2,723 research outputs found
Dark energy records in lensed cosmic microwave background
We consider the weak lensing effect induced by linear cosmological
perturbations on the cosmic microwave background (CMB) polarization
anisotropies. We find that the amplitude of the lensing peak in the BB mode
power spectrum is a faithful tracer of the dark energy dynamics at the onset of
cosmic acceleration. This is due to two reasons. First, the lensing power is
non-zero only at intermediate redshifts between the observer and the source,
keeping record of the linear perturbation growth rate at the corresponding
epoch. Second, the BB lensing signal is expected to dominate over the other
sources. The lensing distortion on the TT and EE spectra do exhibit a similar
dependence on the dark energy dynamics, although those are dominated by primary
anisotropies. We investigate and quantify the effect by means of exact tracking
quintessence models, as well as parameterizing the dark energy equation of
state in terms of the present value () and its asymptotic value in the
past (); in the interval allowed by the present constraints on dark
energy, the variation of induces a significant change in the BB
mode lensing amplitude. A Fisher matrix analysis, under conservative
assumptions concerning the increase of the sample variance due to the lensing
non-Gaussian statistics, shows that a precision of order 10% on both
and is achievable by the future experiments probing a large sky
area with angular resolution and sensitivity appropriate to detect the lensing
effect on the CMB angular power spectrum. These results show that the CMB can
probe the differential redshift behavior of the dark energy equation of state,
beyond its average.Comment: New version including substantial text change, three more figures and
two more table
Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X
We propose a possible way to detect baryons at low redshifts from the
analysis of X-ray absorption spectra of bright AGN pairs. A simple
semi-analytical model to simulate the spectra is presented. We model the
diffuse warm-hot intergalactic medium (WHIM) component, responsible for the
X-ray absorption, using inputs from high-resolution hydro-dynamical simulations
and analytical prescriptions. We show that the number of OVII absorbers per
unit redshift with column density larger than cm -
corresponding to an equivalent width of 1 km/s - which will be possibly
detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it
Constellation-X} will detect OVII absorptions per unit redshift with
an equivalent width of 10 km/s. Our results show that, in a CDM
Universe, the characteristic size of these absorbers at is
Mpc. The filamentary structure of WHIM can be probed by finding
coincident absorption lines in the spectra of background AGN pairs. We estimate
that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to
detect this filamentary structure at a 3 level. Assuming observations
of distant sources using {\it XEUS} for exposure times of 500 ksec, we find
that the minimum source flux to probe the filamentary structure is erg cm s, in the 0.1-2.4 keV energy band. Thus,
most pairs of these extragalactic X-ray bright sources have already been
identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects
by future missions could be a powerful way to search for baryons in the low
redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded.
More optimistic results for Constellation-X. Accepted by MNRA
An improved cosmological bound on the thermal axion mass
Relic thermal axions could play the role of an extra hot dark matter
component in cosmological structure formation theories. By combining the most
recent observational data we improve previous cosmological bounds on the axion
mass m_a in the so-called hadronic axion window. We obtain a limit on the axion
mass m_a < 0.42eV at the 95% c.l. (m_a < 0.72eV at the 99% c.l.). A novel
aspect of the analysis presented here is the inclusion of massive neutrinos and
how they may affect the bound on the axion mass. If neutrino masses belong to
an inverted hierarchy scheme, for example, the above constraint is improved to
m_a < 0.38eV at the 95% c.l. (m_a < 0.67eV at the 99% c.l.). Future data from
experiments as CAST will provide a direct test of the cosmological bound.Comment: 5 Pages, 3 Figure
Electronically excited rubidium atom in a helium cluster or film.
International audienceWe present theoretical studies of helium droplets and films doped with one electronically excited rubidium atom Rb( *) ((2)P). Diffusion and path integral Monte Carlo approaches are used to investigate the energetics and the structure of clusters containing up to 14 helium atoms. The surface of large clusters is approximated by a helium film. The nonpair additive potential energy surface is modeled using a diatomic in molecule scheme. Calculations show that the stable structure of Rb( *)He(n) consists of a seven helium atom ring centered at the rubidium, surrounded by a tirelike second solvation shell. A very different structure is obtained when performing a "vertical Monte Carlo transition." In this approach, a path integral Monte Carlo equilibration starts from the stable configuration of a rubidium atom in the electronic ground state adsorbed to the helium surface after switching to the electronically excited surface. In this case, Rb( *)He(n) relaxes to a weakly bound metastable state in which Rb( *) sits in a shallow dimple. The interpretation of the results is consistent with the recent experimental observations [G. Aubock et al., Phys. Rev. Lett. 101, 035301 (2008)]
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Relativistic effects in Lyman-α forest
We present the calculation of the Lyman-alpha (Lyman-) transmitted
flux fluctuations with full relativistic corrections to the first order. Even
though several studies exist on relativistic effects in galaxy clustering, this
is the first study to extend the formalism to a different tracer of underlying
matter at unique redshift range (). Furthermore, we show a comprehensive
application of our calculations of the Quasar-Lyman- cross-correlation
function. Our results indicate that the signal of relativistic effects is
sizeable at Baryonic Acoustic Oscillation (BAO) scale mainly due to the large
difference in density bias factors of our tracers. We construct an observable,
the anti-symmetric part of the cross-correlation function, that is dominated by
the relativistic signal and offers a new way to measure the relativistic terms
at relatively small scales. The analysis shows that relativistic effects are
important when considering cross-correlations between tracers with very
different biases, and should be included in the data analysis of the current
and future surveys. Moreover, the idea presented in this paper is highly
complementary to other techniques and observable trying to isolate the effect
of the relativistic corrections and thus test the validity of the theory of
gravity beyond the Newtonian regime
The effect of neutrinos on the matter distribution as probed by the Intergalactic Medium
We present a suite of full hydrodynamical cosmological simulations that
quantitatively address the impact of neutrinos on the (mildly non-linear)
spatial distribution of matter and in particular on the neutral hydrogen
distribution in the Intergalactic Medium (IGM), which is responsible for the
intervening Lyman-alpha absorption in quasar spectra. The free-streaming of
neutrinos results in a (non-linear) scale-dependent suppression of power
spectrum of the total matter distribution at scales probed by Lyman-alpha
forest data which is larger than the linear theory prediction by about 25% and
strongly redshift dependent. By extracting a set of realistic mock quasar
spectra, we quantify the effect of neutrinos on the flux probability
distribution function and flux power spectrum. The differences in the matter
power spectra translate into a ~2.5% (5%) difference in the flux power spectrum
for neutrino masses with Sigma m_{\nu} = 0.3 eV (0.6 eV). This rather small
effect is difficult to detect from present Lyman-alpha forest data and nearly
perfectly degenerate with the overall amplitude of the matter power spectrum as
characterised by sigma_8. If the results of the numerical simulations are
normalized to have the same sigma_8 in the initial conditions, then neutrinos
produce a smaller suppression in the flux power of about 3% (5%) for Sigma
m_{\nu} = 0.6 eV (2
sigma C.L.), comparable to constraints obtained from the cosmic microwave
background data or other large scale structure probes.Comment: 38 pages, 21 figures. One section and references added. JCAP in pres
Reionization and galaxy inference from the high-redshift Ly α forest
The transmission of Lyman α (Ly α) in the spectra of distant quasars depends on the density, temperature, and ionization state of the intergalactic medium. Therefore, high-redshift (z > 5) Ly α forests could be invaluable in studying the late stages of the epoch of reionization (EoR), as well as properties of the sources that drive it. Indeed, high-quality quasar spectra have now firmly established the existence of large-scale opacity fluctuations at z > 5, whose physical origins are still debated. Here, we introduce a Bayesian framework capable of constraining the EoR and galaxy properties by forward-modelling the high-z Ly α forest. Using priors from galaxy and cosmic microwave background observations, we demonstrate that the final overlap stages of the EoR (when >95 per cent of the volume was ionized) should occur at z < 5.6, in order to reproduce the large-scale opacity fluctuations seen in forest spectra. However, it is the combination of patchy reionization and the inhomogeneous ultraviolet background that produces the longest Gunn-Peterson troughs. Ly α forest observations tighten existing constraints on the characteristic ionizing escape fraction of galaxies, with the combined observations suggesting fesc â 7+4-3} per cent, and disfavouring a strong evolution with the galaxy's halo (or stellar) mass
Studying the Warm Hot Intergalactic Medium in emission: a reprise
The Warm-Hot Intergalactic Medium (WHIM) is believed to host a significant
fraction of the ``missing baryons'' in the nearby Universe. Its signature has
been detected in the X-ray absorption spectra of distant quasars. However, its
detection in emission, that would allow us to study the WHIM in a systematic
way, is still lacking. Motivated by the possibility to perform these studies
with next generation integral field spectrometers, and thanks to the
availability of a large suite of state-of-the-art hydrodynamic simulations --
the CAMELS suite -- we study here in detail the emission properties of the WHIM
and the possibility to infer its physical properties with upcoming X-ray
missions like Athena. We focused on the two most prominent WHIM emission lines,
the OVII triplet and the OVIII singlet, and build line surface brightness maps
in a lightcone, mimicking a data cube generated through integral field
spectroscopy. We confirm that detectable WHIM emission, even with next
generation instruments, is largely associated to galaxy-size dark matter halos
and that the WHIM properties evolve little from to now. Some
characteristics of the WHIM, like the line number counts as a function of their
brightness, depend on the specific hydrodynamic simulation used, while others,
like the WHIM clustering properties, are robust to this aspect. The large
number of simulations available in the CAMELS datasets allows us to assess the
sensitivity of the WHIM properties to the background cosmology and to the
energy feedback mechanisms regulated by AGN and stellar activity. [ABRIDGED]Comment: 23 pages, 17 figures, 3 table
Primordial non-Gaussianities in the intergalactic medium
We present results from the first high-resolution hydrodynamical simulations of non-Gaussian cosmological models. We focus on the statistical properties of the transmitted Lyman-\u3b1 flux in the high-redshift intergalactic medium. Imprints of non-Gaussianity are present and are larger at high redshifts. Differences larger than 20 per cent at z > 3 in the flux probability distribution function for high-transmissivity regions (voids) are expected for values of the non-linearity parameter fNL = +/-100 when compared to a standard \u39b cold dark matter cosmology with fNL = 0. We also investigate the one-dimensional flux bispectrum: at the largest scales (corresponding to tens of Mpc), we expect deviations in the flux bispectrum up to 20 per cent at z ~ 4 (for fNL = +/-100), significantly larger than deviations of ~3 per cent in the flux power spectrum. We briefly discuss possible systematic errors that can contaminate the signal. Although challenging, a detection of non-Gaussianities in the interesting regime of scales and redshifts probed by the Lyman-\u3b1 forest could be possible with future data sets
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