Do primordial Lithium abundances imply there's no Dark Energy?

Explaining the well established observation that the expansion rate of the universe is apparently accelerating is one of the defining scientific problems of our age. Within the standard model of cosmology, the repulsive 'dark energy' supposedly responsible has no explanation at a fundamental level, despite many varied attempts. A further important dilemma in the standard model is the Lithium problem, which is the substantial mismatch between the theoretical prediction for 7-Li from Big Bang Nucleosynthesis and the value that we observe today. This observation is one of the very few we have from along our past worldline as opposed to our past lightcone. By releasing the untested assumption that the universe is homogeneous on very large scales, both apparent acceleration and the Lithium problem can be easily accounted for as different aspects of cosmic inhomogeneity, without causing problems for other cosmological phenomena such as the cosmic microwave background. We illustrate this in the context of a void model.Comment: 14 pages, 4 figures. v2: minor rearrangements in the text, comments and references expanded, results unchange

Evolution of the Cosmic Gas and the Relic Supernova Neutrino Background

Using the redshift evolution of the cosmic gas, as inferred from QSO absorption line studies, we calculate the past supernova rate and the relic supernova neutrino background. Using this new technique we find the predicted relic neutrino flux at low energies to be at least an order of magnitude below previous estimates. We argue that the evolution of the cosmic gas is consistent with a large decrease in the number of early-type galaxies at redshifts $\sim 3$, and that this evolution is the source of the reduction in the predicted neutrino flux. Additional observational constraints from recent redshift surveys lead us to propose a modified model for the evolution of the cosmic gas in which significant infall at low redshift occurs. We discuss the possible relevance of our calculations to the X-ray emitting metal-enriched gas observed in the intergalactic medium.Comment: minor clarifications added to text. 27 pages, latex, 6 figures, also available at ftp://ftp.cita.utoronto.ca/ftp/cita/malaney Accepted for publication in Astroparticle Physic

Inhomogeneous Neutrino Degeneracy and Big Bang Nucleosynthesis

We examine Big Bang nucleosynthesis (BBN) in the case of inhomogenous neutrino degeneracy, in the limit where the fluctuations are sufficiently small on large length scales that the present-day element abundances are homogeneous. We consider two representive cases: degeneracy of the electron neutrino alone, and equal chemical potentials for all three neutrinos. We use a linear programming method to constrain an arbitrary distribution of the chemical potentials. For the current set of (highly-restrictive) limits on the primordial element abundances, homogeneous neutrino degeneracy barely changes the allowed range of the baryon-to-photon ratio. Inhomogeneous degeneracy allows for little change in the lower bound on the baryon-to-photon ratio, but the upper bound in this case can be as large as 1.1 \times 10^{-8} (only electron neutrino degeneracy) or 1.0 \times 10^{-9} (equal degeneracies for all three neutrinos). For the case of inhomogeneous neutrino degeneracy, we show that there is no BBN upper bound on the neutrino energy density, which is bounded in this case only by limits from structure formation and the cosmic microwave background.Comment: 6 pages, no figure

The Quark-Hadron Phase Transition, QCD Lattice Calculations and Inhomogeneous Big-Bang Nucleosynthesis

We review recent lattice QCD results for the surface tension at the finite temperature quark-hadron phase transition and discuss their implications on the possible scale of inhomogeneities. In the quenched approximation the average distance between nucleating centers is smaller than the diffusion length of a protron, so that inhomogeneities are washed out by the time nucleosynthesis sets in. Consequently the baryon density fluctuations formed by a QCD phase transition in the early universe cannot significantly affect standard big-bang nucleosynthesis calculations and certainly cannot allow baryons to close the universe. At present lattice results are inconclusive when dynamical fermions are included.Comment: 8 pages, LaTe

Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy

We consider the effect of inhomogeneous neutrino degeneracy on Big Bang nucleosynthesis for the case where the distribution of neutrino chemical potentials is given by a Gaussian. The chemical potential fluctuations are taken to be isocurvature, so that only inhomogeneities in the electron chemical potential are relevant. Then the final element abundances are a function only of the baryon-photon ratio $\eta$, the effective number of additional neutrinos $\Delta N_\nu$, the mean electron neutrino degeneracy parameter $\bar \xi$, and the rms fluctuation of the degeneracy parameter, $\sigma_\xi$. We find that for fixed $\eta$, $\Delta N_\nu$, and $\bar \xi$, the abundances of helium-4, deuterium, and lithium-7 are, in general, increasing functions of $\sigma_\xi$. Hence, the effect of adding a Gaussian distribution for the electron neutrino degeneracy parameter is to decrease the allowed range for $\eta$. We show that this result can be generalized to a wide variety of distributions for $\xi$.Comment: 9 pages, 3 figures, added discussion of neutrino oscillations, altered presentation of figure

Scales of the Extra Dimensions and their Gravitational Wave Backgrounds

Circumstances are described in which symmetry breaking during the formation of our three-dimensional brane within a higher-dimensional space in the early universe excites mesoscopic classical radion or brane-displacement degrees of freedom and produces a detectable stochastic background of gravitational radiation. The spectrum of the background is related to the unification energy scale and the the sizes and numbers of large extra dimensions. It is shown that properties of the background observable by gravitational-wave observatories at frequencies $f\approx 10^{-4}$ Hz to $10^3$ Hz contain information about unification on energy scales from 1 to $10^{10}$ TeV, gravity propagating through extra-dimension sizes from 1 mm to $10^{-18}$mm, and the dynamical history and stabilization of from one to seven extra dimensions.Comment: 6 pages, Latex, 1 figure, submitted to Phys. Re

The Supernova Relic Neutrino Background

An upper bound to the supernova relic neutrino background from all past Type II supernovae is obtained using observations of the Universal metal enrichment history. We show that an unambiguous detection of these relic neutrinos by the Super-Kamiokande detector is unlikely. We also analyze the event rate in the Sudbury Neutrino Observatory (where coincident neutrons from anti-nu_e + D --> n + n + e+ might enhance background rejection), and arrive at the same conclusion. If the relic neutrino flux should be observed to exceed our upper bound and if the observations of the metal enrichment history (for z<1) are not in considerable error, then either the Type II supernova rate does not track the metal enrichment history or some mechanism may be responsible for transforming anti-nu_{mu,tau} --> anti-nu_e.Comment: Matches version accepted for publication in Phys. Rev.

Disappearing Dark Matter in Brane World Cosmology: New Limits on Noncompact Extra Dimensions

We explore cosmological implications of dark matter as massive particles trapped on a brane embedded in a Randall-Sundrum noncompact higher dimension $AdS_5$ space. It is an unavoidable consequence of this cosmology that massive particles are metastable and can disappear into the bulk dimension. Here, we show that a massive dark matter particle (e.g. the lightest supersymmetric particle) is likely to have the shortest lifetime for disappearing into the bulk. We examine cosmological constraints on this new paradigm and show that disappearing dark matter is consistent (at the 95% confidence level) with all cosmological constraints, i.e. present observations of Type Ia supernovae at the highest redshift, trends in the mass-to-light ratios of galaxy clusters with redshift, the fraction of X-ray emitting gas in rich clusters, and the spectrum of power fluctuations in the cosmic microwave background. A best $2 \sigma$ concordance region is identified corresponding to a mean lifetime for dark matter disappearance of $15 \le \Gamma^{-1} \le 80$ Gyr. The implication of these results for brane-world physics is discussed.Comment: 7 pages, 7 figures, new cosmological constraints added, accepted for publication in PR

Low-Energy Direct Capture in the 8Li(n,gamma)9Li and 8B(p,gamma)9C Reactions

The cross sections of the 8Li(n,gamma)9Li and 8B(p,gamma)9C capture reactions have been analyzed using the direct capture model. At low energies which is the astrophysically relevant region the capture process is dominated by E1 transitions from incoming s-waves to bound p-states. The cross sections of both mirror reactions can be described simultaneously with consistent potential parameters, whereas previous calculations have overestimated the capture cross sections significantly. However, the parameters of the potential have to be chosen very carefully because the calculated cross section of the 8Li(n,gamma)9Li reaction depends sensitively on the potential strength.Comment: 6 pages, 5 figures, Phys. Rev. C, accepte

Nuclear reaction uncertainties in standard and non-standard cosmologies

We discuss here the uncertainties in the nuclear input data relevant for calculations of standard and non-standard primordial nucleosynthesis. We show how these uncertainties can affect the predictive power of such calculations, and we identify those key nuclear reactions for which improved experimental data on the associated reaction rates is most needed. Such experimental data can lead to more accurate discriminatory tests between the differing primordial nucleosynthesis scenarios. 34 refs., 3 tabs