TY - JOUR

T1 - Consistency of modified gravity with a decreasing Geff (z) in a ΛcDM background

AU - Gannouji, Radouane

AU - Kazantzidis, Lavrentios

AU - Perivolaropoulos, Leandros

AU - Polarski, David

N1 - Funding Information:
The work of R. G. is supported by Fondecyt Project No. 1171384. D. P. aknowledges KASI (Daejeon, Korea) for hospitality. The research of L. K. was implemented under the scholarship program (Grant No 2018-050-0502-11459) by the Hellenic State Scholarship Foundation as part of the Operational Program “Human Resources Development Program, Education and Lifelong Learning—MIS 5000432.”
Publisher Copyright:
© 2018 American Physical Society.

PY - 2018

Y1 - 2018

N2 - Recent analyses [S. Nesseris et al., Phys. Rev. D 96, 023543 (2017)PRVDAQ2470-001010.1103/PhysRevD.96.023543; L. Kazantzidis and L. Pervolaropoulos, Phys. Rev. D 97, 103503 (2018)PRVDAQ2470-001010.1103/PhysRevD.97.103503] have indicated that an effective Newton's constant Geff(z) decreasing with redshift may relieve the observed tension between the Planck15 best fit ΛCDM cosmological background (i.e., Planck15/ΛCDM) and the corresponding ΛCDM background favored by growth fσ8 and weak lensing data. We investigate the consistency of such a decreasing Geff(z) with some viable scalar-tensor models and f(R) theories. We stress that f(R) theories generically cannot lead to a decreasing Geff(z) for any cosmological background. For scalar-tensor models we deduce that in the context of a ΛCDM cosmological background, a decreasing Geff(z) is not consistent with a large Brans-Dicke parameter ωBD,0 today. This inconsistency remains and amplifies in the presence of a phantom dark energy equation of state parameter (w<-1). However, it can be avoided for w>-1. We also find that any modified gravity model with the required decreasing Geff(z) and Geff,0=G would have a characteristic signature in its growth index γ with 0.61γ00.69 and large slopes γ0′, 0.16γ0′0.4, which is a characteristic signature of a decreasing (with z) Geff(z)<G on small redshifts. This is a substantial departure today from the quasistatic behavior in ΛCDM with (γ0,γ0′)≈(0.55,-0.02).

AB - Recent analyses [S. Nesseris et al., Phys. Rev. D 96, 023543 (2017)PRVDAQ2470-001010.1103/PhysRevD.96.023543; L. Kazantzidis and L. Pervolaropoulos, Phys. Rev. D 97, 103503 (2018)PRVDAQ2470-001010.1103/PhysRevD.97.103503] have indicated that an effective Newton's constant Geff(z) decreasing with redshift may relieve the observed tension between the Planck15 best fit ΛCDM cosmological background (i.e., Planck15/ΛCDM) and the corresponding ΛCDM background favored by growth fσ8 and weak lensing data. We investigate the consistency of such a decreasing Geff(z) with some viable scalar-tensor models and f(R) theories. We stress that f(R) theories generically cannot lead to a decreasing Geff(z) for any cosmological background. For scalar-tensor models we deduce that in the context of a ΛCDM cosmological background, a decreasing Geff(z) is not consistent with a large Brans-Dicke parameter ωBD,0 today. This inconsistency remains and amplifies in the presence of a phantom dark energy equation of state parameter (w<-1). However, it can be avoided for w>-1. We also find that any modified gravity model with the required decreasing Geff(z) and Geff,0=G would have a characteristic signature in its growth index γ with 0.61γ00.69 and large slopes γ0′, 0.16γ0′0.4, which is a characteristic signature of a decreasing (with z) Geff(z)<G on small redshifts. This is a substantial departure today from the quasistatic behavior in ΛCDM with (γ0,γ0′)≈(0.55,-0.02).

UR - http://www.scopus.com/inward/record.url?scp=85057830456&partnerID=8YFLogxK

U2 - 10.1103/PhysRevD.98.104044

DO - 10.1103/PhysRevD.98.104044

M3 - Article

AN - SCOPUS:85057830456

VL - 98

JO - Physical Review D

JF - Physical Review D

SN - 2470-0010

IS - 10

M1 - 104044

ER -