Euclid: Forecasts from redshift-space distortions and the Alcock–Paczynski test with cosmic voids

Abstract

Euclid is poised to survey galaxies across a cosmological volume of unprecedented size, providing observations of more than a billion objects distributed over a third of the full sky. Approximately 20 million of these galaxies will have their spectroscopy available, allowing us to map the three-dimensional large-scale structure of the Universe in great detail. This paper investigates prospects for the detection of cosmic voids therein and the unique benefit they provide for cosmological studies. In particular, we study the imprints of dynamic (redshift-space) and geometric (Alcock–Paczynski) distortions of average void shapes and their constraining power on the growth of structure and cosmological distance ratios. To this end, we made use of the Flagship mock catalog, a state-of-the-art simulation of the data expected to be observed with Euclid. We arranged the data into four adjacent redshift bins, each of which contains about 11 000 voids and we estimated the stacked void-galaxy cross-correlation function in every bin. Fitting a linear-theory model to the data, we obtained constraints on f/b and DMH, where f is the linear growth rate of density fluctuations, b the galaxy bias, DM the comoving angular diameter distance, and H the Hubble rate. In addition, we marginalized over two nuisance parameters included in our model to account for unknown systematic effects in the analysis. With this approach, Euclid will be able to reach a relative precision of about 4% on measurements of f/b and 0.5% on DMH in each redshift bin. Better modeling or calibration of the nuisance parameters may further increase this precision to 1% and 0.4%, respectively. Our results show that the exploitation of cosmic voids in Euclid will provide competitive constraints on cosmology even as a stand-alone probe. For example, the equation-of-state parameter, w, for dark energy will be measured with a precision of about 10%, consistent with previous more approximate forecasts.

The halo bias inside cosmic voids

“…Our theoretical modeling of the HMF and halo bias may have several applications in data analyses exploiting the relation between the halo and dark matter distributions inside voids, as for instance measurements of the void size function 4 (Contarini et al 2019;Verza et al 2019;Contarini et al 2022). A further application could be represented by redshift-space distortions around voids, that in ongoing and upcoming galaxy surveys will provide impressive power in constraining the growth of structures and the expansion history of the Universe (Hamaus et al 2020;Nadathur et al 2020;Hamaus et al 2022). Here, the observable is the void profile traced by the galaxy distribution, therefore an accurate halo bias modeling inside voids would help improve the accuracy of this kind of analyses.…”

“…where, for watershed voids, the constant c offset is generally consistent with zero within error margins, or assumed to be negligible (Pollina et al 2017(Pollina et al , 2019Hamaus et al 2020Hamaus et al , 2022. The assumption of a linear relation between halo and dark matter distributions is an empirical approach which parameterizes our ignorance about the way halos populate voids.…”

Dark Energy Survey Year 3 results: Imprints of cosmic voids and superclusters in the Planck CMB lensing map

“…Overall, voids have generated a growing interest when planning future galaxy survey projects with ever-increasing volumes, tracer density, and observational precision (see e.g. Pisani et al 2019;Hamaus et al 2022). Complementing the statistical probes of over-dense environments, the expected gain in cosmological inference associated with voids comes from their ability to unravel extra information from the observed galaxy catalogues (see e.g.…”

Why Cosmic Voids Matter: Nonlinear Structure & Linear Dynamics

“…This opens up a vast observational window to conduct cosmological experiments, allowing us to make use of voids ranging from a few (or possibly less) to hundreds of Mpc in diameter. As linear dynamics holds up in all voids irrespective of their size, void catalogs can not only be extended via enlarging survey volumes towards higher redshifts (as planned for a number of next-generation surveys, such as Euclid [126][127][128]), but also by conducting deeper observations that provide denser tracer samples and hence smaller sub-voids (e.g., as planned for Roman [129] and 4MOST [130]), even at low redshift. This would open up the possibility to maximize the number of observable linear modes of the density field of large-scale structure that can be exploited for the purpose of cosmological inference, far beyond the previously imposed limits.…”