In the first part of this ‘lazy person’s guide to dark matter’, you learned of the unsettling conclusion that four-fifths of the matter in the universe is unidentified, revealing its presence not by emitting or absorbing light but by exerting through gravity an influence on the motion of galaxies and galaxy clusters. All attempts to explain this so-called “dark matter” in terms of known particles and objects– everyday atoms and molecules, dust, dark stars, and rogue planets– have so far failed, and astronomers and physicists have slowly turned to more exotic possibilities to account for dark matter. Although dark matter remains unexplained, observations and calculations of galaxy motion and the structure of the early universe have given astronomers a good idea of its key properties. Particles which are candidates for dark matter must be…
- Dark in the sense that they must emit no light or other forms of detectable radiation, since no unaccounted emission is observed around galaxies or in the space between galaxies in clusters, where most of the dark matter should lie.
- Cold, which means the particles on average must not move too quickly. If they do, they can’t stay together in one place long enough to attract, through gravitation, clumps of ordinary gas to form galaxies and galaxy clusters. That’s why neutrinos, which fly through the universe at nearly the speed of light, were discounted as candidates for the majority of dark matter.
- Weakly interacting with ordinary matter, because there’s no direct evidence of clouds of dark matter smacking into planets or collecting like dust balls in the corners of the solar system. Observations of colliding galaxy clusters such as the Bullet Cluster, for example, show dark matter interacts weakly with clouds of hydrogen gas attending the galaxies. So unlike ordinary matter, dark matter cannot be made from protons, neutrons, and electrons. It is something totally different.
- Abundant and sufficiently massive to account for the ‘missing mass’ in the galaxies and galaxy clusters.
Fortunately, there are a few possibilities for dark matter that have arisen from theories devised by particle physicists and cosmologists. In order from least likely to most likely, as far as scientists know so far, here are three leading candidates…
- Mini Black Holes. As mentioned in the first article, huge numbers of tiny black holes may have been created during the Big Bang. Perhaps these microscopic ‘primordial’ black holes still exist, wandering the universe and exerting gravitational influence while emitting no light. With masses roughly that of a small asteroid, these tiny objects could pass through a star or planet without causing much of a fuss. But so far, there have been no signs of these black holes.
- Axions. As if primordial black holes are not exotic enough, the second leading candidate for dark matter is a bizarre and so-far hypothetical elementary particle called an axion. Proposed to solve a thorny problem in particle physics, axions are thought to have no electrical charge, a very low probability of interacting with ordinary matter, and a very small mass which is offset by their tremendous quantity. Efforts are underway to detect axions, but so far none have been directly observed.
- WIMPs (weakly interacting massive particles). As their name suggests, WIMPS make ideal dark matter candidates. They’re massive, slow (and therefore cold), and interact little with normal matter through the so-called ‘weak nuclear force’ which governs the radioactive decay of atomic nuclei. As fortune has it, candidates for WIMPS are predicted by theories of ‘supersymmetry’, an esoteric theory of particle physics which suggests a new class of particles that are symmetrical counterparts to particles such as neutrinos, photons, quarks, and so on. The lightest of these supersymmetric particles are the longest lived and therefore the most likely candidate for dark matter.
If they exist, huge quantities of WIMPs permeate our galaxy, and our solar system moves continuously through a stream of this dark matter. Dozens of international research groups are searching for WIMPs. Most experiments take place in underground detectors where dark matter particles might collide with a few of the nuclei of atoms in a vat of target material and emit a signal.
A telltale sign of galactic dark matter detection would be the annual variation in the number of dark-matter detection events. As the Earth moves around the Sun, it may spend half the year moving into the stream of galactic dark matter and half the year moving away. This would cause a respective increase and decrease in signal, just as a runner would strike more raindrops running against the wind during a rainstorm than when running with it.
Other groups are searching for signs of dark matter which may be bunched up by gravity at the center of the Earth, the Sun, and the Milky Way itself. Dark matter is not detected directly, but instead by detecting energy emitted when two dark matter particles bump into each. Others hope to create potential dark matter particles directly in high-energy particle accelerators on Earth.
A Nobel Prize is likely assured to the leaders of the first groups to detect dark matter. There has been no definitive detection of dark matter as of mid-2014, but there have been some tantalizing signals of detection of galactic dark matter with the expected annual variation in signal.
Publisher’s Note: If you wish to learn more about dark matter and the massive effort underway to find it, look to the engaging and personal account of the science and search for dark matter in the book “The Cosmic Cocktail” by Katherine Freese, a leading scientist in the field.