2,418 research outputs found
When Did Cosmic Acceleration Start ?
A precise determination, and comparison, of the epoch of the onset of cosmic
acceleration, at redshift z_acc, and of dark energy domination, at z_eq,
provides an interesting measure with which to parameterize dark energy models.
By combining several cosmological datasets we place constraints on the redshift
and age of cosmological acceleration. For a Lambda-CDM model, we find the
constraint z_acc=0.76\pm0.10 at 95% c.l., occurring 6.7\pm0.4 Gyrs ago.
Allowing a constant equation of state but different from -1 changes the
constraints to z_acc=0.81\pm0.12 (6.9\pm0.5 Gyrs ago) and
z_eq=0.48\pm0.14(4.9\pm0.9 Gyrs ago), while dynamical models markedly increase
the error on the constraints with z_acc=0.81\pm0.30 (6.8\pm1.4 Gyrs ago) and
z_eq=0.44\pm0.20 (4.5\pm1.0 Gyrs ago). Unified dark energy models as Silent
Quartessence yield: z_acc=0.80\pm0.16 (6.8\pm0.6 Gyrs ago).Comment: 5 pages, 2 figure
Including birefringence into time evolution of CMB: current and future constraints
We introduce birefringence effects within the propagation history of CMB,
considering the two cases of a constant effect and of an effect that increases
linearly in time, as the rotation of polarization induced by birefringence
accumulates during photon propagation. Both cases result into a mixing of E and
B modes before lensing effects take place, thus leading to the fact that
lensing is acting on spectra that are already mixed because of birefringence.
Moreover, if the polarization rotation angle increases during propagation,
birefringence affects more the large scales that the small scales. We put
constraints on the two cases using data from WMAP 9yr and BICEP 2013 and
compare these results with the constraints obtained when the usual procedure of
rotating the final power spectra is adopted, finding that this dataset
combination is unable to distinguish between effects, but it nevertheless hints
for a non vanishing value of the polarization rotation angle. We also forecast
the sensitivity that will be obtained using data from Planck and PolarBear,
highlighting how this combination is capable to rule out a vanishing
birefringence angle, but still unable to distinguish the different scenarios.
Nevertheless, we find that the combination of Planck and PolarBear is sensitive
enough to highlight the existence of degeneracies between birefringence
rotation and gravitational lensing of CMB photons, possibly leading to false
detection of non standard lensing effects if birefringence is neglected.Comment: 20 pages, 10 figures. New version matching the one accepted by JCAP.
Corrected typos in equations 2.17-2.1
New constraints on primordial gravitational waves from Planck 2015
We show that the new precise measurements of Cosmic Microwave Background
(CMB) temperature and polarization anisotropies made by the Planck satellite
significantly improves previous constraints on the cosmic gravitational waves
background (CGWB) at frequencies Hz. On scales smaller than the
horizon at the time of decoupling, primordial gravitational waves contribute to
the total radiation content of the Universe. Considering adiabatic
perturbations, CGWB affects temperature and polarization CMB power spectra and
matter power spectrum in a manner identical to relativistic particles.
Considering the latest Planck results we constrain the CGWB energy density to
at 95\% CL. Combining CMB power
spectra with lensing, BAO and primordial Deuterium abundance observations, we
obtain at 95\% CL, improving previous
Planck bounds by a factor 3 and the recent direct upper limit from the LIGO and
VIRGO experiments a factor 2. A combined analysis of future satellite missions
as COrE and EUCLID could improve current bound by more than an order of
magnitude.Comment: 9 pages, 1 figure, matching the version published on PL
Constraints on Modified Gravity from ACT and SPT
The Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) have
recently provided new and precise measurements of the Cosmic Microwave
Background anisotropy damping tail. This region of the CMB angular spectra,
thanks to the angular distortions produced by gravitational lensing, can probe
the growth of matter perturbations and provide a test for general relativity.
Here we make use of the ACT and SPT power spectrum measurements (combined with
the recent WMAP9 data) to constrain f(R) gravity theories. Adopting a
parametrized approach, we obtain an upper limit on the lengthscale of the
theory of B_0 < 0.86 at 95% c.l. from ACT, while we get a significantly
stronger bound from SPT with B_0 < 0.14 at 95% c.l..Comment: 6 pages, 4 figures, some sentences correcte
Constraints on cosmological parameters from future cosmic microwave background experiments
The Planck satellite experiment will soon let cosmologists to determine most of the cosmological parameters with unprecedented accuracy. In particular a strong improvement is expected in many parameters of interest, including neutrino mass, the amount of relativistic particles at recombination, the primordial Helium abundance and the injection of extra ionizing photon by dark matter self-annihilation. Here we review the constraints achievable by future experiments and discuss the implications for fundamental physics. © 2010 IOP Publishing Ltd
Cosmological constraints on the neutron lifetime
We derive new constraints on the neutron lifetime based on the recent Planck
2015 observations of temperature and polarization anisotropies of the CMB.
Under the assumption of standard Big Bang Nucleosynthesis, we show that Planck
data constrains the neutron lifetime to at
c.l.. Moreover, by including the direct measurements of primordial
Helium abundance of Aver et al. (2015) and Izotov et al. (2014), we show that
cosmological data provide the stringent constraints and respectively. The latter
appears to be in tension with neutron lifetime value quoted by the Particle
Data Group (). Future CMB surveys as
COrE+, in combination with a weak lensing survey as EUCLID, could constrain the
neutron lifetime up to a precision.Comment: 13 pages, 3 figures. Matching JCAP accepted versio
Breaking Be: a sterile neutrino solution to the cosmological lithium problem
The possibility that the so-called "lithium problem", i.e. the disagreement
between the theoretical abundance predicted for primordial Li assuming
standard nucleosynthesis and the value inferred from astrophysical
measurements, can be solved through a non-thermal BBN mechanism has been
investigated by several authors. In particular, it has been shown that the
decay of a MeV-mass particle, like, e.g., a sterile neutrino, decaying after
BBN not only solves the lithium problem, but also satisfies cosmological and
laboratory bounds, making such a scenario worth to be investigated in further
detail. In this paper, we constrain the parameters of the model with the
combination of current data, including Planck 2015 measurements of temperature
and polarization anisotropies of the CMB, FIRAS limits on spectral distortions,
astrophysical measurements of primordial abundances and laboratory constraints.
We find that a sterile neutrino with mass (at
c.l.), a decay time (at
c.l.) and an initial density (at c.l.) in units of the number density of CMB photons,
perfectly accounts for the difference between predicted and observed Li
primordial abundance. This model also predicts an increase of the effective
number of relativistic degrees of freedom at the time of CMB decoupling at c.l..
The required abundance of sterile neutrinos is incompatible with the standard
thermal history of the Universe, but could be realized in a low reheating
temperature scenario. We provide forecasts for future experiments finding that
the combination of measurements from the COrE+ and PIXIE missions will allow to
significantly reduce the permitted region for the sterile lifetime and density.Comment: 28 pages, 13 figures, 4 tables, matching the published versio
Constraints on the early and late integrated Sachs-Wolfe effects from the Planck 2015 cosmic microwave background anisotropies in the angular power spectra
The Integrated Sachs-Wolfe (ISW) effect predicts additional anisotropies in the Cosmic MicrowaveBackground due to time variation of the gravitational potential when the expansion of the universeis not matter dominated. The ISW effect is therefore expected in the early universe, due to thepresence of relativistic particles at recombination, and in the late universe, when dark energy startsto dominate the expansion. Deviations from the standard picture can be parameterized byAeISWandAlISW, which rescale the overall amplitude of the early and late ISW effects. Analyzing themost recent CMB temperature spectra from the Planck 2015 release, we detect the presence of theearly ISW at high significance withAeISW= 1.06±0.04 at 68% CL and an upper limit for thelate ISW ofAlISW<1.1 at 95% CL. The inclusion of the recent polarization data from the Planckexperiment erases such 1.5σhint forAeISW6= 1. When considering the recent detections of the lateISW coming from correlations between CMB temperature anisotropies and weak lensing, a value ofAlISW= 0.85±0.21 is predicted at 68% CL, showing a 4σevidence. We discuss the stability of ourresult in the case of an extra relativistic energy component parametrized by the effective neutrinonumberNeffand of a CMB lensing amplitudeA
Blue Gravity Waves from BICEP2 ?
We present new constraints on the spectral index n_T of tensor fluctuations
from the recent data obtained by the BICEP2 experiment. We found that the
BICEP2 data alone slightly prefers a positive, "blue", spectral index with
n_T=1.36\pm0.83 at 68 % c.l.. However, when a TT prior on the tensor amplitude
coming from temperature anisotropy measurements is assumed we get
n_T=1.67\pm0.53 at 68 % c.l., ruling out a scale invariant spectrum at
more than three standard deviations. These results are at odds with current
bounds on the tensor spectral index coming from pulsar timing, Big Bang
Nucleosynthesis, and direct measurements from the LIGO experiment. Considering
only the possibility of a "red", n_T<0 spectral index we obtain the lower limit
n_T > -0.76 at 68 % c.l. (n_T>-0.09 when a TT prior is included).Comment: 3 Pages, 4 Figure
Physically based approaches incorporating evaporation for early warning predictions of rainfall-induced landslides
Abstract. In the field of rainfall-induced landslides on sloping covers, models for early warning predictions require an adequate trade-off between two aspects: prediction accuracy and timeliness. When a cover's initial hydrological state is a determining factor in triggering landslides, taking evaporative losses into account (or not) could significantly affect both aspects. This study evaluates the performance of three physically based predictive models, converting precipitation and evaporative fluxes into hydrological variables useful in assessing slope safety conditions. Two of the models incorporate evaporation, with one representing evaporation as both a boundary and internal phenomenon, and the other only a boundary phenomenon. The third model totally disregards evaporation. Model performances are assessed by analysing a well-documented case study involving a 2 m thick sloping volcanic cover. The large amount of monitoring data collected for the soil involved in the case study, reconstituted in a suitably equipped lysimeter, makes it possible to propose procedures for calibrating and validating the parameters of the models. All predictions indicate a hydrological singularity at the landslide time (alarm). A comparison of the models' predictions also indicates that the greater the complexity and completeness of the model, the lower the number of predicted hydrological singularities when no landslides occur (false alarms)
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