Cosmic degeneracies – I. Joint N-body simulations of modified gravity and massive neutrinos

Abstract

We present the first suite of cosmological N-body simulations that simultaneously include the effects of two different and theoretically independent extensions of the standard ΛCDM cosmological scenario -namely an f (R) theory of Modified Gravity (MG) and a cosmological background of massive neutrinos -with the aim to investigate their possible observational degeneracies. We focus on three basic statistics of the large-scale matter distribution, more specifically the nonlinear matter power spectrum, the halo mass function, and the halo bias. Our results show that while these two extended models separately determine very prominent and potentially detectable features in all the three statistics, when we allow them to be simultaneously at work these features are strongly suppressed. In particular, when an f (R) gravity model with f R0 = −1 × 10 −4 is combined with a total neutrino mass of Σ i m νi = 0.4 eV, the resulting matter power spectrum, halo mass function, and bias at z = 0 are found to be consistent with the standard model's predictions at the 10%, 20%, and 5% accuracy levels, respectively. Therefore, our results imply an intrinsic theoretical limit to the effective discriminating power of present and future observational data sets with respect to these widely considered extensions of the standard cosmological scenario.

Beyond the cosmological standard model

“…Additionally, N-body simulations of gravitational collapse and clustering have been carried out in [393][394][395][396][397][398][399][400][401][402][403][404][405]. Functions of other curvature invariants have also been considered [406][407][408][409][410], but f (R) theories are distinguished in that they (and f (Gauss-Bonnet) theories) do not have ghosts.…”

“…This affects various linear-scale observables [884,[902][903][904], such as enhanced large scale bulk flows [396,905], infall velocities [906] and weak-lensing signals [907] as described below. See [393][394][395][396][397][398][399][400][401][402][403][404] for N-body simulations.…”

BeyondΛCDM: Problems, solutions, and the road ahead

“…It is already known that there is a degeneracy between modified gravity effects and these processes [204,205]. Finding ways to break this degeneracy -for instance, by using alternative observables to the matter power spectrum -is a topic of active research.…”

“…As recently pointed out by Baldi et al [204] however, few attempts have so far been made to investigate the effects of baryons and massive neutrinos on the formation and evolution of linear and non-linear cosmic structures in the context of alternative dark energy and modified gravity models. For instance, it has been shown that suitable choices of parameters for a combination of massive neutrinos and modified gravity could in principle result in a large-scale matter distribution that is barely distinguishable from ΛCDM [204].…”

“…For instance, it has been shown that suitable choices of parameters for a combination of massive neutrinos and modified gravity could in principle result in a large-scale matter distribution that is barely distinguishable from ΛCDM [204]. Such results seem to indicate a theoretical limit to the effective discriminating power of cosmological observations, and clearly suggest the necessity of further investigations to properly account for possible degeneracies with small-scale effects.…”