General relativity provides a clear and compelling cosmology, the Friedmann–Lemaître–Robertson–Walker (FLRW) model. The expansion of the Universe discovered by Hubble and Slipher found a natural explanation4 in this context. The picture of a hot Big-Bang cosmology that emerged from this model famously predicted the existence of the 3 degree CMB and the abundances of the light isotopes via BBN.
Within the FLRW framework, we are inexorably driven to infer the existence of both non-baryonic cold dark matter and a non-zero cosmological constant as discussed in Section 2. The resulting concordance CDM model – first proposed in 1995 by Ostriker and Steinhardt  – is encouraged by a wealth of observations: the consistency of the Hubble parameter with the ages of the oldest stars , the consistency between the dynamical mass density of the Universe, that of baryons from BBN (see also discussion in Section 9.2), and the baryon fraction of clusters , as well as the power spectrum of density perturbations [103, 452]. A prediction of the concordance model is that the expansion rate of the Universe should be accelerating; this was confirmed by observations of high redshift Type Ia supernovae [351, 365]. Another successful prediction was the scale of the baryonic acoustic oscillation . Perhaps the most emphatic support for CDM comes from fits to the acoustic power spectrum of temperature fluctuations in the CMB .
For a brief review of the basics and successes of the concordance cosmological model we refer the reader to, e.g., [87, 349] and all references therein. We note that, while most of the cosmological probes in the above list are not uniquely fit by the CDM model on their own, when they are taken together they provide a remarkably tight set of constraints. The success of this now favoured cosmological model on large scales is, thus, remarkable indeed, as there was a priori no reason that such a parameterized cosmology could explain all these completely independent data sets with such outstanding consistency.
In this model, the Hubble constant is (i.e., ), the amplitude of density fluctuations within a top-hat sphere of is , the optical depth to reionization is , the spectral index measuring how fluctuations change with scale is , and the price we pay for the outstanding success of the model is new physics in the form of a dark sector. This dark sector is making up 95% of the mass-energy content of the Universe in CDM: it is composed separately of a dark energy sector and a cold dark matter sector, which we briefly describe below.
Living Rev. Relativity 15, (2012), 10
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