Takashi Tamaki

Chameleon scalar fields in relativistic gravitational backgrounds

We study the field profile of a scalar field that couples to a matter fluid (dubbed a chameleon field) in the relativistic gravitational background of a spherically symmetric spacetime. Employing a linear expansion in terms of the gravitational potential Φc at the surface of a compact object with a constant density, we derive the thin-shell field profile both inside and outside the object, as well as the resulting effective coupling with matter, analytically. We also carry out numerical simulations for the class of inverse power-law potentials V() = M4+n−n by employing the information provided by our analytical solutions to set the boundary conditions around the centre of the object and show that thin-shell solutions in fact exist if the gravitational potential Φc is smaller than 0.3, which marginally covers the case of neutron stars. Thus the chameleon mechanism is present in the relativistic gravitational backgrounds, capable of reducing the effective coupling. Since thin-shell solutions are sensitive to the choice of boundary conditions, our analytic field profile is very helpful to provide appropriate boundary conditions for ΦcO(0.1).

Black hole entropy for the general area spectrum

We consider the possibility that the horizon area is expressed by the general area spectrum in loop quantum gravity when we leave off the semiclassical consideration. To check this idea, we calculate the number of degrees of freedom in spin-network states related to its area. We obtain that logarithm of this number is proportional to its area as in previous works where the simplified area formula has been used. Our result shows that we should be careful in justifying (or falsifying) the area spectrum if we respect to leave off the semiclassical consideration.

Dyonic BIon black hole in string inspired model

We construct static and spherically symmetric particle-like and black hole solutions with magnetic and/or electric charge in the Einstein-Born-Infeld-dilaton-axion system, which is a generalization of the Einstein-Maxwell-dilaton-axion (EMDA) system and of the Einstein-Born-Infeld (EBI) system. They have remarkable properties which are not seen for the corresponding solutions in the EMDA and the EBI system.

Have we already detected astrophysical symptoms of space-time noncommutativity?

We discuss astrophysical implications of

Universal area spectrum in single-horizon black holes

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Particle velocity in noncommutative space-time

-Minkowski space-time, in which there appears space-time noncommutativity. We first derive a velocity formula for particles based on the motion of a wave packet. The result is that a massless particle moves at a constant speed as in the usual Minkowski space-time, which implies that an arrival time analysis by

Ambiguity of black hole entropy in loop quantum gravity

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Post-Newtonian parameters in the tensor-vector-scalar theory

-rays from Markarian (Mk) 421 does not exclude space-time noncommutativity. Based on this observation, we analyze reaction processes in

Considering boundary conditions for black hole entropy in loop quantum gravity

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Continuous area spectrum of a regular black hole

-Minkowski space-time which are related to the puzzling detections of extremely high-energy cosmic rays above the Greisen-Zatsepin-Kuzmin cutoff and of high-energy (