Lambda-CDM model

The Lambda-CDM, Lambda cold dark matter, or ΛCDM model is the standard form of the Big Bang theory of cosmology governing the chronology of the universe. As implied by its name, the model postulates a cosmological constant, denoted by lambda (Λ), associated with dark energy, cold dark matter, denoted by CDM as well as ordinary matter. It is the simplest model that provides a reasonably good account of:

the existence and structure of the cosmic microwave background;

the large-scale structure in the distribution of galaxies;

the observed abundances of hydrogen (including deuterium), helium, and lithium;

the accelerating expansion of the universe observed in the light from distant galaxies and supernovae.

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 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.

The ΛCDM model has been successful in modeling a broad collection of astronomical observations over decades. Remaining issues challenge the assumptions of the ΛCDM model and have led to many alternative models.

Overview

The ΛCDM model is based on three postulates on the structure of spacetime:

The cosmological principle, that the universe is the same everywhere and in all directions, and that it is expanding,

A postulate by Hermann Weyl that the lines of spacetime (geodesics) intersect at only one point, where time along each line can be synchronized; the behavior resembles an expanding perfect fluid,

General relativity that relates the geometry of spacetime to the distribution of matter and energy.

This combination greatly simplifies the equations of general relativity into a form called the Friedmann equations. These equations specify the evolution of the scale factor of the universe in terms of the pressure and density of a perfect fluid.