9 research outputs found
Cosmological constraints on neutrino plus axion hot dark matter
We use observations of the cosmological large-scale structure to derive
limits on two-component hot dark matter consisting of mass-degenerate neutrinos
and hadronic axions, both components having velocity dispersions corresponding
to their respective decoupling temperatures. We restrict the data samples to
the safely linear regime, in particular excluding the Lyman-alpha forest. Using
standard Bayesian inference techniques we derive credible regions in the
two-parameter space of m_a and sum(m_nu). Marginalising over sum(m_nu) provides
m_a < 1.2 eV (95% C.L.). In the absence of axions the same data and methods
give sum(m_nu) < 0.65 eV (95% C.L.). We also derive limits on m_a for a range
of axion-pion couplings up to one order of magnitude larger or smaller than the
hadronic value.Comment: 13 pages, 2 figures, uses iopart.cl
Observational bounds on the cosmic radiation density
We consider the inference of the cosmic radiation density, traditionally
parameterised as the effective number of neutrino species N_eff, from precision
cosmological data. Paying particular attention to systematic effects, notably
scale-dependent biasing in the galaxy power spectrum, we find no evidence for a
significant deviation of N_eff from the standard value of N_eff^0=3.046 in any
combination of cosmological data sets, in contrast to some recent conclusions
of other authors. The combination of all available data in the linear regime
prefers, in the context of a ``vanilla+N_eff'' cosmological model,
1.1<N_eff<4.8 (95% C.L.) with a best-fit value of 2.6. Adding data at smaller
scales, notably the Lyman-alpha forest, we find 2.2<N_eff<5.8 (95% C.L.) with
3.8 as the best fit. Inclusion of the Lyman-alpha data shifts the preferred
N_eff upwards because the sigma_8 value derived from the SDSS Lyman-alpha data
is inconsistent with that inferred from CMB. In an extended cosmological model
that includes a nonzero mass for N_eff neutrino flavours, a running scalar
spectral index and a w parameter for the dark energy, we find 0.8<N_eff<6.1
(95% C.L.) with 3.0 as the best fit.Comment: 23 pages, 3 figures, uses iopart.cls; v2: 1 new figure, references
added, matches published versio
WMAP 5-year constraints on lepton asymmetry and radiation energy density: Implications for Planck
In this paper we set bounds on the radiation content of the Universe and
neutrino properties by using the WMAP-5 year CMB measurements complemented with
most of the existing CMB and LSS data (WMAP5+All),imposing also self-consistent
BBN constraints on the primordial helium abundance. We consider lepton
asymmetric cosmological models parametrized by the neutrino degeneracy
parameter and the variation of the relativistic degrees of freedom, due to
possible other physical processes occurred between BBN and structure formation
epochs. We find that WMAP5+All data provides strong bounds on helium mass
fraction and neutrino degeneracy parameter that rivals the similar bounds
obtained from the conservative analysis of the present data on helium
abundance. We also find a strong correlation between the matter energy density
and the redshift of matter-radiation equality, z_re, showing that we observe
non-zero equivalent number of relativistic neutrinos mainly via the change of
the of z_re, rather than via neutrino anisotropic stress claimed by the WMAP
team. We forecast that the CMB temperature and polarization measurements
observed with high angular resolutions and sensitivities by the future Planck
satellite will reduce the errors on these parameters down to values fully
consistent with the BBN bounds