Moment of inertia -- compactness universal relations in scalar-tensor theories and $\mathcal{R}^2$ gravity

Abstract

We are investigating universal relations between different normalisations of the moment of inertia and the compactness of neutron and strange stars. Slowly rotating as well as rapidly rotating models are studied in General Relativity, $\mathcal{R}^2$ gravity and scalar--tensor theories of gravity. Moment of inertia -- compactness relations are examined for different normalisations of the moment of inertia. It is shown that for all studied cases the deviations from EOS universality are small for the examined equations of state. It turns out that in some of the cases the examined relations are also theory independent to a good extent. Universality in relations between the maximum mass and the moment of inertia for some unstable models is also investigated.