Papers by Steen Hannestad
The Astrophysical Journal, Sep 10, 2009
We perform an autocorrelation study of the Auger data with the aim to constrain the number densit... more We perform an autocorrelation study of the Auger data with the aim to constrain the number density ns of ultrahigh energy cosmic ray (UHECR) sources, estimating at the same time the effect on ns of the systematic energy scale uncertainty and of the distribution of UHECR. The use of global analysis has the advantage that no biases are introduced, either in ns or in the related error bar, by the a priori choice of a single angular scale. The case of continuous, uniformly distributed sources is nominally disfavored at 99% CL and the fit improves if the sources follow the large-scale structure of matter in the universe. The best-fit values obtained for the number density of proton sources are within a factor ~2 around ns sime 1 × 10-4Mpc-3 and depend mainly on the Auger energy calibration scale, with lower densities being preferred if the current scale is correct. The data show no significant small-scale clustering on scales smaller than a few degrees. This might be interpreted as a signature of magnetic smearing of comparable size, comparable with the indication of a ≈3° magnetic deflection coming from cross-correlation results. The effects of some approximations done on the above results are also discussed.
The arrival directions of ultrahigh energy cosmic rays (UHECRs) may show anisotropies on all scal... more The arrival directions of ultrahigh energy cosmic rays (UHECRs) may show anisotropies on all scales, from just above the experimental angular resolution up to medium scales and dipole anisotropies. We find that a global comparison of the two-point auto-correlation function of the data with the one of catalogues of potential sources is a powerful diagnostic tool. In particular, this method is far less sensitive to unknown deflections in magnetic fields than cross-correlation studies while keeping a strong discrimination power among source candidates. We illustrate these advantages by considering ordinary galaxies, gamma ray bursts and active galactic nuclei as possible sources. Already the sparse publicly available data suggest that the sources of UHECRs may be a strongly clustered sub-sample of galaxies or of active galactic nuclei. We present forecasts for various cases of source distributions which can be checked soon by the Pierre Auger Observatory.
Physical Review D Particles and Fields, Jul 6, 2008
The Pierre Auger Collaboration has reported a correlation between ultrahigh energy cosmic rays (U... more The Pierre Auger Collaboration has reported a correlation between ultrahigh energy cosmic rays (UHECR) and nearby active galactic nuclei (AGN) within ˜75Mpc. Two of these events fall within 3 degrees from Centaurus A (Cen A), the nearest AGN, clearly suggesting that this object is a strong UHECR emitter. Here we pursue this hypothesis and forecast the expected rate of ultrahigh energy neutrinos in detectors like IceCube. In our baseline model we find a rate of ˜0.4 0.6yr-1 events above a threshold of 100 TeV, the uncertainty of which is mainly related to the poor knowledge of the physical parameters of the source and details of the model. This situation will improve with detailed high energy gamma ray measurements of Cen A by the upcoming Gamma Ray Large Area Space Telescope (GLAST) satellite. This would make Cen A the first example where the potential of high energy multimessenger astronomy is finally realized.
We have studied bounds on the neutrino mass using new data from the WMAP 3 year data, the Sloan D... more We have studied bounds on the neutrino mass using new data from the WMAP 3 year data, the Sloan Digital Sky Survey measurement of the baryon acoustic peak, the Type Ia supernovae from SNLS, and the Lyman-alpha forest. We find that even in the most general models with a running spectral index where the number of neutrinos and the dark energy equation of state are allowed to vary, the 95% C.L. bound on the sum of neutrino masses is sum m_nu < 0.62 eV (95% C.L.), a bound which we believe to be robust. In the more often used constrained analysis with N_nu =3, w = -1, and alpha_s = 0, we find a bound of 0.48 eV without using the Lyman-alpha data. If Lyman-alpha data is used, the bound shrinks to \sum m_nu < 0.2-0.4 eV (95% C.L.), depending strongly on the Lyman-alpha analysis used.
We perform a global autocorrelation analysis of the Auger data with the aim to constrain the numb... more We perform a global autocorrelation analysis of the Auger data with the aim to constrain the number density ns of ultrahigh energy cosmic ray (UHECR) sources, estimating at the same time the effect of the bias of UHECR sources and of the systematic energy scale uncertainty on ns. The data show no significant small-scale clustering which may be explained by
Astrophys J, 2003
We measure the average gravitational shear profile of six massive clusters (Mvir~1015 Msolar) at ... more We measure the average gravitational shear profile of six massive clusters (Mvir~1015 Msolar) at z=0.3 out to a radius of ~2 h-1 Mpc. The measurements are fitted to a generalized Navarro-Frenk-White-like halo model ρ(r) with an arbitrary r-->0 slope, α. The data are well fitted by such a model with a central cusp with α~0.9-1.6 (68% confidence interval). For the standard Navarro-Frenk-White case α=1.0, we find a concentration parameter cvir that is consistent with recent predictions from high-resolution cold dark matter N-body simulations. Our data are also well fitted by an isothermal sphere model with a softened core. For this model, our 1 σ upper limit for the core radius corresponds to a limit σ*<=0.1 cm2 g-1 on the elastic collision cross section in a self-interacting dark matter model.
Journal of Cosmology and Astroparticle Physics, 2015
We study the potential of a future, large-volume photometric survey to constrain the axion mass m... more We study the potential of a future, large-volume photometric survey to constrain the axion mass m a in the hot dark matter limit. Future surveys such as Euclid will have significantly more constraining power than current observations for hot dark matter. Nonetheless, the lowest accessible axion masses are limited by the fact that axions lighter than ∼ 0.15 eV decouple before the QCD epoch, assumed here to occur at a temperature T QCD ∼ 170 MeV; this leaves an axion population of such low density that its late-time cosmological impact is negligible. For larger axion masses, m a 0.15 eV, where axions remain in equilibrium until after the QCD phase transition, we find that a Euclid-like survey combined with Planck CMB data can detect m a at very high significance. Our conclusions are robust against assumptions about prior knowledge of the neutrino mass. Given that the proposed IAXO solar axion search is sensitive to m a 0.2 eV, the axion mass range probed by cosmology is nicely complementary.
Physical Review D, 2008
The Pierre Auger collaboration has reported a correlation between Ultra-High Energy Cosmic Rays (... more The Pierre Auger collaboration has reported a correlation between Ultra-High Energy Cosmic Rays (UHECR) and nearby Active Galactic Nuclei (AGNs) within ∼75 Mpc. Two of these events fall within 3 degrees from Centaurus A, the nearest AGN, clearly suggesting that this object is a strong UHECR emitter. Here we pursue this hypothesis and forecast the expected rate of ultra-high energy neutrinos in detectors like IceCube. In our baseline model we find a rate of ∼ 0.4-0.6 yr −1 events above a threshold of 100 TeV, the uncertainty of which is mainly related to the poor knowledge of the physical parameters of the source and on the details of the model. This situation will improve with detailed high energy gamma ray measurements of Cen A by the upcoming GLAST satellite. This would make Cen A the first example where the potential of high energy multi-messenger astronomy is finally realized. PACS numbers: 95.85.Ry, 96.50.S-, 98.54.Cm
Physical Review D, 2015
Short baseline neutrino oscillation experiments have shown hints of the existence of additional s... more Short baseline neutrino oscillation experiments have shown hints of the existence of additional sterile neutrinos in the eV mass range. Such sterile neutrinos are incompatible with cosmology because they suppress structure formation unless they can be prevented from thermalising in the early Universe or removed by subsequent decay or annihilation. Here we present a novel scenario in which both sterile neutrinos and dark matter are coupled to a new, light pseudoscalar. This can prevent thermalisation of sterile neutrinos and make dark matter sufficiently self-interacting to have an impact on galactic dynamics and possibly resolve some of the known problems with the standard cold dark matter scenario. Even more importantly it leads to a strongly self-interacting plasma of sterile neutrinos and pseudoscalars at late times and provides an excellent fit to CMB data. The usual cosmological neutrino mass problem is avoided by sterile neutrino annihilation to pseudoscalars. The preferred value of H0 is substantially higher than in standard ΛCDM and in much better agreement with local measurements. PACS numbers: 14.60.St, 14.60.Pq, 98.80.Es, 98.80.Cq
Journal of Cosmology and Astroparticle Physics, 2014
The recent discovery of B-modes in the polarization pattern of the Cosmic Microwave Background by... more The recent discovery of B-modes in the polarization pattern of the Cosmic Microwave Background by the BICEP2 experiment has important implications for neutrino physics. We revisit cosmological bounds on light sterile neutrinos and show that they are compatible with all current cosmological data provided that the mass is relatively low. Using CMB data, including BICEP-2, we find an upper bound of m s < 0.85 eV (2σ Confidence Level). This bound is strengthened to 0.48 eV when HST measurements of H 0 are included. However, the inclusion of SZ cluster data from the Planck mission and weak gravitational measurements from the CFHTLenS project favours a non-zero sterile neutrino mass of 0.44 +0.11 −0.16 eV. Short baseline neutrino oscillations, on the other hand, indicate a new mass state around 1.2 eV. This mass is highly incompatible with cosmological data if the sterile neutrino is fully thermalised (∆χ 2 > 10). However, if the sterile neutrino only partly thermalises it can be compatible with all current data, both cosmological and terrestrial.
We study if the neutrino mixing parameters suggested by the atmospheric neutrino anomaly imply ch... more We study if the neutrino mixing parameters suggested by the atmospheric neutrino anomaly imply chemical equilibrium between µ-and τ -flavored leptons in a supernova (SN) core. The initial flavorconversion rate would indeed be fast if the νµ-ντ -mixing angle were not suppressed by second-order refractive effects. The neutrino diffusion coefficients are different for νµ,νµ, ντ andντ so that neutrino transport will create a net µ and τ 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, µ, and τ lepton number in a SN core on the infall and cooling time scales, even when neutrino mixing angles are large.
Arxiv preprint hep-ph/0702236, 2007
We propose a novel mechanism for dark energy, based on an extended seesaw for scalar fields, whic... more We propose a novel mechanism for dark energy, based on an extended seesaw for scalar fields, which does not require any new physics at energies below the TeV scale. A very light quintessence mass is usually considered to be technically unnatural, unless it is protected by some symmetry broken at the new very light scale. We propose that one can use an extended seesaw mechanism to construct technically natural models for very light fields, protected by SUSY softly broken above a TeV. * [email protected] † [email protected] ‡ [email protected]
The Astrophysical Journal, 2007
We present a measurement of the velocity flow of the local universe relative to the CMB rest fram... more We present a measurement of the velocity flow of the local universe relative to the CMB rest frame, based on the Jha, sample of 133 low redshift type Ia supernovae. At a depth of 4500 km s −1 we find a dipole amplitude of 279 ± 68 km s −1 in the direction l = 285 • ± 18 • , b = −10 • ± 15 • , consistent with earlier measurements and with the assumption that the local velocity field is dominated by the Great Attractor region. At a larger depth of 5900 km s −1 we find a shift in the dipole direction towards the Shapley concentration. We also present the first measurement of the quadrupole term in the local velocity flow at these depths. Finally, we have performed detailed studies based on N-body simulations of the expected precision with which the lowest multipoles in the velocity field can be measured out to redshifts of order 0.1. Our mock catalogues are in good agreement with current observations, and demonstrate that our results are robust with respect to assumptions about the influence of local environment on the type Ia supernova rate. Subject headings:
The Astrophysical Journal, 2008
The arrival directions of ultrahigh energy cosmic rays (UHECRs) may show anisotropies on all scal... more The arrival directions of ultrahigh energy cosmic rays (UHECRs) may show anisotropies on all scales, from just above the experimental angular resolution up to medium scales and dipole anisotropies. We find that a global comparison of the two-point auto-correlation function of the data with the one of catalogues of potential sources is a powerful diagnostic tool. In particular, this method is far less sensitive to unknown deflections in magnetic fields than cross-correlation studies while keeping a strong discrimination power among source candidates. We illustrate these advantages by considering ordinary galaxies, gamma ray bursts and active galactic nuclei as possible sources. Already the sparse publicly available data suggest that the sources of UHECRs may be a strongly clustered sub-sample of galaxies or of active galactic nuclei. We present forecasts for various cases of source distributions which can be checked soon by the Pierre Auger Observatory.
The Astrophysical Journal, 2009
We perform an autocorrelation study of the Auger data with the aim to constrain the number densit... more We perform an autocorrelation study of the Auger data with the aim to constrain the number density n s of ultrahigh energy cosmic ray (UHECR) sources, estimating at the same time the effect on n s of the systematic energy scale uncertainty and of the distribution of UHECR. The use of global analysis has the advantage that no biases are introduced, either in n s or in the related error bar, by the a priori choice of a single angular scale. The case of continuous, uniformly distributed sources is nominally disfavored at 99% C.L. and the fit improves if the sources follow the large-scale structure of matter in the universe. The best fit values obtained for the number density of proton sources are within a factor ∼2 around n s ≃ 1 × 10 −4 /Mpc 3 and depend mainly on the Auger energy calibration scale, with lower densities being preferred if the current scale is correct. The data show no significant small-scale clustering on scales smaller than a few degrees. This might be interpreted as a signature of magnetic smearing of comparable size, comparable with the indication of a ≈ 3 • magnetic deflection coming from cross-correlation results. The effects on the above results of some approximations done is also discussed. Subject headings: cosmic rays -large-scale structure of universe -methods: statistical
Uploads
Papers by Steen Hannestad