| 1. |
- Bradley, Michael, et al.
(author)
-
Density growth in Kantowski-Sachs cosmologies with cosmological constant
- 2012
-
In: Classical and quantum gravity. - Bristol : Institute of Physics (IOP). - 0264-9381 .- 1361-6382. ; 29:9, s. 095023-
-
Journal article (peer-reviewed)abstract
- In this work the growth of density perturbations in Kantowski-Sachs cosmologies with a positive cosmological constant is studied, using the 1+3 and 1+1+2 covariant formalisms. For each wave number we obtain a closed system for scalars formed from quantities that are zero on the background and hence are gauge-invariant. The solutions to this system are then analyzed both analytically andnumerically. In particular the effects of anisotropy and the behaviour close to a bounce in the cosmic scale factor are considered.We find that typically the density gradient in the bouncing direction experiences a local maximum at or slightly after the bounce.
|
|
| 2. |
- Aasi, J., et al.
(author)
-
The characterization of Virgo data and its impact on gravitational-wave searches
- 2012
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 1361-6382 .- 0264-9381. ; 29:15
-
Journal article (peer-reviewed)abstract
- Between 2007 and 2010 Virgo collected data in coincidence with the LIGO and GEO gravitational-wave (GW) detectors. These data have been searched for GWs emitted by cataclysmic phenomena in the universe, by non-axisymmetric rotating neutron stars or from a stochastic background in the frequency band of the detectors. The sensitivity of GW searches is limited by noise produced by the detector or its environment. It is therefore crucial to characterize the various noise sources in a GW detector. This paper reviews the Virgo detector noise sources, noise propagation, and conversion mechanisms which were identified in the three first Virgo observing runs. In many cases, these investigations allowed us to mitigate noise sources in the detector, or to selectively flag noise events and discard them from the data. We present examples from the joint LIGO-GEO-Virgo GW searches to show how well noise transients and narrow spectral lines have been identified and excluded from the Virgo data. We also discuss how detector characterization can improve the astrophysical reach of GW searches.
|
|
| 3. |
- Ames, Ellery, 1984, et al.
(author)
-
On axisymmetric and stationary solutions of the self-gravitating Vlasov system
- 2016
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 33:15
-
Journal article (peer-reviewed)abstract
- Axisymmetric and stationary solutions are constructed to the Einstein-Vlasov and Vlasov-Poisson systems. These solutions are constructed numerically, using finite element methods and a fixed-point iteration in which the total mass is fixed at each step. A variety of axisymmetric stationary solutions are exhibited, including solutions with toroidal, disk-like, spindle-like, and composite spatial density configurations, as are solutions with non-vanishing net angular momentum. In the case of toroidal solutions, we show for the first time, solutions of the Einstein-Vlasov system which contain ergoregions.
|
|
| 4. |
- Andersson, Lars Erik, et al.
(author)
-
Decay of solutions to the Maxwell equation on the Schwarzschild background
- 2016
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 33:8
-
Journal article (peer-reviewed)abstract
- A new Morawetz or integrated local energy decay estimate for Maxwell test fields on the exterior of a Schwarzschild black hole spacetime is proved. The proof makes use of a new superenergy tensor H-ab defined in terms of the Maxwell field and its first derivatives. The superenergy tensor, although not conserved, yields a conserved higher order energy current H-ab (partial derivative(t))(b). The tensor Hab vanishes for the static Coulomb field, and the Morawetz estimate proved here therefore yields integrated decay for the Maxwell field to the Coulomb solution on the Schwarzschild exterior.
|
|
| 5. |
- Andreasson, Håkan, 1966, et al.
(author)
-
A numerical investigation of the stability of steady states and critical phenomena for the spherically symmetric EinsteinVlasov system
- 2006
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 23, s. 3659-3677
-
Journal article (peer-reviewed)abstract
- The stability features of steady states of the spherically symmetric EinsteinVlasov system are investigated numerically. We find support for the conjecture by Zel'dovich and Novikov that the binding energy maximum along a steady state sequence signals the onset of instability, a conjecture which we extend to and confirm for non-isotropic states. The sign of the binding energy of a solution turns out to be relevant for its time evolution in general. We relate the stability properties to the question of universality in critical collapse and find that for Vlasov matter universality does not seem to hold.
|
|
| 6. |
- Andreasson, Håkan, 1966, et al.
(author)
-
A numerical investigation of the steady states of the spherically symmetric Einstein-Vlasov-Maxwell system
- 2009
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 26:14
-
Journal article (peer-reviewed)abstract
- We construct, by numerical means, static solutions of the spherically symmetric Einstein-Vlasov-Maxwell system and investigate various features of the solutions. This extends a previous investigation (Andreasson and Rein 2007 Class. Quantum Grav. 24 1809) of the chargeless case. We study the possible shapes of the energy density profile as a function of the area radius when the electric charge of an individual particle is varied as a parameter. We find profiles which are multi-peaked, where the peaks are separated either by vacuum or a thin atmosphere, and we find that for a sufficiently large charge parameter the solutions break down at a finite radius. Furthermore, we investigate the inequality root M <= root R/3 + root R/9 + Q(2)/3R, which is derived in Andreasson (2009 Commun. Math. Phys. 288 715) for general matter models, and we find that it is sharp for the Einstein-Vlasov-Maxwell system. Here M is the ADM mass, Q is the charge and R is the area radius of the boundary of the static object. We find two classes of solutions with this property, while there is only one in the chargeless case. In particular we find numerical evidence for the existence of arbitrarily thin shell solutions to the Einstein-Vlasov-Maxwell system. Finally, we consider one-parameter families of steady states, and we find spirals in the mass-radius diagram for all examples of the microscopic equation of state which we consider.
|
|
| 7. |
- Andreasson, Håkan, 1966, et al.
(author)
-
Bounds on M/R for charged objects with positive cosmological constant
- 2012
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 29:9
-
Journal article (peer-reviewed)abstract
- We consider charged spherically symmetric static solutions of the Einstein-Maxwell equations with a positive cosmological constant Lambda. If r denotes the area radius, m(g) and q the gravitational mass and charge of a sphere with area radius r respectively, we find that for any solution which satisfies the condition p + 2p(perpendicular to) +/- <= rho, where p >= 0 and p(perpendicular to) are the radial and tangential pressures respectively, rho >= 0 is the energy density, and for which 0 <= q(2)/r(2) + Lambda r(2) <= 1, the inequality m(g)/r <= 2/9 + q(2)/3r(2)-Lambda r(2)/3 + 2/9 root 1 + 3q(2)/r(2) + 3 Lambda r(2) holds. We also investigate the issue of sharpness, and we showthat the inequality is sharp in a few cases but generally this question is open.
|
|
| 8. |
- Andreasson, Håkan, 1966, et al.
(author)
-
Spherically symmetric steady states of John elastic bodies in general relativity
- 2014
-
In: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 31:16
-
Journal article (peer-reviewed)abstract
- We study some properties of static spherically symmetric elastic bodies in general relativity using both analytical and numerical tools. The materials considered belong to the class of John elastic materials and reduce to perfect fluids when the rigidity parameter is set to zero. We find numerical support that such elastic bodies exist with different possible shapes (balls, single shells and multiple shells) and that their gravitational redshift can be very large (z approximate to 2.8) without violating the dominant energy condition. Moreover we show that the elastic body has finite radius even in the case when the constitutive equation of the elastic material is a perturbation of a polytropic fluid without finite radius, thereby concluding that such fluids are structurally unstable within the larger class of elastic matter models under study.
|
|
| 9. |
- Bao, Ling, 1980, et al.
(author)
-
Aspects of higher curvature terms and U-duality
- 2007
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 25:9
-
Journal article (peer-reviewed)abstract
- We discuss various aspects of dimensional reduction of gravity with theEinstein-Hilbert action supplemented by a lowest order deformation formed asthe Riemann tensor raised to powers two, three or four. In the case of R^2 wegive an explicit expression, and discuss the possibility of extended cosetsymmetries, especially SL(n+1,Z) for reduction on an n-torus to threedimensions. Then we start an investigation of the dimensional reduction of R^3and R^4 by calculating some terms relevant for the coset formulation, aiming inparticular towards E_8(8)/(Spin(16)/Z_2) in three dimensions and aninvestigation of the derivative structure. We emphasise some issues concerningthe need for the introduction of non-scalar automorphic forms in order torealise certain expected enhanced symmetries.
|
|
| 10. |
- Bundzik, Daniel, et al.
(author)
-
The geometry of fractional D1-branes
- 2004
-
In: Classical and Quantum Gravity. - : IOP Publishing. - 1361-6382 .- 0264-9381. ; 21:16, s. 3985-3993
-
Journal article (peer-reviewed)abstract
- We find explicit solutions of type IIB string theory on R-4/Z(2) corresponding to the classical geometry of fractional D1-branes. From the supergravity solution obtained, we capture perturbative information about the running of the coupling constant and the metric on the moduli space of N = 4, D = 2 super Yang-Mills.
|
|
