In this communication, we generalize previous work on tensor perturbations in a de Sitter background in terms of Ashtekar variables to cover all complex values of the Immirzi parameter γ (previous work was restricted to imaginary γ). Particular attention is paid to the case of real γ. Following the same approach as in the imaginary case, we can obtain physical graviton states by invoking reality and torsion-free conditions. The Hamiltonian in terms of graviton states has the same form whether γ has a real part or not; however, changes occur for the vacuum energy and fluctuations. Specifically, we observe a γ-dependent chiral asymmetry in the vacuum fluctuations only if γ has an imaginary part. Ordering prescriptions also change this asymmetry. We thus present a measurable result for CMB polarization experiments that could shed light on the workings of quantum gravity.

We derive necessary criteria for the existence of classical meta-stable de Sitter solutions in flux compactifications of type II supergravity down to dimensions higher than 4. We find that the possibilities in higher dimensions are much more restricted than in four dimensions. The only models that satisfy the criteria are derived from O6 compactifications to D = 5, 6 and O5 compactifications to D = 5, and no meta-stable solutions can exist in dimensions higher than 6. All these models have in common that the compact dimensions are negatively curved.

We show that the supersymmetric near horizon black hole geometries of six-dimensional supergravity coupled to any number of scalar and tensor multiplets are either locally AdS ₃ × Σ ³, where Σ ³ is a homology 3-sphere, or , where is a 4-manifold whose geometry depends on the hypermultiplet scalars. In both cases, we find that the tensorini multiplet scalars are constant and the associated 3-form field strengths vanish. We also demonstrate that the AdS ₃ × Σ ³ horizons preserve two, four and eight supersymmetries. For horizons with four supersymmetries, Σ ³ is in addition a non-trivial circle fibration over a topological 2-sphere. The near horizon geometries preserving eig...

In this paper, we investigate the dynamical formation and evolution of (2 + 1)-dimensional charged black holes. We numerically study dynamical collapses of charged matter fields in an anti-de Sitter background and note the formation of black holes using the double-null formalism. Moreover, we include renormalized energy–momentum tensors assuming the S-wave approximation to determine thermodynamical back-reactions to the internal structures. If there are no semi-classical effects, the amount of charge determines the causal structures. If the charge is sufficiently small, the causal structure has a space-like singularity. However, as the charge increases, an inner Cauchy horizon appears. If we have sufficient charge, we see a space-like outer horizon and a time-like inner horizon, and if we give excessive charge, black hole horizons disappear. We have some circumstantial evidence that weak cosmic censorship is still satisfied, even for such excessive charge cases. Also, we confirm ...

In perturbative QED, the approximation is improved by summing more Feynman graphs, while in non-perturbative QCD, by refining the lattice. Here we observe that in quantum gravity, the two procedures may well be the same. We outline the combinatorial structure of spinfoam quantum gravity, define the continuum limit and show that under general conditions, refining foams is the same as summing over them. The conditions bear on the cylindrical consistency of the spinfoam amplitudes and on the presence of appropriate combinatorial factors, related to the implementation of diffeomorphism invariance. Intuitively, the sites of the lattice are points of space: these are themselves quanta of the gravitational field, and thus a lattice discretization is also a Feynman history of quanta.

The observer-dependent tidal effects associated with the electric and magnetic parts of the Riemann tensor with respect to an arbitrary family of observers are discussed in a general spacetime in terms of certain ‘tidal indicators’. The features of such indicators are then explored by specializing our considerations to the family of stationary circularly rotating observers in the equatorial plane of the Kerr spacetime. There exist a number of observer families which are special for several reasons and for each of them such indicators are evaluated. The transformation laws of tidal indicators when passing from one observer to another are also discussed, clarifying the interplay among them. Our analysis shows that no equatorial plane circularly rotating observer in the Kerr spacetime can ever measure a vanishing tidal electric indicator, whereas the family of Carter’s observers measures zero tidal magnetic indicator.

One of the major obstacles to the detection and study of gravitational waves using ground-based laser interferometers is the effect of seismic noise on instrument sensitivity. Environmental disturbances cause motion of the interferometer optics, coupling as noise in the gravitational wave data output whose magnitude can be much greater than that of an astrophysical signal. We present an improved method of identifying times of high seismic noise coupling by tuning a gravitational-wave burst detection algorithm to the low-frequency signature of these events and testing for coincidence with a low-latency compact binary coalescence detection algorithm. This method has been proven highly effective in removing transients of seismic origin, with 60% of all compact binary coalescence candidate events correlated with seismic noise in just 6% of analysis time

We define pure radiation metrics with parallel rays to be n-dimensional pseudo-Riemannian metrics that admit a parallel null line bundle K and whose Ricci tensor vanishes on vectors that are orthogonal to K. We give necessary conditions in terms of the Weyl, Cotton and Bach tensors for a pseudo-Riemannian metric to be conformal to a pure radiation metric with parallel rays. Then, we derive conditions in terms of the tractor calculus that are equivalent to the existence of a pure radiation metric with parallel rays in a conformal class. We also give analogous results for n-dimensional pseudo-Riemannian pp-waves.

We consider the possibility of deriving a decoupled equation in terms of Weyl tensor components for gravitational perturbations of the Schwarzschild–Tangherlini solution. We find that a particular gauge invariant component of the Weyl tensor does decouple and argue that this corresponds to the vector modes of Ishibashi and Kodama. Also, we construct a Hertz potential map for solutions of the electromagnetic and gravitational perturbation equations of a higher-dimensional Kundt background using the decoupled equation of Durkee and Reall. Motivated by the recent work of Guica and Strominger, we use this to construct the asymptotic behaviour of metric perturbations of the NHG of the 5D cohomogeneity-1 Myers–Perry black hole.

The German–British interferometric gravitational wave detector GEO 600 is currently undergoing upgrades within the GEO-HF upgrade program. The goal of this program is to enhance the sensitivity of GEO 600, in particular at frequencies above 500 Hz. At these frequencies, the detector is limited by shot noise. An important element of the upgrade is the implementation of an output mode cleaner (OMC). This filtering cavity suppresses higher order spatial and temporal modes in the interferometer’s output beam, thereby reducing the shot noise and enhancing sensitivity. Fully automated lock acquisition for the OMC was developed in order to ensure a high duty cycle of GEO 600. To maintain optimum sensitivity, the resonance condition of the OMC must be matched to the output beam from the interferometer. This requires continuous control of the resonance frequency of the OMC to the light, and alignment of the beam to the OMC. We describe the design and implementation of the OMC with special...