arXiv subjects

O. Bertolami

Publications and source records attributed to O. Bertolami.

At least 109 records · Page 6Linked to original sources

arXiv2004

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 $δ\sim a$ behaviour.

astro-ph↗

arXiv2004

The Pioneer anomaly in the context of the braneworld scenario

We examine the Pioneer anomaly - a reported anomalous acceleration affecting the Pioneer 10/11, Galileo and Ulysses spacecrafts - in the context of a braneworld scenario. We show that effects due to the radion field cannot account for the anomaly, but that a scalar field with an appropriate potential is able to explain the phenomena. Implications and features of our solution are analyzed.

gr-qc↗

arXiv2004

Time Evolution of the Fine Structure Constant in a Two-Field Quintessence Model

We examine the variation of the fine structure constant in the context of a two-field quintessence model. We find that, for solutions that lead to a transient late period of accelerated expansion, it is possible to fit the data arising from quasar spectra and comply with the bounds on the variation of $α$ from the Oklo reactor, meteorite analysis, atomic clock measurements, Cosmic Microwave Background Radiation and Big Bang Nucleosynthesis. That is more difficult if we consider solutions corresponding to a late period of permanent accelerated expansion.

astro-ph↗

arXiv2004

Latest Supernova data in the framework of Generalized Chaplygin Gas model

We use the most recent Type-Ia Supernova data in order to study the dark energy - dark matter unification approach in the context of the Generalized Chaplygin Gas (GCG) model. Rather surprisingly, we find that data allow models with $α> 1$. We have studied how the GCG adjusts flat and non-flat models, and our results show that GCG is consistent with flat case upto 68% confidence level. Actually this holds even if one relaxes the flat prior assumption. We have also analysed what one should expect from a future experiment such as SNAP. We find that there is a degeneracy between the GCG model and a XCDM model with a phantom-like dark energy component.

astro-ph↗

arXiv2004

Challenges to the Generalized Chaplygin Gas Cosmology

The generalized Chaplygin gas (GCG) model allows for an unified description of the cosmologically recent accelerated expansion of the Universe and of the evolution of energy density perturbations. This dark energy - dark matter unification is achieved through a rather exotic background fluid whose equation of state is given by $p = - A/ρ^α$, where $A$ is a positive constant. Observational constraints arising from bounds on the locations of the first few peaks and troughs of the Cosmic Microwave Background Radiation (CMBR) power spectrum from recent WMAP and BOOMERanG experiments are consistent with the model for $α\lsim 0.6$ assuming that $0 < α\le 1$. Most recent Type-Ia Supernova data indicates however, that the range $α> 1 $ must be considered.

astro-ph↗

arXiv2004

Noncommutative field theory and violation of translation invariance

Noncommutative field theories with commutator of the coordinates of the form $[x^μ,x^ν]=i Λ_{\quad ω}^{μν}x^ω$ are studied. Explicit Lorentz invariance is mantained considering $Λ$ a Lorentz tensor. It is shown that a free quantum field theory is not affected. Since invariance under translations is broken, the conservation of energy-momentum is violated, obeying a new law which is expressed by a Poincaré-invariant equation. The resulting new kinematics is studied and applied to simple examples and to astrophysical puzzles, such as the observed violation of the GZK cutoff. The $λ$$Φ^{4}$ quantum field theory is also considered in this context. In particular, self interaction terms violate the usual conservation of energy-momentum and, hence, the radiative correction to the propagator is altered. The correction to first order in $λ$ is calculated. The usual UV divergent terms are still present, but a new type of term also emerges, which is IR divergent, violates momentum conservation and implies a correction to the dispersion relation.

hep-th↗

arXiv2003

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/ρ^α, where A is a positive constant and 0<α\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.

astro-ph↗

arXiv2003

Expected constraints on the generalized Chaplygin equation of state from future supernova experiments and gravitational lensing statistics

This paper aims to study the use of future SNAP data together with the result of searches for strong gravitational lenses in future large quasar surveys to constrain the Generalized Chaplygin Gas (GCG) model. The GCG is considered as a possible unification scheme for dark matter-dark energy. It is found that both experiments should be able to place important constraints on the model, especially when both tests are used together.

astro-ph↗

arXiv2003

Three Key Tests to Gravity

Three presumably unrelated open questions concerning gravity and the structure of the Universe are here discussed: 1) To which extent is Lorentz invariance an exact symmetry ? 2) What is the equation of state of the Universe ? 3) What is the origin of the so-called Pioneer anomaly ?

gr-qc↗

arXiv2003

Generalized Chaplygin Gas Model: Dark Energy - Dark Matter Unification and CMBR Constraints

The generalized Chaplygin gas (GCG) model allows for an unified description of the recent accelerated expansion of the Universe and the evolution of energy density perturbations. This dark energy - dark matter unification is achieved through an exotic background fluid whose equation of state is given by $p = - A/ρ^α$, where $A$ is a positive constant and $0 < α\le 1$. Stringent constraints on the model parameters can be obtained from recent WMAP and BOOMERanG bounds on the locations of the first few peaks and troughs of the Cosmic Microwave Background Radiation (CMBR) power spectrum as well as SNe Ia data.

gr-qc↗

arXiv2002

Noncommutative Scalar Field Coupled to Gravity

A model for a noncommutative scalar field coupled to gravity is proposed via an extension of the Moyal product. It is shown that there are solutions compatible with homogeneity and isotropy to first non-trivial order in the perturbation of the star-product, with the gravity sector described by a flat Robertson-Walker metric. We show that in the slow-roll regime of a typical chaotic inflationary scenario, noncommutativity has negligible impact.

gr-qc↗

arXiv2002

Supergravity Inflation on the Brane

We study N=1 Supergravity inflation in the context of the braneworld scenario. Particular attention is paid to the problem of the onset of inflation at sub-Planckian field values and the ensued inflationary observables. We find that the so-called $η$-problem encountered in supergravity inspired inflationary models can be solved in the context of the braneworld scenario, for some range of the parameters involved. Furthermore, we obtain an upper bound on the scale of the fifth dimension, $M_5 \lsim 10^{-3} M_P$, in case the inflationary potential is quadratic in the inflaton field, $ϕ$. If the inflationary potential is cubic in $ϕ$, consistency with observational data requires that $M_5 \simeq 9.2 \times 10^{-4} M_P$.

gr-qc↗

arXiv2002

Tachyonic Inflation in the Braneworld Scenario

We consider cosmological inflation driven by the rolling tachyon in the context of the braneworld scenario. We show that sufficient inflation consistent with the observational constraints can be achieved for well defined upper limits on the five-dimensional mass scale, string mass scale and the string coupling for the bosonic string.

hep-th↗

arXiv2002

Generalized Chaplygin gas and CMBR constraints

We study the dependence of the location of the Cosmic Microwave Background Radiation (CMBR) peaks on the parameters of the generalized Chaplygin gas model, whose equation of state is given by $p = - A/ρ^α$, where $A$ is a positive constant and $0 < α\le 1$. We find, in particular, that observational data arising from Archeops for the location of the first peak, BOOMERANG for the location of the third peak, supernova and high-redshift observations allow constraining significantly the parameter space of the model. Our analysis indicates that the emerging model is clearly distinguishable from the $α= 1$ Chaplygin case and the $Λ$CDM model.

astro-ph↗

arXiv2002

Topological Defect Densities in Type-I Superconducting Phase Transitions

We examine the consequences of a cubic term addition to the mean-field potential of Ginzburg-Landau theory to describe first order superconductive phase transitions. Constraints on its existence are obtained from experiment, which are used to assess its impact on topological defect creation. We find no fundamental changes in either the Kibble-Zurek or Hindmarsh-Rajantie predictions.

hep-ph↗

arXiv2002

Generalized Chaplygin gas as a scheme for Unification of Dark Energy and Dark Matter

We study the cosmological scenario arising from the dynamics of a generalized Chaplygin gas. The equation of state of the system is given in terms of the energy density, $ρ$, and pressure, $p$, by the relationship $p = - A/ρ^α$, where $A$ is a positive constant and $0 < α\le 1$. The conditions under which homogeneity arises are discussed and it is shown that this equation of state describes a universe evolving from a phase dominated by non-relativistic matter to a phase dominated by a cosmological constant via an intermediate period where the effective equation of state is $p = αρ$.

astro-ph↗

arXiv2002

Ultracold neutrons, quantum effects of gravity and the Weak Equivalence Principle

We consider an extension of the recent experiment with ultracold neutrons and the quantization of its vertical motion in order to test the Weak Equivalence Principle. We show that an improvement on the energy resolution of the experiment may allow to establish a modest limit to the Weak Equivalence Principle and on the gravitational screening constant. We also discuss the influence of a possible new interaction of Nature.

hep-ph↗

arXiv2002

Hypothetical Gravity Control and Implications for Spacecraft Propulsion

A scientific analysis of the conditions under which gravity could be controlled and the implications that an hypothetical manipulation of gravity would have for known schemes of space propulsion have been the scope of a recent study carried out for the European Space Agency. The underlying fundamental physical principles of known theories of gravity were analysed and shown that even if gravity could be modified it would bring somewhat modest gains in terms of launching of spacecraft and no breakthrough for space propulsion.

gr-qc↗