The puzzle of dark matter is one of the great unsolved problems of science. Dark matter is a theoretical idea first proposed by the Swiss astronomer Fritz Zwicky in 1934. Zwicky was a remarkable man who conceived dark matter, gravitational lenses and supernovae within a span of just three years, Zwicky conceived dark matter to explain the “missing mass” he needed to predict the orbital velocities of galaxies in galactic clusters. His dark matter is “dark” and totally transparent. His dark matter does not interact with electromagnetic forces and is very difficult to detect. Its only known interaction is through the gravitational force.
Fritz Zwicky used dark matter to explain the orbital velocity of galactic clusters. The outer galaxies in the Coma cluster rotated at speeds far greater than expected. It implied a mass some 400 times greater than observed. Zwicky solved the “missing mass problem” by proposing that a “dark halo” of non-interacting matter encircled the galactic group.
Zwicky’s ideas were neglected until Vera Rubin and Kent Ford addressed a meeting of the American Astronomical Society in 1975. Rubin and Ford used sophisticated kit to measure the orbital velocities of spiral galaxies. They found that the outermost stars were rotating faster than expected. The velocities implied that the galaxies had 50% more mass than observed. Rubin and Ford also proposed a halo of dark matter.
In 1980 Rubin and Ford published a definitive paper. Scientists had to either accept that the law of gravitation failed on the large scale or concede that dark matter was a reality. Some continue to investigate quantum gravity and the laws of gravity most scientists accept the existence of dark matter even though it is yet to observed experimentally. Once the influence of distant dust clouds is taken into account it even appears that Rubin’s estimate that galaxies contain 50% dark matter is wrong. Many galaxies are now considered to contain nearly 95% dark matter.
Rather than puzzle over the missing mass in rotational problems scientists have developed an alternative way “to weigh” the universe. The technique relies upon gravitational lensing, a phenomenon that Zwicky first conceived in 1937. A gravitational lens is a very massive object which distorts background light and electromagnetic waves. General relativity relates the extent of distortion to the mass of the lens. The experimental evidence for dark matter is overwhelming, when measured in the this way, the mass of several dozen galactic clusters is consistent with that required from orbital velocity considerations.
In August 2006 a gravitational lens was used to produce the clearest evidence for dark matter yet found. Astronomers used the lens to map the distribution of matter in the Bullet Cluster. The Bullet Cluster formed through the collision of two galactic clusters. The conventional gas cloud slowed down through electromagnetic interactions and settled near the centre of the system. The dark matter has settled in a ring further from the centre as one would expect for particles not subject to electromagnetic breaking.
The experimental evidence for dark matter is both overwhelming and confusing. Some dwarf galaxies need very little dark matter to describe the velocity profile. Conversely the rotation of low brightness dwarf galaxies is explained by a high proportion of dark matter. Even own galaxy, the Milky Way contains 95% dark matter. Researchers from the University of Massachusetts derived this figure in 2005 while trying to explain a buckle in the galactic disc.
Scientists are particularly interested in finding a galaxy with an exceptionally high proportion of dark matter because this would have implications for galactic formation and the nature of dark matter. The VIRGOHI21 gas cloud in the Virgo Cluster is a leading candidate. Based on rotation profiles, the scientists estimate that VIRGOHI21 is 99.% dark matter.
What is dark matter?
At first thought astronomers thought that dark matter was undiscovered conventional matter that had a very low electromagnetic signature. Candidate objects were extremely heavy objects such as black holes, neutron stars, faint old white dwarfs and brown dwarfs, known collectively as Massive Compact Halo Objects (MACHOs) objects and large gas clouds. Large scale galactic black holes can be ruled out because their effects would have been seen through gravitational lensing. Medium sized black holes that were generated at the time of the Big Bang have not been ruled out.
Despite systematic searches astronomers have failed to find sufficient MACHO objects. To date very little dark matter can be explained in this way.
Dark matter interacts with neither electromagnetic nor attractive strong nuclear forces. Each particle of dark matter exists in isolation and is incapable of forming an atom as we known it. Dark matter could be sterile neutrinos, axions or Weakly Interacting Massive Particles (known as WIMPs). These particles are theoretical. Unlike known neutrinos a sterile neutrino can not interact through the weak nuclear force: it only responds to gravitational forces. Unlike other dark matter axions might be observable: when these supersymmetric particles annihilate they liberate photons and neutrinos.
Rather than focus on particle physics observational astrophysicists are interested in the mass and velocity of the dark matter. They have theorized whether the dark matter could be Hot Dark Matter or Warm Dark Matter or Cold Dark Matter. Cold Dark Matter refers to matter travelling at classical speeds, typically less than 10% of the speed of light. Warm Dark Matter travels at relativistic speeds, typically between 10% and 95% of the speed of light. Although no known particles that would satisfy the Warm Dark Matter requirement scientists envisage a slow heavy neutrino. Hot Dark Matter travels at 95% of the speed of light or greater. Hot Dark Matter would probably require the presence of neutrinos. These are tiny particles with mass of less than one hundred thousandth of an electron. Having reviewed the scenarios scientists favour a universe inhabited by cold dark matter.
A successful dark matter theory needs to be compatible with the substantial body of knowledge that has built up around the theory of the Big Bang in which the universe is conjectured to have formed at a single point some 13.7 billion years ago. Most of our knowledge of the Big Bang is derived from the NASA Cosmic Background Explorer (COBE) mission in 1992 and Wilkinson Microwave Anisotropy Probe (WMAP) in 2009. These missions confirm that the universe is bathed in a microwave radiation equivalent to a temperature of 2.7 Kelvin. The probes also provide a contour map of anomalous that are found on a scale of one part in ten thousand. The contour maps provide a picture of the early universe. The background radiation is believed to date back to a time 379,000 years after the Big Bang when atoms formed removing changed particles from the universe allowing photons to roam at will. The original signature has red shifted from several thousands Kelvin due to the expansion of the universe. The contour maps contain details of an era just before the formation of atoms when the universe was dominated by the gravitational effects of dark matter. The small scale nature of the anomalies strongly suggests that they are caused by cold matter. Warm or hot dark matter would travel too fast to form in the requisite clusters.
Large scale structures such as stars, galaxies and galactic clusters are thought to have first formed through gravitational collapse aided by dark matter some 150 million and 1 billion years after the Big Bang. Dark matter was essential for star formation. The gas left over from the Big Bang was too hot. Without the compacting presence of dark matter thermal pressure would have stopped the compression needed to ingite fusion in the core.
In recent years scientists have stepped up their search for WIMPs as favoured by the Cold Dark Matter theory. Many experiments are planned or progressing. Some seek direct evidence from the scattering of dark matter by ordinary matter. Others seek indirect evidence from the annihilation of colliding WIMPS. The direct experiments are difficult requiring deep underground laboratories to avoid contamination by interactions with cosmic rays. The indirect method assumes that an annihilating WIMP pair decays into gamma rays and particle anti-particle pairs. Both methods are supported by work to manufacture WIMPs at the Large Hadron Collider. Some direct experiments have already put forward contenders results for WIMP like behaviour it is too early to draw conclusions. All detection methods are at an early stage.
Dark Matter is a thriving area of astronomical research. Originally theorized to explain the rotational dynamics of large scale objects, such as galaxies and clusters of galaxies, the extent of dark matter in the universe is relevant for galactic structure, evolution and the large scale structure of the universe. Dark matter may have ignited the first stars. The leading theory is that dark matter is cold, made mostly of WIMPs with some contribution from primordial black holes with between 30 and 300,000 solar masses. A considerable research effort aims to pin down the particles responsible in the next few years.