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The **Lambda-CDM model** (**ΛCDM** or **LCDM**)
is a current, widely-used
"working model" of Big Bang/dark matter/dark energy cosmology.
It includes cold dark matter and a
cosmological constant (Lambda or Λ).
The cosmological constant provides an explanation
for the universe's observed accelerating expansion.
Dark matter provides an explanation for
the dynamics of galaxies and galaxy clusters
that appear to have more gravitational attraction
than their light can explain.

Other explanations for these phenomena are some as-yet-unrecognized behavior of gravity or dynamics.

The model relates many observed phenomena to six apparently arbitrary parameters determinable from observation:

parameter | symbol | a current determination |

physical baryon density | Ω_{b}h² | 0.02230 |

physical dark matter density | Ω_{d}h² | 0.1188 |

age of the universe | t_{0} | 13.799 × 10^{9} years |

scalar spectral index | n_{s} | 0.9667 |

curvature fluctuation amplitude | Δ²_{R} | 2.441 × 10^{-9} |

optical depth to the epoch of reionization | τ | 0.066 |

The cited parameter list may differ from the above,
but in combination, they would be equivalent
to the above through relations assumed by the model.
For example, the *optical depth to reionization* represents
a measure of how long ago the EOR took place,
which is also sometimes cited as a redshift (z ~ 11).
**h** (in *h²* above) indicates the **Hubble parameter**,
which is the Hubble constant expressed as a fraction of
100 km s^{-1}Mpc^{-1}.

Some additional values calculable from the six:

item | symbol | a current calculation |

Hubble constant | H_{0} | 67.74 km s^{-1} Mpc^{-1} |

baryon density parameter | Ω_{b} | 0.0486 |

dark matter density parameter | Ω_{d} | 0.2589 |

matter density parameter | Ω_{m} | 0.3089 |

dark energy density parameter | Ω_{Λ} | 0.6911 |

The density parameters are expressed as fractions of the critical density.
The dark matter density parameter is sometimes cited
as Ω_{c}, presumably for *cold* dark matter,
but that symbol is also sometimes used to refer to the
critical density parameter.

astronomical quantities

Bolshoi simulation

cold dark matter (CDM)

Einstein-de Sitter model

epoch of galaxy formation

redshift-angular size relation

standard model