106 research outputs found
Diagnostic Potential of Cosmic-Neutrino Absorption Spectroscopy
Annihilation of extremely energetic cosmic neutrinos on the relic-neutrino
background can give rise to absorption lines at energies corresponding to
formation of the electroweak gauge boson . The positions of the
absorption dips are set by the masses of the relic neutrinos. Suitably intense
sources of extremely energetic ( -- -eV) cosmic neutrinos
might therefore enable the determination of the absolute neutrino masses and
the flavor composition of the mass eigenstates. Several factors--other than
neutrino mass and composition--distort the absorption lines, however. We
analyze the influence of the time-evolution of the relic-neutrino density and
the consequences of neutrino decay. We consider the sensitivity of the
lineshape to the age and character of extremely energetic neutrino sources, and
to the thermal history of the Universe, reflected in the expansion rate. We
take into account Fermi motion arising from the thermal distribution of the
relic-neutrino gas. We also note the implications of Dirac vs. Majorana relics,
and briefly consider unconventional neutrino histories. We ask what kinds of
external information would enhance the potential of cosmic-neutrino absorption
spectroscopy, and estimate the sensitivity required to make the technique a
reality.Comment: 25 pages, 26 figures (in 46 files), uses RevTe
21-cm cosmology
Imaging the Universe during the first hundreds of millions of years remains
one of the exciting challenges facing modern cosmology. Observations of the
redshifted 21 cm line of atomic hydrogen offer the potential of opening a new
window into this epoch. This would transform our understanding of the formation
of the first stars and galaxies and of the thermal history of the Universe. A
new generation of radio telescopes is being constructed for this purpose with
the first results starting to trickle in. In this review, we detail the physics
that governs the 21 cm signal and describe what might be learnt from upcoming
observations. We also generalize our discussion to intensity mapping of other
atomic and molecular lines.Comment: 64 pages, 20 figures, submitted to Reports on Progress in Physics,
comments welcom
Origin and Propagation of Extremely High Energy Cosmic Rays
Cosmic ray particles with energies in excess of 10**(20) eV have been
detected. The sources as well as the physical mechanism(s) responsible for
endowing cosmic ray particles with such enormous energies are unknown. This
report gives a review of the physics and astrophysics associated with the
questions of origin and propagation of these Extremely High Energy (EHE) cosmic
rays in the Universe. After a brief review of the observed cosmic rays in
general and their possible sources and acceleration mechanisms, a detailed
discussion is given of possible "top-down" (non-acceleration) scenarios of
origin of EHE cosmic rays through decay of sufficiently massive particles
originating from processes in the early Universe. The massive particles can
come from collapse and/or annihilation of cosmic topological defects (such as
monopoles, cosmic strings, etc.) associated with Grand Unified Theories or they
could be some long-lived metastable supermassive relic particles that were
created in the early Universe and are decaying in the current epoch. The
highest energy end of the cosmic ray spectrum can thus be used as a probe of
new fundamental physics beyond Standard Model. We discuss the role of existing
and proposed cosmic ray, gamma-ray and neutrino experiments in this context. We
also discuss how observations with next generation experiments of images and
spectra of EHE cosmic ray sources can be used to obtain new information on
Galactic and extragalactic magnetic fields and possibly their origin.Comment: 148 latex pages in tight format, 30 postscript-files and two
gif-files for fig4.14 and fig4.15 included, uses epsf.sty. Considerably
updated version of review to appear in Physics Reports. Links and color ps
version of fig4.14 and fig4.15 at
http://astro.uchicago.edu/home/web/sigl/physrep.htm
Search for high energy neutrinos from our galaxy with IceCube
High energy neutrino telescopes are expected to play a major role in the discovery of the first unambiguous sources of cosmic-rays. With completion in 2011, the IceCube neutrino detector constitutes the most sensitive instrument to sources of high energy neutrinos. Its performance and discovery potential are usually given in the energy range above a few TeV, in order to achieve the best signal to noise for sources following an E^-2 spectrum without an energy cutoff up to at least 1 PeV. However, given the present understanding and multiwavelength picture of our galaxy, we can expect that galactic sources of high energy neutrinos show significant deviations from the E^-2, no cutoff approximation. The common data analysis are therefore not optimal for such galactic scenarios, requiring exposure times of the order of several years, even a decade, to reach a level of sensitivity at which a possible detection starts to be plausible. The main goal of this thesis is to improve the discovery potential of IceCube to galactic sources of high energy neutrinos, aiming to a better understanding of the high energy processes taking place in our galaxy. In order to fulfill this goal, I follow two lines of action: (1) to increase the detection capabilities of IceCube for neutrinos in the energy range between 100 GeV < E < 1 TeV; and (2) to develop a search method which is able to reduce the minimum detectable flux per point source. The improvement of the IceCube performance at energies below 1 TeV is achieved with the use of the combined detector configuration IceCube 22 strings plus AMANDA (Antarctic Muon And Neutrino Detector Array). The data processing scheme is designed in order to keep as many good low energy events as possible. As a result, this analysis achieved the best sensitivity for sources with neutrino spectra steeper than E^-2 and/or an energy cutoff below 1 PeV. The second goal of this thesis is motivated in order to search efficiently for high energy neutrinos from the Cygnus star forming region of the Galaxy. In order to extend the search beyond a single point source, I developed a method based on two-point analysis to detect, within an extended region, event patterns which might go undetected in conventional point source analysis. The results obtained with this method indicate that the minimum detectable flux per point source is reduced by 26%-59% with respect to standard point source analysis, provided there is more than one point source within the region under study. This method was applied on the Cygnus region of the Galaxy using the data sample obtained with the combined detector IceCube 22 strings plus AMANDA, yielding a significance of 2.3-sigma
Primordial black holes and their gravitational-wave signatures
In the recent years, primordial black holes (PBHs) have emerged as one of the
most interesting and hotly debated topics in cosmology. Among other
possibilities, PBHs could explain both some of the signals from binary black
hole mergers observed in gravitational wave detectors and an important
component of the dark matter in the Universe. Significant progress has been
achieved both on the theory side and from the point of view of observations,
including new models and more accurate calculations of PBH formation,
evolution, clustering, merger rates, as well as new astrophysical and
cosmological probes. In this work, we review, analyse and combine the latest
developments in order to perform end-to-end calculations of the various
gravitational wave signatures of PBHs. Different ways to distinguish PBHs from
stellar black holes are emphasized. Finally, we discuss their detectability
with LISA, the first planned gravitational-wave observatory in space.Comment: 161 pages, 47 figures, comments welcom
The Hydra String Method and its Application to High Dimensional Potential Energy Surfaces Arising from Granular Systems
Granular materials are a ubiquitous yet ill-understood class of media. Different approaches and techniques have been developed to understand the many complex behaviors they exhibit, but none have been completely successful. I have instituted a novel means to understand granular materials. This novel method, the Hydra String Method (HSM), is an efficient and autonomous way to trawl the potential energy surfaces (PESs) to enumerate the saddles, minima, and connections between them. I have applied the Hydra String Method to bi-disperse configurations of soft spheres to map out ensembles of pathways between stable packings of the system. These transition pathways are a low-dimensional projection of the larger PES. By understanding these pathways and how they connect to one another may allow for the prediction of the dynamics of a granular system as it moves between stable packings. In this thesis, I show some interesting results about the PESs that arise from systems of Hertzian disks. For example, the basins of attraction around minima of these PESs possess ``tentacles'' that twist and curl through configuration space and the profile of these basins have a characteristic shape, Energy ~ x^(1.7). I have also found novel results about the networks formed by these minima such as: the networks appear to be small-world networks, gamma distribution of the minimum/saddle energies, and the separation in energy space of the various branches of the network. The Hydra String Method is also a useful tool to visualize high dimensional PESs. These surfaces are vast and complex and have not been the subject of much study. This leads me to propose natural extensions of the Climbing String Method and the HSM: the Climbing Kite String Method and the Flying Hydra Method. These two methods enable one to explore more fully the PESs of an arbitrary system. This can potentially be used to study chemical and granular systems that possess too much kinetic energy to be well described as gradient systems.Doctor of Philosoph
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