We analyze the observational constraints on brane-world cosmology whereby the universe is described as a three-brane embedded in a five-dimensional anti-de Sitter space. In this brane-universe cosmology, the Friedmann equation is modified by the appearance of extra terms which derive from existence of the extra dimensions. In the present work we concentrate on the "dark radiation" term which diminishes with cosmic scale factor as a −4 . We show that, although the observational constraints from primordial abundances allow only a small contribution when this term is positive, a much wider range of negative values is allowed. Furthermore, such a negative contribution can reconcile the tension between the observed primordial 4 He and D abundances. We also discuss the possible constraints on this term from the power spectrum of CMB anisotropies in the limit of negligible cosmological perturbation on the brane world. We show that BBN limits the possible contribution from dark radiation just before the e + e − annihilation epoch to lie between −123% and +11% of the background photon energy density. Combining this with the CMB constraint reduces this range to between −41% and +10.5% at the 2σ confidence level.

We analyze the limits on resonant particle production during inflation based upon the power spectrum of fluctuations in matter and the cosmic microwave background. We show that such a model is consistent with features observed in the matter power spectrum deduced from galaxy surveys and damped Lyman-α systems at high redshift. It also provides an alternative explanation for the excess power observed in the power spectrum of the cosmic microwave background fluctuations in the range of 1000 < l < 3500. For our best-fit models, epochs of resonant particle creation reenter the horizon at wave numbers of k * ∼ 0.4 and/or 0.2 (h Mpc −1 ). The amplitude and location of these features correspond to the creation of fermion species of mass ∼ 1 − 2 M pl during inflation with a coupling constant between the inflaton field and the created fermion species of near unity. Although the evidence is marginal, if this interpretation is correct, this could be one of the first observational hints of new physics at the Planck scale.

We discuss effects of fluctuation geometry on primordial nucleosynthesis. For the first time we consider condensed cylinder and cylindrical-shell fluctuation geometries in addition to condensed spheres and spherical shells. We find that a cylindrical shell geometry allows for an appreciably higher baryonic contribution to be the closure density (Ω b h 2 50 < ∼ 0.2) than that allowed in spherical inhomogeneous or standard homogeneous big bang models. This result, which is contrary to some other recent studies, is due to both geometry and recently revised estimates of the uncertainties in the observationally inferred primordial light-element abundances. We also find that inhomogeneous primordial nucleosynthesis in the cylindrical shell geometry can lead to significant Be and B production. In particular, a primordial beryllium abundance as high as [Be] = 12 + log(Be/H) ≈ −3 is possible while still satisfying all of the light-element abundance constraints.

Recently, attempts have been made to understand the apparent near coincidence of the present dark energy and matter energy in terms of a dynamical attractor-like solution for the evolution of a ''quintessence'' scalar field. In these models the field couples with the dominant constituent and only acts like a cosmological constant after the onset of the matter-dominated epoch. A generic feature of such solutions, however, is the possibility of significant energy density in the scalar field during the radiation-dominated epoch. This possibility is even greater if the quintessence field begins in a kinetic-dominated regime generated at the end of ''quintessential inflation.'' As such, these models can affect, and therefore be constrained by, primordial nucleosynthesis and the epoch of photon decoupling. Here we analyze one popular form for the quintessence field ͑with and without a supergravity correction͒ and quantify constraints on the allowed initial conditions and parameters for the effective potential. We also deduce constraints on the epoch of matter creation at the end of quintessential inflation.

We reanalyze the cosmological constraints on the existence of a net universal lepton asymmetry and neutrino degeneracy based upon the latest high resolution CMB sky maps from BOOMERANG, DASI, and MAXIMA-1. We generate likelihood functions by marginalizing over (Ω b h 2 , ξν µ,τ , ξν e , ΩΛ, h, n) plus the calibaration uncertainties. We consider flat ΩM + ΩΛ = 1 cosmological models with two identical degenerate neutrino species, ξν µ,τ ≡ |ξν µ | = |ξν τ | and a small ξν e . We assign weak top-hat priors on the electron-neutrino degeneracy parameter ξν e and Ω b h 2 based upon allowed values consistent with the nucleosynthesis constraints as a function of ξν µ,τ . The change in the background neutrino temperature with degeneracy is also explicitly included, and Gaussian priors for h = 0.72±0.08 and the experimental calibration uncertainties are adopted. The marginalized likelihood functions show a slight (0.5σ) preference for neutrino degeneracy. Optimum values with two equally degenerate µ and τ neutrinos imply ξν µ,τ = 1.