Biomedical subjects

Steen Hannestad

Publications and source records attributed to Steen Hannestad.

2 recordsLinked to original sources

PubMed2005

Neutrino masses and the dark energy equation of state: relaxing the cosmological neutrino mass bound.

At present, cosmology provides the nominally strongest constraint on the masses of standard model neutrinos. However, this constraint is extremely dependent on the nature of the dark energy component of the Universe. When the dark energy equation of state parameter is taken as a free (but constant) parameter, the neutrino mass bound is sigma m(v) < or = 1.48 eV (95% C.L.), compared with sigma m(v) < or = 0.65 eV (95% C.L.) in the standard model where the dark energy is in the form of a cosmological constant. This has important consequences for future experiments aimed at the direct measurement of neutrino masses. We also discuss prospects for future cosmological measurements of neutrino masses.

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PubMed2002

Stringent neutron-star limits on large extra dimensions.

Supernovae (SNe) are copious sources for Kaluza-Klein (KK) gravitons which are generic for theories with large extra dimensions. These massive particles are produced with average velocities approximately 0.5c so that many of them are gravitationally retained by the SN core. Every neutron star thus has a halo of KK gravitons which decay into nu(nu), e(+)e(-), and gammagamma on time scales approximately 10(9) years. The EGRET gamma-flux limits (E(gamma) approximately 100 MeV) for nearby neutron stars constrain the compactification scale for n = 2 extra dimensions to M > or = 500 TeV, and M > or = 30 TeV for n = 3. The requirement that neutron stars are not excessively heated by KK decays implies M > or = 1700 TeV for n = 2, and M > or = 60 TeV for n = 3.

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