Extended matter bounce scenario in ghost free f ( R , G ) gravity compatible with GW170817

摘要

In the context of a ghost freef(R,G)model, an extended matter bounce scenario is studied where the form of the scale factor is given bya(t)=(a0t2+1)n. The ghost free character of the model is ensured by the presence of a Lagrange multiplier, as developed in . The conditions under which, in this model, the speed of gravitational waves becomes equal to the speed of light (equal to one, in natural units), thus becoming compatible with the striking event GW170817, is investigated. It is shown that this happens for a class of Gauss-Bonnet (GB) coupling functions (h(t)) which satisfies a constraint equation of the formh¨=h˙H, withHthe Hubble parameter. This constraint is then imposed on the ghost freef(R,G)gravity theory to be consistent with the GW170817 event, subsequently, the corresponding non-singular bouncing cosmology with the aforementioned scale factor is extensively studied. The forms of the coupling function and Lagrange multiplier in the “low curvature limit” of the theory are reconstructed, yielding a viable approximation forn<1/2. Correspondingly, by solving the cosmological perturbation equation, the main observable quantities, namely the spectral index, tensor to scalar ratio, and the running index are determined and confronted with the latest Planck 2018 data. Consistency with the data is proven for those parametric regimes that which correspond ton<1/2. This makes the low curvature approximation a viable one for calculating the scalar and tensor power spectra.