Abstract
We study cosmology in the bigravity formulation of the dRGT model where matter couples to both metrics. At linear order in perturbation theory two mass scales emerge: an hard one from the dRGT potential, and an environmental dependent one from the coupling of bigravity with matter. At early time, the dynamics is dictated by the second mass scale which is of order of the Hubble scale. The set of gauge invariant perturbations that couples to matter follow closely the same behaviour as in GR . The remaining perturbations show no issue in the scalar sector, while problems arise in the tensor and vector sectors. During radiation domination, a tensor mode grows power-like at super-horizon scales. More dangerously, the only propagating vector mode features an exponential instability on sub-horizon scales. We discuss the consequences of such instabilities and speculate on possible ways to deal with them.
Highlights
Introduction and SummaryThe physical mechanism responsible for the present day acceleration of our universe is unknown
As far as homogeneous configurations are concerned, in bigravity theories the situation drastically improves with respect to massive gravity, and one finds branches of flat FRW solutions
Leaving aside the ones with a curvature singularity at late time with c < 0 (see eq (20)) – as discussed in [18] and more recently in [43, 22, 23] – regular background solutions features an exponential instability in the scalar sector at early time, quickly invalidating perturbation theory already during radiation domination [20]
Summary
The physical mechanism responsible for the present day acceleration of our universe is unknown. It has been proposed to extend the theory of massive (bi)gravity by considering a more general coupling to matter – called doubly matter coupling – in which the physical metric coupled to the matter energy momentum tensor is an appropriate linear combination of the two metrics [24, 25] (see [26, 27] for different approaches) This scenario, problematic for massive gravity [28], is potentially interesting in the bigravity setup since a qualitatively new branch of FRW cosmological solutions exists [29], its cosmological perturbations deserve to be investigated. We speculate on possible extension of the bigravity theory under consideration, that might be able to cure such instability problems
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
