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At least 343 records · Page 19Linked to original sources

arXiv2013

Novel considerations about the error budget of the LAGEOS-based tests of frame-dragging with GRACE geopotential models

A realistic assessment of the uncertainties in the even zonals of a given geopotential model must be made by directly comparing its coefficients with those of a wholly independent solution of superior formal accuracy. Otherwise, a favorable selective bias is introduced in the evaluation of the total error budget of the LAGEOS-based Lense-Thirring tests yielding likely too optimistic figures for it. By applying a novel approach which recently appeared in the literature, the second (L = 4) and the third (L = 6) even zonals turn out to be uncertain at a 2-3 10^-11 (L = 4) and 3-4 10^-11 (L = 6) level, respectively, yielding a total gravitational error of about 27-28%, with an upper bound of 37-39%. The results by Ries et al. themselves yield an upper bound for it of about 33%. The low-degree even zonals are not exclusively determined from the GRACE Satellite-to-Satellite Tracking (SST) range since they affect it with long-period, secular-like signatures over orbital arcs longer than one orbital period: GRACE SST is not accurately sensitive to such signals. Conversely, general relativity affects it with short-period effects as well. Thus, the issue of the a priori "imprinting" of general relativity itself in the GRACE-based models used so far remains open.

gr-qc↗

arXiv2013

Unruh effect in vacua with anisotropic scaling: Applications to multilayer graphene

We extend the calculation of the Unruh effect to the universality classes of quantum vacua obeying topologically protected invariance under anisotropic scaling ${\bf r} \rightarrow b {\bf r}$, $t \rightarrow b^z t$. Two situations are considered. The first one is related to the accelerated detector which detects the electron - hole pairs. The second one is related to the system in external electric field, when the electron - hole pairs are created due to the Schwinger process. As distinct from the Unruh effect in relativistic systems (where $z=1$) the calculated radiation is not thermal, but has properties of systems in the vicinity of quantum criticality. The vacuum obeying anisotropic scaling can be realized, in particular, in multilayer graphene with the rhombohedral stacking. Opportunities of the experimental realization of Unruh effect in this situation are discussed.

cond-mat.mes-hall↗

arXiv2013

On the interaction of the Dirac oscillator with the Aharonov-Casher system in topological defect backgrounds

In this paper, we study the influence of the Aharonov-Casher effect [Y. Aharonov and A. Casher, Phys. Rev. Lett. 53, 319 (1984).] on the Dirac oscillator in three different scenarios of general relativity: the Minkowski spacetime, the cosmic string spacetime and the cosmic dislocation spacetime. In this way, we solve the Dirac equation and obtain the energy levels for bound states and the Dirac spinors for positive-energy solutions. We show that the relativistic energy levels depend on the Aharonov-Casher geometric phase. We also discuss the influence of curvature and torsion on the relativistic energy levels and the Dirac spinors due to the topology of the cosmic string and cosmic dislocation spacetimes.

quant-ph↗

arXiv2013

An Estimate of Λin Resummed Quantum Gravity in the Context of Asymptotic Safety

We show that, by using recently developed exact resummation techniques based on the extension of the methods of Yennie, Frautschi and Suura to Feynman's formulation of Einstein's theory, we get quantum field theoretic descriptions for the UV fixed-point behaviors of the dimensionless gravitational and cosmological constants postulated by Weinberg. Connecting our work to the attendant phenomenological asymptotic safety analysis of Planck scale cosmology by Bonanno and Reuter, we estimate the value of the cosmological constant Λ. We find the encouraging estimate ρ_Λ\equiv \fracΛ{8πG_N} \simeq (2.4\times 10^{-3}eV)^4. While this numerical value is close to recent experimental observations, we caution the reader that the estimate involves a number of model parameters that still possess significant levels of uncertainty, such as the value of the transition time between the Planck scale cosmology era and the Friedmann-Robertson-Walker radiation dominated era, where our current understanding allows for at least two orders of magnitude in its uncertainty and this would change our estimate of ρ_Λby at least four orders of magnitude. We discuss such theoretical uncertainties as well. We show why GUT and EW scale vacuum energies from spontaneous symmetry breaking are suppressed in our approach to the estimation of ρ_Λ. As a bonus, we show how our estimate constrains susy GUTS.

gr-qc↗

arXiv2012

Comments on the Canonical Measure in Cosmology

In the mini-superspace approximation to cosmology, the canonical measure can be used to compute probabilities when a cutoff is introduced in the phase space to regularize the divergent measure. However, the region initially constrained by a simple cutoff evolves non-trivially under the Hamiltonian flow. We determine the deformation of the regularized phase space along the orbits when a cutoff is introduced for the scale factor of the universe or for the Hubble parameter. In the former case, we find that the cutoff for the scale factor varies in the phase space and effectively decreases as one evolves backwards in time. In the later case, we calculate the probability of slow-roll inflation in a chaotic model with a massive scalar, which turns out to be cutoff dependent but not exponentially suppressed. We also investigate the measure problem for non-abelian gauge fields giving rise to inflation.

gr-qc↗

arXiv2012

Entropy and temperatures of Nariai black hole

The statistical entropy of the Nariai black hole in a thermal equilibrium is calculated by using the brick-wall method. Even if the temperature depends on the choice of the time-like Killing vector, the entropy can be written by the ordinary area law which agrees with the Wald entropy. We discuss some physical consequences of this result and the properties of the temperatures.

gr-qc↗

arXiv2013

Perturbative quantum gravity in Batalin-Vilkovisky formalism

In this Letter we consider the perturbative quantum gravity on the super-manifold which remains invariant under absolutely anticommuting BRST and anti-BRST transformations. In addition to that the theory posses one more symmetry known as shift symmetry. The BRST invariant Batalin-Vilkovisky (BV) action for perturbative quantum gravity is realized as a translation in Grassmann coordinate. Further we show that the quantum master equation of the BV quantization method at one-loop order can be translated to have a superfield structure for the action. However, the BRST as well as anti-BRST invariant BV action is constructed in superspace with the help of two Grassmann coordinates.

hep-th↗

arXiv2013

The toroidal Hausdorff dimension of 2d Euclidean quantum gravity

The lengths of shortest non-contractible loops are studied numerically in 2d Euclidean quantum gravity on a torus coupled to conformal field theories with central charge less than one. We find that the distribution of these geodesic lengths displays a scaling in agreement with a Hausdorff dimension given by the formula of Y. Watabiki.

hep-th↗

arXiv2013

Wess-Zumino Inflation in Light of Planck

We discuss cosmological inflation in the minimal Wess-Zumino model with a single massive chiral supermultiplet. With suitable parameters and assuming a plausible initial condition at the start of the inflationary epoch, the model can yield scalar perturbations in the Cosmic Microwave Background (CMB) of the correct strength with a spectral index n_s ~ 0.96 and a tensor-to-scalar perturbation ratio r < 0.1, consistent with the Planck CMB data. We also discuss the possibility of topological inflation within the Wess-Zumino model, and the possibility of combining it with a seesaw model for neutrino masses. This would violate R-parity, but at such a low rate that the lightest supersymmetric particle would have a lifetime long enough to constitute the astrophysical cold dark matter.

astro-ph.CO↗

arXiv2013

Effective $F(T)$ gravity from the higher-dimensional Kaluza-Klein and Randall-Sundrum theories

We explore the four-dimensional effective $F(T)$ gravity with $T$ the torsion scalar in teleparallelism originating from higher-dimensional space-time theories, in particular the Kaluza-Klein (KK) and Randall-Sundrum (RS) theories. First, through the KK dimensional reduction from the five-dimensional space-time, we obtain the four-dimensional effective theory of $F(T)$ gravity with its coupling to a scalar field. Second, taking the RS type-II model in which there exist the five-dimensional Anti-de Sitter (AdS) space-time with four-dimensional Friedmann-Lemaître-Robertson-Walker (FLRW) brane, we find that there will appear the contribution of $F(T)$ gravity on the four-dimensional FLRW brane. It is demonstrated that inflation and the dark energy dominated stage can be realized in the KK and RS models, respectively, due to the effect of only the torsion in teleparallelism without that of the curvature.

gr-qc↗

arXiv2012

New Constraints on Preferred Frame Effects from Binary Pulsars

Preferred frame effects (PFEs) are predicted by a number of alternative gravity theories which include vector or additional tensor fields, besides the canonical metric tensor. In the framework of parametrized post-Newtonian (PPN) formalism, we investigate PFEs in the orbital dynamics of binary pulsars, characterized by the two strong-field PPN parameters, α_1 and α_2. In the limit of a small orbital eccentricity, α_1 and α_2 contributions decouple. By utilizing recent radio timing results and optical observations of PSRs J1012+5307 and J1738+0333, we obtained the best limits of α_1 and α_2 in the strong-field regime. The constraint on α_1 also surpasses its counterpart in the weak-field regime.

gr-qc↗

arXiv2013

Can Dark Energy emerge from quantum effects in compact extra dimension ?

The origin of the observed acceleration of the expansion of the universe is a major problem of modern cosmology and theoretical physics. Simple estimations of the contribution of vacuum to the density energy of the universe in quantum field theory are known to lead to catastrophic large values compared to observations. Such a contribution is therefore generally not regarded as a viable source for the acceleration of the expansion. In this letter we propose that the vacuum contribution actually provides a small positive value to the density energy of the universe. The underlying mechanism is a manifestation of the quantum nature of the gravitational field, through a Casimir-like effect from an additional compact dimension of space. A key ingredient is to assume that only modes with wavelength shorter than the Hubble length contribute to the vacuum. Such a contribution gives a positive energy density, has a Lorentz invariant equation of state in the usual 4D spacetime and hence can be interpreted as a cosmological constant. Its value agrees with observations for a radius of a 5th extra dimension given by $35\,μ$m. This implies a modification of the gravitational inverse square law around this scale, close but below existing limits from experiments testing gravity at short range.

astro-ph.CO↗

arXiv2013

Experimental constraints on the uncoupled Galileon model from SNLS3 data and other cosmological probes

The Galileon model is a modified gravity theory that may provide an explanation for the accelerated expansion of the Universe. This model does not suffer from instabilities or ghost problems (normally associated with higher-order derivative theories), restores local General Relativity -- thanks to the Vainshtein screening effect -- and predicts late time acceleration of the expansion. In this paper, we derive a new definition of the Galileon parameters that allows us to avoid having to choose initial conditions for the Galileon field, and then test this model against precise measurements of the cosmological distances and the rate of growth of cosmic structures. We observe a small tension between the constraints set by growth data and those from distances. However, we find that the Galileon model remains consistent with current observations and is still competitive with the ΛCDM model, contrary to what was concluded in recent publications.

gr-qc↗

arXiv2010

Towards a noncommutative version of Gravitation

Alain Connes' noncommutative theory led to an interesting model including both Standard Model of particle physics and Euclidean Gravity. Nevertheless, an hyperbolic version of the gravitational part would be necessary to make physical predictions, but it is still under research. We shall present the difficulties to generalize the model from Riemannian to Lorentzian Geometry and discuss key ideas and current attempts.

math-ph↗

arXiv2013

Isotropic universe with almost scale-invariant fourth-order gravity

We study a broad class of isotropic vacuum cosmologies in fourth-order gravity under the condition that the gravitational Lagrangian be scale-invariant or almost scale-invariant. The gravitational Lagrangians considered will be of the form L = f(R) + k(G) where R and G are the Ricci and Gauss-Bonnet scalars respectively. Specifically we take f(R) = R^2n and k(G) = G^n or k(G) = G ln G. We find solutions in closed form for a spatially flat Friedmann space-time and interpret their asymptotic early-time and late-time behaviour as well as their inflationary stages. One unique example which we discuss is the case of a very small negative value of the parameter b in the Lagrangian L = R^2 + b G ln G which leads to the replacement of the exact de Sitter solution from L = R^2 (being a local attractor) to a power-law inflation exact solution also representing a local attractor. This shows how one can modify the dynamics from de Sitter to power-law inflation by the addition of the G ln G-term.

gr-qc↗

arXiv2013

Covariant Differential Identities and Conservation Laws in Metric-Torsion Theories of Gravitation. I. General Consideration

Arbitrary diffeomorphically invariant metric-torsion theories of gravity are considered. It is assumed that Lagrangians of such theories contain derivatives of field variables (tensor densities of arbitrary ranks and weights) up to a second order only. The generalized Klein-Noether methods for constructing manifestly covariant identities and conserved quantities are developed. Manifestly covariant expressions are constructed without including auxiliary structures like a background metric. In the Riemann-Cartan space, the following \emph{manifestly generally covariant results} are presented: (a) The complete generalized system of differential identities (the Klein-Noether identities) is obtained. (b) The generalized currents of three types depending on an arbitrary vector field displacements are constructed: they are the canonical Noether current, symmetrized Belinfante current and identically conserved Hilbert-Bergmann current. In particular, it is stated that the symmetrized Belinfante current does not depend on divergences in the Lagrangian. (c) The generalized boundary Klein theorem (third Noether theorem) is proved. (d) The construction of the generalized superpotential is presented in details, and questions related to its ambiguities are analyzed.

gr-qc↗

arXiv2013

The Algebraic Index Theorem and Fedosov Quantization of Lagrange-Finsler and Einstein Spaces

Various types of Lagrange and Finsler geometries and the Einstein gravity theory, and modifications, can be modelled by nonholonomic distributions on tangent bundles/ manifolds when the fundamental geometric objects are adapted to nonlinear connection structures. We can convert such geometries and physical theories into almost Kahler/ Poisson structures on (co)tangent bundles. This allows us to apply the Fedosov quantization formalism to almost symplectic connections induced by Lagrange-Finsler and/or Einstein fundamental geometric objects. There are constructed respective nonholonomic versions of the trace density maps for the zeroth Hochschild homology of deformation quantization of distinguished algebras (in this work, adapted to nonlinear connection structure). Our main result consists in an algebraic index theorem for Lagrange-Finsler and Einstein spaces. Finally, we show how the Einstein field equations for gravity theories and geometric mechanics models can be imbedded into the formalism of deformation quantization and index theorem.

math-ph↗

arXiv2011

Structure Formation in the Symmetron Model

Scalar fields, strongly coupled to matter, can be present in nature and still be invisible to local experiments if they are subject to a screening mechanism. The symmetron is one such mechanism which relies on restoration of a spontaneously broken symmetry in regions of high density to shield the scalar fifth force. We have investigated structure formation in the symmetron model by using N-body simulations and find strong observable signatures in both the linear and nonlinear matter power spectrum and on the halo mass function. The mechanism for suppressing the scalar fifth force in high density regions is also found to work very well.

astro-ph.CO↗