Lambda-CDM Model - History

History

The discovery of the Cosmic Microwave Background in 1965 confirmed a key prediction of the Big Bang cosmology. From that point on it was generally accepted that the universe started in a hot, dense early state, and has been expanding over time. The rate of expansion depends on the types of matter and energy present in the universe, and in particular, whether the total density is above or below the so-called critical density. During the 1970s, most attention focused on pure-baryonic models, but there were serious challenges explaining the formation of galaxies given the small anisotropies in the CMB (upper limits at that time). In the early 1980s, it was realised this could be resolved if cold dark matter dominated over the baryons, and the theory of cosmic inflation motivated models with critical density. During the 1980s, most research focused on cold dark matter with critical density in matter, around 95% CDM and 5% baryons: these showed success at forming galaxies and clusters of galaxies, but problems remained: notably the model required a Hubble constant lower than preferred by observations, and the model under-predicted observed large-scale galaxy clustering. These difficulties sharpened with the discovery of CMB anisotropy by COBE in 1992, and several alternatives including LambdaCDM and mixed cold+hot dark matter came under active consideration. The LambdaCDM model then became the standard following the observations of accelerating expansion in 1998, and was quickly supported by other observations: in 2000, the BOOMERanG microwave background experiment measured the total (matter+energy) density to be close to 100% of critical, while in 2001 the 2dfGRS galaxy survey measured the matter density to be near 25%; the large difference between these supports a positive Λ or dark energy. Much more precise measurements of the microwave background from WMAP in 2003 - 2010 have continued to support and refine the model.

There is currently active research into many aspects of the ΛCDM model, both to refine the parameters and possibly detect deviations. In addition, ΛCDM has no explicit physical theory for the origin or physical nature of dark matter or dark energy; the nearly scale-invariant spectrum of the CMB perturbations, and their image across the celestial sphere, are believed to result from very small thermal and acoustic irregularities at the point of recombination. A large majority of astronomers and astrophysicists support the ΛCDM model or close relatives of it, but Milgrom, McGaugh, and Kroupa are leading critics, attacking the dark matter portions of the theory from the perspective of galaxy formation models and supporting the alternative MOND theory which requires a modification of the Einstein Equations and the Friedmann Equations as seen in proposals such as MOG theory or TeVeS theory. Other proposals by theoretical astrophysicists of cosmological alternatives to Einstein's general relativity that attempt to account for dark energy or dark matter include f(R) gravity, scalar-tensor theories, brane cosmologies, the DGP model, and galileon theories.