1,721,608 research outputs found
Dynamical evolution of axion condensates under stimulated decays into photons
No full textDark matter axion condensates may experience stimulated decays into photon pairs. This effect has been often interpreted as a parametric resonance of photons from the axion-photon coupling, leading to an exponential growth of the photon occupation number in a narrow instability band. Most of the previous literature does not consider the possible evolution of the axion field due to the photon growth. We revisit this effect presenting a mean field solution of the axion-photon kinetic equations, in terms of number of photons and pair correlations. We study the limit of no axion depletion, recovering the known instability. Moreover, we extend the results including a possible depletion of the axion field. In this case we find that the axion condensate exhibits the behavior of an inverted pendulum. We discuss the relevance of these effects for two different cases: a homogeneous axion field at recombination and a localized axion clump and discuss constraints that could result from the induced photon background
Investigating binary-neutron-star mergers as production sites of high-energy neutrinos
Full text linkUltrahigh-Energy Photons as Probes of Lorentz Symmetry Violations in Stringy Space-Time Foam Models
No full textThe time delays between gamma-rays of different energies from extragalactic sources have often been used to probe quantum gravity models in which Lorentz symmetry is violated. It has been claimed that these time delays can be explained by or at least put the strongest available constraints on quantum gravity scenarios that cannot be cast within an effective field theory framework, such as the space-time foam, D-brane model. Here we show that this model would predict too many photons in the ultra-high energy cosmic ray flux to be consistent with observations. The resulting constraints on the space-time foam model are much stronger than limits from time delays and allow for Lorentz violations effects way too small for explaining the observed time delays
Possible cosmogenic neutrino constraints on Planck-scale Lorentz violation
No full textWe study, within an effective field theory framework, O (E-2/M-Pl(2)) Planck-scale suppressed Lorentz invariance violation (LV) effects in the neutrino sector, whose size we parameterize by a dimensionless parameter eta(nu). We find deviations from predictions of Lorentz invariant physics in the cosmogenic neutrino spectrum. For positive O (1) coefficients no neutrino will survive above 10(19) eV. The existence of this cutoff generates a bump in the neutrino spectrum at energies of 10(17) eV. Although at present no constraint can be cast, as current experiments do not have enough sensitivity to detect ultra-high-energy neutrinos, we show that experiments in construction or being planned have the potential to cast limits as strong as eta(nu) less than or similar to 10(-4) on the neutrino LV parameter, depending on how LV is distributed among neutrino mass states. Constraints on eta(nu) < 0 can in principle be obtained with this strategy, but they require a more detailed modeling of how LV affects the neutrino sector
Propagation of ultrahigh energy cosmic rays in extragalactic magnetic fields: a view from cosmological simulations
No full textWe use the crpropa code to simulate the propagation of ultrahigh energy cosmic rays (with energy â¥1018eV and pure proton composition) through extragalactic magnetic fields that have been simulated with the cosmological enzo code. We test both primordial and astrophysical magnetogenesis scenarios in order to investigate the impact of different magnetic field strengths in clusters, filaments and voids on the deflection of cosmic rays propagating across cosmological distances. We also study the effect of different source distributions of cosmic rays around simulated Milky Way-like observers. Our analysis shows that the arrival spectra and anisotropy of events are rather insensitive to the distribution of extragalactic magnetic fields, while they are more affected by the clustering of sources within an ~50 Mpc distance to observers. Finally, we find that in order to reproduce the observed degree of isotropy of cosmic rays at ~EeV energies, the average magnetic fields in cosmic voids must be ~ 0.1 nG, providing limits on the strength of primordial seed fields
Collisional Triggering of Fast Flavor Conversions of Supernova Neutrinos
Full text linkFast flavor conversions of supernova neutrinos, possible near the neutrinosphere, depends on an interesting interplay of collisions and neutrino oscillations. Contrary to naïve expectations, the rate of self-induced neutrino oscillations, due to neutrino-neutrino forward scattering, comfortably exceeds the rate of collisions even deep inside the supernova core. Consistently accounting for collisions and oscillations, we present the first calculations to show that collisions can create the conditions for fast flavor conversions of neutrinos, but oscillations can continue without significant damping thereafter. This may have interesting consequences for supernova explosions and the nature of its associated neutrino emission
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
