Phenomenology of dark energy: general features of large-scale perturbations

Abstract

We present a systematic study of modified gravity (MG) models containing a single scalar field non-minimally coupled to the metric. Despite a large parameter space, exploiting the effective field theory of dark energy (EFT of DE) formulation and imposing simple physical constraints such as stability conditions and (sub-)luminal propagation of perturbations, we arrive at a number of generic predictions about the large scale structures.The goal of this work, in collaboration with F. Piazza, C. Marinoni and L. Hui 1 , is to study the predictability of MG theories aiming at challenging the standard ΛCDM explanation of cosmic acceleration. We use the EFT of DE 2 for it has established a common formalism to describe the widest set of MG theories, those adding a single extra scalar degree of freedom to the Einstein-Hilbert action.Such a unifying description enables MG theories to be parametrized in a common framework in terms of structural functions of time, describing how matter perturbations evolve in the universe. The requirements needed to fully describe an EFT model can be reduced to two constants and three functions of time Ω 0 m , w, µ(t), µ 3 (t), 4 (t) . The three functions are nonminimal couplings, once "turned on" they enable the description theories in the Horndeski class: µ is the Brans-Dicke (BD) non-minimal coupling, adding µ 3 models the cubic galileon (H3 ) term and 4 encodes the 4 and 5 Horndeski (H45 ) Lagrangians. In parallel, the EFT of DE allows one to independently set the background expansion, i.e the Hubble rate. This reduces to fixing the two constants, the present fractional matter density of non-relativistic matter in the perfect fluid approximation Ω 0 m and the background DE equation of state parameter w 3 . They are set by the latest constraints 4 to reproduce a flat ΛCDM background, thus respectively ∼ 0.3 and −1.Another asset of the EFT of DE is to provide a clear and common means of assessing if theories are pathological or not, i.e whether they suffer from ghost or gradient instabilities. The main purposes of this work is to show that despite large degrees of freedom, definite features common to all healthy -stable and with no superluminal propagation of scalar and tensor modes-EFT models arise within the vast Horndeski class. We show this by expanding the non-minimal coupling functions in power series up to second order in the reduced matter density, x, used as time variable. An overall (1 − x) pre-factor ensures the vanishing of the couplings at early times, hence recovering general relativity. We randomly generate the coefficients of the expansions until we obtain 10 4 healthy BD, H3 and H45 theories. a Proceedings of 51st Rencontres de Moriond arXiv:1607.06916v1 [astro-ph.CO]

The Subaru FMOS galaxy redshift survey (FastSound). IV. New constraint on gravity theory from redshift space distortions at z ∼ 1.4

“…The analytic estimation of G eff and the full numerical integration of cosmological perturbations for the covariant Galileon were first carried out by de Felice et al (2011a). In the massless limit (m φ → 0), the effective gravitational coupling can be schematically expressed in the form (Tsujikawa 2015;Pèrenon et al 2015)…”

Effective gravitational couplings for cosmological perturbations in generalized Proca theories

“…In scalar-tensor Horndeski theories, unless the second-order action of tensor perturbations is modified from GR to a large extent, it is difficult to realize G eff < G without ghosts due to the presence of attractive scalar-matter couplings [41] (see also Refs. [42][43][44][45][46][47]). It remains to see whether the existence of the vector field can modify this situation.…”

Testing deviations from ΛCDM with growth rate measurements from six large-scale structure surveys atz= 0.06–1

“…baryon acoustic oscillations (BAO) (Eisenstein et al 2005;Cole et al 2005;Hütsi 2006;Kazin et al 2010;Percival et al 2010;Reid et al 2010;Aubourg et al 2014;Anderson et al 2014a,b), Supernovae (Perlmutter & Schmidt 2003;Conley et al 2011;Goobar & Leibundgut 2011;Suzuki et al 2012;Rodney et al 2014), and CMB (Planck Collaboration et al (2015a), WMAP9 Bennett et al (2013)) observations, it suffers from the coincidence and fine tuning problems (Weinberg 1989;Carroll 2001). Several alternatives to Λ have been proposed in the two decades since the discovery of cosmic acceleration (Riess et al 1998;Perlmutter et al 1999), and they can be roughly divided into two classes. The first class, to which we will refer as modified gravity (MG), corresponds to modifications of the laws of gravity on large scales, designed to achieve self accelerating solutions when matter becomes negligible (Silvestri & Trodden 2009;Clifton et al 2012); alternatively, one can introduce a dynamical degree of freedom, commonly dubbed dark energy (DE; first coined by Huterer & Turner (1999)), which is smoothly distributed and starts to dominate the evolution of the Universe at late times (Copeland et al 2006).…”