Hajime Sotani

Testing general relativity with present and future astrophysical observations

One century after its formulation, Einsteins general relativity (GR) has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that GR should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of GR. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einsteins theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

Probing the equation of state of nuclear matter via neutron star asteroseismology.

We general-relativistically calculate the frequency of fundamental torsional oscillations of neutron star crusts, where we focus on the crystalline properties obtained from macroscopic nuclear models in a way that is dependent on the equation of state of nuclear matter. We find that the calculated frequency is sensitive to the density dependence of the symmetry energy, but almost independent of the incompressibility of symmetric nuclear matter. By identifying the lowest-frequency quasiperiodic oscillation in giant flares observed from soft gamma-ray repeaters as the fundamental torsional mode and allowing for the dependence of the calculated frequency on stellar models, we provide a lower limit of the density derivative of the symmetry energy as L≃50  MeV.

Properties of an electrically charged black hole in Eddington-inspired Born-Infeld gravity

We systematically examine the properties of an electrically charged black hole in Eddington-inspired Born-Infeld gravity with not only the positive but also the negative coupling constant in the theory. As a result, we numerically find that the black hole solution exists even with the negative coupling constant, where the electric charge of black hole can be larger than the black hole mass. We also clarify the parameter space where the black hole solution exists. On the other hand, to examine the particle motion around such black hole, we derive the geodesic equation. The behavior of the effective potential for the radial particle motion is almost the same as that in general relativity, but the radius of the innermost stable circular orbit and the angular momentum giving the innermost stable circular orbit can be changed, depending on the coupling constant. In particular, we find that the radius of innermost stable circular orbit with the specific value of the coupling constant can be smaller than that for the extreme case in general relativity. Such a particle can release the gravitational binding energy more than the prediction in general relativity, which could be important from the observational point of view.

Slowly Rotating Relativistic Stars in Scalar-Tensor Gravity

We consider the slowly rotating relativistic stars with a uniform angular velocity in scalar-tensor gravity, and examine the rotational effect around such compact objects. For this purpose, we derive a second order differential equation describing the frame dragging in scalar-tensor gravity and solve it numerically. As a result, we find that the total angular momentum is proportional to the angular velocity even in scalar-tensor gravity. We also show that one can observe the spontaneous scalarization in rotational effects as well as the other stellar properties, if the cosmological value of the scalar field is zero. On the other hand, if the cosmological value of the scalar field is nonzero, the deviation from general relativity can be seen in a wide range of coupling constants. Additionally, we find that, independently of the cosmological value of the scalar field, the deviation from general relativity becomes larger with more massive stellar models. Thus, via precise observations of astronomical phenomena associated with rotating relativistic stars, one may probe not only the gravitational theory in the strong-field regime, but also the existence of a scalar field.

Strong gravitational lensing by an electrically charged black hole in Eddington-inspired Born-Infeld gravity

We systematically examine the properties of null geodesics around an electrically charged, asymptotically flat black hole in Eddington-inspired Born-Infeld gravity, varying the electric charge of black hole and the coupling constant in the theory. We find that the radius of the unstable circular orbit for massless particle decreases with the coupling constant, if the value of the electrical charge is fixed. Additionally, we consider the strong gravitational lensing around such a black hole. We show that the deflection angle, the position angle of the relativistic images, and the magnification due to the light bending in strong gravitational field are quite sensitive to the parameters determining the black hole solution. Thus, through the accurate observations associated with the strong gravitational lensing, it might be possible to reveal the gravitational theory in a strong field regime.

Possible constraints on the density dependence of the nuclear symmetry energy from quasi-periodic oscillations in soft gamma repeaters

We systematically examine the fundamental frequencies of shear torsional oscillations in neutron star crusts in a manner that is dependent on the parameter

Scalar gravitational waves from relativistic stars in scalar-tensor gravity

L

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

characterizing the poorly known density dependence of the symmetry energy. The identification of the lowest quasiperiodic oscillation (QPO) among the observed QPOs from giant flares in soft-gamma repeaters as the

Stellar oscillations in Eddington-inspired Born-Infeld gravity

/ell=2

Maximum mass limit of neutron stars in scalar-tensor gravity

fundamental torsional oscillations enables us to constrain the parameter