Dark matter: Encyclopedia II - Dark matter - Evidence for dark matter

Dark matter - Evidence for dark matter

In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in 'empty' space". (Ref. See notes)

Dark matter was first hypothesized to exist by the Swiss astrophysicist Fritz Zwicky. In 1933 Zwicky estimated the total amount of mass in a cluster of galaxies, the Coma Cluster, based on the motions of the galaxies near the edge of the cluster. When he compared this mass estimate to one based on the number of galaxies and total brightness of the cluster, he found that there was about 400 times more mass than expected. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some other form of matter existent in the cluster which we have not detected, which provides enough of the mass and gravity to hold the cluster together.

At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. However, dark matter and dark energy are together said to account for 96% of all matter in the universe. This means that only about 4% of all matter can be directly observed. Some hard-to-detect baryonic matter (see baryonic dark matter) makes a contribution to dark matter, but constitutes only a small portion [2] [3].

Since it cannot be directly detected via optical means, many aspects of dark matter remain speculative. The DAMA/NaI experiment has claimed to directly detect dark matter passing through the Earth, though most scientists remain sceptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralinos.

Recent research reported in January 2006 from the University of Massachusetts, Amherst would explain the previously mysterious warp in the disk of the Milky Way by the interaction of the Large and Small Magellanic Clouds and the predicted 20 fold increase in mass of the Milky Way taking into account dark matter.

Dark matter - Galactic rotation

Much of the evidence for dark matter comes from the study of the motions of galaxies. Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. Experimentally, however, it is found to be much greater: in particular, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherical halo of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which impair observations of the rotation curve of outlying stars.

Recently, astronomers from Cardiff University claim to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21 (Wikinews, New Scientist). Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times as much dark matter as hydrogen and has a total mass of about 1/10th that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none have previously been detected. If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter.

Dark matter is believed to affect galaxy clusters as well. The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than 10¹⁴ Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. More info is available here: http://chandra.harvard.edu/photo/2003/abell2029/.

Dark matter - Structure formation

A significant amount of non-baryonic, cold matter is necessary to explain the large-scale structure of the universe. Observations suggest that structure formation in the universe proceeds hierarchically, with the smallest structures, such as stars, forming first, and followed by galaxies and then clusters of galaxies. In the universe, it is thought that the first structures that form are quasars, which are supermassive black holes. This, bottom up model of structure formation requires something like cold dark matter to succeed. Ordinary baryonic matter had too high a temperature, and too much pressure left over from the big bang to collapse and form smaller structures, such as stars, via the Jeans instability.

Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter.

Another important tool for future dark matter observations is gravitational lensing, in particular a technique called weak lensing that allows astrophysicists to characterize the distribution of dark matter by statistical means.

Adapted from the Wikipedia article "Evidence for dark matter", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki