1,721,136 research outputs found
Magnetic field amplification by the small-scale dynamo in the early Universe
No full textIn this paper we show that the Universe is already strongly magnetized at very early epochs during cosmic evolution. Our calculations are based on the efficient amplification of weak magnetic seed fields, which are unavoidably present in the early Universe, by the turbulent small-scale dynamo. We identify two mechanisms for the generation of turbulence in the radiation dominated epoch where velocity fluctuations are produced by the primordial density perturbation and by possible first-order phase transitions at the electroweak or QCD scales. We show that all the necessities for the small-scale dynamo to work are fulfilled. Hence, this mechanism, operating due to primordial density perturbations, guarantees fields with comoving field strength B-0 similar to 10(-6)epsilon(1/2) nG on scales up to lambda(c) similar to 0.1 pc, where e is the saturation efficiency. The amplification of magnetic seed fields could be even larger if there are first-order phase transitions in the early Universe. Where, on scales up to lambda(c) similar to 100 pc, the comoving field strength due to this mechanism will be B-0 similar to 10(-3)epsilon(1/2) nG at the present time. Such fields, albeit on small scales, can play an important role in structure formation and could provide an explanation to the apparently observed magnetic fields in the voids of the large-scale structure
PeV neutrinos from the propagation of ultra-high energy cosmic rays
Full text linkWe discuss the possibility that the PeV neutrinos recently observed by IceCube are produced by the interactions of extragalactic cosmic rays during their propagation through the radiation backgrounds. We show that the fluxes resulting from the decays of neutrons produced in the interactions of cosmic ray protons with the CMB background are suppressed (Eν2dΦν/dE < 10−10 GeV/cm2 s sr), with those resulting from the decays of pions produced in the interactions with the UV/optical/IR backgrounds being the dominant ones at PeV energies. The anti-neutrino fluxes produced by the decay of neutrons resulting from the photodisintegration of heavy nuclei with CMB photons are also shown to be quite suppressed (Eν2dΦν/dE < 10−11 GeV/cm2 s sr), while those produced by photo-pion processes with UV/optical/IR backgrounds may be larger, although they are not expected to be above those achievable in the pure proton case. Scenarios with mixed composition and low cutoff rigidities can lead to PeV neutrino fluxes enhanced with respect to those in the pure Fe scenarios. We also discuss the possible impact of the Glashow resonance for the detection of these scenarios, showing that it plays a moderate role.Fil: Roulet, Esteban. Comision Nacional de Energia Atomica. Gerencia del Area de Investigaciones y Aplicaciones no Nucleares. Gerencia de Fisica (CAB); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sigl, Guenter. Universitat Hamburg; AlemaniaFil: Vliet, Arjen van. Universitat Hamburg; AlemaniaFil: Mollerach, Maria Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energia Atomica. Gerencia del Area de Investigaciones y Aplicaciones no Nucleares. Gerencia de Fisica (CAB); Argentin
On soft limits of large-scale structure correlation functions
No full textWe study soft limits of correlation functions for the density and velocity fields in the theory of structure formation. First, we re-derive the (resummed) consistency conditions at unequal times using the eikonal approximation. These are solely based on symmetry arguments and are therefore universal. Then, we explore the existence of equal-time relations in the soft limit which, on the other hand, depend on the interplay between soft and hard modes. We scrutinize two approaches in the literature: the time-flow formalism, and a background method where the soft mode is absorbed into a locally curved cosmology. The latter has been recently used to set up (angular averaged) ‘equal-time consistency relations’. We explicitly demonstrate that the time-flow relations and ‘equal-time consistency conditions’ are only fulfilled at the linear level, and fail at next-to-leading order for an Einstein de- Sitter universe. While applied to the velocities both proposals break down beyond leading order, we find that the ‘equal-time consistency conditions’ quantitatively approximates the perturbative results for the density contrast. Thus, we generalize the background method to properly incorporate the effect of curvature in the density and velocity fluctuations on short scales, and discuss the reasons behind this discrepancy. We conclude with a few comments on practical implementations and future directions
Electric-magnetic duality invariance implications for axion physics
No full textAxions and axion-like particles are very well-motivated candidates for physics beyond the Standard model, which can be probed by multiple existing and projected experiments and astrophysical observations. Theoretical determination of the relevant low energy axion interactions is essential for guiding the corresponding experimental and observational efforts. In this thesis, we revisit the previous theoretical investigations in this direction. In particular, we show that, contrary to assertions in the literature, the main contribution to the axion-photon coupling need not be quantized in units proportional to . We discuss a loophole in the argument for this quantization and then provide explicit counterexamples. Based on this, we construct a generic axion-photon effective Lagrangian and find that the axion-photon coupling may be dominated by previously unknown Wilson coefficients. We show that this result implies a significant modification of conventional axion electrodynamics and sets new targets for axion experiments. We find that the electromagnetic interactions of axions can violate the CP symmetry and that future experiments could be sensitive to the corresponding coupling. At the core of our theoretical analysis lies a critical reexamination of the interactions between axions and magnetic monopoles. We develop the effective field theory approach to the Zwanziger theory of quantum electromagnetodynamics and show that, contrary to claims in the literature, magnetic monopoles need not give mass to axions. Moreover, we find that a future detection of an axion or axion-like particle with certain parameters can serve as evidence for the existence of magnetically charged matter.Besides studying the structure of the low energy axion interactions in the effective field theory approach, we explicitly construct new theoretical models for the axion which realize the newly found interactions. In these models, the PQ mechanism is realized through a coupling of the Peccei-Quinn complex scalar field to magnetically charged fermions at high energies. We consider both the cases of Abelian and non-Abelian magnetic charges. We show that these models indeed solve the strong CP problem and then integrate out heavy magnetic monopoles using the Schwinger proper time method. We find that the models discussed yield axion couplings to the Standard Model which are drastically different from the ones calculated within the KSVZ/DFSZ-type models. As a consequence, large part of the corresponding parameter space can be probed by various projected experiments. Moreover, the axion we introduce is consistent with the astrophysical hints for axions suggested both by the anomalous TeV-transparency of the Universe and by the excessive cooling of horizontal branch stars in globular clusters. We argue that the leading term for the cosmic axion abundance is not changed compared to the conventional pre-inflationary scenario for an axion decay constant
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
Probing Physics at Extreme Energies with Cosmic Ultra-High Energy Radiation
No full text12 latex pages, 2 postscript figures included; based on invited talk at the WHEPP-7 Conference, Allahabad, India (January, 2002)The highest energy cosmic rays observed possess macroscopic energies and their origin is likely to be associated with the most energetic processes in the Universe. Their existence triggered a flurry of theoretical explanations ranging from conventional shock acceleration to particle physics beyond the Standard Model and processes taking place at the earliest moments of our Universe. Furthermore, many new experimental activities promise a strong increase of statistics at the highest energies and a combination with gamma-ray and neutrino astrophysics will put strong constraints on these theoretical models. We give an overview over this quickly evolving research field with focus on testing new particle physics
- …
