http://arxiv.org/ General Relativity and Quantum Cosmology (gr-qc) updates on the arXiv.org e-print archive en-us 2010-02-08T20:30:00-05:00 www-admin@arxiv.org General Relativity and Quantum Cosmology 1901-01-01T00:00+00:00 1 daily http://arxiv.org/icons/sfx.gif http://arxiv.org/ http://arxiv.org/abs/1002.1361 <p>We discuss the primordial spectrum of a massless and minimally coupled scalar field, produced during the initial anisotropic epoch before the onset of inflation. We consider two models of the anisotropic cosmology, the (planar) Kasner de Sitter solution (Bianchi I) and the Taub-NUT de Sitter solution (Bianchi IX), where the 3-space geometry is initially anisotropic, followed by the de Sitter phase due to the presence of a positive cosmological constant. We discuss the behavior of a quantized, massless and minimally coupled scalar field in the anisotropic stage, which is a counterpart of the inflaton fluctuation. The initial condition is set by the requirement that the scalar field is initially in an adiabatic state. In the Kasner de Sitter model, for one branch of planar solutions there is an adiabatic vacuum unless $k_3\neq 0$, where $k_3$ is the comoving momentum along the third direction, while in the other branch there is no adiabatic state. In the first branch, for the moderate modes, $k_3\sim k$, where $k$ is the total comoving momentum, the scalar power spectrum has an oscillatory behavior and its direction dependence is suppressed. For the planar modes, $k_3\ll k$, in contrast, the direction dependence becomes more important, because of the amplification of the scalar amplitude during this interval of the violation of WKB approximation in the initial anisotropic stage. The qualitative behaviors in the Taub-NUT de Sitter models are very similar to the case of the first branch of the planar Kasner de Sitter model. </p> Hyeong-Chan Kim, Masato Minamitsuji http://arxiv.org/abs/1002.1364 <p>Recently, a strong debate has been pursued about the Newtonian limit (i.e. small velocity and weak field) of fourth order gravity models. According to some authors, the Newtonian limit of $f(R)$-gravity is equivalent to the one of Brans-Dicke gravity with $\omega_{BD} = 0$, so that the PPN parameters of these models turn out to be ill defined. In this paper, we carefully discuss this point considering that fourth order gravity models are dynamically equivalent to the O'Hanlon Lagrangian. This is a special case of scalar-tensor gravity characterized only by self-interaction potential and that, in the Newtonian limit, this implies a non-standard behavior that cannot be compared with the usual PPN limit of General Relativity. </p> <p>The result turns out to be completely different from the one of Brans-Dicke theory and in particular suggests that it is misleading to consider the PPN parameters of this theory with $\omega_{BD} = 0$ in order to characterize the homologous quantities of $f(R)$-gravity. Finally the solutions at Newtonian level, obtained in the Jordan frame for a $f(R)$-gravity, reinterpreted as a scalar-tensor theory, are linked to those in the Einstein frame. </p> S. Capozziello, A. Stabile, A. Troisi http://arxiv.org/abs/1002.1397 <p>GGR News: </p> <p>GGR Program at the APS meeting in Washington D.C., by David Garfinkle </p> <p>we hear that..., by David Garfinkle </p> <p>Research Briefs: </p> <p>Supernovae modelling, by Ian Hawke </p> <p>Conference Reports: </p> <p>ADM-50, by Richard Woodard </p> David Garfinkle http://arxiv.org/abs/1002.1421 <p>A new category of "intrinsic" effects is proposed to be added to the two already known kinematic and dynamical categories. An example of intrinsic effect is predicted, its origin source is established, and a scheme of its experimental detection is proposed. This effect lowers to non-relativistic values the propagation velocity of a plane electromagnetic wave in a vacuum, when a time-independent homogeneous magnetic field is superposed over it. This result, pertaining to the classical Maxwell theory, follows from exact calculations. A critical remark on gravitational waves' detection is given. </p> Nikolai V. Mitskievich http://arxiv.org/abs/1002.1425 <p>The Newtonian regime of a recent nonlocal extension of general relativity (GR) is investigated. Nonlocality is introduced via a scalar "constitutive" kernel in a special case of the translational gauge theory of gravitation, namely, the teleparallel equivalent of GR. In this theory, the nonlocal aspect of gravity simulates dark matter. A nonlocal and nonlinear generalization of Poisson's equation of Newtonian gravitation is presented. The implications of nonlocality for the gravitational physics in the solar system are briefly studied. </p> Hans-Joachim Blome, Carmen Chicone, Friedrich W. Hehl, Bahram Mashhoon http://arxiv.org/abs/1002.1434 <p>The holographic dark energy (HDE) is considered to be the most promising candidate of dark energy. Its definition is originally motivated from the entropy-area relation which depends on the theory of gravity under consideration. Recently a new definition of HDE is proposed with the help of quantum corrections to the entropy-area relation in the setup of loop quantum cosmology. Using this new definition, we investigate the model of interacting dark energy and derive its effective equation of state. Finally we establish a correspondence between generalized Chaplygin gas and entropy-corrected holographic dark energy. </p> Mubasher Jamil, M. Umar Farooq http://arxiv.org/abs/1002.1454 <p>We present, for both minkowskian and euclidean signatures, short derivations of the diagonal Einstein metrics for Bianchi type II, III and V. For the first two cases we show the integrability of the geodesic flow while for the third case a somewhat unusual bifurcation phenomenon takes place: for minkowskian signature elliptic functions are essential in the metric while for euclidean signature only elementary functions appear. </p> Galliano Valent (LPTHE) http://arxiv.org/abs/1002.1471 <p>We analyse the Cauchy problem on a characteristic cone, including its vertex, for the Einstein equations in arbitrary dimensions. We use a wave map gauge, solve the obtained constraints and show gauge conservation. </p> Yvonne Choquet-Bruhat, Piotr T. Chru&#x15b;ciel, Jos&#xe9; M. Mart&#xed;n-Garc&#xed;a http://arxiv.org/abs/1002.1472 <p>We construct solutions of specific asymptotics to the Einstein constraint equations using a cut-off technique. Moreover, we give various examples of vacuum asymptotically flat manifolds whose center of mass and angular momentum fail to exist. </p> Lan-Hsuan Huang http://arxiv.org/abs/1002.1533 <p>It is shown that the vacuum state in presence of Lorentz violation can be followed by a particle-full universe that represents the current status of the universe. In this model the modification in dispersion relation (Lorentz violation) is picked up representing the regime of quantum gravity. The result can be interpreted such that the existence of the particles is an evidence for quantum effects of gravity in the past. It is concluded that only the vacuum state is sufficient to appear the matter fields spontaneously after the process of semi-classical analysis. </p> Nima Khosravi http://arxiv.org/abs/1002.1560 <p>In this work we suggest that higher-dimensional modifications to the matter content in FRW spacetimes can be obtained not only, as first considered by Ponce de Leon, referring to "moving" 4D hypersurfaces non-orthogonal to the time-dependent extra dimension of an embedding 5D manifold, but also referring to "fixed" 4D hypersurfaces orthogonal to a suitable scalar function which defines a static foliation of the 5D manifold and takes the role of the extra dimension in a suitable coordinate system. Results obtained in each approach crucially depend on the method used to identify the 4D metric of our brane universe from the 5D metric of the bulk manifold. </p> M. La Camera http://arxiv.org/abs/0805.0421 <p>It is shown that a first-order cosmological perturbation theory for Friedmann-Lemaitre-Robertson-Walker universes admits one and only one gauge-invariant variable which describes the perturbation to the energy density and which becomes equal to the usual energy density of the Newtonian theory of gravity in the limit that all particle velocities are negligible with respect to the speed of light. The same holds true for the perturbation to the particle number density. A cosmological perturbation theory based on these particular gauge-invariant quantities is more precise than any earlier first-order perturbation theory. In particular, it explains star formation in a satisfactory way, even in the absence of cold dark matter. In a baryon-only universe, the earliest stars, the so-called Population III stars, are found to have masses between 400 and 100,000 solar masses with a peak around 3400 solar masses. If cold dark matter is present then the star masses are between 130 and 13,000 solar masses with a peak around 450 solar masses. They come into existence between 100 Myr and 1000 Myr. At much later times, star formation is possible only in high density regions, for example within galaxies. Late time stars may have much smaller masses than early stars. The smallest stars that can be formed have masses of 0.2--0.8 solar mass, depending on the initial internal relative pressure perturbation. It is demonstrated that the Newtonian theory of gravity cannot be used to study the evolution of cosmological density perturbations. </p> P.G.Miedema, W.A.van Leeuwen http://arxiv.org/abs/0809.3570 <p>The Higher Order Theories of Gravity - $f(R, R_{\alpha\beta}R^{\alpha\beta})$ - theory, where $R$ is the Ricci scalar, $R_{\alpha\beta}$ is the Ricci tensor and $f$ is any analytic function - have recently attracted a lot of interest as alternative candidates to explain the observed cosmic acceleration, the flatness of the rotation curves of spiral galaxies and other relevant astrophysical phenomena. It is a crucial point testing these alternative theories in the so called weak field and newtonian limit of a $f(R, R_{\alpha\beta}R^{\alpha\beta})$ - theory. With this "perturbation technique" it is possible to find spherically symmetric solutions and compare them with the ones of General Relativity. On both approaches we found a modification of General Relativity: the behaviour of gravitational potential presents a modification Yukawa - like in the newtonian case and a massive propagation in the weak field case. When the modification of the theory is removed (i.e. $f(R, R_{\alpha\beta}R^{\alpha\beta}) = R$, Hilbert - Einstein lagrangian) we find the usual outcomes of General Relativity. Also the Noether symmetries technique has been investigated to find some time independent spherically symmetric solutions. </p> Arturo Stabile http://arxiv.org/abs/0903.1185 <p>First we review some of the attempts made to find exact spherically symmetric solutions of Einstein field equations in the presence of scalar fields .Wyman solution in both static and non static scalar field is discussed briefly and it is show that why in the case of non static homogenous matter field, static metric can not be represented in terms of elementary functions. We mention here that if our spacetime be static, according to EFE there is two option for choose scalar field matter: static (time independent) and non static (time dependent). All these solutions are limited to the minimally coupled massless scalar fields and also in the absence of the cosmological constant . Then we show that if we are interesting to have a homogenous isotropic scalar field matter one can construct a series solution in terms of scalar field's mass and cosmological constant. This metric is static and posses a locally flat case as a special chooses of mass of scalar field and can be interpreted as an effective vacuum. Therefore mass of scalar field eliminates any locally gravitational effect as tidal forces. Finally we describe why this system is unstable in the language of dynamical systems. </p> Mohammad Mehrpooya, D. Momeni http://arxiv.org/abs/0906.1385 <p>We investigate the effect of the intrinsic spin of a fundamental spinor field on the surrounding spacetime geometry. We show that despite the lack of a rotating stress-energy source (and despite claims to the contrary) the intrinsic spin of a spin-half fermion gives rise to a frame-dragging effect analogous to that of orbital angular momentum, even in Einstein-Hilbert gravity where torsion is constrained to be zero. This resolves a paradox regarding the counter-force needed to restore Newton's third law in the well known spin-orbit interaction. In addition, the frame-dragging effect gives rise to a {\it long-range} gravitationally mediated spin-spin dipole interaction coupling the {\it internal} spins of two sources. We argue that despite the weakness of the interaction, the spin-spin interaction will dominate over the ordinary inverse square Newtonian interaction in any process of sufficiently high-energy for quantum field theoretical effects to be non-negligible. </p> Andrew Randono http://arxiv.org/abs/0908.1214 <p>We consider the agegraphic models of dark energy in a braneworld scenario with brane-bulk energy exchange. We assume that the adiabatic equation for the dark matter is satisfied while it is violated for the agegraphic dark energy due to the energy exchange between the brane and the bulk. Our study shows that with the brane-bulk interaction, the equation of state parameter of agegraphic dark energy on the brane, $w_D$, can have a transition from normal state where $w_D &gt;-1 $ to the phantom regime where $w_D &lt;-1 $, while the effective equation of state for dark energy always satisfies $w^{\mathrm{eff}}_D\geq-1$. </p> Ahmad Sheykhi http://arxiv.org/abs/0908.2435 <p>Motivated by the increasing interest in models which consider scalar fields as viable dark matter candidates, we have constructed a generalization of relativistic Boson Stars (BS) composed of two coexisting states of the scalar field, the ground state and the first excited state. We have studied the dynamical evolution of these Multi-state Boson Stars (MSBS) under radial perturbations, using numerical techniques. We show that stable MSBS can be constructed, when the number of particles in the first excited state, N2, is smaller than the number of particles in the ground state, N1. On the other hand, when N2 &gt; N1, the configurations are initially unstable. However, they evolve and settle down into stable configurations. In the stabilization process, the initially ground state is excited and ends in a first excited state, whereas the initially first excited state ends in a ground state. During this process, both states emit scalar field radiation, decreasing their number of particles. This behavior shows that even though BS in the first excited state are intrinsically unstable under finite perturbations, the configuration resulting from the combination of this state with the ground state produces stable objects. Finally we show in a qualitative way, that stable MSBS could be realistic models of dark matter galactic halos, as they produce rotation curves that are flatter at large radii than the rotation curves produced by BS with only one state. </p> Argelia Bernal, Juan Barranco, Daniela Alic, Carlos Palenzuela http://arxiv.org/abs/1001.0297 <p>The deflection of a ray of light passing close to a gravitational mass, is generally calculated from the null geodesic which the light ray (photon) follows. However, there is an alternate approach, where the effect of gravitation on the ray of light is estimated by considering the ray to be passing through a material medium. Calculations have been done in this paper, following the later approach, to estimate the amount of deflection due to a static non-rotating mass. The refractive index of such a material medium has been calculated in a more rigorous manner in the present work and the final value of amount of deflection calculated here is claimed to be more accurate than all other values resulting from previous calculations. </p> A. K. Sen http://arxiv.org/abs/1001.1088 <p>After reviewing recently suggested operational "principles of the quantumness", I address the problem on whether Quantum Theory (QT) and Special Relativity (SR) are unrelated theories, or instead, if the one implies the other. I show how SR can be indeed derived from causality of QT, within the computational paradigm "the universe is a huge quantum computer", reformulating QFT as a Quantum-Computational Field Theory (QCFT). In QCFT SR emerges from the fabric of the computational network, which also naturally embeds gauge invariance. In this scheme even the quantization rule and the Planck constant can in principle be derived as emergent from the underlying causal tapestry of space-time. In this way QT remains the only theory operating the huge computer of the universe. Is QCFT only a speculative tautology (theory as simulation of reality), or does it have a scientific value? The answer will come from Occam's razor, depending on the mathematical simplicity of QCFT. Here I will just start scratching the surface of QCFT, analyzing simple field theories, including Dirac's. The number of problems and unmotivated recipes that plague QFT strongly motivates us to undertake the QCFT project, since QCFT makes all such problems manifest, and forces a re-foundation of QFT. </p> Giacomo Mauro D&#x27;Ariano http://arxiv.org/abs/1001.1141 <p>This article will provide the reader with a short introduction to dark spinors, which are ELKO spinors, eingenspinors of the charge conjugation operator, applied to dark matter and dark energy. </p> Christian. G. Boehmer, James Burnett http://arxiv.org/abs/1001.5349 <p>We show that in modified $f(R)$ type gravity models with non-minimal coupling between matter and geometry, both the matter Lagrangian, and the energy-momentum tensor, are completely and uniquely determined by the form of the coupling. This result is obtained by using the variational formulation for the derivation of the equations of motion in the modified gravity models with geometry-matter coupling, and the Newtonian limit for a fluid obeying a barotropic equation of state. The corresponding energy-momentum tensor of the matter in modified gravity models with non-minimal coupling is more general than the usual general-relativistic energy-momentum tensor for perfect fluids, and it contains a supplementary, equation of state dependent term, which could be related to the elastic stresses in the body, or to other forms of internal energy. Therefore, the extra-force induced by the coupling between matter and geometry never vanishes as a consequence of the thermodynamic properties of the system, or for a specific choice of the matter Lagrangian, and it is non-zero in the case of a fluid of dust particles. </p> T. Harko http://arxiv.org/abs/1002.0187 <p>Dissipations, e.g. heat flow and bulk and shear viscosities, cause the transport of energy, momentum and angular momentum, which is the essence of accretion of matters onto celestial objects. Dissipations are usually described by the Fourier and Navier-Stokes laws ("classic laws" of dissipations). However the classic laws result in an infinitely fast propagation of dissipations. In relativistic formulation, the classic laws of dissipations violate the causality, and hence no relativistic theory of accretion flow onto celestial object is formulated. In this short report, we summarize the causal dissipative hydrodynamics, so-called "Extended Irreversible Thermodynamics" (EIT), with a supplemental comment of which the original works of EIT are not aware, and then show two theorems about relativistic dissipative flows around a Schwarzschild black hole. By these theorems, a significant property of EIT in contrast with classic laws of dissipations is clarified, and a dissipative instability of an exact solution of relativistic perfect fluid flow is also obtained. </p> Hiromi Saida, Rohta Takahashi, Hiroki Nagakura http://arxiv.org/abs/gr-qc/0603005 <p>Following a short discussion of some unresolved issues in the standard model of cosmology (considered to be a generic $\Lambda$CDM model with flat geometry and an early period of inflation), we describe the current state of research on the problem of negative action. Arguments are then given to the effect that traditional assumptions concerning the behavior of negative action matter give rise to violations of both the principle of relativity and the principle of inertia. We propose an alternative set of axioms that would govern the behavior of negative action matter if it is to be considered a viable element of physical theories upon which cosmological models are build. We then elaborate a simple framework, based on general relativity and the proposed axioms, which enables the formulation of quantitative predictions concerning the interaction between positive and negative action bodies. Based on those developments, a solution is proposed to the problem of the discrepancy between current experimental and theoretical values of vacuum energy density (in any cosmological model), which is at once also a solution to the problem of the unexplained coincidence between the (model dependent) experimental values of vacuum energy density and present day average matter energy density. We also show how irreversibility naturally arises in cosmological models derived in this context. </p> Christophe Nickner http://arxiv.org/abs/1002.1232 <p>We reanalyse the current data from the supernovae observations including the weak lensing effects caused by inhomogeneities. We compute the lensing probability distribution function for each background solution described by the parameters Omega_M, Omega_Lambda and w in the presence of inhomogeneities, which are designed to mimic the observed large-scale structures. We then perform a likelihood analysis in the space of FLRW-parameters and compare our results with the standard approach. We find that the inclusion of lensing can move the best-fit model significantly towards the cosmic concordance of the flat LCDM model, improving the agreement with the constraints coming from the cosmic microwave background and the baryon acoustic oscillations. </p> Luca Amendola, Kimmo Kainulainen, Valerio Marra, Miguel Quartin http://arxiv.org/abs/1002.1219 <p>We study the bulk corresponding to tidal charged brane-world black holes. We employ a propagating algorithm which makes use of the three-dimensional multipole expansion and analytically yields the metric elements as functions of the five-dimensional coordinates and of the ADM mass, tidal charge and brane tension. Since the projected brane equations cannot determine how the charge depends on the mass, our main purpose is to select the combinations of these parameters for which black holes of microscopic size possess a regular bulk. Our results could in particular be relevant for a better understanding of TeV-scale black holes. </p> R. Casadio, O. Micu http://arxiv.org/abs/1002.0849 <p>We present a comprehensive investigation of cosmological constraints on the class of vector field formulations of modified gravity called Generalized Einstein-Aether models. Using linear perturbation theory we generate cosmic microwave background and large-scale structure spectra for general parameters of the theory, and then constrain them in various ways. We investigate two parameter regimes: a dark-matter candidate where the vector field sources structure formation, and a dark-energy candidate where it causes late-time acceleration. We find that the dark matter candidate does not fit the data, and identify five physical problems that can restrict this and other theories of dark matter. The dark energy candidate does fit the data, and we constrain its fundamental parameters; most notably we find that the theory's kinetic index parameter $n_{\mathrm{ae}}$ can differ significantly from its $\Lambda$CDM value. </p> J. Zuntz, T. G. Zlosnik, F. Bourliot, P. G. Ferreira, G. D. Starkman http://arxiv.org/abs/1002.0834 <p>[Abridged] In a Universe with a detectable nontrivial spatial topology the last scattering surface contains pairs of matching circles with the same distribution of temperature fluctuations -- the so-called circles-in-the-sky. Searches for nearly antipodal circles-in-the-sky in maps of cosmic microwave background radiation have so far been unsuccessful. This negative outcome along with recent theoretical results concerning the detectability of nearly flat compact topologies is sufficient to exclude a detectable nontrivial topology for most observers in very nearly flat positively and negatively curved Universes ($0&lt;|\Omega_{tot}-1| \lesssim 10^{-5}$). Here we investigate the consequences of these searches for observable nontrivial topologies if the Universe turns out to be exactly flat ($\Omega_{tot}=1$) as is often assumed. We demonstrate that in this case the conclusions deduced from such searches can be radically different. We show that for all multiply-connected orientable flat manifolds it is possible to directly study the action of the holonomies in order to obtain an upper bound on the angle that characterizes the deviation from antipodicity of pairs of matching circles associated with the shortest closed geodesic. We also show that in a flat Universe there are observers for whom the circles-in-the-sky searches already undertaken are insufficient to exclude the possibility of a detectable nontrivial spatial topology. It is remarkable how such small variations in the spatial curvature of the Universe, which are effectively indistinguishable geometrically, can have such a drastic effect on the detectability of cosmic topology. </p> B. Mota, M.J. Reboucas, R. Tavakol http://arxiv.org/abs/0903.2882 <p>Recent studies undoubtedly demonstrate that the magnetic field in the photosphere and corona is an intermittent structure, which offers new views on the underlying physics. In particular, such problems as the existence in the corona of localized areas with extremely strong resistivity (required to explain magnetic reconnection of all scales) and the interchange between small and large scales (required in study of the photosphere/corona coupling), to name a few, can be easily captured by the concept of intermittency. This study is focused on simultaneous time variations of intermittency properties derived in the photosphere, chromosphere and corona. We analyzed data for NOAA AR 10930 acquired between Dec 08, 2006 12:00 UT and Dec 13, 2006 18:45 UT. Photospheric intermittency was inferred from Hinode magnetic field measurements, while intermittency in the transition region and corona was derived from Nobeyama 9 GHz radio polarization measurements, high cadence Hinode/XRT/Be-thin data as well as GOES 1-8\AA flux. Photospheric dynamics and its possible relationship with the intermittency variations were also analyzed by calculating the kinetic vorticity. For this case study we found the following chain of events. Intermittency of the photospheric magnetic field peaked after the specific kinetic vorticity of plasma flows in the AR reached its maximum level (4 hour time delay). In turn, gradual increase of coronal intermittency occurred after the peak of the photospheric intermittency. The time delay between the peak of photospheric intermittency and the occurrence of the first strong (X3.4) flare was approximately 1.3 days. Our analysis seems to suggest that the enhancement of intermittency/complexity first occurs in the photosphere and is later transported toward the corona. </p> Valentyna Abramenko, Vasyl Yurchyshyn, Haimin Wang http://arxiv.org/abs/0907.4942 <p>We evaluate the Casimir energy and force for a massive fermionic field in the geometry of two parallel plates on background of Minkowski spacetime with an arbitrary number of toroidally compactified spatial dimensions. The bag boundary conditions are imposed on the plates and periodicity conditions with arbitrary phases are considered along the compact dimensions. The Casimir energy is decomposed into purely topological, single plate and interaction parts. With independence of the lengths of the compact dimensions and the phases in the periodicity conditions, the interaction part of the Casimir energy is always negative. In order to obtain the resulting force, the contributions from both sides of the plates must be taken into account. Then, the forces coming from the topological parts of the vacuum energy cancel out and only the interaction term contributes to the Casimir force. Applications of the general formulae to Kaluza-Klein type models and carbon nanotubes are given. In particular, we show that for finite length metallic nanotubes the Casimir forces acting on the tube edges are always attractive, whereas for semiconducting-type ones they are attractive for small lengths of the nanotube and repulsive for large lengths. </p> S. Bellucci, A. A. Saharian http://arxiv.org/abs/0908.3291 <p>We investigate the Wightman function, the vacuum expectation values of the field squared and the energy-momentum tensor for a massless scalar field with general curvature coupling parameter in spatially flat Friedmann-Robertson-Walker universes with an arbitrary number of toroidally compactified dimensions. The topological parts in the expectation values are explicitly extracted and in this way the renormalization is reduced to that for the model with trivial topology. In the limit when the comoving lengths of the compact dimensions are very short compared to the Hubble length, the topological parts coincide with those for a conformal coupling and they are related to the corresponding quantities in the flat spacetime by standard conformal transformation. In the opposite limit of large comoving lengths of the compact dimensions, in dependence of the curvature coupling parameter, two regimes are realized with monotonic or oscillatory behavior of the vacuum expectation values. In the monotonic regime and for nonconformally and nonminimally coupled fields the vacuum stresses are isotropic and the equation of state for the topological parts in the energy density and pressures is of barotropic type. In the oscillatory regime, the amplitude of the oscillations for the topological part in the expectation value of the field squared can be either decreasing or increasing with time, whereas for the energy-momentum tensor the oscillations are damping. </p> A. A. Saharian, A. L. Mkhitaryan http://arxiv.org/abs/0912.5139 <p>For scalar and electromagnetic fields we evaluate the vacuum expectation value of the energy-momentum tensor induced by a curved boundary in the Robertson--Walker spacetime with negative spatial curvature. In order to generate the vacuum densities we use the conformal relation between the Robertson-Walker and Rindler spacetimes and the corresponding results for a plate moving by uniform proper acceleration through the Fulling--Rindler vacuum. For the general case of the scale factor the vacuum energy-momentum tensor is presented as the sum of the boundary free and boundary induced parts. </p> A. A. Saharian, M. R. Setare http://arxiv.org/abs/1001.4304 <p>What is physics? What are the limits of what physics can say about the world? In seeking ever-broader theoretical `umbrellas' for physical phenomena, we are seeking unifying principles. Emergent phenomena have turned out to be some of the most difficult to explain, causing a `clash of umbrellas' so-to-speak, at the interface between the quantum and classical domains. This essay explores the role of articulation in this particularly vexing problem and ultimately addresses the question of whether the language and mathematics we use to describe the universe is sufficient in its present form and application. </p> Ian T. Durham http://arxiv.org/abs/1002.0062 <p>We consider instanton solutions of Euclidean Horava-Lifshitz gravity in four dimensions satisfying the detailed balance condition. They are described by geometric flows in three dimensions driven by certain combinations of the Cotton and Ricci tensors as well as the cosmological-constant term. The deformation curvature terms can have competing behavior leading to a variety of fixed points. The instantons interpolate between any two fixed points, which are vacua of topologically massive gravity with Lambda &gt; 0, and their action is finite. Special emphasis is placed on configurations with SU(2) isometry associated with homogeneous but generally non-isotropic Bianchi IX model geometries. In this case, the combined Ricci-Cotton flow reduces to an autonomous system of ordinary differential equations whose properties are studied in detail for different couplings. The occurrence and stability of isotropic and anisotropic fixed points are investigated analytically and some exact solutions are obtained. The corresponding instantons are classified and they are all globally R x S^3 and complete spaces. Generalizations to higher-dimensional gravities are also briefly discussed. </p> Ioannis Bakas, Francois Bourliot, Dieter Lust, Marios Petropoulos