Inflationary perturbations in bimetric gravity

Cusin, Giulia; Durrer, Ruth; Guarato, Pietro; Motta, Mariele

doi:10.1088/1475-7516/2015/09/043

Cited by 19 publications

(19 citation statements)

References 65 publications

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“…For massive gravity, it has proved impossible to find consistent and flat FRW background solutions [23] (although solutions which approximate such FRW backgrounds to great accuracy exist). In the singly coupled bigravity case, this obstacle can be overcome [24][25][26], while perturbations in the scalar, vector and tensor sectors can show power law or exponential instabilities [27][28][29][30][31][32][33][34]. Finally, in doubly coupled bigravity models as we are considering here, there are two branches of viable background solutions [35,36] and the perturbative properties of these models have been partially explored in [37,38], with results suggesting that they might be improved with respect to the singly coupled case.…”

Section: Introductionmentioning

confidence: 93%

Dark energy and doubly coupled bigravity

Brax

Davis

Noller

2017

Class. Quantum Grav.

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Abstract:We analyse the late time cosmology and the gravitational properties of doubly coupled bigravity in the constrained vielbein formalism (equivalent to the metric formalism) when the mass of the massive graviton is of the order of the present Hubble rate. We focus on one of the two branches of background cosmology where the ratio between the scale factors of the two metrics is algebraically determined. We find that the late time physics depends on the mass of the graviton, which dictates the future asymptotic cosmological constant. The Universe evolves from a matter dominated epoch to a dark energy dominated era where the equation of state of dark energy can always be made close to -1 now by appropriately tuning the graviton mass. We also analyse the perturbative spectrum of the theory in the quasistatic approximation, well below the strong coupling scale where no instability is present, and we show that there are five scalar degrees of freedom, two vectors and two gravitons. In Minkowski space, where the four Newtonian potentials vanish, the theory manifestly reduces to one massive and one massless graviton. In a cosmological FRW background for both metrics, four of the five scalars are Newtonian potentials which lead to a modification of gravity on large scales. The fifth one gives rise to a ghost which decouples from pressure-less matter in the quasi-static approximation. In this scalar sector, gravity is modified with effects on both the growth of structure and the lensing potential. In particular, we find that the Σ parameter governing the Poisson equation of the weak lensing potential can differ from one in the recent past of the Universe. Overall, the nature of the modification of gravity at low energy, which reveals itself in the growth of structure and the lensing potential, is intrinsically dependent on the couplings to matter and the potential term of the vielbeins. We also find that the time variation of Newton's constant in the Jordan frame can easily satisfy the bound from solar system tests of gravity. Finally we show that the two gravitons present in the spectrum have a non-trivial mass matrix whose origin follows from the potential term of bigravity. This mixing leads to gravitational birefringence.

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Section: Introductionmentioning

confidence: 93%

Dark energy and doubly coupled bigravity

Brax

Davis

Noller

2017

Class. Quantum Grav.

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“…2014a; Lagos and Ferreira 2014; Cusin et al. 2015a, b, 2016; Akrami et al. 2015; Schmidt-May and von Strauss 2016).…”

Section: Types Of Modifications To Gr At Cosmological Scales and Corrmentioning

confidence: 99%

“…Rosen (2011, 2012a,b); Koennig et al (2014b,a); Akrami et al (2015). These theories have a branch of models that admit a limit in which the Planck mass associated to the second metric is small and any scalar instabilities can be pushed to very early times where they are not observable (Koennig et al 2014a;Lagos and Ferreira 2014;Cusin et al 2015a;Akrami et al 2015;Cusin et al 2015b;Schmidt-May and von Strauss 2016;Cusin et al 2016). Even if in this limit the background evolution becomes indistinguishable from that of the ΛCDM, Akrami et al (2015) state that it provides a technically natural value for the effective cosmological constant.…”

Section: Illustrative Example 2: Bimetric Massive Gravity or Bigravitymentioning

confidence: 99%

Testing general relativity in cosmology

2018

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We review recent developments and results in testing general relativity (GR) at cosmological scales. The subject has witnessed rapid growth during the last two decades with the aim of addressing the question of cosmic acceleration and the dark energy associated with it. However, with the advent of precision cosmology, it has also become a well-motivated endeavor by itself to test gravitational physics at cosmic scales. We overview cosmological probes of gravity, formalisms and parameterizations for testing deviations from GR at cosmological scales, selected modified gravity (MG) theories, gravitational screening mechanisms, and computer codes developed for these tests. We then provide summaries of recent cosmological constraints on MG parameters and selected MG models. We supplement these cosmological constraints with a summary of implications from the recent binary neutron star merger event. Next, we summarize some results on MG parameter forecasts with and without astrophysical systematics that will dominate the uncertainties. The review aims at providing an overall picture of the subject and an entry point to students and researchers interested in joining the field. It can also serve as a quick reference to recent results and constraints on testing gravity at cosmological scales.

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“…However, once again, their application to cosmology has shown how non-trivial is to build an IR modification of GR with a stable evolution at the background level, as well as at the level of cosmological perturbations; indeed, massive gravity has difficulties already in obtaining a viable background FRW evolution [108]. In bigravity background FRW solutions exist, but, in a branch of solutions that has a dynamical dark energy, the cosmological perturbations are plagued by instabilities in both the scalar and tensor sectors; in a second branch, taking the limit in which the Planck mass associated to the second metric is small, the scalar instabilities can be pushed to unobservably early times, but in this limit the background evolution becomes indistinguishable from that of ΛCDM [109][110][111][112][113][114][115].…”

Section: Bayesian Parameter Estimation and Model Comparisonmentioning

confidence: 99%