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Comments on ``Spin Connection Resonance in Gravitational General Relativity''

We comment on a recent article of M.W.Evans, Acta Physica Polonica B38 (2007) 2211. We point out that the equations underlying Evans' theory are highly problematic. Moreover, we demonstrate that the so-called ``spin connection resonance'', predicted by Evans, cannot be derived from the equation he used. We provide an exact solution of Evans' corresponding equation and show that is has definitely no resonance solutions.

physics.class-ph

Holographic Dark Energy Model with Modified Generalized Chaplygin Gas

We present a holographic dark energy model of the universe considering modified generalized Chaplygin gas (GCG). The modified GCG behaves as an ordinary barotropic fluid in the early epoch when the universe was tiny but behaves subsequently as a $Λ$CDM model at late epoch. An equivalent model with scalar field is obtained here by constructing the corresponding potential. The holographic dark energy is identified with the modified GCG and we determine the corresponding holographic dark energy field and its potential. The stability of the holographic dark energy in this case is also discussed.

gr-qc

Wormholes with a space- and time-dependent equation of state

The discovery that the Universe is undergoing an accelerated expansion has suggested the existence of an evolving equation of state. This paper discusses various wormhole solutions in a spherically symmetric spacetime with an equation of state that is both space and time dependent. The solutions obtained are exact and generalize earlier results on static wormholes supported by phantom energy.

gr-qc

On the Transfer of Adiabatic Fluctuations through a Nonsingular Cosmological Bounce

We study the transfer of cosmological perturbations through a nonsingular cosmological bounce in a special model in which the parameters of the bounce and the equation of state of matter are chosen such as to allow for an exact calculation of the evolution of the fluctuations. We find that the growing mode of the metric fluctuations in the contracting phase goes over into the growing mode in the expanding phase, a result which is different from what is obtained in analyses in which fluctuations are matched at a singular hypersurface. Consequences for Ekpyrotic cosmology are discussed in a limit when the equation of state of a fluid becomes large.

hep-th

Greybody Factors for d-Dimensional Black Holes

Gravitational greybody factors are analytically computed for static, spherically symmetric black holes in d-dimensions, including black holes with charge and in the presence of a cosmological constant (where a proper definition of greybody factors for both asymptotically dS and AdS spacetimes is provided). This calculation includes both the low-energy case --where the frequency of the scattered wave is small and real-- and the asymptotic case --where the frequency of the scattered wave is very large along the imaginary axis-- addressing gravitational perturbations as described by the Ishibashi-Kodama master equations, and yielding full transmission and reflection scattering coefficients for all considered spacetime geometries. At low frequencies a general method is developed, which can be employed for all three types of spacetime asymptotics, and which is independent of the details of the black hole. For asymptotically dS black holes the greybody factor is different for even or odd spacetime dimension, and proportional to the ratio of the areas of the event and cosmological horizons. For asymptotically AdS black holes the greybody factor has a rich structure in which there are several critical frequencies where it equals either one (pure transmission) or zero (pure reflection, with these frequencies corresponding to the normal modes of pure AdS spacetime). At asymptotic frequencies the computation of the greybody factor uses a technique inspired by monodromy matching, and some universality is hidden in the transmission and reflection coefficients. For either charged or asymptotically dS black holes the greybody factors are given by non-trivial functions, while for asymptotically AdS black holes the greybody factor precisely equals one (corresponding to pure blackbody emission).

hep-th

Magnetized String Cosmological Model in Cylindrically Symmetric Inhomogeneous Universe with Variable Cosmological-Term $\Lambda$

Cylindrically symmetric inhomogeneous magnetized string cosmological model is investigated with cosmological term $\Lambda$ varying with time. To get the deterministic solution, it has been assumed that the expansion ($\theta$) in the model is proportional to the eigenvalue $\sigma^{1}_{1}$ of the shear tensor $\sigma^{i}_{j}$. The value of cosmological constant for the model is found to be small and positive which is supported by the results from recent supernovae Ia observations. The physical and geometric properties of the model are also discussed in presence and absence of magnetic field.

gr-qc

On the Ernst electro-vacuum equations and ergosurfaces

The question of smoothness at the ergosurface of the space-time metric constructed out of solutions (E,phi) of the Ernst electro-vacuum equations is considered. We prove smoothness of those ergosurfaces at which Re(E) provides the dominant contribution to f=-(Re(E)+|phi|^2) at the zero-level-set of f. Some partial results are obtained in the remaining cases: in particular we give examples of leading-order solutions with singular isolated "ergocircles".

gr-qc

Testing modified gravity with motion of satellites around galaxies

A modified gravity (MOG) theory, that has been successfully fitted to galaxy rotational velocity data, cluster data, the Bullet Cluster 1E0657-56 and cosmological observations, is shown to be in good agreement with the motion of satellite galaxies around host galaxies at distances 50-400 kpc.

astro-ph

Green functions and Euclidean fields near the bifurcate Killing horizon

We approximate a Euclidean version of a D+1 dimensional manifold with a bifurcate Killing horizon by a product of a two-dimensional Rindler space and a D-1 dimensional manifold M. We obtain approximate formulas for the Green functions. We study the behaviour of Green functions near the horizon and their dimensional reduction. We show that if M is compact then the massless minimally coupled quantum field contains a zero mode which is a conformal invariant free field on R^2. Then, the Green function near the horizon can be approximated by the Green function of the two-dimensional quantum field theory. The correction term is exponentially small away from the horizon. If the volume of a geodesic ball is growing to infinity with its radius then the Green function cannot be approximated by a two-dimensional one.

hep-th

Lorentz Invariance Violation from String Theory

In this brief, and by no means complete, review I discuss situations in string theory, in which Lorentz Invariance Violation may occur in a way consistent with world-sheet conformal invariance, thereby leading to acceptable, in principle, string backgrounds. In particular, I first discuss spontaneous Lorentz violation in (non supersymmetric) open string field theory. Then, I move onto a discussion of gravity-induced modified dispersion relations in non-critical (Liouville) strings, in the sense of an induced Finsler-like geometry depending on both coordinates and momenta, for string propagation in non-trivial space times (such as D-particle ``foamy situations''). I pay attention to explaining the appearance of bi-metric models from such string theories, which could serve as examples of alternative scenaria to dark matter. Finally, I make some comparisons with similar developments in other contexts, such as critical strings in non-commutative space times, as well as deformed special relativities and theories with reduced Lorentz symmetry, advocated recently, where again Finsler geometry seems to come into play. In this latter respect, I put the emphasis on phenomenology and attempt to answer the question as to whether there is the possibility of experimental disentanglement of the various approaches.

hep-th

Hierarchy Problem, Dilatonic Fifth Force, and Origin of Mass

Based on the fact that the scalar curvature of the internal space determines the mass of the dilaton in higher-dimensional unified theories, we show how the dilaton mass can explain the origin of mass and resolve the hierarchy problem. Moreover, we show that cosmology puts a strong constraint on dilaton mass, and requires the scale of the internal space to be larger than $10^{-9} m$.

hep-ph

On Energy of the Friedman Universes in Conformally Flat Coordinates

Recently many authors have calculated the energy of the Friedman universes by using double index energy-momentum complexes in Cartesian comoving coordinates $(t,x,y,z)$ and concluded that the flat and closed Friedman universes are energy-free. We show in this paper by using Einstein canonical energy-momentum complex and by doing calculations in conformally flat coordinates that such conclusion is incorrect. The results obtained in this paper are compatible with the results of the our previous paper \cite{Gar07} where we have used coordinate-independent averaged relative energy-momentum tensors to analyze Friedman universes.

gr-qc

The Physics of Information

We review of the interface between (theoretical) physics and information for non-experts. The origin of information as related to the notion of entropy is described, first in the context of thermodynamics then in the context of statistical mechanics. A close examination of the foundations of statistical mechanics and the need to reconcile the probabilistic and deterministic views of the world leads us to a discussion of chaotic dynamics, where information plays a crucial role in quantifying predictability. We then discuss a variety of fundamental issues that emerge in defining information and how one must exercise care in discussing concepts such as order, disorder, and incomplete knowledge. We also discuss an alternative form of entropy and its possible relevance for nonequilibrium thermodynamics. In the final part of the paper we discuss how quantum mechanics gives rise to the very different concept of quantum information. Entirely new possibilities for information storage and computation are possible due to the massive parallel processing inherent in quantum mechanics. We also point out how entropy can be extended to apply to quantum mechanics to provide a useful measurement for quantum entanglement. Finally we make a small excursion to the interface betweeen quantum theory and general relativity, where one is confronted with an "ultimate information paradox" posed by the physics of Black Holes. In this review we have limited ourselves; not all relevant topics that touch on physics and information could be covered.

physics.class-ph

Unitarity in the Brout-Englert-Higgs Mechanism for Gravity

Just like the vector bosons in Abelian and non-Abelian gauge theories, gravitons can attain mass by spontaneous local symmetry breaking. The question is whether this can happen in a Lorentz-invariant way. We consider the use of four scalar fields that break coordinate reparametrization invariance, by playing the role of preferred flat coordinates x, y, z, and t. In the unbroken representation, the theory has a (negative) cosmological constant, which is tuned to zero by the scalars in the broken phase. Massive spin 2 bosons and a single massive scalar survive. The theory is not renormalizable, so at best it can be viewed as an effective field theory for massive spin 2 particles. One may think of applications in cosmology, but a more tantalizing idea is to apply it to string theory approaches to QCD: if the gluon sector is to be described by a compactified 26 or 10 dimensional bosonic string theory, then the ideas considered here could be used to describe the mechanism that removes a massless or tachyonic scalar and provides mass to the spin 2 glueball states. The delicate problem of removing indefinite metric and/or negative energy states is addressed. The scalar particle has negative metric, so that unitarity demands that only states with an even number of them are allowed. Various ways are considered to adapt the matter section of the theory such that matter only couples to positive metric states, and we succeed in suppressing the main contributions to unitarity-violating amplitudes, but the exact restoration of unitarity in the spinless sector will continue to be a delicate issue in theories of this sort.

hep-th

Asymptotics of LRS Bianchi type I cosmological models with elastic matter

In this paper we report on results in the study of spatially homogeneous cosmological models with elastic matter. We show that the behavior of elastic solutions is fundamentally different from that of perfect fluid solutions already in the case of locally rotationally symmetric (LRS) Bianchi type I models; this is true even when the elastic material resembles a perfect fluid very closely. In particular, the approach to the initial singularity is characterized by an intricate oscillatory behavior of the scale factors, while the future asymptotic behavior is described by isotropization rates that differ significantly from those of perfect fluids.

gr-qc

Evidence for evolution from pure states to mixed states in pion creation process pi(-) p -> pi(-)pi(+) n on polarized target and its physical interpretation

In 1982 Hawking suggested that quantum fluctuations of space-time metric will induce a non-unitary evolution from pure initial states to mixed final states in particle interactions at any energy. This hypothesis can be tested using existing CERN data on pi(-) p -> pi(-)pi(+) n on polarized target at 17.2 GeV/c. The purity of the final state is controlled by the purity of recoil nucleon polarization. We develop a spin formalism to calculate expressions for recoil nucleon polarization for two specific pure initial states. Imposing conditions of purity on the recoil nuclon polarization we obtain conditions on the amplitudes which are violated by model independent amplitude analysis of CERN data at large t. We conclude that pure states can evolve into mixed states in pi(-) p -> pi(-)pi(+) n. In quantum theory such non-unitary evolution occurs in open quantum systems interacting with an environment. We present evidence to validate the view of the pion creation process as an open quantum system interacting with a quantum environment. The observed process is time-irreversible and violates CPT symmetry.

hep-ph

Non-linear Equation of State, COSMIC Acceleration and Deceleration During Phantom-Dominance

Here, RS-II model of brane-gravity is considerd for phantom universe using non-linear equation of stste. Phantom fluid is known to violate the weak energy condition. In this paper, it is found that this characteristic of phantom energy is affected drastically by the negative brane-tension $λ$ of the RS-II model. It is interesting to see that upto a certain value of energy density $ρ$ satisfying $ρ/λ< 1$, weak energy condition is violated and universe super-accelerates. But as $ρ$ increases more, only strong energy condition is violated and universe accelerates. When $1 < ρ/λ<2$, even strong energy condition is not violated and universe decelerates. Expansion of the universe stops, when $ρ= 2 λ$. This is contrary to earlier results of phantom universe exhibiting acceleration only.

gr-qc