13 research outputs found
Stringent Constraints on Cosmological Neutrino-Antineutrino Asymmetries from Synchronized Flavor Transformation
We assess a mechanism which can transform neutrino-antineutrino asymmetries
between flavors in the early universe, and confirm that such transformation is
unavoidable in the near bi-maximal framework emerging for the neutrino mixing
matrix. We show that the process is a standard Mikheyev-Smirnov-Wolfenstein
flavor transformation dictated by a synchronization of momentum states. We also
show that flavor ``equilibration'' is a special feature of maximal mixing, and
carefully examine new constraints placed on neutrino asymmetries. In
particular, the big bang nucleosynthesis limit on electron neutrino degeneracy
xi_e < 0.04 does not apply directly to all flavors, yet confirmation of the
large-mixing-angle solution to the solar neutrino problem will eliminate the
possibility of degenerate big bang nucleosynthesis.Comment: 11 pages, 6 figures; minor changes to match PRD versio
What is the lowest possible reheating temperature?
We study models in which the universe exits reheating at temperatures in the
MeV regime. By combining light element abundance measurements with cosmic
microwave background and large scale structure data we find a fairly robust
lower limit on the reheating temperature of T_RH > 4 MeV at 95% C.L. However,
if the heavy particle whose decay reheats the universe has a direct decay mode
to neutrinos, there are some small islands left in parameter space where a
reheating temperature as low as 1 MeV is allowed. The derived lower bound on
the reheating temperature also leads to very stringent bounds on models with
large extra dimensions. For n=2 the bound on the compactification scale is
M > 2000 TeV, and for n=3 it is 100 TeV. These are currently the strongest
available bounds on such models.Comment: 9 pages, 7 figures, Revte
Measuring the cosmological background of relativistic with the Wilkinson Microwave Anisotropy Probe
We show that the first year results of the Wilkinson Microwave Anisotropy Probe (WMAP) constrain very efficiently the energy density in relativistic particles in the universe. We derive new bounds on additional relativistic degrees of freedom expressed in terms of an excess in the effective number of light neutrinos Delta Neff. Within the flat LambdaCDM scenario, the allowed range is Delta Neff < 6 (95% CL) using WMAP data only, or -2.6 < Delta Neff < 4 with the prior H0= 72 \pm 8 km/s/Mpc. When other cosmic microwave background and large scale structure experiments are taken into account, the window shrinks to -1.6 < Delta Neff < 3.8. These results are in perfect agreement with the bounds from primordial nucleosynthesis. Non-minimal cosmological models with extra relativistic degrees of freedom are now severely restricted