842 research outputs found

### 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 $R
\times S^3 \times S^d$ 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

### Dark matter as a dynamic effect due to a non-minimal gravitational coupling with matter

In this work the phenomenology of models possessing a non-minimal coupling
between matter and geometry is discussed, with a particular focus on the
possibility of describing the flattening of the galactic rotation curves as a
dynamically generated effect derived from this modification to General
Relativity. Two possibilities are discussed: firstly, that the observed
discrepancy between the measured rotation velocity and the classical prediction
is due to a deviation from geodesic motion, due to a non-(covariant)
conservation of the energy-momentum tensor; secondly, that even if the
principle of energy conservation holds, the dynamical effects arising due to
the non-trivial terms in the Einstein equations of motion can give rise to an
extra density contribution that may be interpreted as dark matter. The
mechanism of the latter alternative is detailed, and a numerical session
ascertaining the order of magnitude of the relevant parameters is undertaken,
with possible cosmological implications discussed.Comment: Talk given at First Mediterranean Conference on Classical and Quantum
Gravity, Kolymbari, Greece, 14-18 September 2009

### 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, $\xi$, and
a dimensionless scale, $s$, 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, $\xi$ and
$s$, 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

### 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

### A mission to test the Pioneer anomaly: estimating the main systematic effects

We estimate the main systematic effects relevant in a mission to test and
characterize the Pioneer anomaly through the flight formation concept, by
launching probing spheres from a mother spacecraft and tracking their motion
via laser ranging.Comment: 9 pages, 1 table, 2 figures; based on a talk presented by one of us
(O.B.) at the 2nd. Pioneer Anomaly Group Meeting at the International Space
Science Institute, 19-23 February 200

### 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 $\delta \sim a$ behaviour.Comment: 6 pages, 4 eps figures, Revtex4 style. New References are added. Some
typos are corrected. Conclusions remain the sam

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