Anisotropic massive strings in the scalar-tensor theory of gravitation

We present the model of an anisotropic universe with string fluid as the source of matter within the framework of the scalar-tensor theory of gravitation. An exact solution of field equations is obtained by applying Berman＇s law of variation to Hubble＇s parameter which yields a constant value of the deceleration parameter. The nature of classical potential is examined for the model under consideration. It has also been found that the massive strings dominate in the early universe and finally disap-pear from the universe. This is in agreement with current astronomical observations. The physical and dynamical properties of the model are also discussed.

Bianchi Type-IX Magnetized Dark Energy Model in Saez-Ballester Theory of Gravitation

The Bianchi type-IX cosmological model with variable ω has been studied in the scalar tensor theory of gravitation proposed by Saez and Ballester [Phys. Lett. A 113: 467, 1985] in the presence and absence of magnetic field of energy densityρb. A special law of variation of Hubble’s parameter proposed by Berman [Nuovo Cimento 74 B, 182, 1983] has been used to solve the field equations. The physical and kinematical properties of the model are also discussed.

Merger estimates for a disformal Kerr black hole in quadratic degenerate higher-order scalar-tensor theories

We investigate the main features of a disformal Kerr black hole merger in quadratic degenerate higher-order scalar-tensor theories.In the ringdown stage of the black hole merger,for the prograde orbit,the real part of the quasinormal modes decreases with an increase in the disformal parameter,and the imaginary part also decreases,except in the Kerr case for a large spin parameter.However,for the retrograde orbit,the real part increases with an increase in the disformal parameter,and the imaginary part always decreases with it.For the approximate final spin,regardless of an equal spin,unequal spin,or generic spin configuration merger,the final black hole spin always increases with an increase in the disformal parameter.Our results show that the disformal parameter in the disformal Kerr solution and the MOG parameter in the Kerr-MOG case have obviously different effects on the black hole merger,which suggests the differences between these two spacetime structures.

Higher derivative scalar-tensor monomials and their classification

We make a full classification of scalar monomials built of the Riemann curvature tensor up to the quadratic order and of the covariant derivatives of the scalar field up to the third order.From the point of view of the effective field theory,the third or even higher order covariant derivatives of the scalar field are of the same importance as the higher curvature terms,and thus should be taken into account.Moreover,the higher curvature terms and the higher order derivatives of the scalar field are complementary to each other,of which novel ghostfree combinations may exist.We make a systematic classification of all the possible monomials,according to the numbers of the Riemann tensor and the higher derivatives of the scalar field in each monomial.A complete basis of monomials at each order is derived,of which the linear combinations may yield novel ghostfree Lagrangians.We also develop diagrammatic representations for the monomials,which may help to simplify the analysis.

Effect of noncircularity on the dynamic behaviors of particles in a disformal rotating black-hole spacetime

A disformal rotating black-hole solution is a black-hole solution in quadratic degenerate higher-order scalar-tensor theories.It breaks the circular condition of spacetime different from the case of the usual Kerr spacetime.This study investigated the dynamic behaviors of the motion of timelike particles in such disformal black-hole spacetime with an extra deformation parameter.Results showed that the characteristics of the particle’s motion depend on the sign of the deformation parameter.For the positive deformation parameter,the motion is regular and orderly.For the negative one,as the deformation parameter changes,the motion of the particles undergoes a series of transitions between the chaotic motion and the regular motion and falls into the horizon or escapes to spatial infinity.This means that the dynamic behavior of timelike particles in the disformal Kerr black-hole spacetime with noncircularity becomes richer than that in the usual Kerr black-hole case.