Change search
Refine search result
1 - 13 of 13
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1. Akrami, Yashar
    et al.
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Solomon, Adam R.
    The nature of spacetime in bigravity: Two metrics or none?2015In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 47, no 1, p. 1838-Article in journal (Refereed)
    Abstract [en]

    The possibility of matter coupling to two metrics at once is considered. This appears natural in the most general ghost-free, bimetric theory of gravity, where it unlocks an additional symmetry with respect to the exchange of the metrics. This double coupling, however, raises the problem of identifying the observables of the theory. It is shown that if the two metrics couple minimally to matter, then there is no physical metric to which all matter would universally couple, and that moreover such an effective metric generically does not exist even for an individual matter species. By studying point particle dynamics, a resolution is suggested in the context of Finsler geometry.

  • 2.
    Bengtsson, Ingemar
    Stockholm University, Faculty of Science, Department of Physics.
    The Hawking energy on photon surfaces2020In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 52, no 5, article id 52Article in journal (Refereed)
    Abstract [en]

    The Hawking energy has a monotonicity property under the inverse mean curvature flow on totally umbilic hypersurfaces with constant scalar curvature in Einstein spaces. It grows if the hypersurface is spacelike, and decreases if it is timelike. Timelike examples include Minkowski and de Sitter hyperboloids, and photon surfaces in Schwarzschild.

  • 3.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    A Toy Penrose Inequality and Its Proof2016In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 48, no 12, article id 156Article in journal (Refereed)
    Abstract [en]

    We formulate and prove a toy version of the Penrose inequality. The formulation mimics the original Penrose inequality in which the scenario is the following: A shell of null dust collapses in Minkowski space and a marginally trapped surface forms on it. Through a series of arguments relying on established assumptions, an inequality relating the area of this surface to the total energy of the shell is formulated. Then a further reformulation turns the inequality into a statement relating the area and the outer null expansion of a class of surfaces in Minkowski space itself. The inequality has been proven to hold true in many special cases, but there is no proof in general. In the toy version here presented, an analogous inequality in (2+1)-dimensional anti-de Sitter space turns out to hold true.

  • 4. Bini, Donato
    et al.
    Gregoris, Daniele
    Stockholm University, Faculty of Science, Department of Physics.
    Rosquist, Kjell
    Stockholm University, Faculty of Science, Department of Physics.
    Succi, Sauro
    Particle motion in a photon gas: friction matters2012In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 44, no 10, p. 2669-2680Article in journal (Refereed)
    Abstract [en]

    The motion of a particle in the Tolman metric generated by a photon gas source is discussed. Both the case of geodesic motion and motion with nonzero friction, due to photon scattering effects, are analyzed. In the Minkowski limit, the particle moves along a straight line segment with a decelerated motion, reaching the endpoint at zero speed. The curved case shows a qualitatively different behavior; the geodesic motion consists of periodic orbits, confined within a specific radial interval. Under the effect of frictional drag, this radial interval closes up in time and in all our numerical simulations the particle ends up in the singularity at the center.

  • 5. Clifton, Timothy
    et al.
    Gregoris, Daniele
    Rosquist, Kjell
    Stockholm University, Faculty of Science, Department of Physics.
    The magnetic part of the Weyl tensor, and the expansion of discrete universes2017In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 49, no 2, article id 30Article in journal (Refereed)
    Abstract [en]

    We examine the effect that the magnetic part of the Weyl tensor has on the large-scale expansion of space. This is done within the context of a class of cosmological models that contain regularly arranged discrete masses, rather than a continuous perfect fluid. The natural set of geodesic curves that one should use to consider the cosmological expansion of these models requires the existence of a non-zero magnetic part of the Weyl tensor. We include this object in the evolution equations of these models by performing a Taylor series expansion about a hypersurface where it initially vanishes. At the same cosmological time, measured as a fraction of the age of the universe, we find that the influence of the magnetic part of the Weyl tensor increases as the number of masses in the universe is increased. We also find that the influence of the magnetic part of the Weyl tensor increases with time, relative to the leading-order electric part, so that its contribution to the scale of the universe can reach values of similar to 1%, before the Taylor series approximation starts to break down.

  • 6. Hu, Shi-Qian
    et al.
    Kuang, Xiao-Mei
    Ong, Yen Chin
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yangzhou University, China; Shanghai Jiao Tong University, China.
    A note on Smarr relation and coupling constants2019In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 51, no 5, article id 55Article in journal (Refereed)
    Abstract [en]

    The Smarr relation plays an important role in black hole thermodynamics. It is often claimed that the Smarr relation can be written down simply by observing the scaling behavior of the various thermodynamical quantities. We point out that this is not necessarily so in the presence of dimensionful coupling constants, and discuss the issues involving the identification of thermodynamical variables.

  • 7. Jolin, Shan W.
    et al.
    Rosquist, Kjell
    Stockholm University, Faculty of Science, Department of Physics.
    Analytic analysis of irregular discrete universes2018In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 50, no 9, article id 115Article in journal (Refereed)
    Abstract [en]

    In this work we investigate the dynamics of cosmological models with spherical topology containing up to 600 Schwarzschild black holes arranged in an irregular manner. We solve the field equations by tessellating the 3-sphere into eight identical cells, each having a single edge which is shared by all cells. The shared edge is enforced to be locally rotationally symmetric, thereby allowing for solving the dynamics to high accuracy along this edge. Each cell will then carry an identical (up to parity) configuration which can however have an arbitrarily random distribution. The dynamics of such models is compared to that of previous works on regularly distributed black holes as well as with the standard isotropic dust models of the FLRW type. The irregular models are shown to have richer dynamics than that of the regular models. The randomization of the distribution of the black holes is done both without bias and also with a certain clustering bias. The geometry of the initial configuration of our models is shown to be qualitatively different from the regular case in the way it approaches the isotropic model.

  • 8. Khosravi, Sh.
    et al.
    Kourkchi, E.
    Mansouri, R.
    Akrami, Yashar
    Stockholm University, Faculty of Science, Department of Physics.
    An inhomogeneous model universe behaving homogeneously2008In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 40, no 5, p. 1047-1069Article in journal (Refereed)
    Abstract [en]

    We present a new model universe based on the junction of FRW to flat Lemaitre-Tolman-Bondi (LTB) solutions of Einstein equations along our past light cone, bringing structures within the FRW models. The model is assumed globally to be homogeneous, i.e. the cosmological principle is valid. Local inhomogeneities within the past light cone are modeled as a flat LTB, whereas those outside the light cone are assumed to be smoothed out and represented by a FRW model. The model is singularity free, always FRW far from the observer along the past light cone, gives way to a different luminosity distance relation as for the CDM/FRW models, a negative deceleration parameter near the observer, and correct linear and non-linear density contrast. As a whole, the model behaves like a FRW model on the past light cone with a special behavior of the scale factor, Hubble and deceleration parameter, mimicking dark energy.

  • 9.
    Koivisto, Tomi S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Tamanini, Nicola
    A note on viability of nonminimally coupled f(R) theory2016In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 48, no 7, article id 97Article in journal (Refereed)
    Abstract [en]

    Consistency conditions for nonminimally coupled f(R) theories have been derived by requiring the absence of tachyons and instabilities in the scalar fluctuations. This note confirms these results and clarifies a subtlety regarding different definitions of sound speeds.

  • 10.
    Ong, Yen Chin
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    The persistence of the large volumes in black holes2015In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 47, no 8, article id 88Article in journal (Refereed)
    Abstract [en]

    Classically, black holes admit maximal interior volumes that grow asymptotically linearly in time. We show that such volumes remain large when Hawking evaporation is taken into account. Even if a charged black hole approaches the extremal limit during this evolution, its volume continues to grow; although an exactly extremal black hole does not have a large interior. We clarify this point and discuss the implications of our results to the information loss and firewall paradoxes.

  • 11.
    Pidokrajt, Narit
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Åman, Jan E.
    Stockholm University, Faculty of Science, Department of Physics.
    Bengtsson, Ingemar
    Stockholm University, Faculty of Science, Department of Physics.
    Geometry of black hole thermodynamics2003In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 35, no 10, p. 1733-1743Article in journal (Refereed)
    Abstract [en]

    The Hessian of the entropy function can be thought of as a metric tensor on the state space. In the context of thermodynamical fluctuation theory Ruppeiner has argued that the Riemannian geometry of this metric gives insight into the underlying statistical mechanical system; the claim is supported by numerous examples. We study this geometry for some families of black holes. It is flat for the BTZ and Reissner-Nordstrom black holes, while curvature singularities occur for the Reissner-Nordstrom-anti-de Sitter and Kerr black holes.

  • 12.
    Rosquist, Kjell
    Stockholm University, Faculty of Science, Department of Physics.
    A unifying coordinate family for the Kerr-Newman metric2009In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 41, p. 2619-2632Article in journal (Refereed)
    Abstract [en]

    A new unified metric form is presented for the Kerr–Newman geometry. The new form is a generalization of the Boyer–Lindquist metric involving an arbitrary gauge function of the spheroidal radial variable. Each choice of the gauge function corresponds to a coordinate system including four of the most important coordinate systems for Kerr–Newman (Boyer–Lindquist, Kerr, Kerr–Schild and Doran coordinates). The representation is given in terms of a single Minkowski frame together with the gauge function. This Minkowski frame arises by boosting a static orthonormal frame which is adapted to spheroidal coordinates. Properties of the boost reflect the rotating nature of the Kerr–Newman solution including an identification of the angular velocities of the disk and the horizon matching previously known values obtained in other ways.

  • 13.
    Åman, Jan E.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bengtsson, Ingemar
    Stockholm University, Faculty of Science, Department of Physics.
    Pidokrajt, Narit
    Stockholm University, Faculty of Science, Department of Physics.
    Flat information geometries in black hole thermodynamics2006In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 38, no 8, p. 1305-1315Article in journal (Refereed)
    Abstract [en]

    The Hessian of either the entropy or the energy function can be regarded as a metric on a Gibbs surface. For two parameter families of asymptotically flat black holes in arbitrary dimension one or the other of these metrics are flat, and the state space is a flat wedge. The mathematical reason for this is traced back to the scale invariance of the Einstein–Maxwell equations. The picture of state space that we obtain makes some properties such as the occurence of divergent specific heats transparent.

1 - 13 of 13
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf