Neutron Stars as Laboratories for Gravity Physics

Abstract

We study the structure of neutron stars in R + alpha R-2 gravity model with perturbative method. We obtain mass-radius relations for four representative equations of state (EoS). We find that, for vertical bar alpha vertical bar similar to 10(9) cm(2), the results differ substantially from the results of general relativity. The effects of modified gravity are seen as mimicking a stiff or soft EoS for neutron stars depending upon whether alpha is negative or positive, respectively. Some of the soft EoS that are excluded within the framework of general relativity can be reconciled for certain values of alpha of this order with the 2 solar mass neutron star recently observed. Indeed, if the EoS is ever established to be soft, modified gravity of the sort studied here may be required to explain neutron star masses as large as 2 M-circle dot. The associated length scale root alpha similar to 10(5) cm is of the order of the the typical radius of neutron stars implying that this is the smallest value we could find by using neutron stars as a probe. We thus conclude that the true value of alpha is most likely much smaller than 10(9) cm(2).