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||Ωbh²||0.02230|
|physical dark matter density||Ωdh²||0.1188|
|age of the universe||t0||13.799 × 109 years|
|scalar spectral index||ns||0.9667|
|curvature fluctuation amplitude||Δ²R||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-1Mpc-1.
Some additional values calculable from the six:
|item||symbol||a current calculation|
|Hubble constant||H0||67.74 km s-1 Mpc-1|
|baryon density parameter||Ωb||0.0486|
|dark matter density parameter||Ωd||0.2589|
|matter density parameter||Ωm||0.3089|
|dark energy density parameter||ΩΛ||0.6911|
The density parameters are expressed as fractions of the critical density. The dark matter density parameter is sometimes cited as Ωc, presumably for cold dark matter, but that symbol is also sometimes used to refer to the critical density parameter.