arXiv subjects

Marian Douspis

Publications and source records attributed to Marian Douspis.

53 records · Page 3Linked to original sources

arXiv2009

Cross-correlation Study between the Cosmological 21-cm Signal and the kinetic Sunyaev-Zel'dovich effect

The Universe's Epoch of Reionization (EoR) can be studied using a number of observational probes that provide complementary or corroborating information. Each of these probes suffers from its own systematic and statistical uncertainties. It is therefore useful to consider the mutual information that these data sets contain. In this paper we present a cross-correlation study between the kinetic Sunyaev-Zel'dovich effect (kSZ) -- produced by the scattering of CMB photons off free electrons produced during the reionization process -- and the cosmological 21cm signal -- which reflects the neutral hydrogen content of the Universe, as a function of redshift. The study is carried out using a simulated reionization history in 100Mpc/h scale N-body simulations with radiative transfer. In essence we find that the two probes anti-correlate. The significance of the anti-correlation signal depends on the extent of the reionization process, wherein extended histories result in a much stronger signal compared to instantaneous cases. Unfortunately however, once the primary CMB fluctuations are included into our simulation they serve as a source of large correlated noise that renders the cross-correlation signal insignificant, regardless of the reionization scenario.

astro-ph.CO↗

arXiv2009

Biases on the cosmological parameters and thermal SZ residuals

We examine the biases induced on cosmological parameters when the presence of secondary anisotropies is not taken into account in Cosmic Microwave Background analyses. We first develop an exact analytical expression for computing the biases on parameters when any additive signal is neglected in the analysis. We then apply it in the context of the forthcoming Planck experiment. For illustration, we investigate the effect of the sole residual thermal Sunyaev--Zel'dovich signal that remains after cluster extraction. We find in particular that analyses neglecting the presence of this contribution introduce on the cosmological parameters n_s and tau biases, at least 6.5 and 2.9 times their one sigma confidence intervals. The Omega_b parameter is also biased to a lesser extent.

astro-ph↗

arXiv2009

Implications of bias evolution on measurements of the integrated Sachs-Wolfe effect: errors and biases in parameter estimation

The subject of this paper is a quantification of the impact of uncertainties in bias and bias evolution on the interpretation of measurements of the integrated Sachs-Wolfe effect, in particular on the estimation of cosmological parameters. We carry out a Fisher-matrix analysis for quantifying the degeneracies between the parameters of a dark energy cosmology and bias evolution, for the combination of the PLANCK microwave sky survey with the EUCLID main galaxy sample, where bias evolution b(a)=b_0+(1-a)b_a is modelled with two parameters b_0 and b_a. Using a realistic bias model introduces a characteristic suppression of the iSW-spectrum on large angular scales, due to the altered distance-weighting functions. The errors in estimating cosmological parameters if the data with evolving bias is interpreted in the framework of cosmologies with constant bias is quantified in an extended Fisher-formalism. We find that the best-fit values of all parameters are shifted by an amount comparable to the statistical accuracy: The estimation bias in units of the statistical accuracy amounts to 1.19 for Omega_m, 0.27 for sigma_8, and 0.72 for w for bias evolution with b_a=1. Leaving b_a open as a free parameter deteriorates the statistical accuracy, in particular on Omega_m and w.

astro-ph.CO↗

arXiv2009

The cross-correlation of the CMB polarisation and the 21-cm line fluctuations from cosmic reionisation

The cosmic microwave background (CMB) polarisation and the 21 cm line fluctuations are powerful probes of cosmological reionisation. We study how the cross-correlation between the CMB polarisation (E-modes) and the 21 cm line fluctuations can be used to gain further understanding of the reionisation history, within the framework of inhomogeneous reionisation. Since the E-mode polarisation reflects the amplitude of the quadrupole component of the CMB temperature fluctuations, the angular power spectrum of the cross-correlation exhibits oscillations at all multipoles. The first peak of the power spectrum appears at the scale corresponding to the quadrupole at the redshift that is probed by the 21 cm line fluctuations. The peak reaches its maximum value in redshift when the average ionisation fraction of the universe is about half. On the other hand, on small scales, there is a damping that depends on the duration of reionisation. Thus, the cross-correlation between the CMB polarisation and the 21 cm line fluctuations has the potential to constrain accurately the epoch and the duration of reionisation.

astro-ph↗

arXiv2008

Summary of the DUNE Mission Concept

The Dark UNiverse Explorer (DUNE) is a wide-field imaging mission concept whose primary goal is the study of dark energy and dark matter with unprecedented precision. To this end, DUNE is optimised for weak gravitational lensing, and also uses complementary cosmolo gical probes, such as baryonic oscillations, the integrated Sachs-Wolf effect, a nd cluster counts. Immediate additional goals concern the evolution of galaxies, to be studied with groundbreaking statistics, the detailed structure of the Milky Way and nearby galaxies, and the demographics of Earth-mass planets. DUNE is a medium class mission consisting of a 1.2m telescope designed to carry out an all-sky survey in one visible and three NIR bands (1deg$^2$ field-of-view) which will form a unique legacy for astronomy. DUNE has been selected jointly with SPACE for an ESA Assessment phase which has led to the Euclid merged mission concept.

astro-ph↗

arXiv2008

What can be learned about dark energy evolution?

We examine constraints obtained from SNIa surveys on a two parameter model of dark energy in which the equation of state $w (z) = P(z) / ρ(z)$ undergoes a transition over a period significantly shorter than the Hubble time. We find that a transition between $w \sim -0.2$ and $w \sim -1$ (the first value being somewhat arbitrary) is allowed at redshifts as low as 0.1, despite the fact that data extend beyond $z \sim 1$. Surveys with the precision anticipated for space experiments should allow only slight improvement on this constraint, as a transition occurring at a redshift as low as $\sim 0.17$ could still remain undistinguishable from a standard cosmological constant. The addition of a prior on the matter density $Ω_\MAT = 0.3$ only modestly improves the constraints. Even deep space experiments would still fail to identify a rapid transition at a redshift above 0.5. These results illustrate that a Hubble diagram of distant SNIa alone will not reveal the actual nature of dark energy at a redshift above 0.2 and that only the local dynamics of the quintessence field can be infered from a SNIa Hubble diagram. Combinations, however, seem to be very efficient: we found that the combination of present day CMB data and SNIa already excludes a transition at redshifts below 0.8.

astro-ph↗

arXiv2008

Optimising large galaxy surveys for ISW detection

We report on investigations of the power of next generation cosmic microwave background and large scale structure surveys in constraining the nature of dark energy through the cross-correlation of the Integrated Sachs Wolfe effect and the galaxy distribution. First we employ a signal to noise analysis to find the most appropriate properties of a survey in order to detect the correlated signal at a level of more than 4 sigma: such a survey should cover more than 35% of the sky, the galaxy distribution should be probed with a median redshift higher than 0.8, and the number of galaxies detected should be higher than a few per squared arcmin. We consider the forthcoming surveys DUNE, LSST, SNAP, PanSTARRS. We then compute the constraints that the DUNE survey can put on the nature of dark energy (through different parametrizations of its equation of state) with a standard Fisher matrix analysis. We confirm that, with respect to pure CMB constraints, cross-correlation constraints help in breaking degeneracies among the dark energy and the cosmological parameters. Naturally, the constraining capability is not independent of the choice of the dark energy model. Despite being weaker than some other probes (like Gravitational Weak-Lensing), these constraints are complementary to them, being sensitive to the high-redshift behaviour of the dark energy.

astro-ph↗

arXiv2006

Small scale contributions to CMB: A coherent analysis

We reanalyse Cosmic Microwave Background data from experiments probing both large and small scales. We assume that measured anisotropies are due not only to primary fluctuations but also, especially at small scales, to secondary effects (namely the Sunyaev-Zel'dovich effect) and possible point source contaminations. We first consider primary and secondary anisotropies only. For the first time in such analyses, the cosmological dependence of secondary fluctuations is fully taken into account. We show in that case that a higher value of the normalisation $σ\_8$ is preferred, as found by previous studies, but also higher values of the optical depth $τ$ and power spectrum index $n\_s$ are needed. In the second part of our analysis, we further include possible contaminations from unresolved and unremoved point sources. Under these considerations, we discuss the effects on the cosmological parameters. We further obtain the best combination of relative contributions of the three kinds of sources to the measured microwave power on small scales at each frequency. Our method allows us to simultaneously obtain cosmological parameters and explain the so-called small scale power excess in a consistent way.

astro-ph↗

arXiv2006

Large-scale galaxy correlations as a test for dark energy

We have shown earlier that, contrary to popular belief, Einstein--de Sitter (E--deS) models can still fit the {\sl WMAP} data on the cosmic microwave background provided one adopts a low Hubble constant and relaxes the usual assumption that the primordial density perturbation is scale-free. The recent {\sl SDSS} measurement of the large-scale correlation function of luminous red galaxies at $z \sim 0.35$ has however provided a new constraint by detecting a `baryon acoustic peak'. Our best-fit E--deS models do possess a baryonic feature at a similar physical scale as the best-fit $Λ$CDM concordance model, but do not fit the new observations as well as the latter. In particular the shape of the correlation function in the range $\sim 10-100 h^{-1}$ Mpc cannot be reproduced properly without violating the CMB angular power spectrum in the multipole range $l \sim 100-1000$. Thus, the combination of the CMB fluctuations and the shape of the correlation function up to $\sim 100 h^{-1}$Mpc, if confirmed, does seem to require dark energy for a homogeneous cosmological model based on (adiabatic) inflationary perturbations.

astro-ph↗

arXiv2005

Evidence for new physics from clusters ?

The abundance of local clusters is a traditional way to derive the amplitude of matter fluctuations. In the present work, by assuming that the observed baryon content of clusters is representative of the universe, we show that the mass temperature relation (M-T) can be specified for any cosmological model. This approach allows one to remove most of the uncertainty coming from M-T relation, and to provide an estimation of sigma\_8 whose uncertainty is essentially statistical. The values we obtain are fortuitously almost independent of the matter density of the Universe (sigma\_8 ~ 0.6-0.63) with an accuracy better than 5%. Quite remarkably, the amplitude of matter fluctuations can be also tightly constrained to similar accuracy from existing CMB measurements alone. However, the amplitude inferred in this way in a concordance model (Lambda-CDM) is significantly larger than the value derived from the above method based on X-ray clusters. Such a discrepancy would almost disappear if the actual optical thickness of the Universe was 0 but could also be alleviated from more exotic solutions: the existence of a new dark component in the Universe as massive neutrinos. However, recent other indications of sigma\_8 favor a high normalization. In this case, the assumption that the baryonic content observed in clusters actually reflects the primordial value has to be relaxed : either there exists a large baryonic dark component in the Universe or baryons in clusters have undergone a large depletion during the formation of these structures. We concluded that the baryon fraction in clusters is not representative and therefore that an essential piece of the physics of baryons in clusters is missing in standard structure formation scenario.

astro-ph↗

arXiv2004

A new search for features in the primordial power spectrum

We develop a new approach toward a high resolution non-parametric reconstruction of the primordial power spectrum using WMAP cosmic microwave background temperature anisotropies that we confront with SDSS large-scale structure data in the range k~0.01-0.1 h/Mpc. We utilise the standard LambdaCDM cosmological model but we allow the baryon fraction to vary. In particular, for the concordance baryon fraction, we compare indications of a possible feature at k~0.05 h/Mpc in WMAP data with suggestions of similar features in large scale structure surveys.

astro-ph↗

arXiv2004

New Physics In Clusters of Galaxy: A New Dark Matter Component ?

The latest cosmological observables analyses seem to converge to a concordant view of the cosmological model: namely the power law Lambda-CDM. The recent WMAP results comfort this new standard model. Nevertheless, some degeneracy in the CMB physics do not allow one to exclude alternative models. A combined analysis with other cosmological observations is thus needed. An example of such work is shown here, focusing on the abundance of local clusters. The latter is a traditional way to derive the amplitude of matter fluctuations, but which suffers from a lack of accurate knowledge of their masses. Here we show that the mass temperature relation can be specified for any cosmological model from consistency arguments, removing most of this uncertainty. This allows to obtain an estimation of the amplitude of matter fluctuations with an accuracy of 5%. Quite remarkably, this amplitude can be also tightly constrained from existing CMB measurements. However, the amplitude inferred in this way in a concordance model (Lambda-CDM) is significantly larger than the value derived from x-ray clusters. Such a discrepancy may reveal the existence of a new dark component in the Universe. It may alternatively reveal a significant depletion of the gas during the formation of clusters. In all cases, an essential element of clusters formation history seems to be missing.

astro-ph↗

arXiv2004

Initial power spectrum shape

Mainly used to set constraints on cosmological parameters, CMB anisotropies can give information on primordial universe. Unfortunately, initial conditions and late time cosmology are degenerated. I focus on the effect of the shape of the initial power spectrum of fluctuations on the cosmological parameter estimation from CMB temperature anisotropies. Strong constraints obtained using a priori knowledge of the initial spectrum disappear when the latter is let free.

astro-ph↗

arXiv2004

Cosmological Parameter Estimation: Method

CMB anisotropy data could put powerful constraints on theories of the evolution of our Universe. Using the observations of the large number of CMB experiments, many studies have put constraints on cosmological parameters assuming different frameworks. Assuming for example inflationary paradigm, one can compute the confidence intervals on the different components of the energy densities, or the age of the Universe, inferred by the current set of CMB observations. The aim of this note is to present some of the available methods to derive the cosmological parameters with their confidence intervals from the CMB data, as well as some practical issues to investigate large number of parameters.

astro-ph↗

arXiv2004

Cosmological implications from the observed properties of CMB

Observations of the cosmic microwave background represent a remarkable source of information for modern cosmology. Besides providing impressive support for the Big Bang model itself, they quantify the overall framework, or background, for the formation of large scale structure. Most exciting, however, is the potential access these observations give to the first moments of cosmic history and to the physics reigning at such exceptionally high energies, which will remain beyond the reach of the laboratory in any foreseeable future. Upcoming experiments, such as the Planck mission, thus offer a window onto the Physics of the Third Millennium.

astro-ph↗

arXiv2003

Scale of Homogeneity of the Universe from WMAP

We review the physics of the Grishchuck-Zel'dovich effect which describes the impact of large amplitude, super-horizon gravitational field fluctuations on the Cosmic Microwave Background anisotropy power spectrum. Using the latest determination of the spectrum by WMAP, we infer a lower limit on the present length-scale of such fluctuations of 3927 times the cosmological particle horizon (at the 95% confidence level).

astro-ph↗

arXiv2003

An alternative to the cosmological 'concordance model'

Precision measurements of the cosmic microwave background by WMAP are believed to have established a flat $Λ$-dominated universe, seeded by nearly scale-invariant adiabatic primordial fluctuations. However by relaxing the hypothesis that the fluctuation spectrum can be described by a single power law, we demonstrate that an Einstein-de Sitter universe with {\em zero} cosmological constant can fit the data as well as the best concordance model. Moreover unlike a $Λ$-dominated universe, such an universe has no strong integrated Sachs-Wolfe effect, so is in better agreement with the low quadrupole seen by WMAP. The main problem is that the Hubble constant is required to be rather low: $H_0 \simeq 46$ km/s/Mpc; we discuss whether this can be consistent with observations. Furthermore for universes consisting only of baryons and cold dark matter, the amplitude of matter fluctuations on cluster scales is too high, a problem which seems generic. However, an additional small contribution ($Ω_X \sim 0.1$) of matter which does not cluster on small scales, e.g. relic neutrinos with mass of order eV or a `quintessence' with $w \sim 0$, can alleviate this problem. Such models provide a satisfying description of the power spectrum derived from the 2dF galaxy redshift survey and from observations of the Ly-$α$ forest. We conclude that Einstein-de Sitter models can indeed accommodate all data on the large scale structure of the Universe, hence the Hubble diagram of distant Type Ia supernovae remains the only {\em direct} evidence for a non-zero cosmological constant.

astro-ph↗