Browsing by Author "Braglia, Matteo"

Now showing 1 - 3 of 3
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    Cosmological constraints on post-Newtonian parameters in effectively massless scalar-tensor theories of gravity
    (2019) Rossi, Massimo; Ballardini, Mario; Braglia, Matteo
    We study the cosmological constraints on the variation of Newton’s constant and on post-Newtonian parameters for simple models of the scalar-tensor theory of gravity beyond the extended Jordan-BransDicke theory. We restrict ourselves to an effectively massless scalar field with a potential V ∝ F2, where FðσÞ ¼ N2 pl þ ξσ2 is the coupling to the Ricci scalar considered. We derive the theoretical predictions for cosmic microwave background anisotropies and matter power spectra by requiring that the effective gravitational strength at present is compatible with the one measured in a Cavendish-like experiment and by assuming an adiabatic initial condition for scalar fluctuations. When comparing these models with Planck 2015 and a compilation of baryonic acoustic oscillations data, all these models accommodate a marginalized value for H0 higher than in ΛCDM. We find no evidence for a statistically significant deviation from Einstein’s general relativity. We find ξ < 0.064 (jξj < 0.011) at 95% CL for ξ > 0 (for ξ < 0, ξ ≠ −1=6). In terms of post-Newtonian parameters, we find 0.995 < γPN < 1 and 0.99987 < βPN < 1 (0.997 < γPN < 1 and 1 < βPN < 1.000011) for ξ > 0 (for ξ < 0). For the particular case of the conformal coupling, i.e., ξ ¼ −1=6, we find constraints on the post-Newtonian parameters of similar precision to those within the Solar System.
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    Larger value for H0 by an evolving gravitational constant
    (American Physical Society, 2020) Ballardini, Mario; Braglia, Matteo; Emond, William T.
    We provide further evidence that a massless cosmological scalar field with a nonminimal coupling to the Ricci curvature of the type M 2 pl ( 1 + ξ σ n / M n pl ) alleviates the existing tension between local measurements of the Hubble constant and its inference from cosmic microwave background anisotropies and baryonic acoustic oscillations data in the presence of a cosmological constant. In these models, the expansion history is modified compared to Λ CDM at early time, mimicking a change in the effective number of relativistic species, and gravity weakens after matter-radiation equality. Compared to Λ CDM , a quadratic ( n = 2 ) coupling increases the Hubble constant when Planck 2018 (alone or in combination with BAO and SH0ES) measurements data are used in the analysis. Negative values of the coupling, for which the scalar field decreases, seem favored and consistency with the Solar System can be naturally achieved for a large portion of the parameter space without the need of any screening mechanism. We show that our results are robust to the choice of n , also presenting the analysis for n = 4 .
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    Scalar-tensor theories of gravity, neutrino physics, and the H0 tension
    (IOP Publishing, 2020) Ballardini, Mario; Braglia, Matteo; Finelli, Fabio
    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) = N2 pl + ξσ2 . 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 Npl = 0 (induced gravity or equivalently extended Jordan-Brans-Dicke) and (Npl √ 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 H0 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.