In the last 30 days

• Open/close Results from the LISA phase measurement system project

  David Summers and David Hoyland 2005 Class. Quantum Grav. 22 S249 Tag this article

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  This paper presents some of the more topical results of a study into the LISA phase measurement system. This system is responsible for measuring the phase of the heterodyne signal caused by the interference of the laser beams between the local and far spacecraft. Interactions with the LISA systems that surround the phase measurement system imply additional non-trivial requirements on the phase measurement system.

• Open/close Killing tensors in pp-wave spacetimes

  Aidan J Keane and Brian O J Tupper 2010 Class. Quantum Grav. 27 245011 Tag this article

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  The formal solution of the second-order Killing tensor equations for the general pp-wave spacetime is given. The Killing tensor equations are integrated fully for some specific pp-wave spacetimes. In particular, the complete solution is given for the conformally flat plane wave spacetimes and we find that irreducible Killing tensors arise for specific classes. The maximum number of independent irreducible Killing tensors admitted by a conformally flat plane wave spacetime is shown to be six. It is shown that every pp-wave spacetime that admits an homothety will admit a Killing tensor of Koutras type and, with the exception of the singular scale-invariant plane wave spacetimes, this Killing tensor is irreducible.

• Open/close Emergence of general relativity from loop quantum gravity: a summary

  Chun-Yen Lin 2012 Class. Quantum Grav. 29 082001 Tag this article

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  A model is proposed to demonstrate that classical general relativity can emerge from loop quantum gravity in a relational description of gravitational field in terms of the coordinates given by matter. Local Dirac observables and coherent states are defined to explore the physical content of the model. Expectation values of commutators between the observables for the coherent states recover the four-dimensional diffeomorphism algebra and the large-scale dynamics of the gravitational field relative to the matter coordinates. Both results conform with general relativity up to calculable corrections near singularities.

• Open/close String theory: a perspective over the last 25 years

  Sunil Mukhi 2011 Class. Quantum Grav. 28 153001 Tag this article

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  This review provides some historical background and then reviews developments in string theory over the last 25 years or so. Both perturbative and non-perturbative approaches to string theory are surveyed and their impact on how we view quantum gravity is analysed.

• Open/close Gravitational waves from core collapse supernovae

  Konstantin N Yakunin et al 2010 Class. Quantum Grav. 27 194005 Tag this article

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  We present the gravitational wave signatures for a suite of axisymmetric core collapse supernova models with progenitor masses between 12 and 25 M . These models are distinguished by the fact that they explode and contain essential physics (in particular, multi-frequency neutrino transport and general relativity) needed for a more realistic description. Thus, we are able to compute complete waveforms (i.e. through explosion) based on non-parameterized, first-principles models. This is essential if the waveform amplitudes and time scales are to be computed more precisely. Fourier decomposition shows that the gravitational wave signals we predict should be observable by AdvLIGO across the range of progenitors considered here. The fundamental limitation of these models is in their imposition of axisymmetry. Further progress will require counterpart three-dimensional models.

• Open/close Mass, gauge conditions and spectral properties of the Sen–Witten and 3-surface twistor operators in closed universes

  László B Szabados 2012 Class. Quantum Grav. 29 095001 Tag this article

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  A non-negative expression, built from the norm of the 3-surface twistor operator and the energy–momentum tensor of the matter fields on a spacelike hypersurface, is found which, in the asymptotically flat/hyperboloidal case, provides a lower bound for the ADM/Bondi–Sachs mass, while on closed hypersurfaces coincides with the first eigenvalue of the Sen–Witten operator. Also in the closed case, its vanishing is equivalent to the existence of non-trivial solutions of Witten’s gauge condition. Moreover, it is vanishing if and only if the closed data set is in a flat spacetime with spatial topology S ¹ × S ¹ × S ¹. Thus, it provides a positive definite measure of the strength of the gravitational field (with physical dimension mass) on closed hypersurfaces, i.e. some sort of the total mass of closed universes.

• Open/close DC readout experiment in Enhanced LIGO

  Tobin T Fricke et al 2012 Class. Quantum Grav. 29 065005 Tag this article

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  The two 4 km long gravitational wave detectors operated by the Laser Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to read out the gravitational wave channel using the DC readout form of homodyne detection and to include an optical filter cavity at the output of the detector. As part of the upgrade to Enhanced LIGO, these modifications replaced the radio-frequency (RF) heterodyne system used previously. We describe the motivations for and the implementation of DC readout and the output mode cleaner in Enhanced LIGO. We present characterizations of the system, including measurements and models of the couplings of the noises from the laser source to the gravitational wave readout channel. We show that noise couplings using DC readout are improved over those for RF readout, and we find that the achieved shot-noise-limited sensitivity is consistent with modeled results.

• Open/close Loop quantum gravity: the first 25 years

  Carlo Rovelli 2011 Class. Quantum Grav. 28 153002 Tag this article

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  I give a synthetic presentation of loop quantum gravity. I present the aims of the theory and compare the results obtained with the initial hopes that motivated the early interest in this research direction. I give my own perspective on the status of the program and attempt for a critical evaluation of its successes and limitations.

• Open/close Comment on superluminality in general relativity

  Dieter Lüst and Marios Petropoulos 2012 Class. Quantum Grav. 29 085013 Tag this article

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  General relativity provides an appropriate framework for addressing the issue of sub- or superluminality as an apparent effect. It is shown that even though a massless particle travels on the light cone, its average velocity over a finite path measured by different observers is not necessarily equal to the velocity of light, as a consequence of the time dilation or contraction in gravitational fields. Contrary to the naïve expectation, this phenomenon occurs in either direction (increase or depletion) according to the location of the observer and irrespective of the details and strength of the gravitational interaction.

• Open/close A new symmetry of the relativistic wave equation

  F T Falciano and E Goulart 2012 Class. Quantum Grav. 29 085011 Tag this article

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  In this paper we show that there exists a new symmetry in the relativistic wave equation for a scalar field in arbitrary dimensions. This symmetry is related to redefinitions of the metric tensor which implement a map between non-equivalent manifolds. It is possible to interpret these transformations as a generalization of the conformal transformations. In addition, one can show that this set of manifolds together with the transformation connecting its metrics forms a group. As long as the scalar field dynamics is invariant under these transformations, there immediately appears an ambiguity concerning the definition of the underlying background geometry.