Dark energy and doubly coupled bigravity

Brax, Philippe; Davis, Anne-Christine; Noller, Johannes

doi:10.1088/1361-6382/aa6856

Cited by 8 publications

(32 citation statements)

References 59 publications

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“…(119) Thus, the mode V −i shows a gradient instability on subhorizon scales in the radiation and matter eras, wherē p =p γ > 0, and we recover the results obtained in [13][14][15].…”

Section: Its Equation Of Motion Readssupporting

confidence: 85%

“…We can obtain their equations of motion from the nonlinear Einstein equations (19), starting at the level of the vierbeins. In the case of a constant scalar field ϕ we recover the results obtained from the quadratic action at the level of the metrics in [13][14][15]. This gives for the first graviton h 1ij and the equation of motion of the second graviton h 2ij is given by the permutation 1 ↔ 2.…”

Section: G Solutions With Different Conformal Timessupporting

confidence: 72%

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Self-acceleration in scalar-bimetric theories

Brax

Valageas

2018

Phys. Rev. D

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We describe scalar-bimetric theories where the dynamics of the Universe are governed by two separate metrics, each with an Einstein-Hilbert term. In this setting, the baryonic and dark matter components of the Universe couple to metrics which are constructed as functions of these two gravitational metrics. More precisely, the two metrics coupled to matter are obtained by a linear combination of their vierbeins, with scalar-dependent coefficients. The scalar field, contrary to dark-energy models, does not have a potential whose role is to mimic a late-time cosmological constant. The late-time acceleration of the expansion of the Universe can be easily obtained at the background level in these models by appropriately choosing the coupling functions appearing in the decomposition of the vierbeins for the baryonic and dark matter metrics. We explicitly show how the concordance model can be retrieved with negligible scalar kinetic energy. This requires the scalar coupling functions to show variations of order unity during the accelerated expansion era. This leads in turn to deviations of order unity for the effective Newton constants and a fifth force that is of the same order as Newtonian gravity, with peculiar features. The baryonic and dark matter self-gravities are amplified although the gravitational force between baryons and dark matter is reduced and even becomes repulsive at low redshift. This slows down the growth of baryonic density perturbations on cosmological scales, while dark matter perturbations are enhanced. These scalarbimetric theories have a perturbative cutoff scale of the order of one astronomical unit (a.u.), which prevents a precise comparison with Solar System data. On the other hand, we can deduce strong requirements on putative Ultra Violet (UV) completions by analyzing the stringent constraints in the Solar System. Hence, in our local environment, the upper bound on the time evolution of Newton's constant requires an efficient screening mechanism that both damps the fifth force on small scales and decouples the local value of Newton constant from its cosmological value. This cannot be achieved by a quasistatic chameleon mechanism, and requires going beyond the quasistatic regime and probably using derivative screenings, such as Kmouflage or Vainshtein screening, on small scales.

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“…(119) Thus, the mode V −i shows a gradient instability on subhorizon scales in the radiation and matter eras, wherē p =p γ > 0, and we recover the results obtained in [13][14][15].…”

Section: Its Equation Of Motion Readssupporting

confidence: 85%

Section: G Solutions With Different Conformal Timessupporting

confidence: 72%

Self-acceleration in scalar-bimetric theories

Brax

Valageas

2018

Phys. Rev. D

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“…In Refs. [76,77] the interaction sector has been written in terms of the constrained metric vierbeins as , m f f f g = β 3 24…”

Section: Discussionmentioning

confidence: 99%

“…This doubly coupled theory has been shown to provide viable and interesting cosmological solutions at the background level [35,73], with linear perturbations that are stable at least around specific cosmological backgrounds [80] (see also Refs. [69,[76][77][78]). In particular, in contrast to the singly coupled theory, this double coupling admits combinations of proportional metrics at the background level, and interestingly, the effective metric always corresponds to the massless fluctuations around such backgrounds, i.e.…”

Section: Introductionmentioning

confidence: 99%

Neutron star merger GW170817 strongly constrains doubly coupled bigravity

et al. 2018

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We study the implications of the recent detection of gravitational waves emitted by a pair of merging neutron stars and their electromagnetic counterpart, events GW170817 and GRB170817A, on the viability of the doubly coupled bimetric models of cosmic evolution, where the two metrics couple directly to matter through a composite, effective metric. We demonstrate that the bounds on the speed of gravitational waves place strong constraints on the doubly coupled models, forcing either the two metrics to be proportional at the background level or the models to become singly coupled. Proportional backgrounds are particularly interesting as they provide stable cosmological solutions with phenomenologies equivalent to that of ΛCDM at the background level as well as for linear perturbations, while nonlinearities are expected to show deviations from the standard model.

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“…Hence if the low l plateau characteristic of a massive graviton were observed, a shift in the position of the first few peaks would be representative of the presence of another graviton mode. On the other hand, these models also suffer from a gradient instability [17,32,33], possibly lethal, in the vector sector whose study is left for future work.…”

Section: Introductionmentioning

confidence: 99%

Signatures of graviton masses on the CMB

Brax

Céspedes

Davis

2018

J. Cosmol. Astropart. Phys.

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The impact of the existence of gravitons with non-vanishing masses on the B modes of the Cosmic Microwave Background (CMB) is investigated. We also focus on putative modifications to the speed of the gravitational waves. We find that a change of the graviton speed shifts the acoustic peaks of the CMB and then could be easily constrained. For the case of massive gravity, we show analytically how the B modes are sourced in a manner differing from the massless case leading to a plateau at low l in the CMB spectrum. We also study the case when there are more than one graviton, and when pressure instabilities are present. The latter would occur in doubly coupled bigravity in the radiation era. We focus on the case where a massless graviton becomes tachyonic in the radiation era whilst a massive one remains stable. As the unstable mode decouples from matter in the radiation era, we find that the effects of the instability is largely reduced on the spectrum of B-modes as long as the unstable graviton does not grow into the non-linear regime. In all cases when both massless and massive gravitons are present, we find that the B-mode CMB spectrum is characterised by a low l plateau together with a shifted position for the first few peaks compared to a purely massive graviton spectrum, a shift which depends on the mixing between the gravitons in their coupling to matter and could serve as a hint in favour of the existence of multiple gravitons.

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Dark energy and doubly coupled bigravity

Cited by 8 publications

References 59 publications

Self-acceleration in scalar-bimetric theories

Self-acceleration in scalar-bimetric theories

Neutron star merger GW170817 strongly constrains doubly coupled bigravity

Signatures of graviton masses on the CMB

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