Testing gravity on cosmic scales: A case study of JordanBransDicke theory
Abstract
We provide an endtoend exploration of a distinct modified gravitational theory in JordanBransDicke (JBD) gravity, from an analytical and numerical description of the background expansion and linear perturbations, to the nonlinear regime captured with a hybrid suite of N body simulations, to the cosmological constraints from existing probes of the expansion history, the largescale structure, and the cosmic microwave background (CMB). We have focused on JBD gravity as it both approximates a wider class of Horndeski scalartensor theories on cosmological scales and allows us to adequately model the nonlinear corrections to the matter power spectrum. In a combined analysis of the Planck 2018 CMB temperature, polarization, and lensing reconstruction, together with Pantheon supernova distances and the Baryon Oscillation Spectroscopic Survey (BOSS) measurements of baryon acoustic oscillation distances, the AlcockPaczynski effect, and the growth rate, we constrain the JBD coupling constant to ω_{BD}>970 (95% confidence level; C.L.) in agreement with the General Relativistic expectation given by ω_{BD}→∞ . In the unrestricted JBD model, where the effective gravitational constant at present, G_{matter}/G , is additionally varied, increased dataset concordance (e.g., within 1 σ agreement in S_{8}=σ_{8}√{Ω_{m}/0.3 }) enables us to further include the combined ("3 ×2 pt ") dataset of cosmic shear, galaxygalaxy lensing, and overlapping redshiftspace galaxy clustering from the Kilo Degree Survey and the 2degree Field Lensing Survey (KiDS ×2 dFLenS ). In analyzing the weak lensing measurements, the nonlinear corrections due to baryons, massive neutrinos, and modified gravity are simultaneously modeled and propagated in the cosmological analysis for the first time. In the joint analysis of all datasets, we constrain ω_{BD}>1540 (95% C.L.), G_{matter}/G =0.997 ±0.029 , the sum of neutrino masses, ∑m_{ν}<0.12 eV (95% C.L.), and the baryonic feedback amplitude, B <2.8 (95% CL), all in agreement with the standard model expectation. In fixing the sum of neutrino masses, the lower bound on the coupling constant strengthens to ω_{BD}>1460 and ω_{BD}>2230 (both at 95% C.L.) in the restricted and unrestricted JBD models, respectively. We explore the impact of the JBD modeling choices, and show that a more restrictive parametrization of the coupling constant degrades the neutrino mass bound by up to a factor of three. In addition to the improved concordance between KiDS ×2 dFLenS and Planck, the tension in the Hubble constant between Planck and the direct measurement of Riess et al. (2019) is reduced to ∼3 σ ; however, we find no substantial model selection preference for JBD gravity relative to Λ CDM . We further show that a positive shift in the effective gravitational constant suppresses the CMB damping tail, which might complicate future inferences of smallscale physics, given its degeneracy with the primordial helium abundance, the effective number of neutrinos, and the running of the spectral index.
 Publication:

Physical Review D
 Pub Date:
 February 2022
 DOI:
 10.1103/PhysRevD.105.043522
 arXiv:
 arXiv:2010.15278
 Bibcode:
 2022PhRvD.105d3522J
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology
 EPrint:
 53 pages, 26 figures, results unchanged, version accepted for publication by PRD. Abstract abridged. Our MCMC chains, likelihood code, and data products are public at https://github.com/sjoudaki/CosmoJBD