arXiv subjects

Alvise Raccanelli

Publications and source records attributed to Alvise Raccanelli.

86 records · Page 5Linked to original sources

arXiv2015

Cosmology with SKA Radio Continuum Surveys

Radio continuum surveys have, in the past, been of restricted use in cosmology. Most studies have concentrated on cross-correlations with the cosmic microwave background to detect the integrated Sachs-Wolfe effect, due to the large sky areas that can be surveyed. As we move into the SKA era, radio continuum surveys will have sufficient source density and sky area to play a major role in cosmology on the largest scales. In this chapter we summarise the experiments that can be carried out with the SKA as it is built up through the coming decade. We show that the SKA can play a unique role in constraining the non-Gaussianity parameter to σ(f_NL) ~ 1, and provide a unique handle on the systematics that inhibit weak lensing surveys. The SKA will also provide the necessary data to test the isotropy of the Universe at redshifts of order unity and thus evaluate the robustness of the cosmological principle.Thus, SKA continuum surveys will turn radio observations into a central probe of cosmological research in the coming decades.

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arXiv2014

Optimization of spectroscopic surveys for testing non-Gaussianity

We investigate optimization strategies to measure primordial non-Gaussianity with future spectroscopic surveys. We forecast measurements coming from the 3D galaxy power spectrum and compute constraints on primordial non-Gaussianity parameters f_NL and n_NG. After studying the dependence on those parameters upon survey specifications such as redshift range, area, number density, we assume a reference mock survey and investigate the trade-off between number density and area surveyed. We then define the observational requirements to reach the detection of f_NL of order 1. Our results show that while power spectrum constraints on non-Gaussianity from future spectroscopic surveys can be competitive with current CMB limits, measurements from higher-order statistics will be needed to reach a sub unity precision in the measurements of the non-Gaussianity parameter f_NL.

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arXiv2014

Large-scale 3D galaxy correlation function and non-Gaussianity

We investigate the properties of the 2-point galaxy correlation function at very large scales, including all geometric and local relativistic effects -- wide-angle effects, redshift space distortions, Doppler terms and Sachs-Wolfe type terms in the gravitational potentials. The general three-dimensional correlation function has a nonzero dipole and octupole, in addition to the even multipoles of the flat-sky limit. We study how corrections due to primordial non-Gaussianity and General Relativity affect the multipolar expansion, and we show that they are of similar magnitude (when f_NL is small), so that a relativistic approach is needed. Furthermore, we look at how large-scale corrections depend on the model for the growth rate in the context of modified gravity, and we discuss how a modified growth can affect the non-Gaussian signal in the multipoles.

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arXiv2013

Constraints on Large-Scale Dark Acoustic Oscillations from Cosmology

If all or a fraction of the dark matter (DM) were coupled to a bath of dark radiation (DR) in the early Universe we expect the combined DM-DR system to give rise to acoustic oscillations of the dark matter until it decouples from the DR. Much like the standard baryon acoustic oscillations, these dark acoustic oscillations (DAO) imprint a characteristic scale, the sound horizon of dark matter, on the matter power spectrum. We compute in detail how the microphysics of the DM-DR interaction affects the clustering of matter in the Universe and show that the DAO physics also gives rise to unique signatures in the temperature and polarization spectra of the cosmic microwave background (CMB). We use cosmological data from the CMB, baryon acoustic oscillations (BAO), and large-scale structure to constrain the possible fraction of interacting DM as well as the strength of its interaction with DR. Like nearly all knowledge we have gleaned about dark matter since inferring its existence this constraint rests on the betrayal by gravity of the location of otherwise invisible dark matter. Although our results can be straightforwardly applied to a broad class of models that couple dark matter particles to various light relativistic species, in order to make quantitative predictions, we model the interacting component as dark atoms coupled to a bath of dark photons. We find that linear cosmological data and CMB lensing put strong constraints on existence of DAO features in the CMB and the large-scale structure of the Universe. Interestingly, we find that at most ~5% of all DM can be very strongly interacting with DR. We show that our results are surprisingly constraining for the recently proposed Double-disk DM model, a novel example of how large-scale precision cosmological data can be used to constrain galactic physics and sub-galactic structure.

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arXiv2013

Growth of Cosmic Structure: Probing Dark Energy Beyond Expansion

The quantity and quality of cosmic structure observations have greatly accelerated in recent years. Further leaps forward will be facilitated by imminent projects, which will enable us to map the evolution of dark and baryonic matter density fluctuations over cosmic history. The way that these fluctuations vary over space and time is sensitive to the nature of dark matter and dark energy. Dark energy and gravity both affect how rapidly structure grows; the greater the acceleration, the more suppressed the growth of structure, while the greater the gravity, the more enhanced the growth. While distance measurements also constrain dark energy, the comparison of growth and distance data tests whether General Relativity describes the laws of physics accurately on large scales. Modified gravity models are able to reproduce the distance measurements but at the cost of altering the growth of structure (these signatures are described in more detail in the accompanying paper on Novel Probes of Gravity and Dark Energy). Upcoming surveys will exploit these differences to determine whether the acceleration of the Universe is due to dark energy or to modified gravity. To realize this potential, both wide field imaging and spectroscopic redshift surveys play crucial roles. Projects including DES, eBOSS, DESI, PFS, LSST, Euclid, and WFIRST are in line to map more than a 1000 cubic-billion-light-year volume of the Universe. These will map the cosmic structure growth rate to 1% in the redshift range 0<z<2, over the last 3/4 of the age of the Universe.

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arXiv2013

Relativistic corrections and non-Gaussianity in radio continuum surveys

Forthcoming radio continuum surveys will cover large volumes of the observable Universe and will reach to high redshifts, making them potentially powerful probes of dark energy, modified gravity and non-Gaussianity. Here we extend recent works by analyzing the general relativistic (GR) corrections to the angular power spectrum. These GR corrections to the standard Newtonian analysis of the power spectrum become significant on scales near and beyond the Hubble scale at each redshift. We consider the continuum surveys with LOFAR, WSRT and ASKAP, and examples of continuum surveys with the SKA. We find that the GR corrections are at most percent-level in LOFAR, WODAN and EMU surveys, but they can produce $O(10%)$ changes for high enough sensitivity SKA continuum surveys. The signal is however dominated by cosmic variance, and multiple-tracer observations will be needed to overcome this problem. The GR corrections are suppressed in continuum surveys because of the integration over redshift -- we expect that GR corrections will be enhanced for future SKA HI surveys in which the source redshifts will be known. We also provide predictions for the angular power spectra in the case where the primordial perturbations have local non-Gaussianity. We find that non-Gaussianity dominates over GR corrections, and rises above cosmic variance when $f_{\rm NL}\gtrsim5$ for SKA continuum surveys.

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arXiv2012

The Clustering of Galaxies in SDSS-III DR9 Baryon Oscillation Spectroscopic Survey: Constraints on Primordial Non-Gaussianity

We analyze the density field of 264,283 galaxies observed by the Sloan Digital Sky Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) and included in the SDSS data release nine (DR9). In total, the SDSS DR9 BOSS data includes spectroscopic redshifts for over 400,000 galaxies spread over a footprint of more than 3,000 deg^2. We measure the power spectrum of these galaxies with redshifts 0.43 < z < 0.7 in order to constrain the amount of local non-Gaussianity, f_NL,local, in the primordial density field, paying particular attention to the impact of systematic uncertainties. The BOSS galaxy density field is systematically affected by the local stellar density and this influences the ability to accurately measure f_NL,local. In the absence of any correction, we find (erroneously) that the probability that f_NL,local is greater than zero, P(f_NL,local >0), is 99.5%. After quantifying and correcting for the systematic bias and including the added uncertainty, we find -45 < f_NL,local < 195 at 95% confidence, and P(f_NL,local >0) = 91.0%. A more conservative approach assumes that we have only learned the k-dependence of the systematic bias and allows any amplitude for the systematic correction; we find that the systematic effect is not fully degenerate with that of f_NL,local, and we determine that -82 < f_NL,local < 178 (at 95% confidence) and P(f_NL,local >0) = 68%. This analysis demonstrates the importance of accounting for the impact of Galactic foregrounds on f_NL,local measurements. We outline the methods that account for these systematic biases and uncertainties. We expect our methods to yield robust constraints on f_NL,local for both our own and future large-scale-structure investigations.

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arXiv2012

Impact of Redshift Information on Cosmological Applications with Next-Generation Radio Surveys

In this paper, we explore how the forthcoming generation of large-scale radio continuum surveys, with the inclusion of some degree of redshift information, can constrain cosmological parameters. By cross-matching these radio surveys with shallow optical to near-infrared surveys, we can essentially separate the source distribution into a low- and a high-redshift sample, thus providing a constraint on the evolution of cosmological parameters such as those related to dark energy. We examine two radio surveys, the Evolutionary Map of the Universe (EMU) and the Westerbork Observations of the Deep APERTIF Northern sky (WODAN). A crucial advantage is their combined potential to provide a deep, full-sky survey. The surveys used for the cross-identifications are SkyMapper and SDSS, for the southern and northern skies, respectively. We concentrate on the galaxy clustering angular power spectrum as our benchmark observable, and find that the possibility of including such low redshift information yields major improvements in the determination of cosmological parameters. With this approach, and provided a good knowledge of the galaxy bias evolution, we are able to put strict constraints on the dark energy parameters, i.e. w_0=-0.9+/-0.041 and w_a=-0.24+/-0.13, with type Ia supernovae and CMB priors (with a one-parameter bias in this case); this corresponds to a Figure of Merit (FoM) > 600, which is twice better than what is obtained by using only the cross-identified sources and greater than four time better than the case without any redshift information at all.

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arXiv2012

Testing Gravity Using Large-Scale Redshift-Space Distortions

We use Luminous Red Galaxies from the Sloan Digital Sky Survey II to test the cosmological structure growth in two alternatives to the standard LCDM+GR cosmological model. We compare observed three-dimensional clustering in SDSS DR7 with theoretical predictions for the standard vanilla LCDM+GR model, Unified Dark Matter cosmologies and the normal branch DGP. In computing the expected correlations in UDM cosmologies, we derive a parameterized formula for the growth factor in these models. For our analysis we apply the methodology tested in Raccanelli et al. 2010 and use the measurements of Samushia et al. 2011, that account for survey geometry, non-linear and wide-angle effects and the distribution of pair orientation. We show that the estimate of the growth rate is potentially degenerate with wide-angle effects, meaning that extremely accurate measurements of the growth rate on large scales will need to take such effects into account. We use measurements of the zeroth and second order moments of the correlation function from SDSS DR7 data and the Large Suite of Dark Matter Simulations, and perform a likelihood analysis to constrain the parameters of the models. Using information on the clustering up to r_max = 120 Mpc/h, and after marginalizing over the bias, we find, for UDM models, a speed of sound < 6.1e-4, and, for the nDGP model, a cross-over scale r_c > 340 Mpc, at 95% confidence level.

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arXiv2011

Interpreting large-scale redshift-space distortion measurements

The simplest theory describing large-scale redshift-space distortions (RSD), based on linear theory and distant galaxies, depends on the growth of cosmological structure, suggesting that strong tests of General Relativity can be constructed from galaxy surveys. As data sets become larger and the expected constraints more precise, the extent to which the RSD follow the simple theory needs to be assessed in order that we do not introduce systematic errors into the tests by introducing inaccurate simplifying assumptions. We study the impact of the sample geometry, non-linear processes, and biases induced by our lack of understanding of the radial galaxy distribution on RSD measurements. Using LasDamas simulations of the Sloan Digital Sky Survey II (SDSS-II) Luminous Red Galaxy (LRG) data, these effects are shown to be important at the level of 20 per cent. Including them, we can accurately model the recovered clustering in these mock catalogues on scales 30 -- 200 Mpc/h. Applying this analysis to robustly measure parameters describing the growth history of the Universe from the SDSS-II data, gives $f(z=0.25)σ_8(z=0.25)=0.3512\pm0.0583$ and $f(z=0.37)σ_8(z=0.37)=0.4602\pm0.0378$ when no prior is imposed on the growth-rate, and the background geometry is assumed to follow a $Λ$CDM model with the WMAP + SNIa priors. The standard WMAP constrained $Λ$CDM model with General Relativity predicts $f(z=0.25)σ_8(z=0.25)=0.4260\pm0.0141$ and $f(z=0.37)σ_8(z=0.37)=0.4367\pm0.0136$, which is fully consistent with these measurements.

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arXiv2011

Cosmological Measurements with Forthcoming Radio Continuum Surveys

We present forecasts for constraints on cosmological models which can be obtained by forthcoming radio continuum surveys: the wide surveys with the LOw Frequency ARray (LOFAR), Australian Square Kilometre Array Pathfinder (ASKAP) and the Westerbork Observations of the Deep APERTIF Northern sky (WODAN). We use simulated catalogues appropriate to the planned surveys to predict measurements obtained with the source auto-correlation, the cross-correlation between radio sources and CMB maps (the Integrated Sachs-Wolfe effect), the cross-correlation of radio sources with foreground objects due to cosmic magnification, and a joint analysis together with the CMB power spectrum and supernovae. We show that near future radio surveys will bring complementary measurements to other experiments, probing different cosmological volumes, and having different systematics. Our results show that the unprecedented sky coverage of these surveys combined should provide the most significant measurement yet of the Integrated Sachs-Wolfe effect. In addition, we show that using the ISW effect will significantly tighten constraints on modified gravity parameters, while the best measurements of dark energy models will come from galaxy auto-correlation function analyses. Using the combination of EMU and WODAN to provide a full sky survey, it will be possible to measure the dark energy parameters with an uncertainty of \{$σ(w_0) = 0.05$, $σ(w_a) = 0.12$\} and the modified gravity parameters \{$σ(η_0) = 0.10$, $σ(μ_0) = 0.05$\}, assuming Planck CMB+SN(current data) priors. Finally, we show that radio surveys would detect a primordial non-Gaussianity of $f_{\rm NL}$ = 8 at 1-$σ$ and we briefly discuss other promising probes.

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arXiv2011

CMB-Galaxy correlation in Unified Dark Matter Scalar Field Cosmologies

We present an analysis of the cross-correlation between the CMB and the large-scale structure (LSS) of the Universe in Unified Dark Matter (UDM) scalar field cosmologies. We work out the predicted cross-correlation function in UDM models, which depends on the speed of sound of the unified component, and compare it with observations from six galaxy catalogues (NVSS, HEAO, 2MASS, and SDSS main galaxies, luminous red galaxies, and quasars). We sample the value of the speed of sound and perform a likelihood analysis, finding that the UDM model is as likely as the LambdaCDM, and is compatible with observations for a range of values of c_\infinity (the value of the sound speed at late times) on which structure formation depends. In particular, we obtain an upper bound of c_\infinity^2 \leq 0.009 at 95% confidence level, meaning that the LambdaCDM model, for which c_\infinity^2 = 0, is a good fit to the data, while the posterior probability distribution peaks at the value c_\infinity^2=10^(-4) . Finally, we study the time dependence of the deviation from LambdaCDM via a tomographic analysis using a mock redshift distribution and we find that the largest deviation is for low-redshift sources, suggesting that future low-z surveys will be best suited to constrain UDM models.

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arXiv2010

Simulating Redshift-Space Distortions for Galaxy Pairs with Wide Angular Separation

The analysis of Redshift-Space Distortions (RSD) within galaxy surveys provides constraints on the amplitude of peculiar velocities induced by structure growth, thereby allowing tests of General Relativity on extremely large scales. The next generation of galaxy redshift surveys, such as the Baryon Oscillation Spectroscopic Survey (BOSS), and the Euclid experiment will survey galaxies out to z=2, over 10,000--20,000 sq deg. In such surveys, galaxy pairs with large comoving separation will preferentially have a wide angular separation. In standard plane-parallel theory the displacements of galaxy positions due to RSD are assumed to be parallel for all galaxies, but this assumption will break down for wide-angle pairs. Szapudi 2004 and Papai & Szapudi 2008 provided a methodology, based on tripolar spherical harmonics expansion, for computing the redshift-space correlation function for all angular galaxy pair separations. In this paper we introduce a new procedure for analysing wide-angle effects in numerical simulations. We are able to separate, demonstrate, and fit each of the effects described by the wide-angle RSD theory. Our analysis highlights some of the nuances of dealing with wide-angle pairs, and shows that the effects are not negligible even for relatively small angles. This analysis will help to ensure the full exploitation of future surveys for RSD measurements, which are currently confined to pair separations less than \sim80 Mpc/h out to z\simeq 0.5.

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arXiv2010

A model for the cosmological evolution of low frequency radio sources

We present a new evolutionary model that describes the population properties of radio sources at frequencies <5 GHz, thus complementing the De Zotti et al. (2005) model, holding at higher frequencies. We find that simple analytic luminosity evolution is still sufficient to fit the wealth of available data on local luminosity functions, multi-frequency source counts, and redshift distributions. However, the fit requires a luminosity-dependent decline of source luminosities at high redshifts, at least for steep-spectrum sources, thus confirming earlier indications of a "downsizing" also for radio sources. The upturn of source counts at sub-mJy levels is accounted for by a straightforward extrapolation, using the empirical far-IR/radio correlation, of evolutionary models matching the far-IR counts and redshift distributions of star-forming galaxies. We also discuss the implications of the new model for the interpretation of data on large-scale clustering of radio sources and on the Integrated Sachs-Wolfe (ISW) effect, and for the investigation of the contribution of discrete sources to the extragalactic background. As for the ISW effect, a new analysis exploiting a very clean CMB map, yields at a substantially higher significance than reported before.

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