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S. H. Hansen

Publications and source records attributed to S. H. Hansen.

At least 19 recordsLinked to original sources
arXiv2017

Infall near clusters of galaxies: comparing gas and dark matter velocity profiles

We consider the dynamics in and near galaxy clusters. Gas, dark matter and galaxies are presently falling into the clusters between approximately 1 and 5 virial radii. At very large distances, beyond 10 virial radii, all matter is following the Hubble flow, and inside the virial radius the matter particles have on average zero radial velocity. The cosmological parameters are imprinted on the infall profile of the gas, however, no method exists, which allows a measurement of it. We consider the results of two cosmological simulations (using the numerical codes RAMSES and Gadget) and find that the gas and dark matter radial velocities are very similar. We derive the relevant dynamical equations, in particular the generalized hydrostatic equilibrium equation, including both the expansion of the Universe and the cosmological background. This generalized gas equation is the main new contribution of this paper. We combine these generalized equations with the results of the numerical simulations to estimate the contribution to the measured cluster masses from the radial velocity: inside the virial radius it is negligible, and inside two virial radii the effect is below 40%, in agreement the earlier analyses for DM. We point out how the infall velocity in principle may be observable, by measuring the gas properties to distance of about two virial radii, however, this is practically not possible today.

astro-ph.CO↗
arXiv2017

Zeldovich pancakes in observational data are cold

The present day universe consists of galaxies, galaxy clusters, one-dimensional filaments and two-dimensional sheets or pancakes, all of which combine to form the cosmic web. The so called "Zeldovich pancakes", are very difficult to observe, because their overdensity is only slightly greater than the average density of the universe. Falco et al (2014) presented a method to identify Zeldovich pancakes in observational data, and these were used as a tool for estimating the mass of galaxy clusters. Here we expand and refine that observational detection method. We study two pancakes on scales of 10 Mpc, identified from spectroscopically observed galaxies near the Coma cluster, and compare with twenty numerical pancakes. We find that the observed structures have velocity dispersion about 100 km/sec, which is relatively low compared to typical groups and filaments. These velocity dispersions are consistent with those found for the numerical pancakes. We also confirm that the identified structures are in fact two-dimensional structures. Finally, we estimate the stellar to total mass of the observational pancakes to be $2 \times 10^{-4}$, within one order of magnitude, which is smaller than that of clusters of galaxies.

astro-ph.CO↗
arXiv2017

A White Paper on keV Sterile Neutrino Dark Matter

We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved - cosmology, astrophysics, nuclear, and particle physics - in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

hep-ph↗
arXiv2013

Why does the Jeans Swindle work?

When measuring the mass profile of any given cosmological structure through internal kinematics, the distant background density is always ignored. This trick is often refereed to as the "Jeans Swindle". Without this trick a divergent term from the background density renders the mass profile undefined, however, this trick has no formal justification. We show that when one includes the expansion of the Universe in the Jeans equation, a term appears which exactly cancels the divergent term from the background. We thereby establish a formal justification for using the Jeans Swindle.

astro-ph.CO↗
arXiv2007

The impact of going beyond the Maxwell distribution in direct dark matter detection rates

We consider direct dark matter detection rates and investigate the difference between a standard Maxwell-Boltzmann velocity distribution and a "realistic" distribution like the ones extracted from numerical N-body simulations. Sizable differences are observed when such results are compared to the standard Maxwell-Boltzmann distribution. For a light target both the total rate and the annual modulation are reduced by ~25%. For a heavy target the total rate is virtually unchanged, whereas the annual modulation is modified by up to 50%, depending on the WIMP mass and detector energy threshold. We also consider the effect of a possible velocity anisotropy, and the effect is found to be largest for a light target For the realistic velocity distribution the anisotropy may reduce the annual modulation, in contrast to the Maxwell-Boltzmann case.

astro-ph↗
arXiv2005

Neutrino statistics and big bang nucleosynthesis

Neutrinos may possibly violate the spin-statistics theorem, and hence obey Bose statistics or mixed statistics despite having spin half. We find the generalized equilibrium distribution function of neutrinos which depends on a single fermi-bose parameter, κ, and interpolates continuously between the bosonic and fermionic distributions when κchanges from -1 to +1. We consider modification of the Big Bang Nucleosynthesis (BBN) in the presence of bosonic or partly bosonic neutrinos. For pure bosonic neutrinos the abundances change (in comparison with the usual Fermi-Dirac case) by -3.2% for 4He (which is equivalent to a decrease of the effective number of neutrinos by ΔN_ν= - 0.6), +2.6% for 2H and -7% for 7Li. These changes provide a better fit to the BBN data. Future BBN studies will be able to constrain the fermi-bose parameter to κ> 0.5, if no deviation from fermionic nature of neutrinos is found. We also evaluate the sensitivity of future CMB and LSS observations to the fermi-bose parameter.

astro-ph↗
arXiv2002

Observational constraint on the fourth derivative of the inflaton potential

We consider the flow-equations for the 3 slow-roll parameters n_S (scalar spectral index), r (tensor to scalar ratio), and dn_S/dlnk (running of the spectral index). We show that the combination of these flow-equations with the observational bounds from cosmic microwave background and large scale structure allows one to put a lower bound on the fourth derivative of the inflationary potential, M_P^4(V''''/V) > -0.02.

hep-ph↗
arXiv2002

The impact of relativistic corrections and component separation in the measurement of the SZ effect and on the small angular scale non-Gaussianity of the CMB

We study the effect of imperfect subtraction of the Sunyaev-Zel'dovich effect (SZE) using a robust and non-parametric method to estimate the SZE residual in the Planck channels. We include relativistic corrections to the SZE, and present a simple fitting formula for the SZE temperature dependence for the Planck channels. We show how the relativistic corrections constitute a serious problem for the estimation of the kinematic SZE component from Planck data, since the key channel to estimate the kinematic component of the SZE, at 217 GHz, will be contaminated by a non-negligible thermal SZE component. The imperfect subtraction of the SZE will have an effect on both the Planck cluster catalogue and the recovered CMB map. In the cluster catalogue, the relativistic corrections are not a major worry for the estimation of the total cluster flux of the thermal SZE component, however, they must be included in the SZE simulation when calculating the selection function and completeness level. The power spectrum of the residual at 353 GHz, where the intensity of the thermal SZE is maximum, does not contribute significantly to the power spectrum of the CMB. We calculate the non-Gaussian signal due to the SZE residual in the 353 GHz CMB map using a simple Gaussianity estimator, and this estimator detects a 4.25-sigma non-Gaussian signal at small scales, which could be mistaken for a primordial non-Gaussian signature. The other channels do not show any significant departure from Gaussianity with our estimator.

astro-ph↗
arXiv2002

Cosmological bounds on neutrino degeneracy improved by flavor oscillations

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 theta_13 is close to the experimental limit tan^2(theta_13)<0.065. In the LMA case, the BBN limit on the nu_e degeneracy parameter, |xi_nu|< 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.

hep-ph↗
arXiv2001

The Impact of an Extra Background of Relativistic Particles on the Cosmological Parameters derived from Microwave Background Anisotropies

Recent estimates of cosmological parameters derived from Cosmic Microwave Background (CMB) anisotropies are based on the assumption that we know the precise amount of energy density in relativistic particles in the universe, $ω_{rel}$, at all times. There are, however, many possible mechanisms that can undermine this assumption. In this paper we investigate the effect that removing this assumption has on the determination of the various cosmological parameters. We obtain fairly general bounds on the redshift of equality, $z_{eq}= ω_{m}/ω_{rel}=3100_{-400}^{+600}$. We show that $ω_{rel}$ is nearly degenerate with the amount of energy in matter, $ω_m$, and that its inclusion in CMB parameter estimation also affects the present constraints on other parameters such as the curvature or the scalar spectral index of primordial fluctuations. This degeneracy has the effect of limiting the precision of parameter estimation from the MAP satellite, but it can be broken by measurements on smaller scales such as those provided by the Planck satellite mission.

astro-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

Producing massive sterile neutrinos as warm Dark Matter

This paper has been withdrawn by the authors. This paper started with a wrong term describing elastic scattering. The correct form of the kinetic equations for the density matrix are derived e.g. in A. Dolgov, Yad. Fiz. 33 (1981) 1309; K. Enqvist, K. Kainulainen and J. Maalampi, Nucl. Phys. B 349 (1991) 754; G. Sigl and G. Raffelt, Nucl. Phys. B 406 (1993) 423. When using the correct form of the kinetic equations, the result is in good agreement with old results, A. Dolgov and S. Hansen, hep-ph/0009083.

hep-ph↗
arXiv2001

Constraining neutrino physics with BBN and CMBR

We perform a likelihood analysis of the recent results on the anisotropy of Cosmic Microwave Background Radiation from the BOOMERanG and DASI experiments to show that they single out an effective number of neutrinos in good agreement with standard Big Bang Nucleosynthesis. We also consider degenerate Big Bang Nucleosynthesis to provide new bounds on effective relativistic degrees of freedom $N_ν$ and, in particular, on neutrino chemical potential $ξ_α$. When including Supernova Ia data we find, at $2σ$, $N_ν\leq 7$ and $-0.01 \leq ξ_e \leq 0.22$, $|ξ_{μ,τ}|\leq 2.6$.

astro-ph↗
arXiv2001

Massive sterile neutrinos as warm Dark Matter

We show that massive sterile neutrinos mixed with the ordinary ones may be produced in the early universe in the right amount to be natural warm dark matter particles. Their mass should be below 40 keV and the corresponding mixing angles sin^2 2θ> 10^{-11} for mixing with ν_μor ν_τ, while mixing with ν_e is slightly stronger bounded with mass less than 30 keV.

hep-ph↗
arXiv2001

Spectral distortion of cosmic background radiation by scattering on hot electrons. Exact calculations

The spectral distortion of the cosmic background radiation produced by the inverse Compton scattering on hot electrons in clusters of galaxies (thermal Sunyaev-Zeldovich effect) is calculated for arbitrary optical depth and electron temperature. The distortion is found by a numerical solution of the exact Boltzmann equation for the photon distribution function. In the limit of small optical depth and low electron temperature our results confirm the previous analyses. In the opposite limits, our method is the only one that permits to make accurate calculations.

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↗