51 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
Neutrinos in Non-linear Structure Formation - a Simple SPH Approach
We present a novel method for implementing massive neutrinos in N-body
simulations. Instead of sampling the neutrino velocity distribution by
individual point particles we take neutrino free-streaming into account by
treating it as an effective redshift dependent sound speed in a perfect
isothermal fluid, and assume a relation between the sound speed and velocity
dispersion of the neutrinos. Although the method fails to accurately model the
true neutrino power spectrum, it is able to calculate the total matter power
spectrum to the same accuracy as more complex hybrid neutrino methods, except
on very small scales. We also present an easy way to update the publicly
available Gadget-2 version with this neutrino approximation.Comment: 13 pages, 7 figure
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
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
Asymmetric Dark Matter and Dark Radiation
Asymmetric Dark Matter (ADM) models invoke a particle-antiparticle asymmetry,
similar to the one observed in the Baryon sector, to account for the Dark
Matter (DM) abundance. Both asymmetries are usually generated by the same
mechanism and generally related, thus predicting DM masses around 5 GeV in
order to obtain the correct density. The main challenge for successful models
is to ensure efficient annihilation of the thermally produced symmetric
component of such a light DM candidate without violating constraints from
collider or direct searches. A common way to overcome this involves a light
mediator, into which DM can efficiently annihilate and which subsequently
decays into Standard Model particles. Here we explore the scenario where the
light mediator decays instead into lighter degrees of freedom in the dark
sector that act as radiation in the early Universe. While this assumption makes
indirect DM searches challenging, it leads to signals of extra radiation at BBN
and CMB. Under certain conditions, precise measurements of the number of
relativistic species, such as those expected from the Planck satellite, can
provide information on the structure of the dark sector. We also discuss the
constraints of the interactions between DM and Dark Radiation from their
imprint in the matter power spectrum.Comment: 22 pages, 5 figures, to be published in JCAP, minor changes to match
version to be publishe
Isocurvature perturbations in extra radiation
Recent cosmological observations, including measurements of the CMB
anisotropy and the primordial helium abundance, indicate the existence of an
extra radiation component in the Universe beyond the standard three neutrino
species. In this paper we explore the possibility that the extra radiation has
isocurvatrue fluctuations. A general formalism to evaluate isocurvature
perturbations in the extra radiation is provided in the mixed inflaton-curvaton
system, where the extra radiation is produced by the decay of both scalar
fields. We also derive constraints on the abundance of the extra radiation and
the amount of its isocurvature perturbation. Current observational data favors
the existence of an extra radiation component, but does not indicate its having
isocurvature perturbation. These constraints are applied to some particle
physics motivated models. If future observations detect isocurvature
perturbations in the extra radiation, it will give us a hint to the origin of
the extra radiation.Comment: 41 pages, 8 figures; version accepted for publication in JCA
Measuring the neutrino mass from future wide galaxy cluster catalogues
We present forecast errors on a wide range of cosmological parameters
obtained from a photometric cluster catalogue of a future wide-field
Euclid-like survey. We focus in particular on the total neutrino mass as
constrained by a combination of the galaxy cluster number counts and
correlation function. For the latter we consider only the shape information and
the Baryon Acoustic Oscillations (BAO), while marginalising over the spectral
amplitude and the redshift space distortions. In addition to the cosmological
parameters of the standard LCDM+nu model we also consider a non-vanishing
curvature, and two parameters describing a redshift evolution for the dark
energy equation of state. For completeness, we also marginalise over a set of
"nuisance" parameters, representing the uncertainties on the cluster mass
determination. We find that combining cluster counts with power spectrum
information greatly improves the constraining power of each probe taken
individually, with errors on cosmological parameters being reduced by up to an
order of magnitude. In particular, the best improvements are for the parameters
defining the dynamical evolution of dark energy, where cluster counts break
degeneracies. Moreover, the resulting error on neutrino mass is at the level of
\sigma(M_\nu)\sim 0.9 eV, comparable with that derived from present Ly-alpha
forest measurements and Cosmic Microwave background (CMB) data in the framework
of a non-flat Universe. Further adopting Planck priors and reducing the number
of free parameters to a LCDM+nu cosmology allows to place constraints on the
total neutrino mass of \sigma(M_\nu) \sim 0.08 eV, close to the lower bound
enforced by neutrino oscillation experiments. [abridged]Comment: 25 pages, 2 figures, 2 tables, matches the JCAP accepted versio
Towards testing the theory of gravity with DESI: summary statistics, model predictions and future simulation requirements
Large scale structure and cosmolog
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