Adam R. Solomon

Stable and unstable cosmological models in bimetric massive gravity

Nonlinear, ghost-free massive gravity has two tensor fields; when both are dynamical, the mass of the graviton can lead to cosmic acceleration that agrees with background data, even in the absence of a cosmological constant. Here the question of the stability of linear perturbations in this bimetric theory is examined. Instabilities are presented for several classes of models, and simple criteria for the cosmological stability of massive bigravity are derived. In this way, we identify a particular self-accelerating bigravity model, infinite-branch bigravity (IBB), which exhibits both viable background evolution and stable linear perturbations. We discuss the modified gravity parameters for IBB, which do not reduce to the standard

Bimetric gravity is cosmologically viable

\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}

Linear growth of structure in massive bigravity

result at early times, and compute the combined likelihood from measured growth data and type Ia supernovae. IBB predicts a present matter density

The nature of spacetime in bigravity: Two metrics or none?

{\mathrm{\ensuremath{\Omega}}}_{m0}=0.18

Integrated Sachs-Wolfe effect in massive bigravity

and an equation of state

Inflationary power asymmetry from primordial domain walls

w(z)=\ensuremath{-}0.79+0.21z/(1+z)

Instabilities in tensorial nonlocal gravity

. The growth rate of structure is well approximated at late times by

Solitons in generalized Galileon theories

f(z)\ensuremath{\approx}{\mathrm{\ensuremath{\Omega}}}_{m}^{0.47}[1+0.21z/(1+z)]

Superfluids and the Cosmological Constant Problem

. The implications of the linear instability for other bigravity models are discussed: the instability does not necessarily rule these models out, but rather presents interesting questions about how to extract observables from them when linear perturbation theory does not hold.

WKB approximation and tunneling in theories with noncanonical kinetic terms

Bimetric theory describes gravitational interactions in the presence of an extra spin-2 field. Previous work has suggested that its cosmological solutions are generically plagued by instabilities. ...