Neutrino mass and kinetic gravity braiding degeneracies

Journal of Cosmology and Astroparticle Physics

We use Planck 2018 data to constrain the simplest models of scalar-tensor theories characterized by a coupling to the Ricci scalar of the type F (σ)R with F (σ) = N 2 pl + ξσ 2. We update our results with previous Planck and BAO data releases obtaining the tightest constraints to date on the coupling parameters, that is ξ < 5.5 × 10 −4 for N pl = 0 (induced gravity or equivalently extended Jordan-Brans-Dicke) and (N pl √ 8πG) − 1 < 1.8 × 10 −5 for ξ = −1/6 (conformal coupling), both at 95% CL. Because of a modified expansion history after radiation-matter equality compared to the ΛCDM model, all these dynamical models accommodate a higher value for H 0 and therefore alleviate the tension between Planck/BAO and distance-ladder measurement from SNe Ia data from 4.4σ at best to 2.7-3.2σ with CMB alone and 3.5-3.6σ including BAO data. We show that all these results are robust to changes in the neutrino physics. In comparison to the ΛCDM model, partial degeneracies between neutrino physics and the coupling to the Ricci scalar allow for smaller values N eff ∼ 2.8, 1σ lower compared to the standard N eff = 3.046, and relax the upper limit on the neutrino mass up to 40%.

In a recent work, Baldi et al. highlighted the issue of cosmic degeneracies, consisting in the fact that the standard statistics of the large-scale structure might not be sufficient to conclusively test cosmological models beyond $\Lambda $CDM when multiple extensions of the standard scenario coexist in nature. In particular, it was shown that the characteristic features of an $f(R)$ Modified Gravity theory and of massive neutrinos with an appreciable total mass $\Sigma _{i}m_{\nu _{i}}$ are suppressed in most of the basic large-scale structure observables for a specific combination of the main parameters of the two non-standard models. In the present work, we explore the possibility that the mean specific size of the supercluster spines -- which was recently proposed as a non-standard statistics by Shim and Lee to probe gravity at large scales -- can help to break this cosmic degeneracy. By analyzing the halo samples from N-body simulations featuring various combinations of $f(R)$ ...

arXiv: Cosmology and Nongalactic Astrophysics, 2019

The presence of massive neutrinos affects the growth of large-scale structure in the universe, leaving a potentially observable imprint on the abundance and properties of massive dark matter-dominated halos. Cosmological surveys detect large numbers of these halos in the form of rich groups and clusters, using the information as an input to constraining the properties of dark energy. We use a suite of N-body simulations that include the effects of massive neutrinos as well as of dynamical dark energy to study the properties of the mass function. As in our previous work, we follow an approach valid at low neutrino mass, where the neutrino overdensities are assumed to be too small to act as a significant nonlinear source term for gravity. We study how well a universal form for the halo mass function describes our numerical results, finding that the use of an appropriate linear power spectrum within the formalism yields a good match to the simulation results, correctly accounting for t...

The European Physical Journal C, 2019

It has been pointed out that there exists a tension in σ 8 − Ω m measurement between CMB and LSS observation. In this paper we show that σ 8 − Ω m observations can be used to test the dark energy theories. We study two models, (1) Hu-Sawicki (HS) Model of f (R) gravity and (2) Chavallier-Polarski-Linder (CPL) parametrization of dynamical dark energy (DDE), both of which satisfy the constraints from supernovae. We compute σ 8 consistent with the parameters of these models. We find that the well known tension in σ 8 between Planck CMB and large scale structure (LSS) observations is (1) exacerbated in the HS model and (2) somewhat alleviated in the DDE model. We illustrate the importance of the σ 8 measurements for testing modified gravity models. Modified gravity models change the matter power spectrum at cluster scale which also depends upon the neutrino mass. We present the bound on neutrino mass in the HS and DDE model.

Physical Review Letters, 2013

It is shown that the tension between recent neutrino oscillation experiments, favoring sterile neutrinos with mass of order of 1 eV, and cosmological data which impose stringent constraints on neutrino masses from the free streaming suppression of density fluctuations, can be resolved in models of the present accelerated expansion of the Universe based on f (R) gravity.

"We study the effects of introducing modifications to general relativity (“GR”) at large scales as an alternative to exotic forms of matter required to replicate the observed cosmic acceleration. We survey the effects on cosmology and solar-system tests of Dvali–Gabadadze–Porrati (“DGP”) gravity, f(R) gravity and Modified-Source Gravity (“MSG”). We find that, in addition to the changes to the background expansion history of the universe, these modifications have substantial impact on structure formation and its observable predictions. For DGP, we develop a scaling approximation for the behaviour of perturbations off the brane, for which the predicted integrated Sachs-Wolf (“ISW”) effect is much stronger than observed, requiring new physics at around horizon scale to bring it into agreement with data. We develop a test based on cross-correlating galaxies and the ISW effect which is independent of the initial power spectrum for perturbations and is a smoking-gun test for DGP gravity. For f(R) models, we find that, for the expansion history to resemble that of LCDM, it is required that f,RR> 0. This also ensures that GR-like high-curvature solutions are stable. We then find the conditions on f(R) which allow this subset of models to pass solar-system tests. Provided that gravity behave like GR in the galaxy, these constraints are weak. However, for a model to allow large deviations from GR in the cosmology, the galactic halo must differ significantly from that predicted by structure evolution in GR. We then discuss the effect that these models have on structure formation, and find that even in the most conservative of models, percent-level deviations in the matter power spectrum will exist and should be detectable in the future. Finally, for MSG, we investigate the cosmology of a theory of gravity with a modified constraint structure. The acceleration era can easily be replicated in these models; however, linear perturbations become unstable as the universe begins to accelerate. Once the perturbations become non-linear, the model reverts to GR, regaining stability. This should leave a significant imprint on the structure-formation probes, but one which we cannot calculate."

Monthly Notices of the Royal Astronomical Society, 2014

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.

Physical Review D, 2006

In this paper we investigate the cosmological effects of modified gravity with string curvature corrections added to Einstein-Hilbert action in the presence of a dynamically evolving scalar field coupled to Riemann invariants. The scenario exhibits several features of cosmological interest for late universe. It is shown that higher order stringy corrections can lead to a class of dark energy models consistent with recent observations. The model can give rise to quintessence without recourse to scalar field potential. The detailed treatment of reconstruction program for general scalar-Gauss-Bonnet gravity is presented for any given cosmology. The explicit examples of reconstructed scalar potentials are given for accelerated (quintessence, cosmological constant or phantom) universe. Finally, the relation with modified F (G) gravity is established on classical level and is extended to include third order terms on curvature. PACS numbers: 11.25.-w, 95.36.+x, 98.80.-k

Progress of Theoretical Physics, 2010

The effect of massive neutrinos on matter power spectrum is discussed in the context of f (R) gravity. It is shown that the anomalous growth of density fluctuations on small scales due to the scalaron force can be compensated by free streaming of neutrinos. As a result, models which predict observable deviation of the equation-of-state parameter w DE from w DE = −1 can be reconciled with observations of matter clustering if the total neutrino mass is O(0.5 eV).