Observables and unobservables in dark energy cosmologies

Abstract

The aim of this paper is to answer the following two questions: (1) Given cosmological observations of the expansion history and linear perturbations in a range of redshifts and scales as precise as is required, which of the properties of dark energy could actually be reconstructed without imposing any parameterization? (2) Are these observables sufficient to rule out not just a particular dark energy model, but the entire general class of viable models comprising a single scalar field?This paper bears both good and bad news. On one hand, we find that the goal of reconstructing dark energy models is fundamentally limited by the unobservability of the present values of the matter density Ωm0, the perturbation normalization σ8 as well as the present matter power spectrum. On the other, we find that, under certain conditions, cosmological observations can nonetheless rule out the entire class of the most general single scalar-field models, i.e. those based on the Horndeski Lagrangian.

BeyondΛCDM: Problems, solutions, and the road ahead

“…For example, without a parameterization for w, it is impossible to measure Ω M from distances directly; they are only sensitive to the total expansion rate, H(z)/H 0 , and a function w(a) can always be chosen to mimic the same distances for a different dark matter fraction. This is called the dark degeneracy [463,464]. Thus it is only by measuring the properties of large-scale structure that this degeneracy can be broken and Ω M determined [465].…”

Model-independent limits and constraints on extended theories of gravity from cosmic reconstruction techniques

“…It appears that we are left with four independent parameters (besides Ω m ), namely F i (i = 1, 2, 3, 4), which allow us to only obtain three cosmographic parameters (q, j, s). However, rescaling all quantities as follows: 12) we are able to eliminate F 1 from (3.11) entirely, finding…”

“…In particular, among all plausible modifications, several approaches successfully reproduce the cosmic evolution with the same accuracy as the Concordance cosmological ΛCDM model, leading to a degeneracy problem [11][12][13][14]. A possible way of alleviating such a degeneracy is to combine different measurements with the aim of reducing the phase space of free parameters.…”

Transforming gravity: From derivative couplings to matter to second-order scalar-tensor theories beyond the Horndeski Lagrangian

“…A very important advantage of the Horndeski Lagrangian is that it contains many interesting physical theories (see [18] for a summary) and allows for a systematic study of their properties. Such a general approach has been applied to cosmological dynamics [19,20], compatibility with cosmological observations [21][22][23], inflationary mechanisms [24] and screening modifications of gravity [25,26] and the effective cosmological constant [27]. Besides General Relativity (G 2 = G 3 = G 5 = 0, G 4 = 1/16πG), the best known class of theories contained in L H are Jordan-Brans-Dicke theories [28], for which G 3 = G 5 = 0, G 4 = f (φ)/16πG and G 2 = X/ω(φ) − V (φ).…”