Abstract
The recently proposed chameleonic extension of bigravity theory, by including a scalar field dependence in the graviton potential, avoids several fine-tunings found to be necessary in usual massive bigravity. In particular it ensures that the Higuchi bound is satisfied at all scales, that no Vainshtein mechanism is needed to satisfy solar-system experiments, and that the strong coupling scale is always above the scale of cosmological interest all the way up to the early universe. This paper extends the previous work by presenting a stable example of cosmology in the chameleon bigravity model. We find a set of initial conditions and parameters such that the derived stability conditions on general flat Friedmann background are satisfied at all times. The evolution goes through radiation dominated, matter dominated, and de Sitter eras. We argue that the parameter space allowing for such a stable evolution may be large enough to encompass an observationally viable evolution. We also argue that our model satisfies all known constraints due to gravitational wave observations so far and thus can be considered as a unique testing ground of gravitational wave phenomenologies in bimetric theories of gravity.
Highlights
Bimetric theories are an intensively studied class of massive gravity theories considered as an alternative to general relativity (GR)
We find a set of initial conditions and parameters such that the derived stability conditions on general flat Friedmann background are satisfied at all times
Following the recent proposal [9] of an extended massive bigravity theory supplemented by a chameleon scalar field as a means to cure or evade the fine-tunings of the original theory and improve its applicability, we have found it important to study further its validity and implications
Summary
Bimetric theories are an intensively studied class of massive gravity theories considered as an alternative to general relativity (GR). In response to these practical issues, it has been recently proposed to add a new chameleonlike degree of freedom to the theory [9] In this model, the constant coefficients appearing in the graviton potential are promoted to be general functions of the new scalar field φ, and matter is coupled to gravity through a φ-dependent effective metric. The constant coefficients appearing in the graviton potential are promoted to be general functions of the new scalar field φ, and matter is coupled to gravity through a φ-dependent effective metric In this way, the effective graviton mass mT becomes environment dependent, so that m2T scales as the local energy density of matter ρ. We present a viable set of parameters and initial conditions that upon numerical integration leads to a stable cosmological evolution, including radiation-dominated, matter-dominated, and de Sitter phases.
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