Cosmological singularity theorems for $f(R)$ gravity theories

TL;DR

This paper extends Hawking's singularity theorems to $f(R)$ gravity theories, establishing conditions under which spacetime geodesic incompleteness and singularities occur, applicable to models like Hu-Sawicki and Starobinsky.

Contribution

It generalizes classical singularity theorems to $f(R)$ theories, providing new conditions for geodesic incompleteness in these modified gravity models.

Findings

Generalized singularity theorems for $f(R)$ gravity.

Applicable to specific models like Hu-Sawicki and Starobinsky.

Conditions under which spacetime is geodesically incomplete.

Abstract

In the present work some generalizations of the Hawking singularity theorems in the context of $f(R)$ theories are presented. The assumptions are of these generalized theorems is that the matter fields satisfy the conditions $\bigg(T_{ij}-\frac{g_{ij}}{2} T\bigg)k^i k^j\geq 0$ for any generic unit time like field, that the scalaron takes bounded positive values during its evolution, and that the resulting space time is globally hyperbolic. Then, if there exist a Cauchy hyper surface $\Sigma$ for which the expansion parameter $\theta$ of the geodesic congruence emanating orthogonally from $\Sigma$ satisfies some specific conditions, it may be shown that the resulting space time is geodesically incomplete. Some mathematical results of reference \cite{fewster} are very important for proving this. The generalized theorems presented here apply directly some specific models such as the Hu-Sawicki or Starobinsky ones \cite{especif3}, \cite{capoziello4}. However, for other scenarios, some extra assumptions should be implemented for the geodesic incompleteness to take place. However, the negation of the hypothesis of these results does not necessarily imply that a singularity is absent, but that other mathematical results should be considered to prove that.