**Cold Dark Matter** is an abbreviation for **Lambda-Cold Dark Matter**. Cold Dark Matter represents the current concordance model of Big Bang cosmology that explains cosmic microwave background observations, as well as large scale structure observations and supernovae observations of the accelerating expansion of the universe. Cold Dark Matter is the simplest known model that is in general agreement with observed phenomena.

- (Lambda) stands for the cosmological constant which is a Dark energy term that allows for the current accelerating expansion of the universe. The cosmological constant is often described in terms of O
_{?}, the fraction of the energy density of a flat Universe in the form of the cosmological constant. Currently, , implying 74% of the energy density of the present universe is in this form.

- cold dark matter is the model where the Dark matter is explained as being cold (i.e. not thermalized), non-baryonic, collisionless dust. This component makes up 22% of the energy density of the present universe. The remaining 4% is all of the matter and energy that makes up the atoms and photons that are the building blocks of planets, stars, and gas clouds in the universe.

- The model assumes a nearly scale-invariant spectrum of primordial perturbations and a universe without spatial curvature. It also assumes that it has no observable topology, so that the universe is much larger than the observable particle horizon. These are predictions of cosmic inflation.

These are the simplest assumptions for a consistent, physical model of cosmology. However, ?CDM is a *model*. Cosmologists anticipate that all of these assumptions will not be borne out exactly, after more is learned about the applicable fundamental physics. In particular, cosmic inflation predicts spatial curvature at the level of 10^{-4} to 10^{-5}. It would also be surprising if the temperature of dark matter were absolute zero. Moreover, ?CDM says nothing about the fundamental physical origin of dark matter, dark energy and the nearly scale-invariant spectrum of primordial curvature perturbations: in that sense, it is merely a useful parameterization of ignorance.

**Parameters of Cold Dark Matter.**

The model has six parameters. The Hubble constant determines the rate of expansion of the universe, as well as the critical density for closure of the universe, ?_{0}. Densities for baryons, dark matter and dark energy are given as Os, which are the ratio of the true density to the critical density: *e.g.*O_{b} = ?_{b} / ?_{0}. Since the ?CDM model assumes a flat universe, these densities sum to one, and the density of dark energy is not a free parameter. The optical depth to reionization determines the red shift of reionization. Information about the density fluctuations is determined by the amplitude of the primordial fluctuations (from cosmic inflation) and the spectral index, which measures how the fluctuations change with scale (*n*_{s} = 1 corresponds to a scale-invariant spectrum).

The errors quoted are 1s: that is, there is statistically a 68% likelihood that the true value falls within the upper and lower error bounds. The errors are not gaussian, and they have been derived using a Markov chain Monte Carlo analysis by the Wilkinson Microwave Anisotropy Probe collaboration (Spergel *et al.* 2006) which also uses Sloan Digital Sky Survey and Type Ia supernova data.

Parameter | Value | Description |
---|---|---|

Basic parameters | ||

H_{0} | km s^{-1} Mpc^{-1} | Hubble parameter |

O_{b} | Baryon density | |

O_{m} | Total matter density (baryons + dark matter) | |

t | Optical depth to reionization | |

A_{s} | Scalar fluctuation amplitude | |

n_{s} | Scalar spectral index | |

Derived parameters | ||

?_{0} | kg/m^{3} | Critical density |

O_{?} | Dark energy density | |

z_{ion} | Reionization red-shift | |

s_{8} | Galaxy fluctuation amplitude | |

t_{0} | years | Age of the universe |

**Extended models of Cold Dark Matter.**

Possible extensions of the simplest "vanilla" ?CDM model are to allow quintessence rather than a cosmological constant. In this case, the equation of state of dark energy is different from -1. Cosmic inflation predicts tensor fluctuations (gravitational waves). Their amplitude is parameterized by the tensor-to-scalar ratio, which is determined by the energy scale of inflation. Other modifications allow for spatial curvature or a running spectral index, which are generally viewed as inconsistent with cosmic inflation.

Allowing these parameters will generally *increase* the errors in the vanilla parameters quoted above, and may also shift the observed values somewhat.

Parameter | Value | Description |
---|---|---|

w | Equation of state | |

r | < 0.55 (2s) | Tensor-to-scalar ratio |

O_{k} | Spatial curvature | |

a | Running of the spectral index | |

Sm_{?} | < 0.87 eV (2s) | Summed Neutrino masses |

These are consistent with a cosmological constant,*w* = - 1, and no spatial curvature O_{k} = 0. There is some evidence for a running spectral index, but it is not statistically significant. Theoretical expectations suggest that the tensor-to-scalar ratio r should be between 0 and 0.3, and so should be tested in the near future.

Go To Original Article