848 research outputs found
Naturalness and stability of the generalized Chaplygin gas in the seesaw cosmon scenario
The seesaw mechanism is conceived on the basis that a mass scale, , and
a dimensionless scale, , can be fine-tuned in order to control the dynamics
of active and sterile neutrinos through cosmon-type equations of motion: the
seesaw cosmon equations. This allows for sterile neutrinos to be a dark matter
candidate. In this scenario, the dynamical masses and energy densities of
active and sterile neutrinos can be consistently embedded into the generalized
Chaplygin gas (GCG), the unified dark sector model. In addition, dark matter
adiabatically coupled to dark energy allows for a natural decoupling of the
(active) mass varying neutrino (MaVaN) component from the dark sector. Thus
MaVaN's turn into a secondary effect. Through the scale parameters, and
, the proposed scenario allows for a convergence among three distinct
frameworks: the cosmon scenario, the seesaw mechanism for mass generation and
the GCG model. It is found that the equation of state of the perturbations is
the very one of the GCG background cosmology so that all the results from this
approach are maintained, being smoothly modified by active neutrinos.
Constrained by the seesaw relations, it is shown that the mass varying
mechanism is responsible for the stability against linear perturbations and is
indirectly related to the late time cosmological acceleration.Comment: 24 pages, 6 figure
The Revival of the Unified Dark Energy-Dark Matter Model ?
We consider the generalized Chaplygin gas (GCG) proposal for unification of
dark energy and dark matter and show that it admits an unique decomposition
into dark energy and dark matter components once phantom-like dark energy is
excluded. Within this framework, we study structure formation and show that
difficulties associated to unphysical oscillations or blow-up in the matter
power spectrum can be circumvented. Furthermore, we show that the dominance of
dark energy is related to the time when energy density fluctuations start
deviating from the linear behaviour.Comment: 6 pages, 4 eps figures, Revtex4 style. New References are added. Some
typos are corrected. Conclusions remain the sam
Weak Lensing, Shear and the Cosmic Virial Theorem in a Model with a Scale-Dependent Gravitational Coupling
It is argued that, in models where the gravitational coupling is
scale-dependent, predictions concerning weak gravitational lensing and shear
are essentially similar to the ones derived from General Relativity. This is
consistent with recent negative results of observations of the MS1224, CL2218
and A1689 systems aimimg to infer from those methods the presence of dark
matter. It is shown, however, that the situation is quite different when an
analysis based on the Cosmic Virial Theorem is concerned.Comment: Footnote and references added. Version to in Gen. Relativity and
Gravitation Vol. 29 (1997
Perturbative approach for mass varying neutrinos coupled to the dark sector in the generalized Chaplygin gas scenario
We suggest a perturbative approach for generic choices for the universe
equation of state and introduce a novel framework for studying mass varying
neutrinos (MaVaN's) coupled to the dark sector. For concreteness, we examine
the coupling between neutrinos and the underlying scalar field associated with
the generalized Chaplygin gas (GCG), a unification model for dark energy and
dark matter. It is shown that the application of a perturbative approach to
MaVaN mechanisms translates into a constraint on the coefficient of a linear
perturbation, which depends on the ratio between a neutrino energy dependent
term and scalar field potential terms. We quantify the effects on the MaVaN
sector by considering neutrino masses generated by the seesaw mechanism. After
setting the GCG parameters in agreement with general cosmological constraints,
we find that the squared speed of sound in the neutrino-scalar GCG fluid is
naturally positive. In this scenario, the model stability depends on previously
set up parameters associated with the equation of state of the universe. Our
results suggest that the GCG is a particularly suitable candidate for
constructing a stable MaVaN scenario.Comment: 27 pages, 9 figure
Compactification, Vacuum Energy and Quintessence
We study the possibility that the vacuum energy density of scalar and
internal-space gauge fields arising from the process of dimensional reduction
of higher dimensional gravity theories plays the role of quintessence. We show
that, for the multidimensional Einstein-Yang-Mills system compactified on a topology, there are classically stable solutions such
that the observed accelerated expansion of the Universe at present can be
accounted for without upsetting structure formation scenarios or violating
observational bounds on the vacuum energy density.Comment: 15 pages, Latex, Third Award in 1999 Essay Competition of the Gravity
Research Foundatio
Stability Conditions For a Noncommutative Scalar Field Coupled to Gravity via the Positive Energy Theorem
The stability requirements for a noncommutative scalar field coupled to
gravity is investigated through the positive energy theorem. It is shown that
for a noncommutative scalar with a polynomial potential, the stability
conditions are similar to the ones for the commutative case. This result
remains valid even whether the space-time has horizons.Comment: 6 pages. Talk presented by C. Z. at the "Invisible Universe
International Conference", Paris, Palais de l'UNESCO, June 29- July 3. To
appear in the Proceeding
WMAP Constraints on the Generalized Chaplygin Gas Model
The generalized Chaplygin gas (GCG) model explains the recent accelerated
expansion of the Universe via an exotic background fluid whose equation of
state is given by p=-A/\rho^\alpha, where A is a positive constant and
0<\alpha\le 1. The model is an interesting alternative to scenarios involving
scalar field potentials, with the ensuing unnatural fine tuning conditions for
the underlying particle physics theories. We derive constraints on the
parameter space of the model from bounds on the location of the first few peaks
and troughs of the the Cosmic Microwave Background Radiation (CMBR) power
spectrum arising from recent WMAP and BOOMERanG data.Comment: 18 pages, 5 figures, version to appear in Phys. Lett.
Wormhole Effects on Yang-Mills Theory
In this paper wormhole effects on YM theory are examined. The
wormhole wave functions for the scalar, the vector and the tensor expansion
modes are computed assuming a small gauge coupling which leads to an effective
decoupling of gravity and YM theory. These results are used to determine the
wormhole vertices and the corresponding effective operators for the lowest
expansion mode of each type. For the lowest scalar mode we find a
renormalization of the gauge coupling from the two point function and the
operators \tr (F^3), \tr (F^2\tilde{F}) from the three point function. The
two point function for the lowest vector mode contributes to the gauge coupling
renormalization only whereas the lowest tensor mode can also generate higher
derivative terms.Comment: 15 pages, TUM--TH--165/9
A Curvature Principle for the interaction between universes
We propose a Curvature Principle to describe the dynamics of interacting
universes in a multi-universe scenario and show, in the context of a simplified
model, how interaction drives the cosmological constant of one of the universes
toward a vanishingly small value. We also conjecture on how the proposed
Curvature Principle suggests a solution for the entropy paradox of a universe
where the cosmological constant vanishes.Comment: Essay selected for an honorable mention by the Gravity Research
Foundation, 2007. Plain latex, 8 page
- …