$f(R,{T_{\mu\nu} T^{\mu\nu}})$ gravity and Cardassian-like expansion as one of its consequences

TL;DR

This paper introduces a modified gravity model replacing the Ricci scalar with a function of R and the energy-momentum tensor norm, leading to non-geodesic motion, potential explanations for cosmic acceleration, and implications for early universe nucleosynthesis.

Contribution

The paper proposes a novel $f(R,T_{ u u}T^{ u u})$ gravity model with unique field equations and explores its cosmological and astrophysical consequences, including accelerated expansion without dark energy.

Findings

Test particles experience non-geodesic motion due to extra force.

Model can account for accelerated universe without cosmological constant.

Deviation from GR in perihelion precession unless energy density is constant.

Abstract

We propose a new model of gravity where the Ricci scalar (R) in Einstein-Hilbert action is replaced by an arbitrary function of R and of the norm of energy-momentum tensor i.e., $f(R,T_{\mu\nu}T^{\mu\nu})$. Field equations are derived in the metric formalism. We find that the equation of motion of massive test particles is non-geodesic and these test particles are acted upon by a force which is orthogonal to the four-velocity of the particles. We also find the Newtonian limit of the model to calculate the extra acceleration which can affect the perihelion of Mercury. There is a deviation from the general relativistic(GR) result unless the energy density of fluid is constant. Arranging $\alpha$ parameter gives an opportunity to cure the inconsistency between the observational values for the abundance of light elements and the standard Big Bang Nucleosynthesis results. Even the dust dominated universe undergoes an accelerated expansion without using a cosmological constant in Model II. With this specific choice of $f(R,T_{\mu\nu}T^{\mu\nu})$, we get the a Cardassian-like expansion.