Constraining the $f$-mode oscillations frequency in Neutron Stars through Universal Relations in the realm of Energy-Momentum Squared Gravity

Abstract: Neutron stars (NSs), superdense objects with exceptionally strong gravitational fields, provide an ideal laboratory for probing general relativity (GR) in the high-curvature regime. They also present an exciting opportunity to explore new gravitational physics beyond the traditional framework of GR. Thus, investigating alternative theories of gravity in the context of superdense stars is intriguing and essential for advancing our understanding of gravitational phenomena in extreme environments. Energy-Momentum Squared Gravity (EMSG) is a modified theory of gravity that extends GR by including nonlinear terms involving the energy-momentum tensor $T_{\mu \nu}$. This study examines the effects of EMSG on the properties and behaviour of NSs by varying the free parameter $\alpha$. The hydrostatic equilibrium equations in the EMSG framework are derived and solved numerically to obtain mass-radius relations for soft, stiff, and intermediate equations of state (EOS). Observational measurements of NS masses and radii are used to constrain the fundamental-mode ($f$-mode) oscillation frequency through its universal relation with the tidal Love number and compactness. Results indicate that the Stiff EOS undergoes a phase transition at the highest energy densities and pressures, followed by the Intermediate and Soft EOSs, highlighting the distinctive characteristics of these models. Additionally, the study explores the impact of EOS choice on the sound speed profile of NSs, reaffirming the physical validity of the models across varying $\alpha$ values.