Observational discrimination of Eddington-inspired Born-Infeld gravity from general relativity

Direct observations of neutron stars could tell us an imprint of modified gravity. However, it is generally difficult to resolve the degeneracy due to the uncertainties in equation of state (EOS) of neutron star matter and in gravitational theories. In this paper, we are successful to find the observational possibility to distinguish Eddington-inspired Born-Infeld gravity (EiBI) from general relativity. We show that the radii of neutron stars with $0.5M_{sun}$ are strongly correlated with the neutron skin thickness of ${}^{208}$Pb independently of EOS, while this correlation depends on the coupling constant in EiBI. As a result, via the direct observations of radius of neutron star with $0.5M_{sun}$ and the measurements of neutron skin thickness of ${}^{208}$Pb by the terrestrial experiments, one could not only discriminate EiBI from general relativity but also estimate the coupling constant in EiBI.Comment: accepted for publication in PR

Stellar oscillations in Eddington-inspired Born-Infeld gravity

We consider the stellar oscillations of relativistic stars in the Eddington-inspired Born-Infeld gravity (EiBI). In order to examine the specific frequencies, we derive the perturbation equations governing the stellar oscillations in EiBI by linearizing the field equations, and numerically determine the oscillation frequencies as changing the coupling parameter in EiBI, $\kappa$, and stellar models. As a result, we find that the frequencies depend strongly on the value of $\kappa$, where the frequencies in EiBI with negative $\kappa$ become higher and those with positive $\kappa$ become lower than the expectations in general relativity. We also find that, via the observation of the fundamental frequency, one could distinguish EiBI with $8\pi\epsilon_0|\kappa|\gtrsim 0.03$ from general relativity, independently of the equation of state (EOS) for neutron star matter, where $\epsilon_0$ denotes the nuclear saturation density and $\epsilon_0\kappa$ become dimensionless parameter. With the further constraints on EOS, one might distinguish EiBI even with $8\pi\epsilon_0|\kappa|\lesssim 0.03$ from general relativity.Comment: accepted for publication in PR

Alfv\'en Polar Oscillations of Relativistic Stars

We study polar Alfv\'en oscillations of relativistic stars endowed with a strong global poloidal dipole magnetic field. Here we focus only on the axisymmetric oscillations which are studied by evolving numerically the two-dimensional perturbation equations. Our study shows that the spectrum of the polar Alfv\'{e}n oscillations is discrete in contrast to the spectrum of axial Alfv\'{e}n oscillations which is continuous. We also show that the typical fluid modes, such as the f and p modes, are not significantly affected by the presence of the strong magnetic field.Comment: 10 pages, 5 figure