A Bayesian Estimate of the Primordial Helium Abundance

We introduce a new statistical method to estimate the primordial helium abundance, Y_p from observed abundances in a sample of galaxies which have experienced stellar helium enrichment. Rather than using linear regression on metal abundance we construct a likelihood function using a Bayesian prior, where the key assumption is that the true helium abundance must always exceed the primordial value. Using a sample of measurements compiled from the literature we find estimates of Y_p between 0.221 and 0.236, depending on the specific subsample and prior adopted, consistent with previous estimates either from a linear extrapolation of the helium abundance with respect to metallicity, or from the helium abundance of the lowest metallicity HII region, I Zw 18. We also find an upper limit which is insensitive to the specific subsample or prior, and estimate a model-independent bound Y_p < 0.243 at 95% confidence, favoring a low cosmic baryon density and a high primordial deuterium abundance. The main uncertainty is not the model of stellar enrichment but possible common systematic biases in the estimate of Y in each individual HII region.Comment: 14 pages, latex, 3 ps figure

Numerical simulation evidence of dynamical transverse Meissner effect and moving Bose glass phase

We present 3D numerical simulation results of moving vortex lattices in presence of 1D correlated disorder at zero temperature. Our results with field tilting confirm the theoritical predictions of a moving Bose glass phase, characterized by transverse pinning and dynamical transverse Meissner effect, the moving flux lines being localized along the correlated disorder direction. Beyond a critical transverse field, vortex lines exhibit along all their length a "kink" structure resulting from an effective static "tin roof" pinning potential in the transverse direction.Comment: 5 pages, 4 figure

Cosmological Cosmic Rays and the observed Li6 plateau in metal poor halo stars

Very recent observations of the Li6 isotope in halo stars reveal a Li6 plateau about 1000 times above the predicted BBN abundance. We calculate the evolution of Li6 versus redshift generated from an initial burst of cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that the pregalactic production of the Li6 isotope can account for the Li6 plateau observed in metal poor halo stars without additional over-production of Li7. The derived relation between the amplitude of the CCR energy spectra and the redshift of the initial CCR production puts constraints on the physics and history of the objects, such as pop III stars, responsible for these early cosmic rays. Consequently, we consider the evolution of Li6 in the Galaxy. Since Li6 is also produced in Galactic cosmic ray nucleosynthesis, we argue that halo stars with metallicities between [Fe/H] = -2 and -1, must be somewhat depleted in Li6.Comment: 8 pages, 6 figures, version accepted for publication in Ap

Numerical simulations on the 4d Heisenberg spin glass

We study the 4d Heisenberg spin glass model with Gaussian nearest-neighbor interactions. We use finite size scaling to analyze the data. We find a behavior consistent with a finite temperature spin glass transition. Our estimates for the critical exponents agree with the results from epsilon-expansion.Comment: 11 pages, LaTeX, preprint ROMA1 n. 105

Neutrinos And Big Bang Nucleosynthesis

The early universe provides a unique laboratory for probing the frontiers of particle physics in general and neutrino physics in particular. The primordial abundances of the relic nuclei produced during the first few minutes of the evolution of the Universe depend on the electron neutrinos through the charged-current weak interactions among neutrons and protons (and electrons and positrons and neutrinos), and on all flavors of neutrinos through their contributions to the total energy density which regulates the universal expansion rate. The latter contribution also plays a role in determining the spectrum of the temperature fluctuations imprinted on the Cosmic Background Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer and helium-4 as a chronometer, the predictions of BBN and the CBR are compared to observations. The successes of, as well as challenges to the standard models of particle physics and cosmology are identified. While systematic uncertainties may be the source of some of the current tensions, it could be that the data are pointing the way to new physics. In particular, BBN and the CBR are used to address the questions of whether or not the relic neutrinos were fully populated in the early universe and, to limit the magnitude of any lepton asymmetry which may be concealed in the neutrinos.Comment: Accepted for publication in the Proceedings of Nobel Symposium 129, "Neutrino Physics"; to appear in Physics Scripta, eds., L Bergstrom, O. Botner, P. Carlson, P. O. Hulth, and T. Ohlsso

Low Mass Stars and the He3 Problem

The prediction of standard chemical evolution models of higher abundances of He3 at the solar and present-day epochs than are observed indicates a possible problem with the yield of He3 for stars in the range of 1-3 solar masses. Because He3 is one of the nuclei produced in Big Bang Nucleosynthesis (BBN), it is noted that galactic and stellar evolution uncertainties necessarily relax constraints based on He3. We incorporate into chemical evolution models which include outflow, the new yields for He3 of Boothroyd & Malaney (1995) which predict that low mass stars are net destroyers of He3. Since these yields do not account for the high \he3/H ratio observed in some planetary nebulae, we also consider the possibility that some fraction of stars in the 1 - 3 solar mass range do not destroy their He3 in theirpost main-sequence phase. We also consider the possibility that the gas expelled by stars in these mass ranges does not mix with the ISM instantaneously thus delaying the He3 produced in these stars, according to standard yields, from reaching the ISM. In general, we find that the Galactic D and He3 abundances can be fit regardless of whether the primordial D/H value is high (2 x 10^{-4}) or low (2.5 x 10^{-5}).Comment: 20 pages, latex, 9 ps figure

Update on the Direct Detection of Supersymmetric Dark Matter

We compare updated predictions for the elastic scattering of supersymmetric neutralino dark matter with the improved experimental upper limit recently published by CDMS II. We take into account the possibility that the \pi-nucleon \Sigma term may be somewhat larger than was previously considered plausible, as may be supported by the masses of exotic baryons reported recently. We also incorporate the new central value of m_t, which affects indirectly constraints on the supersymmetric parameter space, for example via calculations of the relic density. Even if a large value of \Sigma is assumed, the CDMS II data currently exclude only small parts of the parameter space in the constrained MSSM (CMSSM) with universal soft supersymmetry-breaking Higgs, squark and slepton masses. None of the previously-proposed CMSSM benchmark scenarios is excluded for any value of \Sigma, and the CDMS II data do not impinge on the domains of the CMSSM parameter space favoured at the 90 % confidence level in a recent likelihood analysis. However, some models with non-universal Higgs, squark and slepton masses and neutralino masses \lappeq 700 GeV are excluded by the CDMS II data.Comment: 25 pages, 28 eps figure

Cosmological Bounds on Spatial Variations of Physical Constants

We derive strong observational limits on any possible large-scale spatial variation in the values of physical 'constants' whose space-time evolution is driven by a scalar field. The limits are imposed by the isotropy of the microwave background on large angular scales in theories which describe space and time variations in the fine structure constant, the electron-proton mass ratio, and the Newtonian gravitational constant, G. Large-scale spatial fluctuations in the fine structure constant are bounded by 2x10^-9 and 1.2x10^-8 in the BSBM and VSL theories respectively, fluctuations in the electron-proton mass ratio by 9x10^-5 in the BM theory and fluctuations in G by 3.6x10^-10 in Brans-Dicke theory. These derived bounds are significantly stronger than any obtainable by direct observations of astrophysical objects at the present time.Comment: 13 pages, 1 table, typos corrected, refs added. Published versio

On the Stability of the Classical Vacua in a Minimal SU(5) 5-D Supergravity Model

We consider a five-dimensional supergravity model with SU(5) gauge symmetry and the minimal field content. Studying the arising scalar potential we find that the gauging of the $U(1)_R$ symmetry of the five-dimensional supergravity causes instabilities. Lifting the instabilities the vacua are of Anti-de-Sitter type and SU(5) is broken along with supersymmetry. Keeping the $U(1)_R$ ungauged the potential has flat directions along which supersymmetry is unbroken.Comment: 24 pages, 2 figure