Abstract:
In this article, we discuss the space-time of a global monopole field as a candidate for galactic dark matter in the context of scalar tensor theory.

Abstract:
In recent past, W.A.Hiscock [ Class.Quan.Grav. (1990) 7,6235 ] studied the semi classical gravitational effects around global monopole. He obtained the vacuum expectation value of the stress-energy tensor of an arbitrary collection of conformal mass less free quantum fields (scalar, spinor and vectors) in the space time of a global monopole. With this stress-energy tensor, we study the semi classical gravitational effects of a global monopole in the context of Brans-Dicke theory of gravity.

Abstract:
In Brans-Dicke theory of gravity we explain how the extra constant value in the formula for rotation velocities of stars in a galactic halo can be obtained due to the global monopole field. We argue on a few points of the preceding Comment and discuss improvement of our model.

Abstract:
The electrogravity transformation is defined by an interchange of the ``active'' and ``passive'' electric parts of the Riemann tensor. Such a transformation has been used to find new solutions that are ``dual'' to the Kerr family of black hole spacetimes in general relativity. In such a case, the dual solution is a similar black hole spacetime endowed with a global monopole charge. Here, we extend this formalism to obtain solutions dual to the static, spherically symmetric solutions of two different scalar-tensor gravity theories. In particular, we first study the duals of the charged black hole solutions of a four-dimensional low-energy effective action of heterotic string theory. Next, we study dual of the Xanthopoulos-Zannias solution in Brans-Dicke theory, which contains a naked singularity. We show that, analogous to general relativity, in these scalar-tensor gravity theories the dual solutions are similar to the original spacetimes, but with a global monopole charge.

Abstract:
In this paper we calculate the effects produced by temperature in the renormalized vaccum expectation value of the square of the massless scalar field in the pointlike global monopole spacetime. In order to develop this calculation, we had to construct the Euclidean thermal Green function associated with this field in this background. We also calculate the high-temperature limit for the thermal average of the zero-zero component of the energy-momentum tensor.

Abstract:
In this paper we analyze the vacuum polarization effects of a massless scalar field in the background of a global monopole considering a inner structure to it. Specifically we investigate the effect of its structure on the vacuum expectation value of the square of the field operator, $<\hat{\Phi}^2(x)>$, admitting a non-minimal coupling between the field with the geometry: $\xi {\cal{R}}\hat{\Phi}^2$. Also we calculate the corrections on the vacuum expectation value of the energy-momentum tensor, $<\hat{T}_{\mu\nu}>$, due to the inner structure of the monopole. In order to develop these analysis, we calculate the Euclidean Green function associated with the system for points in the region outside the core. As we shall see, for specific value of the coupling parameter $\xi$, the corrections caused by the inner structure of the monopole can provide relevant contributions on these vacuum polarizations.

Abstract:
In this study, FRW-cosmologies with some matter groups such as monopole-domain wall, monopole-Chaplygin gas and monopole-strange quark matter in the scalar theory of gravitation based on Lyra geometry are investigated. We expand two exact models as static case and time-depended case for each matter groups in order to solve field equations in the scalar theory. For each matter groups, the solutions are introduced as the models of expanding universe, exponentially. Hubble parameters in the case of k=0,-1,1 are obtained for these models. Furthermore, we realize interesting result which the well-known relation between scalar theory based on Lyra geometry and Einstein's theory is an incomplete idea. In opposition to the well accepted idea in the literature, we suggest that Einstein's theory with no cosmological constant is equivalent of scalar theory based on Lyra geometry with zero displacement vector, completely. If the components of displacement vector in the scalar theory are any constant functions, the scalar theory couldn't correspond to Einstein's theory, identically. Even if the components of displacement vector in the scalar theory are the constant, the field equations and their solutions contain the Einstein's field equations and their solutions, but they are variously more general than Einstein's theory of gravitation. So, coefficients of constant displacement vector don't play the role of cosmological constant in keeping with the Einstein's theory. Finally, the results have been discussed.

Abstract:
To understand the accelerating universe discovered observationally in 1998, we develop the scalar-tensor theory of gravitation originally due to Jordan, extended only minimally. The unique role of the conformal transformation and frames is discussed particularly from a physical point of view. We show the theory to provide us with a simple and natural way of understanding the core of the measurements, Λ obs ～ t 0 ？2 for the observed values of the cosmological constant and today’s age of the universe both expressed in the Planckian units. According to this scenario of a decaying cosmological constant, Λ obs is this small only because we are old, not because we fine-tune the parameters. It also follows that the scalar field is simply the pseudo Nambu–Goldstone boson of broken global scale invariance, based on the way astronomers and astrophysicists measure the expansion of the universe in reference to the microscopic length units. A rather phenomenological trapping mechanism is assumed for the scalar field around the epoch of mini-inflation as observed, still maintaining the unmistakable behavior of the scenario stated above. Experimental searches for the scalar field, as light as ～ 10 ？9 eV, as part of the dark energy, are also discussed.

Abstract:
We present a solution of the coupled Einstein and rank-two antisymmetric tensor field equations where Lorentz symmetry is spontaneously broken, and we discuss its observational signatures. Especially, the deflection angles have important qualitative differences between tensor and scalar monopoles. If a monopole were to be detected, it would be discriminated whether or not to correspond to a tensor one. This phenomenon might open up new direction in the search of Lorentz violation with future astrophysical observations.

Abstract:
We consider the ground state energy of scalar massive field in the spacetime of a pointlike global monopole. Using zeta function regularization method we obtain the heat kernel coefficients for this system. We show that the coefficient $B_1$ contains additional contribution due to the non-trivial topological structure of the spacetime. Taking into account the heat kernel coefficients we obtain that the ground state energy of the scalar field is zero. We also discuss our result using dimensional considerations.