Alvise Raccanelli

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 Large Suite of Dark Matter Simulations of the Sloan Digital Sky Survey II (SDSS-II) luminous red galaxy 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 h−1 Mpc. Applying this analysis to robustly measure parameters describing the growth history of the Universe from the SDSS-II data gives f(z= 0.25)σ8(z= 0.25) = 0.3512 ± 0.0583 and f(z= 0.37)σ8(z= 0.37) = 0.4602 ± 0.0378 when no prior is imposed on the growth rate, and the background geometry is assumed to follow a Λ cold dark matter (ΛCDM) model with the Wilkinson Microwave Anisotropy Probe (WMAP)+Type Ia supernova priors. The standard WMAP constrained ΛCDM model with general relativity predicts f(z= 0.25)σ8(z= 0.25) = 0.4260 ± 0.0141 and f(z= 0.37)σ8(z= 0.37) = 0.4367 ± 0.0136, which is fully consistent with these measurements.

Did LIGO detect dark matter

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 20M_{⊙}≲M_{bh}≲100M_{⊙} where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will rapidly spiral inward due to the emission of gravitational radiation and ultimately will merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2-53  Gpc^{-3} yr^{-1} rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have neither optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next-generation experiments will be invaluable in performing these tests.

EMU: Evolutionary Map of the Universe

EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms ~10 μJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as +30° declination, with a resolution of 10 arcsec. EMU is expected to detect and catalogue about 70 million galaxies, including typical star-forming galaxies up to z ~ 1, powerful starbursts to even greater redshifts, and active galactic nuclei to the edge of the visible Universe. It will undoubtedly discover new classes of object. This paper defines the science goals and parameters of the survey, and describes the development of techniques necessary to maximise the science return from EMU.

The clustering of galaxies in the SDSS-III DR9 Baryon Oscillation Spectroscopic Survey: constraints on primordial non-Gaussianity

We analyse the density field of 264 283 galaxies observed by the Sloan Digital Sky Surve (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) and included in the SDSS Data Release 9 (DR9). In total, the SDSS DR9 BOSS data include spectroscopic redshifts for over 400 000 galaxies spread over a footprint of more than 3000 deg^2. We measure the power spectrum of these galaxies with redshifts 0.43 0), is 99.5 per cent. After quantifying and correcting for the systematic bias and including the added uncertainty, we find −45 0) = 91.0 per cent. A more conservative approach assumes that we have only learnt 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^(local)_(NL) , and we determine that −82 0) = 68 per cent. This analysis demonstrates the importance of accounting for the impact of Galactic foregrounds on f^(local)_(NL) measurements. We outline the methods that account for these systematic biases and uncertainties. We expect our methods to yield robust constraints on f^(local)_(NL) for both our own and future large-scale structure investigations.

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, 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 DM since inferring its existence this constraint rests on the betrayal by gravity of the location of otherwise invisible DM. 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 the 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 subgalactic structure.

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. 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, multifrequency 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-infrared (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 cosmic microwave background map, yields at a substantially higher significance than reported before.

Radio Continuum Surveys with Square Kilometre Array Pathfinders

In the lead-up to the Square Kilometre Array (SKA) project, several next-generation radio telescopes and upgrades are already being built around the world. These include APERTIF (The Netherlands), ASKAP (Australia), e-MERLIN (UK), VLA (USA), e-EVN (based in Europe), LOFAR (The Netherlands), MeerKAT (South Africa), and the Murchison Widefield Array. Each of these new instruments has different strengths, and coordination of surveys between them can help maximise the science from each of them. A radio continuum survey is being planned on each of them with the primary science objective of understanding the formation and evolution of galaxies over cosmic time, and the cosmological parameters and large-scale structures which drive it. In pursuit of this objective, the different teams are developing a variety of new techniques, and refining existing ones. To achieve these exciting scientific goals, many technical challenges must be addressed by the survey instruments. Given the limited resources of the global radio-astronomical community, it is essential that we pool our skills and knowledge. We do not have sufficient resources to enjoy the luxury of re-inventing wheels. We face significant challenges in calibration, imaging, source extraction and measurement, classification and cross-identification, redshift determination, stacking, and data-intensive research. As these instruments extend the observational parameters, we will face further unexpected challenges in calibration, imaging, and interpretation. If we are to realise the full scientific potential of these expensive instruments, it is essential that we devote enough resources and careful study to understanding the instrumental effects and how they will affect the data. We have established an SKA Radio Continuum Survey working group, whose prime role is to maximise science from these instruments by ensuring we share resources and expertise across the projects. Here we describe these projects, their science goals, and the technical challenges which are being addressed to maximise the science return.

Growth of cosmic structure: Probing dark energy beyond expansion

The quantity and quality of cosmic structure observations have greatly accelerated in recent years, and further leaps forward will be facilitated by imminent projects. These 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 several pieces of fundamental physics: the primordial perturbations generated by GUT-scale physics; neutrino masses and interactions; the nature of dark matter and dark energy. We focus on the last of these here: the ways that combining probes of growth with those of the cosmic expansion such as distance-redshift relations will pin down the mechanism driving the acceleration of the Universe.

Beyond the plane-parallel and Newtonian approach: Wide-angle redshift distortions and convergence in general relativity

We extend previous analyses of wide-angle correlations in the galaxy power spectrum in redshift space to include all general relativistic effects. These general relativistic corrections to the standard approach become important on large scales and at high redshifts, and they lead to new terms in the wide-angle correlations. We show that in principle the new terms can produce corrections of nearly 10% on Gpc scales over the usual Newtonian approximation. General relativistic corrections will be important for future large-volume surveys such as SKA and Euclid, although the problem of cosmic variance will present a challenge in observing this.

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 th e 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 crosscorrelation between radio sources and CMB maps (the Integrated Sachs-Wolfe effect), the cross-correlation of radio sources with foreground object s 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 modifie d 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(w0) = 0.05, σ(wa) = 0.12} and the modified gravity parameters {σ(η0) = 0.10, σ(µ0) = 0.05}, assuming Planck CMB+SN(current data) priors. Finally, we show that radio surveys would detect a primordial non-Gaussianity of fNL = 8 at 1-σ and we briefly discuss other promising probes.