2,142 research outputs found
Primordial Non-Gaussian Signatures in the Sky
The presence of non-Gaussian features in the CMB radiation maps represents
one of the most long-awaited clues in the search for the actual structure of
the primordial radiation. These features could shed some light on the non
trivial task of distinguishing the real source of the primeval perturbations
leading to large scale structure. In the present paper we briefly review recent
work towards finding analytical estimates of the three- and four-point
correlation functions and of their zero-lag limits, namely, the skewness and
kurtosis, respectively. - Contributed talk to appear in the proceedings of
"Birth of the Universe & Fundamental Physics", May 18-21, 1994, Rome, Italy.Comment: 4pp; PostScript fil
The positron excess and supersymmetric dark matter
Using a new instrument, the HEAT collaboration has confirmed the excess of
cosmic ray positrons that they first detected in 1994. We explore the
possibility that this excess is due to the annihilation of neutralino dark
matter in the galactic halo. We confirm that neutralino annihilation can
produce enough positrons to make up the measured excess only if there is an
additional enhancement to the signal. We quantify the `boost factor' that is
required in the signal for various models in the Minimal Supersymmetric
Standard Model parameter space, and find that a boost factor >30 provides good
fits to the HEAT data. Such an enhancement in the signal could arise if we live
in a clumpy halo.Comment: 6 pages, LaTeX, proceedings of the 4th International Workshop on
Identification of Dark Matter (idm2002), York, England, 2-6 September, 200
Dark matter and the first stars: a new phase of stellar evolution
A mechanism is identified whereby dark matter (DM) in protostellar halos
dramatically alters the current theoretical framework for the formation of the
first stars. Heat from neutralino DM annihilation is shown to overwhelm any
cooling mechanism, consequently impeding the star formation process and
possibly leading to a new stellar phase. A "dark star'' may result: a giant
( AU) hydrogen-helium star powered by DM annihilation instead of
nuclear fusion. Observational consequences are discussed.Comment: 5 pages, 2 figures; replaced with accepted versio
Dark matter powered stars: Constraints from the extragalactic background light
The existence of predominantly cold non-baryonic dark matter is unambiguously
demonstrated by several observations (e.g., structure formation, big bang
nucleosynthesis, gravitational lensing, and rotational curves of spiral
galaxies). A candidate well motivated by particle physics is a weakly
interacting massive particle (WIMP). Self-annihilating WIMPs would affect the
stellar evolution especially in the early universe. Stars powered by
self-annihilating WIMP dark matter should possess different properties compared
with standard stars. While a direct detection of such dark matter powered stars
seems very challenging, their cumulative emission might leave an imprint in the
diffuse metagalactic radiation fields, in particular in the mid-infrared part
of the electromagnetic spectrum. In this work the possible contributions of
dark matter powered stars (dark stars; DSs) to the extragalactic background
light (EBL) are calculated. It is shown that existing data and limits of the
EBL intensity can already be used to rule out some DS parameter sets.Comment: Accepted for publication in ApJ; 7 pages, 5 figure
Mapping Nanoscale Metal-Insulator Phase Transition in NdNiO3 and Molecular Beam Epitaxy of SmTiO3 Thin Films
The subject of this thesis is the growth and characterization of quantum materials. Quantum materials are those in which correlated electron interactions result in functional emergent
properties like high-temperature superconductivity, colossal magnetoresistance, ferromagnetism, and metal-insulator transitions. 3d transition metal oxides, particularly perovskites,
have been found to be a fertile area of investigation in quantum materials. Rare earth nickelates and titanates fall into this category and are examined here. The bulk of this work
concerns soft x-ray spectroscopy and imaging of a free-standing NdNiO3 thin film. NdNiO3
displays a metal-insulator transition as well as a magnetic transition, with the respective
Curie and Néel temperatures coinciding.
We employ x-ray absorption spectroscopy to characterize the electronic transition from
metallic to insulating on cooling the sample, noting the magnitude of the hysteresis in the
process; x-ray magnetic scattering to characterize the magnetic transition on heating; and
scanning transmission x-ray spectromicroscopy to search for the formation of distinct domains of metallic and insulating phases during the progress of the electronic phase transition.
Although these electronic domains were not observed, the sum of our experiments present
confirmation that freestanding films of this novel configuration possess similar magnetic and
electronic properties to those observed in their bulk counterparts. This finding is significant
as it indicates that films of this type could be integrated into device applications in the same
manner as bulk nickelates.
A secondary thrust of this work is the development of the capability to synthesize thin
films by molecular beam epitaxy (MBE) at the Canadian Light Source for future studies of
quantum materials with an emphasis on interface effects and heterostructures. We briefly
present results of an effort to grow SmTiO3 thin films by MBE with characterization by
electron diffraction and spectroscopy
Probing the Evolution of the Dark Energy Density with Future Supernova Surveys
The time dependence of the dark energy density can be an important clue to
the nature of dark energy in the universe. We show that future supernova data
from dedicated telescopes (such as SNAP), when combined with data of nearby
supernovae, can be used to determine how the dark energy density
depends on redshift, if is not too close to a constant. For
quantitative comparison, we have done an extensive study of a number of dark
energy models. Based on these models we have simulated data sets in order to
show that we can indeed reconstruct the correct sign of the time dependence of
the dark energy density, outside of a degeneracy region centered on (where is the maximum redshift of the survey, e.g.,
for SNAP). We emphasize that, given the same data, one can obtain
much more information about the dark energy density directly (and its time
dependence) than about its equation of state.Comment: submitted to PR
Slow nucleation rates in Chain Inflation with QCD Axions or Monodromy
The previous proposal (by two of us) of chain inflation with the QCD axion is
shown to fail. The proposal involved a series of fast tunneling events, yet
here it is shown that tunneling is too slow. We calculate the bubble nucleation
rates for phase transitions in the thick wall limit, approximating the barrier
by a triangle. A similar problem arises in realization of chain inflation in
the string landscape that uses series of minima along the monodromy staircase
around the conifold point. The basic problem is that the minima of the
potential are too far apart to allow rapid enough tunneling in these two
models. We entertain the possibility of overcoming this problem by modifying
the gravity sector to a Brans-Dicke theory. However, one would need extremely
small values for the Brans-Dicke parameter. Many successful alternatives exist,
including other "axions" (with mass scales not set by QCD) or potentials with
comparable heights and widths that do not suffer from the problem of slow
tunneling and provide successful candidates for chain inflation.Comment: 6 pages, 1 figur
A 24-hour food list for dietary assessment in large-scale epidemiological studies
The validity of dietary assessment in large-scale cohort studies has been questioned. Combining different instruments for the assessment of consumption probability and amounts consumed might be feasible and improve the estimation of usual dietary intake in such studies. Thus, the objectives were (a) to develop a web-based 24-hour food list (24 h FL) for Germany to assess the consumption probability of foods during the previous 24 hours, (b) to evaluate the performance of the new questionnaire in a feasibility study, (c) to identify determinants of consumption-day amounts in order to derive person-specific standard consumption-day amounts and (d) to evaluate their relevance for the estimation of usual dietary intake distributions. Data from the German National Nutrition Survey II (NVS II) was used to develop a finite list of food items for the 24 h FL applying stepwise linear regression analysis. In addition, NVS II data was analyzed for determinants of consumption-day amounts across 22 food groups. A total of 508 individuals participating in the pilot study for the German National Cohort (GNC) were invited to fill in the 24 h FL via Internet up to three times during a three to six month period. In addition, GNC pilot study participants were asked to evaluate the questionnaire using a brief online evaluation form. Finally, usual intake distributions were compared that were either derived by using a parsimonious or a comprehensive prediction model. In total, 246 items were identified for the item list of the 24 h FL, reflecting more than 75% of variation in intake of 27 nutrients and four major food groups. Among individuals of the GNC pilot study invited, 64% participated in the feasibility study. Of these, 100%, 85%, and 68% of participants completed the 24 h FL one, two, or three times, respectively. The average time needed to complete the questionnaire was nine minutes and its acceptability in terms of understandability, usability, completeness and visual presentation was rated as high. Relevant determinants for consumption-day amounts of food groups were sex, age, body mass index (BMI), smoking status, years of education, household net income, living with a partner and employment status. However, the use of a comprehensive prediction model (sex, age, BMI, smoking status, years of education, household net income) compared to a parsimonious prediction model (sex, age, BMI) seemed to be important for less frequently consumed foods only. The 24 h FL represents a promising new dietary assessment tool, which can be employed as part of a blended approach combining multiple data sources for estimation of usual dietary intake in large-scale cohort studies
Satisfiability in multi-valued circuits
Satisfiability of Boolean circuits is among the most known and important
problems in theoretical computer science. This problem is NP-complete in
general but becomes polynomial time when restricted either to monotone gates or
linear gates. We go outside Boolean realm and consider circuits built of any
fixed set of gates on an arbitrary large finite domain. From the complexity
point of view this is strictly connected with the problems of solving equations
(or systems of equations) over finite algebras.
The research reported in this work was motivated by a desire to know for
which finite algebras there is a polynomial time algorithm that
decides if an equation over has a solution. We are also looking for
polynomial time algorithms that decide if two circuits over a finite algebra
compute the same function. Although we have not managed to solve these problems
in the most general setting we have obtained such a characterization for a very
broad class of algebras from congruence modular varieties. This class includes
most known and well-studied algebras such as groups, rings, modules (and their
generalizations like quasigroups, loops, near-rings, nonassociative rings, Lie
algebras), lattices (and their extensions like Boolean algebras, Heyting
algebras or other algebras connected with multi-valued logics including
MV-algebras).
This paper seems to be the first systematic study of the computational
complexity of satisfiability of non-Boolean circuits and solving equations over
finite algebras. The characterization results provided by the paper is given in
terms of nice structural properties of algebras for which the problems are
solvable in polynomial time.Comment: 50 page
Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red
dwarfs, stars just massive enough to burn Hydrogen. The authors of this search
(Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less
than 6\% of the galactic halo. First, we extrapolate this result to include
brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a
mass function. Then the total mass of red dwarfs and brown dwarfs
is 18\% of the halo. This result is consistent with microlensing results
assuming a popular halo model. However, using new stellar models and parallax
observations of low mass, low metallicity stars, we obtain much tighter bounds
on low mass stars. We find the halo red dwarf density to be of the halo,
while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the
halo mass density of red dwarfs drops to 16-40 times less than the reported
result of Bahcall et al (1994). For a mass function, this suggests
a total density of red dwarfs and brown dwarfs of 0.25-0.67\% of the
halo, \ie , (0.9-2.5)\times 10^9\msun out to 50 kpc. Such a low result would
conflict with microlensing estimates by the \macho\ group (Alcock \etal
1995a,b).Comment: 13 pages, 2 figures. Figure one only available via fax or snail-mail
To be published in ApJL. fig. 2 now available in postscript. Some minor
changes in dealing with disk forground. Some cosmetic changes. Updated
reference
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