1,354 research outputs found
Evidence for extra radiation? Profile likelihood versus Bayesian posterior
A number of recent analyses of cosmological data have reported hints for the
presence of extra radiation beyond the standard model expectation. In order to
test the robustness of these claims under different methods of constructing
parameter constraints, we perform a Bayesian posterior-based and a likelihood
profile-based analysis of current data. We confirm the presence of a slight
discrepancy between posterior- and profile-based constraints, with the
marginalised posterior preferring higher values of the effective number of
neutrino species N_eff. This can be traced back to a volume effect occurring
during the marginalisation process, and we demonstrate that the effect is
related to the fact that cosmic microwave background (CMB) data constrain N_eff
only indirectly via the redshift of matter-radiation equality. Once present CMB
data are combined with external information about, e.g., the Hubble parameter,
the difference between the methods becomes small compared to the uncertainty of
N_eff. We conclude that the preference of precision cosmological data for
excess radiation is "real" and not an artifact of a specific choice of
credible/confidence interval construction.Comment: 10 pages, 4 figures; v2: discussion section expanded and references
added, version accepted for publication by JCA
Kondo Behavior of U in CaB
Replacing U for Ca in semiconducting CaB at the few at.% level induces
metallic behaviour and Kondo-type phenomena at low temperatures, a rather
unusual feature for U impurities in metallic hosts. For
CaUB, the resistance minimum occurs at = 17 K. The
subsequent characteristic logarithmic increase of the resistivity with
decreasing temperature merges into the expected dependence below 0.8 K.
Data of the low-temperature specific heat and the magnetization are analyzed by
employing a simple resonance-level model. Analogous measurements on LaB
with a small amount of U revealed no traces of Kondo behavior, above 0.4 K.Comment: 4 pages, 4 figures, submitted for publication to Europhysics Letter
Fundamental Parameters of four Massive Eclipsing Binaries in Westerlund 1
We present fundamental parameters of 4 massive eclipsing binaries in the
young massive cluster Westerlund 1. The goal is to measure accurate masses and
radii of their component stars, which provide much needed constraints for
evolutionary models of massive stars. Accurate parameters can further be used
to determine a dynamical lower limit for the magnetar progenitor and to obtain
an independent distance to the cluster. Our results confirm and extend the
evidence for a high mass for the progenitor of the magnetar.Comment: 2 pages, to appear in the proceedings of the IAUS 282 on "From
Interacting Binaries to Exoplanets:Essential Modelling Tools" (Tatranska
Lomnica, July 18-22, 2011), Cambridge University Pres
Model selection applied to reconstruction of the Primordial Power Spectrum
The preferred shape for the primordial spectrum of curvature perturbations is
determined by performing a Bayesian model selection analysis of cosmological
observations. We first reconstruct the spectrum modelled as piecewise linear in
\log k between nodes in k-space whose amplitudes and positions are allowed to
vary. The number of nodes together with their positions are chosen by the
Bayesian evidence, so that we can both determine the complexity supported by
the data and locate any features present in the spectrum. In addition to the
node-based reconstruction, we consider a set of parameterised models for the
primordial spectrum: the standard power-law parameterisation, the spectrum
produced from the Lasenby & Doran (LD) model and a simple variant
parameterisation. By comparing the Bayesian evidence for different classes of
spectra, we find the power-law parameterisation is significantly disfavoured by
current cosmological observations, which show a preference for the LD model.Comment: Minor changes to match version accepted by JCA
Reconstructing the primordial power spectrum from the CMB
We propose a straightforward and model independent methodology for
characterizing the sensitivity of CMB and other experiments to wiggles,
irregularities, and features in the primordial power spectrum. Assuming that
the primordial cosmological perturbations are adiabatic, we present a function
space generalization of the usual Fisher matrix formalism, applied to a CMB
experiment resembling Planck with and without ancillary data. This work is
closely related to other work on recovering the inflationary potential and
exploring specific models of non-minimal, or perhaps baroque, primordial power
spectra. The approach adopted here, however, most directly expresses what the
data is really telling us. We explore in detail the structure of the available
information and quantify exactly what features can be reconstructed and at what
statistical significance.Comment: 43 pages Revtex, 23 figure
Thermalisation of light sterile neutrinos in the early universe
Recent cosmological data favour additional relativistic degrees of freedom
beyond the three active neutrinos and photons, often referred to as 'dark'
radiation. Light sterile neutrinos is one of the prime candidates for such
additional radiation. However, constraints on sterile neutrinos based on the
current cosmological data have been derived using simplified assumptions about
thermalisation of the sterile neutrino at the Big Bang Nucleosynthesis (BBN)
epoch. These assumptions are not necessarily justified and here we solve the
full quantum kinetic equations in the (1 active + 1 sterile) scenario and
derive the number of thermalised species just before BBN begins (T~1MeV) for
null (L=0) and large (L=0.01) initial lepton asymmetry and for a range of
possible mass-mixing parameters. We find that the full thermalisation
assumption during the BBN epoch is justified for initial small lepton asymmetry
only. Partial or null thermalisation occurs when the initial lepton asymmetry
is large.Comment: 19 pages, several figures. Identical to published version, only minor
changes to original arXiv versio
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
Influence of uniaxial tensile stress on the mechanical and piezoelectric properties of short-period ferroelectric superlattice
Tetragonal ferroelectric/ferroelectric BaTiO3/PbTiO3 superlattice under
uniaxial tensile stress along the c axis is investigated from first principles.
We show that the calculated ideal tensile strength is 6.85 GPa and that the
superlattice under the loading of uniaxial tensile stress becomes soft along
the nonpolar axes. We also find that the appropriately applied uniaxial tensile
stress can significantly enhance the piezoelectricity for the superlattice,
with piezoelectric coefficient d33 increasing from the ground state value by a
factor of about 8, reaching 678.42 pC/N. The underlying mechanism for the
enhancement of piezoelectricity is discussed
Increasing Neff with particles in thermal equilibrium with neutrinos
Recent work on increasing the effective number of neutrino species (Neff) in
the early universe has focussed on introducing extra relativistic species
(`dark radiation'). We draw attention to another possibility: a new particle of
mass less than 10 MeV that remains in thermal equilibrium with neutrinos until
it becomes non-relativistic increases the neutrino temperature relative to the
photons. We demonstrate that this leads to a value of Neff that is greater than
three and that Neff at CMB formation is larger than at BBN. We investigate the
constraints on such particles from the primordial abundance of helium and
deuterium created during BBN and from the CMB power spectrum measured by ACT
and SPT and find that they are presently relatively unconstrained. We forecast
the sensitivity of the Planck satellite to this scenario: in addition to
dramatically improving constraints on the particle mass, in some regions of
parameter space it can discriminate between the new particle being a real or
complex scalar.Comment: 10 pages, 5 figures v2 matches version to appear in JCA
Sterile neutrinos with eV masses in cosmology -- how disfavoured exactly?
We study cosmological models that contain sterile neutrinos with eV-range
masses as suggested by reactor and short-baseline oscillation data. We confront
these models with both precision cosmological data (probing the CMB decoupling
epoch) and light-element abundances (probing the BBN epoch). In the minimal
LambdaCDM model, such sterile neutrinos are strongly disfavoured by current
data because they contribute too much hot dark matter. However, if the
cosmological framework is extended to include also additional relativistic
degrees of freedom -- beyond the three standard neutrinos and the putative
sterile neutrinos, then the hot dark matter constraint on the sterile states is
considerably relaxed. A further improvement is achieved by allowing a dark
energy equation of state parameter w<-1. While BBN strongly disfavours extra
radiation beyond the assumed eV-mass sterile neutrino, this constraint can be
circumvented by a small nu_e degeneracy. Any model containing eV-mass sterile
neutrinos implies also strong modifications of other cosmological parameters.
Notably, the inferred cold dark matter density can shift up by 20 to 75%
relative to the standard LambdaCDM value.Comment: 14 pages, 6 figures, v2: minor changes, matches version accepted for
publication in JCA
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