arXiv subjects

S. Hannestad

Publications and source records attributed to S. Hannestad.

24 records · Page 2Linked to original sources

arXiv2007

The Signature of Large Scale Structures on the Very High Energy Gamma-Ray Sky

If the diffuse extragalactic gamma ray emission traces the large scale structures of the universe, peculiar anisotropy patterns are expected in the gamma ray sky. In particular, because of the cutoff distance introduced by the absorption of 0.1-10 TeV photons on the infrared/optical background, prominent correlations with the local structures within a range of few hundreds Mpc should be present. We provide detailed predictions of the signal based on the PSCz map of the local universe. We also use mock N-body catalogues complemented with the halo model of structures to study some statistical features of the expected signatures. The results are largely independent from cosmological details, and depend mostly on the index of correlation (or bias) of the sources with respect to the large scale distribution of galaxies. For instance, the predicted signal in the case of a quadratic correlation (as it may happen for a dark matter annihilation contribution to the diffuse gamma flux) differs substantially from a linear correlation case, providing a complementary tool to unveil the nature of the sources of the diffuse gamma ray emission. The chances of the present and future space and ground based observatories to measure these features are discussed.

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arXiv2005

New cosmological mass limit on thermal relic axions

Observations of the cosmological large-scale structure provide well-established neutrino mass limits. We extend this argument to thermal relic axions. We calculate the axion thermal freeze-out temperature and thus their cosmological abundance on the basis of their interaction with pions. For hadronic axions we find a new mass limit $m_a<1.05$ eV (95% CL), corresponding to a limit on the axion decay constant of $f_a>5.7\times 10^6$ GeV. For other models this constraint is significantly weakened only if the axion-pion coupling is strongly suppressed. For comparison we note that the same approach leads to $\sum m_ν<0.65$ eV (95% CL) for neutrinos.

hep-ph↗

arXiv2001

Constraints on inflation from CMB and Lyman-alpha forest

We constrain the spectrum of primordial curvature perturbations by using recent Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) data. Specifically, we consider CMB data from the COBE, Boomerang and Maxima experiments, the real space galaxy power spectrum from the IRAS PSCz survey, and the linear matter power spectrum inferred from Ly-alpha forest spectra. We study the case of single field slow roll inflationary models, and we extract bounds on the scalar spectral index, n, the tensor to scalar ratio, r, and the running of the scalar spectral index, dlnk, for various combinations of the observational data. We find that CMB data, when combined with data from Lyman-alpha forest, place strong constraints on the inflationary parameters. Specifically, we obtain n \approx 0.9, r < 0.3 and dlnk \approx 0, indicating that single field hybrid models are ruled out.

astro-ph↗

arXiv2001

Probing the power spectrum bend with recent CMB data

We constrain the primordial fluctuation spectrum P(k) by using the new data on the Cosmic Microwave Background (CMB) from the Boomerang and MAXIMA experiments. Our study is based on slow-roll inflationary models, and we consider the possibility of a running spectral index. Specifically, we expand the power spectrum P(k) to second order in ln(k), thus allowing the power spectrum to ``bend'' in k-space. We show that allowing the power spectrum to bend erases the ability of the present data to measure the tensor to scalar perturbation ratio. Moreover, if the primordial baryon density Ω_b h^2 is as low as found from Big Bang nucleosynthesis (BBN), the data favor a negative bending of the power spectrum, corresponding to a bump-like feature in the power spectrum around a scale of k=0.004 Mpc^-1.

astro-ph↗

arXiv2000

Electron-, Mu-, and Tau-Number Conservation in a Supernova Core

We study if the neutrino mixing parameters suggested by the atmospheric neutrino anomaly imply chemical equilibrium between mu- and tau-flavored leptons in a supernova (SN) core. The initial flavor-conversion rate would indeed be fast if the nu_mu-nu_tau-mixing angle were not suppressed by second-order refractive effects. The neutrino diffusion coefficients are different for nu_mu, anti-nu_mu, nu_tau and anti-nu_tau so that neutrino transport will create a net mu and tau lepton number density. This will typically lead to a situation where the usual first-order refractive effects dominate, further suppressing the rate of flavor conversion. Altogether, neutrino refraction has the nontrivial consequence of guaranteeing the separate conservation of e, mu, and tau lepton number in a SN core on the infall and cooling time scales, even when neutrino mixing angles are large.

astro-ph↗

arXiv1997

Tau Neutrino Decays and Big Bang Nucleosynthesis

We investigate the non-radiative decay during nucleosynthesis of a massive tau neutrino with mass 0.1 - 1 MeV into an electron neutrino and a scalar or pseudoscalar particle, $ϕ$. The full Boltzmann equation is used and shown to give markedly different results than the usual non-relativistic formalism for relativistic or semi-relativistic neutrino decays. Indeed, the region we investigate is where the formalism that has previously been applied to solving this problem is expected to break down. We also compare the nucleosynthesis predictions from this scenario with results from the standard model and with some of the available observational determinations of the primordial abundances. It is found that for relativistic or semi-relativistic decays the helium abundance can be significantly lowered without changing other light element abundances. Since a problem with the standard model of Big Bang nucleosynthesis is that helium appears to be overproduced, a decay of the type we discuss can be a possible solution.

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