Lambda-CDM model

The Lambda-CDM model (ΛCDM or LCDM) is a current, widely-used "working model" (standard model) of Big Bang/dark matter/dark energy cosmology. It includes cold dark matter and presumes a cosmological constant (Lambda aka Λ) in Einstein's field equation. The cosmological constant provides an explanation for the universe's observed accelerating expansion. The dark matter provides an explanation for the dynamics of galaxies and galaxy clusters that appear to have more gravitational attraction than their EMR can explain. (Other explanations for these phenomena include deviations in the behavior of gravity and/or dynamics from accepted models.) The model also presumes inflation. Lambda-CDM relates many observed phenomena to six apparently arbitrary parameters. Some values determined in 2015 using Planck observation data:

(Values from other determinations do not match these exactly, but the differences are small.) These are not constants but are the current values for quantities that slowly shift with the passage of time (which is obvious for "age of the universe"). The six parameters cited may not be those listed 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 sometimes indicated by the redshift of reionization (z ~ 11). h (in h² above) indicates the (dimensionless) Hubble parameter, which is the universe's expansion expressed as a fraction of 100 km s^-1Mpc^-1, which varies with time. Some additional values calculable from the six:

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.