Do sewn singularities falsify the Palatini cosmology?

TL;DR

This paper analyzes the dynamics of the Starobinsky $R+ extgamma R^2$ model in Palatini formalism with Chaplygin gas, revealing singularities and bifurcations that challenge negative $ extgamma$ scenarios and align with $ extLambda$CDM.

Contribution

It introduces a dynamical systems approach to classify singularities and phase space trajectories in Palatini $f(R)$ cosmology with Chaplygin gas, highlighting the impact of $ extgamma$ sign on universe evolution.

Findings

Degenerate freeze singularities are common for positive $ extgamma$.

Negative $ extgamma$ leads to bounce solutions and sudden singularities.

Negative $ extgamma$ is inconsistent with observational data and $ extLambda$CDM.

Abstract

We investigate further (cf. arXiv:1512.01199, JCAP01 (2016) 040) Starobinsky cosmological model $R+\gamma R^2$ in the Palatini formalism with Chaplygin gas and baryonic matter as a source. For this aim we use dynamical system theory. The dynamics is reduced to the 2D sewn dynamical system of a Newtonian type (a piecewise-smooth dynamical system). We classify all evolutional paths in the model as well as trajectories in the phase space. We demonstrate the presence of a degenerate freeze singularity (glued freeze type singularities) for the positive $\gamma$. In this case it is a generic feature of early evolution of the universe. We point out that a degenerate type III of singularity can be considered as an endogenous model of inflation between the matter dominating epoch and the dark energy phase. We also investigate cosmological models with negative $\gamma$. It is demonstrated that $\gamma$ equal zero is a bifurcation parameter and dynamics qualitatively changes in comparison to positive $\gamma$. Instead of the big bang the sudden bounce singularity of a finite scale factor appears and there is a generic class of bouncing solutions sewn along the line $a=a_{\text{sing}}$. And we argue that the presence of sudden singularities in an evolutional scenario of the Universe falsifies the negative $\gamma$ in the Palatini cosmology. Only very small values of $\Omega_{\gamma}$ parameter are admissible if we requires that agreements physics with the $\Lambda$CDM model. From the statistical analysis of astronomical observations, we deduce that the case of negative values of $\Omega_\gamma$ can be rejected even if it may fit better to the data.