Steen H. Hansen

Cosmological bounds on neutrino degeneracy improved by flavor oscillations

Abstract We study three-flavor neutrino oscillations in the early universe in the presence of neutrino chemical potentials. We take into account all effects from the background medium, i.e., collisional damping, the refractive effects from charged leptons, and in particular neutrino self-interactions that synchronize the neutrino oscillations. We find that effective flavor equilibrium between all active neutrino species is established well before the big-bang nucleosynthesis (BBN) epoch if the neutrino oscillation parameters are in the range indicated by the atmospheric neutrino data and by the large mixing angle (LMA) MSW solution of the solar neutrino problem. For the other solutions of the solar neutrino problem, partial flavor equilibrium may be achieved if the angle θ13 is close to the experimental limit tan2θ13≲0.065. In the LMA case, the BBN limit on the νe degeneracy parameter, |ξν|≲0.07, now applies to all flavors. Therefore, a putative extra cosmic radiation contribution from degenerate neutrinos is limited to such low values that it is neither observable in the large-scale structure of the universe nor in the anisotropies of the cosmic microwave background radiation. Existing limits and possible future measurements, for example in KATRIN, of the absolute neutrino mass scale will provide unambiguous information on the cosmic neutrino mass density, essentially free of the uncertainty of the neutrino chemical potentials.

Early-universe constraints on dark energy

In the past years quintessence models have been considered which can produce the accelerated expansion in the universe suggested by recent astronomical observations. One of the key differences between quintessence and a cosmological constant is that the energy density in quintessence,

Constraining the window on sterile neutrinos as warm dark matter

Omega_phi

Non-equilibrium corrections to the spectra of massless neutrinos in the early universe

, could be a significant fraction of the overall energy even in the early universe, while the cosmological constant will be dynamically relevant only at late times. We use standard Big Bang Nucleosynthesis and the observed abundances of primordial nuclides to put constraints on

Constraining neutrino physics with big bang nucleosynthesis and cosmic microwave background radiation

Omega_phi

The impact of an extra background of relativistic particles on the cosmological parameters derived from the cosmic microwave background

at temperatures near

Heavy sterile neutrinos: Bounds from big-bang nucleosynthesis and SN 1987A

T sim 1MeV

Interacting dark matter disguised as warm dark matter

. We point out that current experimental data does not support the presence of such a field, providing the strong constraint

Cosmological and astrophysical bounds on a heavy sterile neutrino and the KARMEN anomaly

Omega_phi(MeV)<0.045

First Measurement of Cluster Temperature Using the Thermal Sunyaev-Zel’dovich Effect

at