Cold dark matter
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Cold dark matter (or CDM) is a refinement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material which cannot be observed by its electromagnetic radiation and hence is dark while at the same time the particles making up this matter are slowly moving and hence are cold. As of 2004, most cosmologists favor the cold dark matter theory as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation), to the lumpy distribution of galaxies and their clusters we see today — the large-scale structure of the universe.
In the cold dark matter theory, structure grows hierarchically, with small objects collapsing first and merging in a continuous hierarchy to form more and more massive objects. In the hot dark matter paradigm, popular in the early eighties, structure does not form hierarchically (bottom-up), but rather forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way. The predictions of hot dark matter strongly disagree with observations of large-scale structure, whereas the cold dark matter paradigm is in general agreement with the observations.
Two important discrepancies between the predictions of the cold dark matter paradigm and observations of galaxies and their clustering in space have arisen, however, creating a potential crisis for the cold dark matter picture.
- The cuspy halo problem is that cold dark matter predicts that the rotation curves of halos be peaked much more strongly than what is observed in galaxies.
- The missing satellites problem is that cold dark matter predicts large numbers of small dwarf galaxies about one thousandth the mass of the Milky Way. These are not observed.
Both of these problems have a number of proposed solutions, some more promising than others. It remains unclear as to how intractable these problems are; whether they represent a crisis or simply a nuisance is a matter of some dispute in the cosmological community.
The CDM theory makes no predictions about exactly what the cold dark matter particles are, and one large weakness in the cold dark matter theory is that it is unclear what the dark matter consists of. The candidates fall into two categories which are humourously named.
- WIMPs or Weakly Interacting Massive Particles assumes that the dark matter is some sort of heavy unknown particle. Unfortunately, there is no known particle with the required properties. The search for these involves particle accelerators.
- MACHOs or Massive Compact Halo Objects assumes that the dark matter consists of condensed objects such as black holes, neutron stars, white dwarfs, very faint stars, or non-luminous objects like planets. The search for these consists of using gravitational lensing to see the effect of these objects on background galaxies.