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In cosmology and physics, cold dark matter (CDM) is a hypothetical form of matter (a kind of dark matter) whose particles moved slowly compared to the speed of light (the ''cold'' in CDM) since the universe was approximately one year old (a time when the cosmic particle horizon contained the mass of one typical galaxy); and interact very weakly with ordinary matter and electromagnetic radiation (the ''dark'' in CDM). It is believed that approximately 80% of matter in the Universe is dark matter, with only a small fraction being the ordinary baryonic matter that composes stars, planets and living organisms. Since the late 1980s or 1990s, most cosmologists favor the cold dark matter theory (specifically the modern Lambda-CDM model) 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. The theory sees the role that dwarf galaxies played as crucial, as they are thought to be natural building blocks that form larger structures, created by small-scale density fluctuations in the early Universe. The theory was originally published in 1982 by three independent groups of cosmologists; James Peebles at Princeton, J. Richard Bond, Alex Szalay and Michael Turner; and George Blumenthal, H. Pagels and Joel Primack. An influential review article in 1984 by Blumenthal, Sandra Moore Faber, Primack and British scientist Martin Rees developed the details of the theory. In the cold dark matter theory, structure grows hierarchically, with small objects collapsing under their self-gravity first and merging in a continuous hierarchy to form larger and more massive objects. In the hot dark matter paradigm, popular in the early 1980s, 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 the hot dark matter theory disagree with observations of large-scale structures, whereas the cold dark matter paradigm is in general agreement with the observations. == Composition == Dark matter is detected through its gravitational interactions with ordinary matter and radiation. As such, it is very difficult to determine what the constituents of cold dark matter are. The candidates fall roughly into three categories: * Axions are very light particles with a specific type of self-interaction that makes them a suitable CDM candidate. Axions have the theoretical advantage that their existence solves the Strong CP problem in QCD, but have not been detected. * MACHOs or ''Massive Compact Halo Objects'' are large, 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. Most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate.〔. p. 3: "MACHOs can only account for a very small percentage of the nonluminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no..."〕〔Gianfranco Bertone, "The moment of truth for WIMP dark matter," Nature 468, 389–393 (18 November 2010)〕 * WIMPs: Dark matter is composed of ''Weakly Interacting Massive Particles''. There is no currently known particle with the required properties, but many extensions of the standard model of particle physics predict such particles. The search for WIMPs involves attempts at direct detection by highly sensitive detectors, as well as attempts at production by particle accelerators. WIMPs are generally regarded as the most promising dark matter candidates.〔〔〔 The DAMA/NaI experiment and its successor DAMA/LIBRA have claimed to directly detect dark matter particles passing through the Earth, but many scientists remain skeptical, as no results from similar experiments seem compatible with the DAMA results. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cold dark matter」の詳細全文を読む スポンサード リンク
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