111 research outputs found
Current cosmological bounds on neutrino masses and relativistic relics
We combine the most recent observations of large-scale structure (2dF and
SDSS galaxy surveys) and cosmic microwave anisotropies (WMAP and ACBAR) to put
constraints on flat cosmological models where the number of massive neutrinos
and of massless relativistic relics are both left arbitrary. We discuss the
impact of each dataset and of various priors on our bounds. For the standard
case of three thermalized neutrinos, we find an upper bound on the total
neutrino mass sum m_nu < 1.0 (resp. 0.6) eV (at 2sigma), using only CMB and LSS
data (resp. including priors from supernovae data and the HST Key Project), a
bound that is quite insensitive to the splitting of the total mass between the
three species. When the total number of neutrinos or relativistic relics N_eff
is left free, the upper bound on sum m_nu (at 2sigma, including all priors)
ranges from 1.0 to 1.5 eV depending on the mass splitting. We provide an
explanation of the parameter degeneracy that allows larger values of the masses
when N_eff increases. Finally, we show that the limit on the total neutrino
mass is not significantly modified in the presence of primordial gravitational
waves, because current data provide a clear distinction between the
corresponding effects.Comment: 13 pages, 6 figure
Lightest sterile neutrino abundance within the nuMSM
We determine the abundance of the lightest (dark matter) sterile neutrinos
created in the Early Universe due to active-sterile neutrino transitions from
the thermal plasma. Our starting point is the field-theoretic formula for the
sterile neutrino production rate, derived in our previous work [JHEP
06(2006)053], which allows to systematically incorporate all relevant effects,
and also to analyse various hadronic uncertainties. Our numerical results
differ moderately from previous computations in the literature, and lead to an
absolute upper bound on the mixing angles of the dark matter sterile neutrino.
Comparing this bound with existing astrophysical X-ray constraints, we find
that the Dodelson-Widrow scenario, which proposes sterile neutrinos generated
by active-sterile neutrino transitions to be the sole source of dark matter, is
only possible for sterile neutrino masses lighter than 3.5 keV (6 keV if all
hadronic uncertainties are pushed in one direction and the most stringent X-ray
bounds are relaxed by a factor of two). This upper bound may conflict with a
lower bound from structure formation, but a definitive conclusion necessitates
numerical simulations with the non-equilibrium momentum distribution function
that we derive. If other production mechanisms are also operative, no upper
bound on the sterile neutrino mass can be established.Comment: 34 pages. v2: clarifications and a reference added; published
version. v3: erratum appende
Do many-particle neutrino interactions cause a novel coherent effect?
We investigate whether coherent flavor conversion of neutrinos in a neutrino
background is substantially modified by many-body effects, with respect to the
conventional one-particle effective description. We study the evolution of a
system of interacting neutrino plane waves in a box. Using its equivalence to a
system of spins, we determine the character of its behavior completely
analytically. We find that, if the neutrinos are initially in flavor
eigenstates, no coherent flavor conversion is realized, in agreement with the
effective one-particle description. This result does not depend on the size of
the neutrino wavepackets and therefore has a general character. The validity of
the several important applications of the one-particle formalism is thus
confirmed.Comment: 25 pages, 1 figur
Cosmological and astrophysical limits on brane fluctuations
We consider a general brane-world model parametrized by the brane tension
scale and the branon mass . For low tension compared to the fundamental
gravitational scale, we calculate the relic branon abundance and its
contribution to the cosmological dark matter. We compare this result with the
current observational limits on the total and hot dark matter energy densities
and derive the corresponding bounds on and . Using the nucleosynthesis
bounds on the number of relativistic species, we also set a limit on the number
of light branons in terms of the brane tension. Finally, we estimate the bounds
coming from the energy loss rate in supernovae explosions due to massive branon
emission.Comment: 26 pages, 6 figures, LaTeX. Final version with minor corrections. To
appear in Phys. Rev.
Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy
We consider the effect of inhomogeneous neutrino degeneracy on Big Bang
nucleosynthesis for the case where the distribution of neutrino chemical
potentials is given by a Gaussian. The chemical potential fluctuations are
taken to be isocurvature, so that only inhomogeneities in the electron chemical
potential are relevant. Then the final element abundances are a function only
of the baryon-photon ratio , the effective number of additional neutrinos
, the mean electron neutrino degeneracy parameter , and
the rms fluctuation of the degeneracy parameter, . We find that for
fixed , , and , the abundances of helium-4,
deuterium, and lithium-7 are, in general, increasing functions of .
Hence, the effect of adding a Gaussian distribution for the electron neutrino
degeneracy parameter is to decrease the allowed range for . We show that
this result can be generalized to a wide variety of distributions for .Comment: 9 pages, 3 figures, added discussion of neutrino oscillations,
altered presentation of figure
Neutrino flavor conversion in a neutrino background: single- versus multi-particle description
In the early Universe, or near a supernova core, neutrino flavor evolution
may be affected by coherent neutrino-neutrino scattering. We develop a
microscopic picture of this phenomenon. We show that coherent scattering does
not lead to the formation of entangled states in the neutrino ensemble and
therefore the evolution of the system can always be described by a set of
one-particle equations. We also show that the previously accepted formalism
overcounts the neutrino interaction energy; the correct one-particle evolution
equations for both active-active and active-sterile oscillations contain
additional terms. These additional terms modify the index of refraction of the
neutrino medium, but have no effect on oscillation physics.Comment: 12 pages, 3 figures, minor typos correcte
Measuring neutrino masses with a future galaxy survey
We perform a detailed forecast on how well a Euclid-like photometric galaxy
and cosmic shear survey will be able to constrain the absolute neutrino mass
scale. Adopting conservative assumptions about the survey specifications and
assuming complete ignorance of the galaxy bias, we estimate that the minimum
mass sum of sum m_nu ~ 0.06 eV in the normal hierarchy can be detected at 1.5
sigma to 2.5 sigma significance, depending on the model complexity, using a
combination of galaxy and cosmic shear power spectrum measurements in
conjunction with CMB temperature and polarisation observations from Planck.
With better knowledge of the galaxy bias, the significance of the detection
could potentially reach 5.4 sigma. Interestingly, neither Planck+shear nor
Planck+galaxy alone can achieve this level of sensitivity; it is the combined
effect of galaxy and cosmic shear power spectrum measurements that breaks the
persistent degeneracies between the neutrino mass, the physical matter density,
and the Hubble parameter. Notwithstanding this remarkable sensitivity to sum
m_nu, Euclid-like shear and galaxy data will not be sensitive to the exact mass
spectrum of the neutrino sector; no significant bias (< 1 sigma) in the
parameter estimation is induced by fitting inaccurate models of the neutrino
mass splittings to the mock data, nor does the goodness-of-fit of these models
suffer any significant degradation relative to the true one (Delta chi_eff ^2<
1).Comment: v1: 29 pages, 10 figures. v2: 33 pages, 12 figures; added sections on
shape evolution and constraints in more complex models, accepted for
publication in JCA
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