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Orfeu Bertolami

Publications and source records attributed to Orfeu Bertolami.

At least 145 records · Page 8Linked to original sources
arXiv2007

Spontaneous Symmetry Breaking in the Bulk as a Localization Mechanism of Fields on the Brane

We consider real and complex scalar fields non-minimally coupled to gravity in the bulk spacetime and study their impact on the brane upon acquiring a non-vanishing vacuum expectation value. When examining the case of the complex scalar field, a minimally coupled U(1) gauge field is also considered so that spontaneous symmetry breaking can take place. Particular attention is paid to the terms arising from the junction conditions, which act towards reinforcing the mechanism of spontaneous symmetry breaking. We find that the embedding of a braneworld universe in a bulk spacetime endowed with matter fields can induce this mechanism at very high energies, which implies the localization on the brane of the bulk fields.

hep-th↗
arXiv2007

Cosmology in Portugal: The First 20 Years

We reminisce on the first steps of the cosmology community in Portugal, which can be traced back to about 20 years ago, and discuss its achievements and current specificities. We also reflect on the aspirations, hopes and challenges for the future.

physics.hist-ph↗
arXiv2007

Lorentz symmetry from Lorentz violation in the bulk

We consider the mechanism of spontaneous symmetry breaking of a bulk vector field to study signatures of bulk dimensions invisible to the standard model confined to the brane. By assigning a non-vanishing vacuum expectation value to the vector field, a direction is singled out in the bulk vacuum, thus breaking the bulk Lorentz symmetry. We present the condition for induced Lorentz symmetry on the brane, as phenomenologically required, noting that it is related to the value of the observed cosmological constant.

astro-ph↗
arXiv2006

Weyl-Wigner Formulation of Noncommutative Quantum Mechanics

We address the phase space formulation of a noncommutative extension of quantum mechanics in arbitrary dimension, displaying both spatial and momentum noncommutativity. By resorting to a covariant generalization of the Weyl-Wigner transform and to the Seiberg-Witten map we construct an isomorphism between the operator and the phase space representations of the extended Heisenberg algebra. This map provides a systematic approach to derive the entire structure of noncommutative quantum mechanics in phase space. We construct the extended starproduct, Moyal bracket and propose a general definition of noncommutative states. We study the dynamical and eigenvalue equations of the theory and prove that the entire formalism is independent of the particular choice of Seiberg-Witten map. Our approach unifies and generalizes all the previous proposals for the phase space formulation of noncommutative quantum mechanics. For concreteness we rederive these proposals by restricting our formalism to some 2-dimensional spaces.

hep-th↗
arXiv2006

Lorentz Symmetry Derived from Lorentz Violation in the Bulk

We consider bulk fields coupled to the graviton in a Lorentz violating fashion. We expect that the overly tested Lorentz symmetry might set constraints on the induced Lorentz violation in the brane, and hence on the dynamics of the interaction of bulk fields on the brane. We also use the requirement for Lorentz symmetry to constrain the cosmological constant observed on the brane.

gr-qc↗
arXiv2006

The adventures of Spacetime

We discuss how developments in physics often imply in the need that spacetime acquires an increasingly richer and complex structure. General Relativity was the first theory to show us the way to connect space and time with the physical world. Since then, scrutinizing the ways spacetime might exist is, in a way, the very essence of physics. Physics has thus given substance to the pioneering work of scores of brilliant mathematicians who speculated on the geometry and topology of spaces.

gr-qc↗
arXiv2006

Dark Energy, Dark Matter and Gravity

We discuss the motivation for high accuracy relativistic gravitational experiments in the Solar System and complementary cosmological tests. We focus our attention on the issue of distinguishing a generic scalar-theory of gravity as the underlying physical theory from the usual general relativistic picture, where one expects the presence of fundamental scalar fields associated, for instance, to inflation, dark matter and dark energy.

astro-ph↗
arXiv2006

General Theory of Relativity: Will it survive the next decade?

The nature of gravity is fundamental to our understanding of our own solar system, the galaxy and the structure and evolution of the Universe. Einstein's general theory of relativity is the standard model that is used for almost ninety years to describe gravitational phenomena on these various scales. We review the foundations of general relativity, discuss the recent progress in the tests of relativistic gravity, and present motivations for high-accuracy gravitational experiments in space. We also summarize the science objectives and technology needs for the laboratory experiments in space with laboratory being the entire solar system. We discuss the advances in our understanding of fundamental physics anticipated in the near future and evaluate discovery potential for the recently proposed gravitational experiments.

gr-qc↗
arXiv2006

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

The vacuum solutions arising from a spontaneous breaking of Lorentz symmetry due to the acquisition of a vacuum expectation value by a vector field are derived. These include the purely radial Lorentz symmetry breaking (LSB), radial/temporal LSB and axial/temporal LSB scenarios. It is found that the purely radial LSB case gives rise to new black hole solutions. Whenever possible, Parametrized Post-Newtonian (PPN) parameters are computed and compared to observational bounds, in order to constrain the Lorentz symmetry breaking scale.

gr-qc↗
arXiv2005

Dark Energy - Dark Matter Unification: Generalized Chaplygin Gas Model

We review the main features of the generalized Chaplygin gas (GCG) proposal for unification of dark energy and dark matter and discuss how it admits an unique decomposition into dark energy and dark matter components once phantom-like dark energy is excluded. In the context of this approach we consider structure formation and show that unphysical oscillations or blow-up in the matter power spectrum are not present. Moreover, we demonstrate that the dominance of dark energy occurs about the time when energy density fluctuations start evolving away from the linear regime.

astro-ph↗
arXiv2005

Noncommutative scalar field minimally coupled to gravity

A model for noncommutative scalar fields coupled to gravity based on the generalization of the Moyal product is proposed. Solutions compatible with homogeneous and isotropic flat Robertson-Walker spaces to first non-trivial order in the perturbation of the star-product are presented. It is shown that in the context of a typical chaotic inflationary scenario, at least in the slow-roll regime, noncommutativity yields no observable effect.

gr-qc↗
arXiv2004

What is fair to ask Society to fund ?

Our world is strongly driven by technological developments that continuously create new markets and shape individual taste and choice. The scale of dissemination of the most recent technological breakthroughs and the capability of creating them is what divides wealthy societies from poor ones. The increasing sophistication of these developments make it harder and harder for developing countries to actively contribute to these breakthroughs as well as to acquire the right to benefit from them through the exchange of products obtained through low-tech agriculture, industry and services. Thus, as new technological developments rely often on discoveries in fundamental science and technology that have their origins, in some instances, many decades ago, it is only through fostering the acquisition of the skills necessary for innovation in basic science and technology that economical and social progress can be insured on a long term basis. In this perspective, funding of research is an investment for the future and clearly, just buying technology is no short cut to create sustainable wealth. Furthermore, Society should fund fundamental research for cultural and training reasons. Indeed, as a representative of a country or of a culture in a given historical period, activities assertive of cultural and national identities should be funded. These involve necessarily scientific developments too. Moreover, experience shows that the best training for innovation is acquired from individuals and institutions that have an active interest in basic science and on its application to technology.

physics.soc-ph↗
arXiv2004

Translational Invariance and Noncommutative Field Theories

Implications of noncommutative field theories with commutator of the coordinates of the form $[x^μ,x^ν]=i Λ_{\quad ω}^{μν}x^ω$with nilpotent structure constants are investigated. It is shown that a free quantum field theory is not affected by noncommutativity, but that invariance under translations is broken and hence the energy-momentum conservation is not respected. The new energy-momentum law is expressed by a Poincaré-invariant equation and the resulting kinematics is developed and applied to the astrophysical puzzle related with the observed violation of the GZK cutoff.

gr-qc↗
arXiv2003

Cosmological acceleration, varying couplings, and Lorentz breaking

Many candidate fundamental theories contain scalar fields that can acquire spacetime-varying expectation values in a cosmological context. Such scalars typically obey Lorentz-violating effective dispersion relations. We illustrate this fact within a simple supergravity model that also exhibits the observed late-time cosmological acceleration and implies varying electromagnetic couplings.

astro-ph↗
arXiv2003

Threshold Effects and Lorentz Symmetry

Evidence on the violation of Lorentz symmetry arises from the observation of cosmic rays with energies beyond the GZK cutoff, $E_{GZK} \simeq 4 \times 10^{19} eV$, from the apparent transparency of the Universe to the propagation of high energy gamma radiation and from the stability of pions in air showers. These three paradoxes can be explained through deformations of the relativistic dispersion relation. Theoretical ideas aimed to understand how Lorentz symmetry may be broken and phenomenologically interesting deformations of the relativistic dispersion relation may arise are briefly discussed.

hep-ph↗
arXiv1999

Modular Quantum Cosmology

We study solutions of the Wheeler-DeWitt equation corresponding to an S-modular invariant N=1 supergravity model and a closed homogeneous and isotropic Friedmann-Robertson-Walker spacetime. The issues of inflation and of the cosmological constant problem are addressed with the help of the relevant wave functions. We find that topological type inflation is consistent from the quantum mechanical point of view and that a solution for the cosmological constant problem along the lines of the strong CP problem naturally arises.

gr-qc↗