1,475 research outputs found

    A New Look At Neutrino Limits From Big Bang Nucleosynthesis

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    We take a fresh look at the limits on the number of neutrino flavors derived from big bang nucleosynthesis. In particular, recent measurements of the \he4 abundance enable one to estimate the primordial \he4 mass fraction at Yp=0.232±.003(stat)±.005(syst)Y_p = 0.232 \pm .003(stat) \pm .005(syst). For a baryon to photon ratio, η\eta, consistent with the other light elements, this leads to a best fit for the number of neutrino flavors Nν<3N_\nu < 3 (the precise number depends on η\eta) indicating a very strong upper limit to NνN_\nu. Here, we derive new upper limits on NνN_\nu, paying special attention to the fact that the best estimate may lie in an unphysical region (Nν<3N_\nu < 3 if all three neutrino flavors are light or massless; the lower bound to NνN_\nu may even be as low as 2, if the small window for a ντ\nu_\tau mass is exploited.) Our resulting upper limits therefore depend on whether Nν2N_\nu \ge 2 or 3 is assumed. We also explore the sensitivity of our results to the adopted value of η\eta and the assumed systematic errors in YpY_p.Comment: 11 pages, latex, four uuencoded ps figures include

    The Humanities and the Public Soul 1

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    Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73120/1/j.1467-8330.2008.00615.x.pd

    Magnetic fields in the early universe in the string approach to MHD

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    There is a reformulation of magnetohydrodynamics in which the fundamental dynamical quantities are the positions and velocities of the lines of magnetic flux in the plasma, which turn out to obey equations of motion very much like ideal strings. We use this approach to study the evolution of a primordial magnetic field generated during the radiation-dominated era in the early Universe. Causality dictates that the field lines form a tangled random network, and the string-like equations of motion, plus the assumption of perfect reconnection, inevitably lead to a self-similar solution for the magnetic field power spectrum. We present the predicted form of the power spectrum, and discuss insights gained from the string approximation, in particular the implications for the existence or not of an inverse cascade.Comment: 12 pages, 2 figure
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