975 research outputs found
Dark spinor inflation -- theory primer and dynamics
Inflation driven by a single dark spinor field is discussed. We define the
notion of a dark spinor field and derive the cosmological field equations for
such a matter source. The conditions for inflation are determined and an
exactly solvable model is presented. We find the power spectrum of the quantum
fluctuation of this field and compare the results with scalar field inflation.Comment: 13 pages; typo in Eq. (12) corrected, minor improvement
Bounds on the basic physical parameters for anisotropic compact general relativistic objects
We derive upper and lower limits for the basic physical parameters
(mass-radius ratio, anisotropy, redshift and total energy) for arbitrary
anisotropic general relativistic matter distributions in the presence of a
cosmological constant. The values of these quantities are strongly dependent on
the value of the anisotropy parameter (the difference between the tangential
and radial pressure) at the surface of the star. In the presence of the
cosmological constant, a minimum mass configuration with given anisotropy does
exist. Anisotropic compact stellar type objects can be much more compact than
the isotropic ones, and their radii may be close to their corresponding
Schwarzschild radii. Upper bounds for the anisotropy parameter are also
obtained from the analysis of the curvature invariants. General restrictions
for the redshift and the total energy (including the gravitational
contribution) for anisotropic stars are obtained in terms of the anisotropy
parameter. Values of the surface redshift parameter greater than two could be
the main observational signature for anisotropic stellar type objects.Comment: 18 pages, no figures, accepted for publication in CQ
Perfect fluid spheres with cosmological constant
We examine static perfect fluid spheres in the presence of a cosmological
constant. New exact matter solutions are discussed which require the Nariai
metric in the vacuum region. We generalize the Einstein static universe such
that neither its energy density nor its pressure is constant throughout the
spacetime. Using analytical techniques we derive conditions depending on the
equation of state to locate the vanishing pressure surface. This surface can in
general be located in regions with decreasing area group orbits. We use
numerical methods to integrate the field equations for realistic equations of
state and find consistent results.Comment: 15 pages, 6 figures; added new references, removed one figure,
improved text, accepted for publication in PR
A new two-sphere singularity in general relativity
The Florides solution, proposed as an alternative to the interior
Schwarzschild solution, represents a static and spherically symmetric geometry
with vanishing radial stresses. It is regular at the center, and is matched to
an exterior Schwarzschild solution. The specific case of a constant energy
density has been interpreted as the field inside an Einstein cluster. In this
work, we are interested in analyzing the geometry throughout the permitted
range of the radial coordinate without matching it to the Schwarzschild
exterior spacetime at some constant radius hypersurface. We find an interesting
picture, namely, the solution represents a three-sphere, whose equatorial
two-sphere is singular, in the sense that the curvature invariants and the
tangential pressure diverge. As far as we know, such singularities have not
been discussed before. In the presence of a large negative cosmological
constant (anti-de Sitter) the singularity is removed.Comment: 17 pages, 3 figure
On the relation between mass of pion, fundamental physical constants and cosmological parameters
In this article we reconsider the old mysterious relation, advocated by Dirac
and Weinberg, between the mass of the pion, the fundamental physical constants,
and the Hubble parameter. By introducing the cosmological density parameters,
we show how the corresponding equation may be written in a form that is
invariant with respect to the expansion of the Universe and without invoking a
varying gravitational "constant", as was originaly proposed by Dirac. It is
suggest that, through this relation, Nature gives a hint that virtual pions
dominante the "content" of the quantum vacuum
Dynamics of dark energy with a coupling to dark matter
Dark energy and dark matter are the dominant sources in the evolution of the
late universe. They are currently only indirectly detected via their
gravitational effects, and there could be a coupling between them without
violating observational constraints. We investigate the background dynamics
when dark energy is modelled as exponential quintessence, and is coupled to
dark matter via simple models of energy exchange. We introduce a new form of
dark sector coupling, which leads to a more complicated dynamical phase space
and has a better physical motivation than previous mathematically similar
couplings.Comment: 11 pages, 4 figures, revtex, references adde
Dark spinors with torsion in cosmology
We solve one of the open problems in Einstein-Cartan theory, namely we find a
natural matter source whose spin angular momentum tensor is compatible with the
cosmological principle. We analyze the resulting evolution equations and find
that an epoch of accelerated expansion is an attractor. The torsion field
quickly decays in that period. Our results are interpreted in the context of
the standard model of cosmology.Comment: 7 pages, 3 figures; reference added, minor improvement
Quintessence with quadratic coupling to dark matter
We introduce a new form of coupling between dark energy and dark matter that
is quadratic in their energy densities. Then we investigate the background
dynamics when dark energy is in the form of exponential quintessence. The three
types of quadratic coupling all admit late-time accelerating critical points,
but these are not scaling solutions. We also show that two types of coupling
allow for a suitable matter era at early times and acceleration at late times,
while the third type of coupling does not admit a suitable matter era.Comment: 11 pages, 8 figures, revte
Physics of dark energy particles
We consider the astrophysical and cosmological implications of the existence
of a minimum density and mass due to the presence of the cosmological constant.
If there is a minimum length in nature, then there is an absolute minimum mass
corresponding to a hypothetical particle with radius of the order of the Planck
length. On the other hand, quantum mechanical considerations suggest a
different minimum mass. These particles associated with the dark energy can be
interpreted as the ``quanta'' of the cosmological constant. We study the
possibility that these particles can form stable stellar-type configurations
through gravitational condensation, and their Jeans and Chandrasekhar masses
are estimated. From the requirement of the energetic stability of the minimum
density configuration on a macroscopic scale one obtains a mass of the order of
10^55 g, of the same order of magnitude as the mass of the universe. This mass
can also be interpreted as the Jeans mass of the dark energy fluid. Furthermore
we present a representation of the cosmological constant and of the total mass
of the universe in terms of `classical' fundamental constants.Comment: 10 pages, no figures; typos corrected, 4 references added; 1
reference added; reference added; entirely revised version, contains new
parts, now 14 page
Solar system tests of brane world models
The classical tests of general relativity (perihelion precession, deflection
of light, and the radar echo delay) are considered for the Dadhich, Maartens,
Papadopoulos and Rezania (DMPR) solution of the spherically symmetric static
vacuum field equations in brane world models. For this solution the metric in
the vacuum exterior to a brane world star is similar to the Reissner-Nordstrom
form of classical general relativity, with the role of the charge played by the
tidal effects arising from projections of the fifth dimension. The existing
observational solar system data on the perihelion shift of Mercury, on the
light bending around the Sun (obtained using long-baseline radio
interferometry), and ranging to Mars using the Viking lander, constrain the
numerical values of the bulk tidal parameter and of the brane tension.Comment: 14 pages, to appear in Classical and Quantum Gravity. V2, minor
corrections and references adde
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