Limits of space-times in five dimensions and their relation to the Segre Types

A limiting diagram for the Segre classification in 5-dimensional space-times is obtained, extending a recent work on limits of the energy-momentum tensor in general relativity. Some of Geroch's results on limits of space-times in general relativity are also extended to the context of five-dimensional Kaluza-Klein space-times.Comment: Late

Spontaneous CP Violation in the Next-to-Minimal Supersymmetric Standard Model Revisited

We re-examine spontaneous CP violation at the tree level in the context of the next-to-minimal supersymmetric standard model (NMSSM) with two Higgs doublets and a gauge singlet field. We analyse the most general Higgs potential without a discrete Z_3 symmetry, and derive an upper bound on the mass of the lightest neutral Higgs boson consistent with present experimental data. We investigate, in particular, its dependence on the admixture and CP-violating phase of the gauge singlet field, as well as on tan(beta). To assess the viability of the spontaneous CP violation scenario, we estimate epsilon_K by applying the mass insertion approximation. We find that a non-trivial flavour structure in the soft-breaking A terms is required to account for the observed CP violation in the neutral kaon sector. Furthermore, combining the minimisation conditions for spontaneous CP violation with the constraints coming from K0-K0bar mixing, we find that the upper bound on the lightest Higgs-boson mass becomes stronger. We also point out that the electric dipole moments of electron and neutron are a serious challenge for SUSY models with spontaneous CP violation.Comment: 19 pages, LaTeX2e, 5 figures; matches the published versio

Topological Reverberations in Flat Space-times

We study the role played by multiply-connectedness in the time evolution of the energy E(t) of a radiating system that lies in static flat space-time manifolds M_4 whose t=const spacelike sections M_3 are compact in at least one spatial direction. The radiation reaction equation of the radiating source is derived for the case where M_3 has any non-trivial flat topology, and an exact solution is obtained. We also show that when the spacelike sections are multiply-connected flat 3-manifolds the energy E(t) exhibits a reverberation pattern with discontinuities in the derivative of E(t) and a set of relative minima and maxima, followed by a growth of E(t). It emerges from this result that the compactness in at least one spatial direction of Minkowski space-time is sufficient to induce this type of topological reverberation, making clear that our radiating system is topologically fragile. An explicit solution of the radiation reaction equation for the case where M_3 = R^2 x S^1 is discussed, and graphs which reveal how the energy varies with the time are presented and analyzed.Comment: 16 pages, 4 figures, REVTEX; Added five references and inserted clarifying details. Version to appear in Int. J. Mod. Phys. A (2000

Spikes in Cosmic Crystallography

If the universe is multiply connected and small the sky shows multiple images of cosmic objects, correlated by the covering group of the 3-manifold used to model it. These correlations were originally thought to manifest as spikes in pair separation histograms (PSH) built from suitable catalogues. Using probability theory we derive an expression for the expected pair separation histogram (EPSH) in a rather general topological-geometrical-observational setting. As a major consequence we show that the spikes of topological origin in PSH's are due to translations, whereas other isometries manifest as tiny deformations of the PSH corresponding to the simply connected case. This result holds for all Robertson-Walker spacetimes and gives rise to two basic corollaries: (i) that PSH's of Euclidean manifolds that have the same translations in their covering groups exhibit identical spike spectra of topological origin, making clear that even if the universe is flat the topological spikes alone are not sufficient for determining its topology; and (ii) that PSH's of hyperbolic 3-manifolds exhibit no spikes of topological origin. These corollaries ensure that cosmic crystallography, as originally formulated, is not a conclusive method for unveiling the shape of the universe. We also present a method that reduces the statistical fluctuations in PSH's built from simulated catalogues.Comment: 25 pages, LaTeX2e. References updated. To appear in Int. J. Mod. Phys. D (2002) in the present for

Photospheric properties and fundamental parameters of M dwarfs

M dwarfs are an important source of information when studying and probing the lower end of the Hertzsprung-Russell (HR) diagram, down to the hydrogen-burning limit. Being the most numerous and oldest stars in the galaxy, they carry fundamental information on its chemical history. The presence of molecules in their atmospheres, along with various condensed species, complicates our understanding of their physical properties and thus makes the determination of their fundamental stellar parameters more challenging and difficult. The aim of this study is to perform a detailed spectroscopic analysis of the high-resolution H-band spectra of M dwarfs in order to determine their fundamental stellar parameters and to validate atmospheric models. The present study will also help us to understand various processes, including dust formation and depletion of metals onto dust grains in M dwarf atmospheres. The high spectral resolution also provides a unique opportunity to constrain other chemical and physical processes that occur in a cool atmosphere The high-resolution APOGEE spectra of M dwarfs, covering the entire H-band, provide a unique opportunity to measure their fundamental parameters. We have performed a detailed spectral synthesis by comparing these high-resolution H-band spectra to that of the most recent BT-settl model and have obtained fundamental parameters such as effective temperature, surface gravity, and metallicity (Teff, log g and [Fe/H]) respectively.Comment: 15 pages, 10 figures, accepted for publication in A&

Innovative solar concentration systems and its potential application in Angola

Energy demands have been increasing worldwide, endangering the future supply–demand energy balance. To provide a sustainable solution for future generations and to comply with the international goal to achieve Carbon Neutrality by 2050, renewable energies have been at the top of the international discussions, actively contributing to the energy transition and climatic policies. To achieve the international goal, Angola proposed a long-term strategy that promotes a fair and sustainable development of the national territory by means of improving the electric sector. Among all the renewable resources, solar energy is found to be the most promising solution since it has the second major renewable energy potential in Angola. However, the main problem related to solar energy is the efficiency of the solar systems and the electrical and thermal energy storage. As part of the solution, Concentration Solar Power (CSP) can make a sounder contribution to the transformation of the Angolan energy sector since it enables a significant increase in energy intensity through the concentration of solar energy. Moreover, the large applicability of this technology can contribute to the development of the rural regions which still struggle for energy equity. By considering the potential of CSP, this work presents the status of the Angolan energy sector, and focus is provided on the solar potential of the country. The advantages of the CSP technologies with emphasis on the parabolic dish systems are presented, and the contribution and innovative solutions for the enhancement of thermal efficiency are presented.This research was funded by FCT-Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020 (ALGORITMI Center) and Project Scope UIDB/00319/2020 (METRICS Center)