Speaker
Dr
Ryo Namba
(Kavli IPMU, University of Tokyo)
Description
A fully non-linear massive gravity was recently constructed by
de Rham, Gabadadze and Tolley (dRGT), removing the unwanted
extra degree
of freedom condemned by ghost instability, known as
Boulware-Deser (BD)
ghost. It was, however, soon found that homogeneous and
isotropic
solutions with accelerated expansion in this theory suffers
new-type
ghost instabilities at the non-linear level. The bigravity
theories as
an extension of the dRGT massive gravity promote the
non-dynamical
fiducial metric in the dRGT theory to a dynamical quantity.
The class of
these theories with each matter sector coupling to only one
of the
physical and fiducial metrics preserves the structure that
guarantees
the absence of the BD ghost, and it has been shown to have some
parameter space for cosmological solutions. While these
solutions are
stable against perturbations for the graviton mass greater
than the
Hubble expansion rate, a linear analysis exhibits ghosts for
the scalar
graviton in the opposite regime, indicating an instability
in the early
universe. We extend the analysis including non-linear
effects from self
interactions of the scalar graviton. We study the stability
of the
non-linear solutions, exploring the possibility to evade the
instability
at both early and late times.
