Lorenzo Sebastiani

Class of viable modified f(R) gravities describing inflation and the onset of accelerated expansion

A general approach to viable modified f(R) gravity is developed in both the Jordan and the Einstein frames. A class of exponential, realistic modified gravities is introduced and investigated with care. Special focus is made on step-class models, most promising from the phenomenological viewpoint and which provide a natural way to classify all viable modified gravities. One- and two-step models are explicitly considered, but the analysis is extensible to N-step models. Both inflation in the early universe and the onset of recent accelerated expansion arise in these models in a natural, unified way. Moreover, it is demonstrated that models in this category easily pass all local tests, including stability of spherical body solution, nonviolation of Newtons law, and generation of a very heavy positive mass for the additional scalar degree of freedom.

Finite-time future singularities in modified Gauss–Bonnet and ℱ(R,G) gravity and singularity avoidance

We study all four types of finite-time future singularities emerging in the late-time accelerating (effective quintessence/phantom) era from ℱ(R,G)-gravity, where R and G are the Ricci scalar and the Gauss–Bonnet invariant, respectively. As an explicit example of ℱ(R,G)-gravity, we also investigate modified Gauss–Bonnet gravity, so-called F(G)-gravity. In particular, we reconstruct the F(G)-gravity and ℱ(R,G)-gravity models where accelerating cosmologies realizing the finite-time future singularities emerge. Furthermore, we discuss a possible way to cure the finite-time future singularities in F(G)-gravity and ℱ(R,G)-gravity by taking into account higher-order curvature corrections. The example of non-singular realistic modified Gauss–Bonnet gravity is presented. It turns out that adding such non-singular modified gravity to singular Dark Energy makes the combined theory a non-singular one as well.

Nonsingular exponential gravity: A simple theory for early- and late-time accelerated expansion

A theory of exponential modified gravity which explains both early-time inflation and late-time acceleration, in a unified way, is proposed. The theory successfully passes the local tests and fulfills the cosmological bounds and, remarkably, the corresponding inflationary era is proven to be unstable. Numerical investigation of its late-time evolution leads to the conclusion that the corresponding dark energy epoch is not distinguishable from the one for the {Lambda}CDM model. Several versions of this exponential gravity, sharing similar properties, are formulated. It is also shown that this theory is nonsingular, being protected against the formation of finite-time future singularities. As a result, the corresponding future universe evolution asymptotically tends, in a smooth way, to de Sitter space, which turns out to be the final attractor of the system.

Trace-anomaly driven inflation in modified gravity and the BICEP2 result

We explore conformal-anomaly driven inflation in

F(R)-gravity and inflation

F(R)

Inflationary universe from higher-derivative quantum gravity

gravity without invoking the scalar-tensor representation. We derive the stress-energy tensor of the quantum anomaly in the flat homogeneous and isotropic universe. We investigate a suitable toy model of exponential gravity plus the quantum contribution due to the conformal anomaly, which leads to the de Sitter solution. It is shown that in

Mimetic gravity: a review of recent developments and applications to cosmology and astrophysics

F(R)

Cosmic history of viable exponential gravity: Equation of state oscillations and growth index from inflation to dark energy era

gravity model, the curvature perturbations with its enough amplitude consistent with the observations are generated during inflation. We also evaluate the number of

Energy issue for a class of modified higher order gravity black hole solutions

e

f(ϕ)R-models for inflation

-folds at the inflationary stage and the spectral index