A quantitative analysis of singular inflation with scalar-tensor and modified gravity

TL;DR

This paper analyzes singular inflation within scalar-tensor and modified gravity frameworks, examining how finite-time singularities, especially Type IV, influence inflationary parameters and their consistency with observational data.

Contribution

It provides a detailed quantitative analysis of singular inflation, classifies finite-time singularities, and explores their effects on inflationary observables in scalar-tensor theories.

Findings

Type IV singularities can align with observational data if occurring at or after inflation.

Singularities during inflation lead to divergent observational indices, invalidating the model.

Type IV singularities can be incorporated into late-time cosmological evolution.

Abstract

We provide a detailed quantitative description of singular inflation. Its close analogy with finite-time future singularity which is associated to dark energy era is described. Calling and classifying the singularities of such inflation as finite-time cosmological singularities we investigate their occurrence, with special emphasis on the Type IV singularity. The study is performed in the context of a general non-canonical scalar-tensor theory. In addition, the impact of finite time singularities on the slow-roll parameters is also investigated. Particularly, we study three cases, in which the singularity occurs during the inflationary era, at the end, and also we study the case that the singularity occurs much more later than inflation ends. Using the obtained slow-roll parameters, for each case, we calculate explicitly the spectral index of primordial curvature perturbations $n_s$, the associated running of the spectral index $a_s$ and of the scalar-to-tensor ratio $r$ and compare the resulting values to the Planck and BICEP2 data. As we demonstrate, in some cases corresponding to the Type IV singularity, there might be the possibility of agreement with the observational data, when the singularity occurs at the end, or after inflation. However, absolute concordance of all observational indices is not achieved. On the contrary, if the singularity occurs during the inflationary era, this is catastrophic for the theory, since the observational indices become divergent. We also show how a Type IV singularity may be consistently accommodated in the Universe's late time evolution.