Constraints on energy-momentum squared gravity from neutron stars and its cosmological implications.

Deviations from the predictions of general relativity due to energy-momentum squared gravity (EMSG) are expected to become pronounced in the high density cores of neutron stars. We derive the hydrostatic equilibrium equations in EMSG and solve them numerically to obtain the neutron star mass-radius relations for four different realistic equations of state. We use the existing observational measurements of the masses and radii of neutron stars to constrain the free parameter, $\alpha$, that characterises the coupling between matter and spacetime in EMSG. We show that $-10^{-38}\,cm^{3}/erg<\alpha <+10^{-37}\,cm^{3}/erg$. Under this constraint, we discuss how EMSG alters the dynamics of the early universe from the predictions of the standard cosmological model. We show that EMSG leaves the standard cosmology safely unaltered back to $t\sim 10^{-4}$ seconds at which the energy density of the universe is $\sim 10^{34}\,erg\,cm^{-3}$.