The first structures we consider are cosmic voids. Cosmic voids are defined as large underdense regions of the cosmic web, they are the largest structures in the Universe and make up most of its volume (Cautun et al. 2014; Falck & Neyrinck 2015). Historically, their existence was one of the earliest predictions of the concordance cosmological model (Hausman, Olson & Roth 1983), and their observational detection goes back to roughly 40 yr ago (Gregory, Thompson & Tifft 1978; Kirshner et al. 1981). Voids are in particular extremely underdense near their centres, and their spherically averaged density profile shows a characteristic shape (Colberg et al. 2005; Ricciardelli, Quilis & Planelles 2013; Hamaus, Sutter & Wandelt 2014a; Nadathur et al. 2014b; Ricciardelli, Quilis & Varela 2014). Recently, cosmic voids are becoming a promising cosmological probes: first they could represent a population of statistically ideal spheres with a homogeneous distribution at different redshifts which size evolution could be used to probe the expansion of the Universe using Alcock & Paczynski tests (Alcock & Paczynski 1979; Lavaux & Wandelt 2012; Sutter et al. 2012; Sutter et al. 2014b; Hamaus et al. 2015; Hamaus et al. 2016; Mao et al. 2017; Hamaus et al. 2022). Moreover, due to their low density, voids are naturally sensitive to dark energy and thus the interest to use them as probe of alternative Dark Energy models and modified gravity scenarios is increasing (Odrzywołek 2009; Lavaux & Wandelt 2010; D’Amico et al. 2011; Li 2011; Bos et al. 2012; Clampitt, Cai & Li 2013; Gibbons et al. 2014; Barreira et al. 2015; Cai, Padilla & Li 2015; Pisani et al. 2015; Zivick et al. 2015; Pollina et al. 2016; Baldi & Villaescusa-Navarro 2018), as well as the possibility of using them to put constraints on neutrinos masses (Massara et al. 2015; Kreisch et al. 2019; Contarini et al. 2021). Their imprint on the observed Cosmic Microwave Background (CMB) is also becoming an encouraging new cosmological probe, either through their Integrated Sachs-Wolfe (ISW) imprint (Baccigalupi, Amendola & Occhionero 1997; Baccigalupi 1999; Granett, Neyrinck & Szapudi 2008; Cai et al. 2014; Granett, Kovács & Hawken 2015; Hotchkiss et al. 2015; Ade et al. 2016; Nadathur & Crittenden 2016; Kovács et al. 2017; Kovács et al. 2019; Hang et al. 2021), or their lensing imprint (Cai et al. 2017; Raghunathan et al. 2020; Vielzeuf et al. 2021). Furthermore, the observed cold spot of the CMB could be explained as the imprint of the ISW sourced by very large voids along the line of sight (Rees, Sciama & Stobbs 1968; Kovac et al. 2013; Finelli et al. 2014; Nadathur et al. 2014a). Moreover, some works such as Jamieson & Loverde (2019) studied the properties of the voids via the separate universe technique. Finally, some studies tried to link high redshift intergalactic voids in the transmitted Lyman-α flux to the gas density (Viel, Colberg & Kim 2008). Because they are almost empty regions, their evolution during cosmic history is at most weakly non-linear and their properties could possibly be impacted by the primordial density fields from which they formed. This fact motivates us to investigate the effects of baryon-CDM relative perturbations on these objects and their statistics.
Citation: Gibbons et al. 2014
Citation function: Motivation