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The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda (Greek Λ), associated with dark energy, and cold dark matter (abbreviated CDM). It is frequently referred to as the standard model of Big Bang cosmology because it is the simplest model that provides a reasonably good account of the following properties of the cosmos:
The model assumes that general relativity is the correct theory of gravity on cosmological scales. It emerged in the late 1990s as a concordance cosmology, after a period of time when disparate observed properties of the universe appeared mutually inconsistent, and there was no consensus on the makeup of the energy density of the universe.
Some alternative models challenge the assumptions of the ΛCDM model. Examples of these are modified Newtonian dynamics, modified gravity and theories of large-scale variations in the matter density of the universe.
Most modern cosmological models are based on the cosmological principle, which states that our observational location in the universe is not unusual or special; on a large-enough scale, the universe looks the same in all directions (isotropy) and from every location (homogeneity).
The model includes an expansion of metric space that is well documented both as the red shift of prominent spectral absorption or emission lines in the light from distant galaxies and as the time dilation in the light decay of supernova luminosity curves. Both effects are attributed to a Doppler shift in electromagnetic radiation as it travels across expanding space. Although this expansion increases the distance between objects that are not under shared gravitational influence, it does not increase the size of the objects (e.g. galaxies) in space. It also allows for distant galaxies to recede from each other at speeds greater than the speed of light; local expansion is less than the speed of light, but expansion summed across great distances can collectively exceed the speed of light.
|Physical baryon density parameter||Ωbh2||30±0.000140.022|
|Physical dark matter density parameter||Ωch2||±0.00100.1188|
|Age of the universe||t0||±0.021 × 109 years 13.799|
|Scalar spectral index||ns||±0.00400.9667|
|Curvature fluctuation amplitude, k0 = 0.002 Mpc−1||Δ2
|Reionization optical depth||τ||±0.0120.066|
|Total density parameter||Ωtot||1|
|Equation of state of dark energy||w||−1|
|Sum of three neutrino masses||∑ mν||0.06 eV/c2|
|Effective number of relativistic degrees of freedom||Neff||3.046|
|Running of spectral index||dns / d ln k||0|
|Hubble constant||H0||±0.46 km s−1 67.74Mpc−1|
|Baryon density parameter||Ωb||±0.00100.0486|
|Dark matter density parameter||Ωc||±0.00570.2589|
|Matter density parameter||Ωm||±0.00620.3089|
|Dark energy density parameter||ΩΛ||±0.00620.6911|
|Critical density||ρcrit||±0.12)×10−27 kg/m3(8.62|
|Fluctuation amplitude at 8h−1 Mpc||σ8||±0.00860.8159|
|Redshift at decoupling||z∗||089.90±0.231|
|Age at decoupling||t∗||700±3200 years 377|
|Redshift of reionization (with uniform prior)||zre||+1.0
|Total density parameter||Ωtot||+0.0056
|Equation of state of dark energy||w||±0.053−0.980|
|Tensor-to-scalar ratio||r||< 0.11, k0 = 0.002 Mpc−1 (2σ)|
|Running of the spectral index||dns / d ln k||±0.020, k0 = 0.002 Mpc−1 −0.022|
|Physical neutrino density parameter||Ωνh2||< 0.0062|
|Sum of three neutrino masses||∑ mν||< 0.58 eV/c2 (2σ)|
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