy, an even more mysterious component that permeates the universe.)
The last time something was called “dark” in physics was in the mid-1800s, when Urbain-Jean-Joseph Leverrier of France proposed an unseen dark planet, which he named Vulcan. Leverrier’s goal was to explain the peculiar trajectory of the planet Mercury. Leverrier, along with John C. Adams of England, had previously deduced the existence of Neptune based on its effects on the planet Uranus. Yet he was wrong about Mercury. It turned out that the reason for Mercury’s strange orbit was much more dramatic than the existence of another planet. The explanation could be found only with Einstein’s theory of relativity. The first confirmation that the theory of general relativity was correct came when Einstein proved it could be used it to accurately predict Mercury’s orbit.
It could turn out that dark matter presages a similar paradigm change. Even so, I’d say that it is very likely to have a more conventional explanation, consistent with the type of physical laws we now know. After all, even if novel matter acts in accordance with force laws similar to those we know, why should all matter behave exactly like familiar matter? To put it more succinctly, why should all matter interact with light? If the history of science has taught us anything, it should be the shortsightedness of believing that what we see is all there is.
Many people ﬁnd dark matter’s existence very strange and ask how it can possibly be that most matter—about six times the amount we see —is something we can’t detect with conventional telescopes. In fact, we know something with dark matter’s properties has to be there. We know it exists by the extensive observational evidence of its gravitational effects in the cosmos.
The first clue of dark matter’s existence came from the speed with which stars rotated in galaxy clusters. Much more solid evidence for dark matter came from Vera Rubin, an observational astronomer, who in the late 1960s and early 1970s made detailed quantitative measurements of stars rotating in galaxies. The properties of a galaxy, such as the rate at which its stars orbit, depend on how much matter it contains. With only visible matter present, one would expect those stars well beyond the galaxy to be rather insensitive to its gravitational pull. Yet Rubin found that stars far from the luminous central matter rotated with the same velocity as stars one-tenth the distance from the galaxy’s center. This implied that the mass density did not fall off with distance, at least to the distances Rubin observed. Astronomers concluded that galaxies consisted primarily of unseen dark matter. Supplementary evidence comes from studies of gravitational lensing.
We now know the density of dark matter, that it is “cold” (which is to say, it moves slowly relative to the speed of light), and that it interacts extremely weakly and certainly has no significant interaction with light. But that’s about it. Dark matter could be small black holes or objects from other dimensions. Most likely, though, it is simply a new elementary particle that doesn’t have the usual interactions associated with the standard model, the reigning physics theory that so far explains the known forces governing the fundamental particles of ordinary matter.
The Dark Matter Factory
Many connections exist between particle physics and cosmology, but one of the most intriguing is that dark matter might actually be produced at the energies explored by the world’s most powerful particle accelerator, the Large Hadron Collider (LHC). The LHC contains an enormous, 16.5-mile-long circular tunnel that crosses the French-Swiss border deep underground. Electric fields inside this tunnel accelerate two beams—each consisting of billions of protons, which belong to a class of particles called hadrons, hence the collider’s name—about 11,000 times a second as they circle the track.
When Lyn Evans, the LHC’s chief engineer, spoke at the California LHC/Dark Matter conference in January 2010, he closed by teasing the audience. “You theorists have been thrashing around in the dark [sector],” he said. “Now I understand why I spent the last 15 years building the LHC.” Evans’s comments referred to the paucity of high-energy data over the previous couple decades. But they were also hints about the possibility that LHC discoveries might shed light on dark matter.
The intriguing possibility of producing dark matter is among the reasons cosmologists are curious about what the LHC might ﬁnd. The LHC has just the right energy to search for a hypothetical dark particle called a WIMP, or weakly interacting massive particle. Even if that’s the case, however, the dark matter particle won’t necessarily be discovered there. After all, dark matter doesn’t interact a lot, so dark matter particles certainly won’t be produced directly in a detector, and even if produced indirectly, they will just ﬂy through. Nonetheless, the LHC might produce other particles with stronger interactions that subsequently decay into dark matter, which could then carry away momentum and energy, providing proof that the dark particles were there.Finding evidence of the existence of dark matter would certainly be a major accomplishment. However, to establish that a particle indirectly detected at the LHC indeed constitutes dark matter would require further substantiation. That is what detectors on the ground and in space might provide.
Dark Matter Detectors
Current searches rely on a leap of faith that dark matter, despite its near invisibility, nonetheless interacts feebly with matter we know (and can build detectors out of). The only trace of a dark matter particle passing through a detector would be the consequences of its hitting nuclei in the detector and changing their energy by a minuscule amount. Dark matter detectors search for the tiny amounts of heat or recoil energy created when dark matter particles pass through. The detectors are designed to be either very cold or very sensitive in order to record the small heat or energy deposits from dark matter particles subtly ricocheting off.
The very cold devices, known as cryogenic detectors, are made with a crystalline absorber such as germanium. Experiments of this sort include the Cryogenic Dark Matter Search (CDMS), CRESST, and EDELWEISS. The other class of experiments involves detectors made of liquefied noble gases. Even though dark matter doesn’t directly interact with light, the energy added to an atom of xenon or argon when a dark matter particle hits it can lead to a ﬂash of characteristic scintillation. Experiments with xenon include XENON100 and LUX, and the other proposed noble liquid experiments include ZEPLIN and ArDM.
With these extraordinarily difﬁcult dark matter experiments, the devil is in the details. CDMS has hockey-puck-size pieces of germanium or silicon topped by a delicate recording device, a phonon sensor. The detector operates at very low temperature—low enough to be just at the border between superconducting and nonsuperconducting. If even a small amount of energy from phonons (the sound units that carry the energy through the germanium or silicon, much as photons are the units of light) hit the detector, it can be enough to make the device lose superconductivity and register a potential dark matter event through a device called a superconducting quantum interference device, or SQUID. These devices are extraordinarily sensitive and measure the energy deposition extremely well.
But recording an event isn’t the end of the story. The experimenters need to establish that the detector is recording dark matter—not just background radiation. The problem is that everything radiates. We radiate. The magazine (or electronic device) you’re reading radiates. The sweat from a single experimenter’s ﬁnger is enough to swamp any dark matter signal. And that doesn’t even take into account all the primordial and man-made radioactive substances. The environment and the air as well as the detector itself carry radiation. Cosmic rays can hit the detector. Low-energy neutrons in rock can mimic dark matter. There are about 1,000 times as many background electromagnetic events as there are predicted signal events.
So the name of the game for dark matter experiments is shielding and discrimination. Shielding is accomplished in part by performing the experiments deep in mines. XENON100—as well as CRESST, a detector that uses tungsten—is set up in the Gran Sasso laboratory, situated in a tunnel in Italy about 1,400 meters underground. The CDMS experiment is in the Soudan mine in Minnesota, more than 700 meters underground. The experiments further shield the actual detectors in a variety of ways. CDMS has a layer of surrounding polyethylene that will light up if something too strongly interacting to be dark matter comes through from the outside. Even more memorable is the surrounding lead from an 18th-century sunken French galleon. Older lead that has been underwater for centuries has had time to shed its radioactivity. It is a dense absorbing material that is perfect for shielding the detector from incoming radiation.
Impressive as CDMS is, though, noble gas experiments like XENON100 are currently the most sensitive detectors for dark matter. Liquid xenon is dense and homogenous, has a large mass per atom (enhancing the dark matter interaction rate), scintillates well, ionizes fairly readily when energy is deposited, and is relatively cheap. XENON100 uses special light-detecting phototubes that were designed to work in the low-temperature, high-pressure environment of the detector to measure the scintillation. Noble gas experiments have become a lot better as they have gotten bigger, and they should continue to do so. With more material, not only is detection more likely, but the outer part of the detector can shield the inner part of the detector more efficiently, helping assure a significant result.
The strange state of affairs today is that one scintillation experiment—DAMA, located at Gran Sasso—has actually seen a signal. DAMA, unlike the experiments described so far, has no internal discrimination between signal and background. Instead it relies on identifying dark matter signal events by their time dependence. Due to Earth’s orbit around the sun, the speed of dark matter relative to us (and hence the energy deposited) depends on the time of year, making it easier to see a signal at some times of year (summer) than at others (winter). The DAMA experiment looks for an annual modulation in the event rate that accords with this prediction. The researchers’ data indicate they have found such a signal. But people are skeptical because no other experiment has confirmed the result, although other experiments are beginning to see hints of a signal.
Although confusing, this is the sort of thing that makes science interesting. The result encourages us to think whether dark matter might have properties that make it easier for DAMA to see it than for other experiments to do so.
Dark Matter Observatories
The third way to detect dark matter is through indirect detection in the sky or on Earth. Dark matter is dilute, but nonetheless dark particles might sometimes collide and annihilate, releasing energy and other particles in the process. Annihilation doesn’t happen enough to significantly affect the overall density, but it might be enough to produce a measurable signal. Depending on its nature, dark matter annihilation could sometimes yield detectable particles and antiparticles, such as electrons and positrons, or pairs of photons.
The instruments that search for these products of dark matter annihilation were conceived as telescopes or detectors to look at particles and photons emitted by galaxies and the exotic objects that lie within them. But such observations might also illuminate the nature of dark matter. Since antimatter particles are relatively rare in the universe and since the distribution of photon energies could exhibit distinctive and identifiable properties, such detections could eventually be associated with dark matter. The spatial distribution of these particles might help distinguish dark matter annihilation products from more common astrophysical backgrounds.