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 ($\gtrsim 1$ 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 $\rho_X(z)$ depends on redshift, if $\rho_X(z)$ 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 $1+w_0 = -w_1 z_{max}/3$ (where $z_{max}$ is the maximum redshift of the survey, e.g., $z_{max}=1.7$ 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 $\mathbf A$ there is a polynomial time algorithm that decides if an equation over $\mathbf A$ 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 $1/{\cal M}$ mass function. Then the total mass of red dwarfs and brown dwarfs is $\leq$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 $<1\%$ 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 $1/{\cal M}$ mass function, this suggests a total density of red dwarfs and brown dwarfs of $\sim$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