Astrophysics (Index)About

Lambda-CDM model

(ΛCDM, LCDM, standard model)
(Big-Bang cosmological model with a cosmological constant and cold dark matter)

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 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 EMR can explain. (Other explanations for these phenomena include deviations in the behavior of gravity and/or dynamics from accepted models.) Lambda-CDM relates many observed phenomena to six apparently arbitrary parameters determinable from observation. Some values determined in 2015 using Planck data:

parametersymboldetermined value
physical baryon densityΩb0.02230
physical dark matter densityΩd0.1188
age of the universet013.799 × 109 years
scalar spectral indexns0.9667
curvature fluctuation amplitudeΔ²R2.441 × 10-9
optical depth to the epoch of reionization (EOR)τ0.066

(Values from other determinations do not match these exactly, but the differences are small.) 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 also sometimes cited as a redshift (z ~ 11). h (in 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:

itemsymbolcalculated value
Hubble constantH067.74 km s-1 Mpc-1
baryon density parameterΩb0.0486
dark matter density parameterΩd0.2589
matter density parameterΩm0.3089
dark energy density parameterΩΛ0.6911

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.

(astrophysics,model,cosmology,dark matter)
Further reading:

Referenced by pages:
alternative cosmologies
angular power spectrum
astronomical quantities
Bolshoi simulation
cold dark matter (CDM)
core-cusp problem
cosmological simulation
dark matter
dark matter halo
dwarf galaxy problem
Einstein-de Sitter model
epoch of galaxy formation
galaxy formation
initial fluctuations
redshift-angular size relation
standard model