Constraint closure and gravitational-wave content of shear-free cosmologies in metric f(R) gravity

Abstract

We derive the consistency conditions governing the propagating content of linear shear-free perturbations of Friedmann-Lema\^ıtre-Robertson-Walker cosmologies in metric $f(R)$ gravity. The divergence of the shear-free constraint is shown to be the total momentum-conservation equation and therefore does not imply geodesic flow. Its projected time derivative instead supplies a nontrivial integrability condition. Scalar, vector, and tensor sectors must then be tested separately. Shear-freeness removes the independent transverse-traceless electric-magnetic Weyl pair, so no ordinary $+$ or $\times$ tensor gravitational wave survives. The scalaron is not removed algebraically, but its four-variable harmonic system must remain in the time-dependent consistent subspace generated by the shear-free constraint and all of its time derivatives. For a geodesic shear-free congruence in the spatially flat expanding de Sitter patch, this excludes every nonzero fixed comoving scalar harmonic, including the healthy $R+αR^2-2Λ$ model with $α,Λ>0$. Vector modes obey a curvature-dependent global eigenvalue condition whose right-hand side is negative for positive-density matter with $f&#39;>0$ and $1+w>0$; hence no nonzero vector harmonic survives on that branch. The familiar coasting $R^3$ example evades this sign obstruction only because it lies on an $f&#39;<0$ branch. Thus tensor radiation is absent in the imposed shear-free sector, whereas scalar radiation is not excluded kinematically but remains a model- and background-dependent constraint-closure question.