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Cosmological models, nonideal fluids and viscous forces in general relativity
Stockholm University, Faculty of Science, Department of Physics.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis addresses the open questions of providing a cosmological model describing an accelerated expanding Universe without violating the energy conditions or a model that contributes to the physical interpretation of the dark energy. The former case is analyzed considering a closed model based on a regular lattice of black holes using the Einstein equation in vacuum. In the latter case I will connect the dark energy to the Shan-Chen equation of state. A comparison between these two proposals is then discussed. As a complementary topic I will discuss the motion of test particles in a general relativistic spacetime undergoing friction effects. This is modeled following the formalism of Poynting-Robertson whose link with the Stokes’ formula is presented. The cases of geodesic and non-geodesic motion are compared and contrasted for Schwarzschild, Tolman, Pant-Sah and Friedman metrics respectively.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2014. , 94 p.
Keyword [en]
gravity, exact model, black hole, friction
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-107130ISBN: 978-91-7447-982-9 (print)OAI: oai:DiVA.org:su-107130DiVA: diva2:743162
Public defence
2014-11-07, room FP41, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2014-10-14 Created: 2014-09-03 Last updated: 2015-03-17Bibliographically approved
List of papers
1. Radiation pressure vs. friction effects in the description of the Poynting-Robertson scattering process
Open this publication in new window or tab >>Radiation pressure vs. friction effects in the description of the Poynting-Robertson scattering process
2012 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 97, no 4, 40007- p.Article in journal (Refereed) Published
Abstract [en]

The motion of a massive test particle inside a thermal (test) photon gas is studied near a Schwarzschild black hole, leading to a novel description of the effect of radiation scattering on the particle trajectory, responsible for half of the Poynting-Robertson effect: the azimuthal radiation drag. Lacking the outward directed radiation pressure of the latter effect, gravitationally bound orbits always decay, leading to capture by the black hole or the central object generating the exterior Schwarzschild field in which this discussion takes place. Copyright (C) EPLA, 2012

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-107178 (URN)10.1209/0295-5075/97/40007 (DOI)000300844100007 ()
Note

AuthorCount:3;

Available from: 2014-09-05 Created: 2014-09-05 Last updated: 2017-12-05Bibliographically approved
2. Particle motion in a photon gas: friction matters
Open this publication in new window or tab >>Particle motion in a photon gas: friction matters
2012 (English)In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 44, no 10, 2669-2680 p.Article in journal (Refereed) Published
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.

Keyword
Tolman metric, Accelerated motion, Friction forces
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-81270 (URN)10.1007/s10714-012-1425-5 (DOI)000308226700017 ()
Note

AuthorCount:4;

Available from: 2012-10-15 Created: 2012-10-15 Last updated: 2017-12-07Bibliographically approved
3. Effects of friction forces on the motion of objects in smoothly matched interior/exterior spacetimes
Open this publication in new window or tab >>Effects of friction forces on the motion of objects in smoothly matched interior/exterior spacetimes
2013 (English)In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 30, no 2, 025009- p.Article in journal (Refereed) Published
Abstract [en]

We investigate the non-geodesic motion of a test particle inside a fluid in equilibrium, as described by the interior Schwarzschild solution, in the presence of a Poynting-Robertson-like dissipative force. The accelerated motion in the interior spacetime is (regularly) connected to the geodesic one in the corresponding external Schwarzschild background, mimicking physical conditions often occurring in spacetime regions close to stellar or compact objects. A similar analysis is repeated for the case in which the interior Schwarzschild solution is given by a Pant-Sah perfect-fluid solution. This latter case, supporting a polytropic equation of state for the interior fluid source, appears to be more appropriate for modeling situations of either stellar or cosmological interest.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-87698 (URN)10.1088/0264-9381/30/2/025009 (DOI)000313097300009 ()
Note

AuthorCount:4;

Available from: 2013-02-14 Created: 2013-02-14 Last updated: 2017-12-06Bibliographically approved
4. Friction forces in cosmological models
Open this publication in new window or tab >>Friction forces in cosmological models
2013 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 2, 2334- p.Article in journal (Refereed) Published
Abstract [en]

We investigate the dynamics of test particles undergoing friction forces in a Friedmann-Robertson-Walker (FRW) spacetime. The interaction with the background fluid is modeled by introducing a Poynting-Robertson-like friction force in the equations of motion, leading to measurable (at least in principle) deviations of the particle trajectories from geodesic motion. The effect on the peculiar velocities of the particles is investigated for various equations of state of the background fluid and different standard cosmological models. The friction force is found to have major effects on particle motion in closed FRW universes, where it turns the time-asymptotic value (approaching the recollapse) of the peculiar particle velocity from ultra-relativistic (close to light speed) to a co-moving one, i.e., zero peculiar speed. On the other hand, for open or flat universes the effect of the friction is not so significant, because the time-asymptotic peculiar particle speed is largely non-relativistic also in the geodesic case.

Keyword
SUPERNOVAE
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-90802 (URN)10.1140/epjc/s10052-013-2334-9 (DOI)000318289600034 ()
Funder
EU, FP7, Seventh Framework Programme, 2011-1640
Note

AuthorCount:4;

Available from: 2013-06-12 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
5. Dark energy from cosmological fluids obeying a Shan-Chen nonideal equation of state
Open this publication in new window or tab >>Dark energy from cosmological fluids obeying a Shan-Chen nonideal equation of state
2013 (English)In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 88, no 6, 063007- p.Article in journal (Refereed) Published
Abstract [en]

We consider a Friedmann-Robertson-Walker universe with a fluid source obeying a nonideal equation of state with asymptotic freedom, namely ideal gas behavior (pressure changes directly proportional to density changes) both at low and high density regimes, following a fluid dynamical model due to Shan and Chen. It is shown that, starting from an ordinary energy density component, such fluids naturally evolve towards a universe with a substantial dark energy component at the present time, with no need of invoking any cosmological constant. Moreover, we introduce a quantitative indicator of darkness abundance, which provides a consistent picture of the actual matter-energy content of the Universe.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-94731 (URN)10.1103/PhysRevD.88.063007 (DOI)000324237800001 ()
Note

AuthorCount:3;

Funding Agencies:

EACEA of the European Commission 2011-1640 

Available from: 2013-10-10 Created: 2013-10-09 Last updated: 2017-12-06Bibliographically approved
6. Exact evolution of discrete relativistic cosmological models
Open this publication in new window or tab >>Exact evolution of discrete relativistic cosmological models
2013 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 11, 010- p.Article in journal (Refereed) Published
Abstract [en]

We study the effects of inhomogeneities on the evolution of the Universe, by considering a range of cosmological models with discretized matter content. This is done using exact and fully relativistic methods that exploit the symmetries in and about submanifolds of spacetimes that themselves possess no continuous global symmetries. These methods allow us to follow the evolution of our models throughout their entire history, far beyond what has previously been possible. We find that while some space-like curves collapse to anisotropic singularities in finite time, others remain non-singular forever. The resulting picture is of a cosmological spacetime in which some behaviour remains close to Friedmann-like, while other behaviours deviate radically. In particular, we find that large-scale acceleration is possible without any violation of the energy conditions.

Keyword
gravity, cosmological simulations, GR black holes
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-99903 (URN)10.1088/1475-7516/2013/11/010 (DOI)000327843900011 ()
Note

AuthorCount:4;

Funding agencies:

STFC, ST/J001546/1; Erasmus Mundus Joint Doctorate Program from the EACEA of the European Commission, 2011-1640 

Available from: 2014-01-23 Created: 2014-01-20 Last updated: 2017-12-06Bibliographically approved
7. Scalar field inflation and Shan-Chen fluid models
Open this publication in new window or tab >>Scalar field inflation and Shan-Chen fluid models
2014 (English)In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 90, no 4, 044021- p.Article in journal (Refereed) Published
Abstract [en]

A scalar field equivalent to a nonideal “dark energy fluid” obeying a Shan-Chen-like equation of state is used as the background source of a flat Friedmann-Robertson-Walker cosmological spacetime to describe the inflationary epoch of our Universe. Within the slow-roll approximation, a number of interesting features are presented, including the possibility to fulfill current observational constraints as well as a graceful exit mechanism from the inflationary epoch.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-107179 (URN)10.1103/PhysRevD.90.044021 (DOI)000341255200005 ()
Available from: 2014-09-05 Created: 2014-09-05 Last updated: 2017-12-05Bibliographically approved
8. Piecewise silence in discrete cosmological models
Open this publication in new window or tab >>Piecewise silence in discrete cosmological models
2014 (English)In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 31, no 10, 105012- p.Article in journal (Refereed) Published
Abstract [en]

We consider a family of cosmological models in which all mass is confined to a regular lattice of identical black holes. By exploiting the reflection symmetry about planes that bisect these lattices into identical halves, we are able to consider the evolution of a number of geometrically distinguished surfaces that exist within each of them. We find that the evolution equations for the reflection symmetric surfaces can be written as a simple set of Friedmann-like equations, with source terms that behave like a set of interacting effective fluids. We then show that gravitational waves are effectively trapped within small chambers for all time, and are not free to propagate throughout the space-time. Each chamber therefore evolves as if it were in isolation from the rest of the universe. We call this phenomenon 'piecewise silence'.

Keyword
cosmology, cosmological models, mathematical models
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-105191 (URN)10.1088/0264-9381/31/10/105012 (DOI)000336092700013 ()
Note

AuthorCount:3;

Available from: 2014-07-04 Created: 2014-06-24 Last updated: 2017-12-05Bibliographically approved

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