3 A Brief Overview of the ΛCDM Cosmological Model

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 ΛCDM model – first proposed in 1995 by Ostriker and Steinhardt [344Jump To The Next Citation Point] – is encouraged by a wealth of observations: the consistency of the Hubble parameter with the ages of the oldest stars [344], 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 [486], as well as the power spectrum of density perturbations [103, 452Jump To The Next Citation Point]. 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 [134]. Perhaps the most emphatic support for ΛCDM comes from fits to the acoustic power spectrum of temperature fluctuations in the CMB [229Jump To The Next Citation Point].

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 ΛCDM 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 H0 = 70 km s− 1 Mpc −1 (i.e., h = 0.7), the amplitude of density fluctuations within a top-hat sphere of 8h −1 Mpc is σ = 0.8 8, the optical depth to reionization is τ = 0.08, the spectral index measuring how fluctuations change with scale is ns = 0.97, 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 ΛCDM: it is composed separately of a dark energy sector and a cold dark matter sector, which we briefly describe below.

 3.1 Dark Energy (Λ)
 3.2 Cold Dark Matter (CDM)

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