Hypergeometric viable models in $f(R)$ gravity

TL;DR

This paper develops a hypergeometric model in $f(R)$ gravity that aligns with $ ext{Lambda}$CDM cosmology, analyzing its viability through phase space and differential equations, and relates known models to a general hypergeometric framework.

Contribution

It introduces a general hypergeometric $f(R)$ model encompassing known viable models, providing a new perspective on model construction and viability analysis.

Findings

Starobinsky and Hu-Sawicki models are special cases of the hypergeometric model.

The model's parameter space was analyzed for viability.

Differential equations relate deviations from General Relativity to hypergeometric functions.

Abstract

A cosmologically viable hypergeometric model in the modified gravity theory $f(R)$ is found from the need for asintoticity towards $\Lambda$CDM, the existence of an inflection point in the $f(R)$ curve, and the conditions of viability given by the phase space curves $(m, r)$, where $m$ and $r$ are characteristic functions of the model. To analyze the constraints associated with the viability requirements, the models were expressed in terms of a dimensionless variable, i.e. $R\to x$ and $f(R)\to y(x)=x+h(x)+\lambda$, where $h(x)$ represents the deviation of the model from General Relativity. Using the geometric properties imposed by the inflection point, differential equations were constructed to relate $h'(x)$ and $h''(x)$, and the solutions found were Starobinsky (2007) and Hu-Sawicki type models, nonetheless, it was found that these differential equations are particular cases of a hypergeometric differential equation, so that these models can be obtained from a general hypergeometric model. The parameter domains of this model were analyzed to make the model viable.