arXiv subjects

O. Bertolami

Publications and source records attributed to O. Bertolami.

At least 91 records · Page 5Linked to original sources

arXiv2006

Generalized Chaplygin gas model, supernovae and cosmic topology

In this work we study to which extent the knowledge of spatial topology may place constraints on the parameters of the generalized Chaplygin gas (GCG) model for unification of dark energy and dark matter. By using both the Poincaré dodecahedral and binary octahedral spaces as the observable spatial topologies, we examine the current type Ia supernovae (SNe Ia) constraints on the GCG model parameters. We show that the knowledge of spatial topology does provide additional constraints on the $A_s$ parameter of the GCG model but does not lift the degeneracy of the $α$ parameter.

gr-qc↗

arXiv2006

Scaling of variables and the relation between noncommutative parameters in Noncommutative Quantum Mechanics

We consider Noncommutative Quantum Mechanics with phase space noncommutativity. In particular, we show that a scaling of variables leaves the noncommutative algebra invariant, so that only the self-consistent effective parameters of the model are physically relevant. We also discuss the recently proposed relation of direct proportionality between the noncommutative parameters, showing that it has a limited applicability.

hep-th↗

arXiv2006

Gravitational Quantum Well

We discuss the implications of a model of noncommutative Quantum Mechanics where noncommutativity is extended to the phase space. We analyze how this model affects the problem of the two-dimensional gravitational quantum well and use the latest experimental results for the energy states of neutrons in the Earth's gravitational field to establish an upper bound on the fundamental momentum scale introduced by noncommutativity. We show that the configuration space noncommutativity has, in leading order, no effect on the problem and that in the context of the model, a correction to the presently accepted value of Planck's constant to 1 part in $10^{24}$ arises. We also study the transition between quantum and classical behaviour of particles in a gravitational quantum well and analyze how an increase in the particles mass turns the energy spectrum into a continuous one. We consider these effects and argue that they could be tested by through experiments with atoms and fullerene-type molecules.

hep-th↗

arXiv2006

Gamma Ray Bursts as Cosmological Probes

We discuss the prospects of using Gamma Ray Bursts (GRBs) as high-redshift distance estimators, and consider their use in the study of two dark energy models, the Generalized Chaplygin Gas (GCG), a model for the unification of dark energy and dark matter, and the XCDM model, a model where a generic dark energy fluid like component is described by the equation of state, $p= ωρ$. Given that the GRBs range of redshifts is rather high, it turns out that they are not very sensitive to the dark energy component, being however, fairly good estimators of the amount of dark matter in the Universe.

astro-ph↗

arXiv2005

The Chaplygin dark star

We study the general properties of a spherically symmetric body described through the generalized Chaplygin equation of state. We conclude that such object, dubbed generalized Chaplygin dark star, should exist within the context of the generalized Chaplygin gas model of unification of dark energy and dark matter, and derive expressions for its size and expansion velocity. A criteria for the survival of the perturbations in the GCG background that give origin to the dark star are developed, and its main features are analyzed.

astro-ph↗

arXiv2005

Supernovae constraints on dark energy and modified gravity models

We use the Type Ia Supernova gold sample to constrain the parameters of dark energy models namely the Cardassian, Dvali-Turner (DT) and generalized Chaplygin gas (GCG) models. In our best fit analysis for these dark energy proposals we consider flat and the non-flat priors. For all models, we find that relaxing the flatness condition implies that data favors a positive curvature; moreover, the GCG model is nearly flat, as required by Cosmic Microwave Background (CMB) observations.

astro-ph↗

arXiv2005

Quantum and Classical divide: the gravitational case

We study the transition between quantum and classical behavior of particles in a gravitational quantum well. We analyze how an increase in the particles mass turns the energy spectrum into a continuous one, from an experimental point of view. We also discuss the way these effects could be tested by conducting experiments with atoms and fullerene-type molecules.

hep-ph↗

arXiv2005

Fundamental Physics with the Laser Astrometric Test Of Relativity

The Laser Astrometric Test Of Relativity (LATOR) is a joint European-U.S. Michelson-Morley-type experiment designed to test the pure tensor metric nature of gravitation - a fundamental postulate of Einstein's theory of general relativity. By using a combination of independent time-series of highly accurate gravitational deflection of light in the immediate proximity to the Sun, along with measurements of the Shapiro time delay on interplanetary scales (to a precision respectively better than 0.1 picoradians and 1 cm), LATOR will significantly improve our knowledge of relativistic gravity. The primary mission objective is to i) measure the key post-Newtonian Eddington parameter γwith accuracy of a part in 10^9. (1-γ) is a direct measure for presence of a new interaction in gravitational theory, and, in its search, LATOR goes a factor 30,000 beyond the present best result, Cassini's 2003 test. The mission will also provide: ii) first measurement of gravity's non-linear effects on light to ~0.01% accuracy; including both the Eddington βparameter and also the spatial metric's 2nd order potential contribution (never measured before); iii) direct measurement of the solar quadrupole moment J2 (currently unavailable) to accuracy of a part in 200 of its expected size; iv) direct measurement of the "frame-dragging" effect on light by the Sun's gravitomagnetic field, to 1% accuracy. LATOR's primary measurement pushes to unprecedented accuracy the search for cosmologically relevant scalar-tensor theories of gravity by looking for a remnant scalar field in today's solar system. We discuss the mission design of this proposed experiment.

gr-qc↗

arXiv2005

Hypothetical Gravity Control and Possible Influence on Space Propulsion

In nearly all concepts invoked or proposed to change or shield gravity it is intuitively assumed that manipulation of gravity automatically leads to a breakthrough for propulsion. In this study it is shown, that even if gravity could be hypothetically controlled along the manipulation schemes outlined, the gains in terms of propulsion would be modest and lead to no breakthrough. Although the manipulation schemes presented are not exhaustive, they include the most straightforward ones from the current physics point of view.

physics.space-ph↗

arXiv2005

Gamma-ray bursts as dark energy-matter probes in the context of the generalized Chaplygin gas model

In this paper we consider the use of Gamma Ray Bursts (GRBs) as distance markers to study the unification of dark energy and dark matter in the context of the so-called Generalized Chaplygin Gas (GCG) model. We consider that the GRB luminosity may be estimated from its variability, time-lag, and also use the so-called Ghirlanda relation. We evaluate the improvements one may expect once more GRBs and their redshift become available. We show that although GRBs allow for extending the Hubble diagram to higher redshifts, its use as a dark energy probe is limited when compared to SNe Ia. We find that the information from GRBs can provide some bounds on the amount of dark matter and dark energy independently of the equation of state. This is particularly evident for XCDM-type models, which are, for low-redshifts ($z\leq2$), degenerate with the GCG.

astro-ph↗

arXiv2005

A Mission to Explore the Pioneer Anomaly

The Pioneer 10 and 11 spacecraft yielded the most precise navigation in deep space to date. These spacecraft had exceptional acceleration sensitivity. However, analysis of their radio-metric tracking data has consistently indicated that at heliocentric distances of $\sim 20-70$ astronomical units, the orbit determinations indicated the presence of a small, anomalous, Doppler frequency drift. The drift is a blue-shift, uniformly changing with a rate of $\sim(5.99 \pm 0.01)\times 10^{-9}$ Hz/s, which can be interpreted as a constant sunward acceleration of each particular spacecraft of $a_P = (8.74 \pm 1.33)\times 10^{-10} {\rm m/s^2}$. This signal has become known as the Pioneer anomaly. The inability to explain the anomalous behavior of the Pioneers with conventional physics has contributed to growing discussion about its origin. There is now an increasing number of proposals that attempt to explain the anomaly outside conventional physics. This progress emphasizes the need for a new experiment to explore the detected signal. Furthermore, the recent extensive efforts led to the conclusion that only a dedicated experiment could ultimately determine the nature of the found signal. We discuss the Pioneer anomaly and present the next steps towards an understanding of its origin. We specifically focus on the development of a mission to explore the Pioneer Anomaly in a dedicated experiment conducted in deep space.

gr-qc↗

arXiv2005

Supernovae constraints on models of dark energy revisited

We use the Type Ia Supernova gold sample data of Riess {\it et al} in order to constrain three models of dark energy. We study the Cardassian model, the Dvali-Turner gravity modified model and the generalized Chaplygin gas model of dark energy - dark matter unification. In our best fit analysis for these three dark energy proposals we consider flat model and the non-flat model priors. We also discuss the degeneracy of the models with the XCDM model through the computation of the so-called jerk parameter.

astro-ph↗

arXiv2005

Bounds on Cubic Lorentz-Violating Terms in the Fermionic Dispersion Relation

We study the recently proposed Lorentz-violating dispersion relation for fermions and show that it leads to two distinct cubic operators in the momentum. We compute the leading order terms that modify the non-relativistic equations of motion and use experimental results for the hyperfine transition in the ground state of the ${}^9\textrm Be^+$ ion to bound the values of the Lorentz-violating parameters $η_1$ and $η_2$ for neutrons. The resulting bounds depend on the value of the Lorenz-violating background four-vector in the laboratory frame.

hep-ph↗

arXiv2005

Noncommutative Gravitational Quantum Well

We study noncommutative geometry at the Quantum Mechanics level by means of a model where noncommutativity of both configuration and momentum spaces is considered. We analyze how this model affects the problem of the two-dimensional gravitational quantum well and use the latest experimental results for the two lowest energy states of neutrons in the Earth's gravitational field to establish an upper bound on the fundamental momentum scale introduced by noncommutativity, namely $\sqrtη\lesssim1\ \mathrm{meV/c}$, a value that can be improved in the future by up to 3 orders of magnitude. We show that the configuration space noncommutativity has, in leading order, no effect on the problem. We also analyze some features introduced by the model, specially a correction to the presently accepted value of Planck's constant to 1 part in $10^{24}$.

hep-th↗

arXiv2005

The flight of the bumblebee: vacuum solutions of a gravity model with vector-induced spontaneous Lorentz symmetry breaking

We study the vacuum solutions of a gravity model where Lorentz symmetry is spontaneously broken once a vector field acquires a vacuum expectation value. Results are presented for the purely radial Lorentz symmetry breaking (LSB), radial/temporal LSB and axial/temporal LSB. The purely radial LSB result corresponds to new black hole solutions. When possible, Parametrized Post-Newtonian (PPN) parameters are computed and observational boundaries used to constrain the Lorentz symmetry breaking scale.

hep-th↗

arXiv2005

Varying Electromagnetic Coupling and Primordial Magnetic Fields

We study the effect of variations of the electromagnetic coupling on the process of generation of primordial magnetic fields. We find that only through a significant growth of the electromagnetic coupling minimum seed fields can be produced. We also show that, if through some process in the early Universe the photon acquires a mass that leads, thanks to inflation, to the generation of primordial magnetic fields, then the influence of variations of the electromagnetic coupling amounts essentially to the results due to the photon effective mass alone.

astro-ph↗

arXiv2004

Pioneer's Final Riddle

Launched more than thirty years ago and now drifting in space with no further contact, the Pioneer 10 and 11 spacecraft are currently at the center of a small but developing concern: are they under the influence of an anomalous acceleration that reflects new, yet unknown physics, or merely experiencing some unexpected technical problem ? The proposals vary from basic engineering principles to extra spatial dimensions, but most probably an answer can only be obtained after a dedicated mission is underway.

gr-qc↗

arXiv2004

Astrophysical Constraints on Scalar Field Models

We use stellar structure dynamics arguments to extract bounds on the relevant parameters of scalar field models: the putative scalar field mediator of a fifth force with a Yukawa potential, the new variable mass particle (VAMP) models, and a phenomenologically viable model to explain the Pioneer anomaly. We consider the polytropic gas model to estimate the effect of these models on the hydrostatic equilibrium equation and fundamental quantities such as the central temperature. The current bound on the solar luminosity is used to constrain the relevant parameters of each model.

astro-ph↗