1,721,020 research outputs found
Constraining the primordial black hole abundance through big-bang nucleosynthesis
No full textWe investigate the scenario in which primordial black holes (PBHs) with masses Mpbh < 10^9 g undergo Hawking evaporation, around the Big-Bang nucleosynthesis (BBN) epoch. The evaporation process modifies the Universe's expansion rate and the baryon-to-photon ratio, leading to an alteration of the primordial abundance of light nuclei. We present numerical solutions for the set of equations describing this physics, considering different values of PBH masses and abundances at their formation, showing how their evaporation impacts the abundances of light nuclei, obtained by incorporating the non-standard Hubble rate and baryon-to-photon ratio into the BBN code PArthENoPE. The results are then used to place upper bounds for the PBH relative abundance at formation in the range 10^8 g < Mpbh < 10^9 g, providing the strongest constraints existing to-date in this mass range
Gravitational waves and neutrino oscillations in Chern-Simons axion gravity
No full textWe investigate the modifications in the neutrino flavor oscillations under the influence of a stochastic gravitational wave background (SGWB), in a scenario in which General Relativity is modified by an additional Chern-Simons (CS) term. Assuming that the dark matter halo is in the form of axions, the CS coupling modifies the pattern of the neutrino flavor oscillations at Earth up to a total suppression in some frequency range. At the same time, the SGWB in the halo could stimulate the axion decay into gravitons over a narrow frequency range, leading to a potentially detectable resonance peak in the enhanced SGWB strain. A consistent picture would require these features to potentially show up in neutrino detection from supernovae, gravitational wave detectors, and experiments aimed at the search for axions in the Milky Way halo
Astrophysical neutrino oscillations after pulsar timing array analyses
Full text linkThe pattern of neutrino flavor oscillations could be altered by the influence
of noisy perturbations such as those arising from a gravitational wave
background (GWB). A stochastic process that is consistent with a GWB has been
recently reported by the independent analyses of pulsar timing array (PTA) data
sets collected over a decadal timescale by the North American Nanohertz
Observatory for Gravitational Waves (NANOGrav), the European Pulsar Timing
Array (EPTA), the Parkes Pulsar Timing Array (PPTA), and the Chinese Pulsar
Timing Array (CPTA) collaborations. We investigate the modifications in the
neutrino flavor oscillations under the influence of the GWB reported by the PTA
collaborations and we discuss how such effects could be potentially revealed in
near-future neutrino detectors, possibly helping the discrimination of
different models for the GWB below the nHz frequency range.Comment: 6 pages, 3 figures. Matches the version published on Phys. Rev.
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
Light axion-like dark matter must be present during inflation
Full text linkAxion-like particles (ALPs) might constitute the totality of the cold dark
matter (CDM) observed. The parameter space of ALPs depends on the mass of the
particle and on the energy scale of inflation , the latter being
bound by the non-detection of primordial gravitational waves. We show that the
bound on HI implies the existence of a mass scale , depending on the ALP susceptibility , such that
the energy density of ALPs of mass smaller than is too low to explain
the present CDM budget, if the ALP field has originated after the end of
inflation. This bound affects Ultra-Light Axions (ULAs), which have recently
regained popularity as CDM candidates. Light () ALPs can then be
CDM candidates only if the ALP field has already originated during the
inflationary period, in which case the parameter space is constrained by the
non-detection of axion isocurvature fluctuations. We comment on the effects on
these bounds from additional physics beyond the Standard Model, besides ALPs.Comment: 12 pages, 3 figures. Improved reference
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
(Non-)Thermal Production of WIMPs during Kination
No full textUnderstanding the nature of the Dark Matter (DM) is one of the current challenges in modern astrophysics and cosmology. Knowing the properties of the DM particle would shed light on physics beyond the Standard Model and even provide us with details of the early Universe. In fact, the detection of such a relic would bring us information from the pre-Big Bang Nucleosynthesis (BBN) period, an epoch from which we have no direct data, and could even hint at inflation physics. In this work, we assume that the expansion rate of the Universe after inflation is governed by the kinetic energy of a scalar field phi, in the so-called "kination" model. Adding to previous work on the subject, we assume that the phi field decays into both radiation and DM particles, which we take to be Weakly Interacting Massive Particles (WIMPs). The present abundance of WIMPs is then fixed during the kination period through either a thermal "freeze-out" or "freeze-in" mechanism, or through a non-thermal process governed by the decay of phi. We explore the parameter space of this theory with the requirement that the present WIMP abundance provides the correct relic budget. Requiring that BBN occurs during the standard cosmological scenario sets a limit on the temperature at which the kination period ends. Using this limit and assuming the WIMP has a mass m(chi) = 100 GeV, we obtain that the thermally averaged WIMP annihilation cross section has to satisfy the constraints 4 x 10(-16) GeV-2 less than or similar to <sigma upsilon > less than or similar to 2 x 10(-5) GeV-2 in order for having at least one of the production mechanism to yield the observed amount of DM. This result shows how the properties of the WIMP particle, if ever measured, can yield information on the pre-BBN content of the Universe
Doctor of Philosophy
Full text linkdissertationThe subjects of this thesis are the invisible axion and the more general family of axion-like particles. The invisible axion is a hypothetical elementary particle and a cold dark matter candidate. I present an improved computation of the constraints on the parameter space of the cold dark matter axion in the standard cosmology, that includes the contributions from anharmonicities in the axion potential and from the decay of axionic strings. In this scenario, I update the value of the mass of the cold dark matter axion, fi nding the value (67 ± 17)µeV, approximately one order of magnitude larger than previous computations. The eff ect of nonstandard cosmological scenarios on the parameter space of axion cold dark matter is studied for the first time. In particular, I consider the cases of low-temperature reheating and kination cosmologies, and I show that the mass of the cold dark matter axion can differ from the value in the standard cosmological scenario by orders of magnitude. Finally, I consider the family of axion-like particles, assuming that these particles serve as the inflation in the context; of warm inflation. I find that the axion energy scale f, which in the standard inflation scenario is of the order of the Planck mass, can be lowered to the much safer Grand Uni cation Theory scale f ~ 1016GeV. I also constrain the parameter space and the amount of gravitational waves from this model, using results from the Wilkinson Microwave Anisotropy Probe 7-year data
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