Curing singularities in cosmological evolution of F(R) gravity

TL;DR

Problem: Weak curvature singularities and scalaron instabilities in viable $F(R)$ dark-energy models. Approach: introduce a small $R^2$ correction at large $R$ to cure the singularity, bound the scalaron mass, and remove scalaron overabundance, enabling a combined inflationary and late-time de Sitter stage. Findings: the $R^2$ term stabilizes high-curvature dynamics, requires adjustments in the low-$R$ and negative-$R$ regime to maintain stability, and alters reheating and the primordial power spectrum relative to the classic $R^2$ inflation model because the effective gravitational constant differs for positive and negative $R$. Significance: demonstrates a viable, testable route to unify primordial and present-day acceleration within $F(R)$ gravity and to predict modified early-universe observables.

Abstract

We study $F(R)$ modified gravity models which are capable of driving the accelerating epoch of the Universe at the present time whilst not destroying the standard Big Bang and inflationary cosmology. Recent studies have shown that a weak curvature singularity with $|R|\to\infty$ can arise generically in viable $F(R)$ models of present dark energy (DE) signaling an internal incompleteness of these models. In this work we study how this problem is cured by adding a quadratic correction with a sufficiently small coefficient to the $F(R)$ function at large curvatures. At the same time, this correction eliminates two more serious problems of previously constructed viable $F(R)$ DE models: unboundedness of the mass of a scalar particle (scalaron) arising in $F(R)$ gravity and the scalaron overabundance problem. Such carefully constructed models can also yield both an early time inflationary epoch and a late time de Sitter phase with vastly different values of $R$. The reheating epoch in these combined models of primordial and present dark energy is completely different from that of the old $R + R^{2}/6M^{2}$ inflationary model, mainly due to the fact that values of the effective gravitational constant at low and intermediate curvatures are different for positive and negative $R$. This changes the number of e-folds during the observable part of inflation that results in a different value of the primordial power spectrum index.