Non-singular bouncing cosmology in f(T,T) gravity with energy condition violations

Abstract

The singularity and inflationary problems have posed significant challenges for understanding the universe’s origin and evolution. Bouncing cosmology has emerged as a promising alternative to standard cosmological models, offering a non-singular approach to early universe dynamics by facilitating a ”bounce” rather than a singular beginning. In this study, we explore the feasibility of modeling specific bouncing scenarios within the framework of $f(T,T)$ gravity, allowing for a comprehensive coupling between the torsion scalar $T$ and the trace of the energy–momentum tensor $T$. We analyze two $f(T,T)$ models: a linear model $f(T,T)=\alpha T+\beta T$ and a non-linear model $f(T,T)=\alpha \sqrt{-T}+\beta T$, with a parameterized scale factor $a\left(t\right)=\sqrt{a_0^2+{\gamma }^2{t}^2}$ to capture the bounce behavior. The analysis confirms a cosmic bounce at $t=0$, where the Hubble parameter $H=0$ signals a transition from contraction to expansion. A crucial condition for achieving the bounce is the violation of the null energy condition (NEC) near the bounce, enabling the equation of state (EoS) parameter to enter the phantom region ($\omega <-1$). Both models exhibit an increase in energy density as the universe approaches the bounce, peaking at the bounce epoch and then decreasing post-bounce. Pressure remains negative throughout, with the EoS parameter crossing into the phantom region near the bounce in both positive and negative time zones. Our findings show that NEC and strong energy condition (SEC) violations are essential for the non-singular bounce, while the dominant energy condition (DEC) is satisfied, ensuring a consistent matter distribution. These results indicate that both linear and non-linear $f(T,T)$ models effectively replicate the critical features of a bouncing cosmology, offering valuable approaches for addressing the singularity and inflationary challenges in cosmology.