Cosmology with voids from the Nancy Grace Roman Space Telescope

TL;DR

This paper forecasts the cosmological constraints attainable from Roman HLSS void statistics, focusing on the void size function (VSF) and the void-galaxy cross-correlation function (VGCF) across $Λ$CDM, $w$CDM, and $w_0 w_{a}$CDM models. Using a realistic 2000 deg$^{2}$ H$\alpha$ mock lightcone with over ${8\times10^{4}}$ voids, the authors develop a Voronoi-threshold void catalog and a moving-barrier VSF model, paired with a linear VGCF model that includes deprojection, redshift-space distortions, and AP effects, calibrated via MCMC. They show that VSF and VGCF provide complementary cosmological information and that their joint analysis yields strong constraints on $Ω_m$, $σ_8$, $h$, and dark-energy parameters, illustrating the power of Roman voids as an independent probe. The work also discusses methodological avenues to improve robustness (e.g., more realistic galaxy-halo connection models, survey masks) and highlights the potential of Roman voids to constrain dynamical dark energy and neutrino masses when combined with other large-scale structure probes.

Abstract

We provide an accurate forecast of the expected constraining power from the main void statistics -- the void size function and the void-galaxy cross-correlation function -- to be measured by the Roman reference High Latitude Spectroscopic Survey from the Nancy Grace Roman Space Telescope. Relying on a realistic galaxy mock lightcone, covering 2000 square degrees, we find more than $8\times 10^4 $ voids and explore their constraining power in the framework of three different cosmological models: $Λ$CDM, $w$CDM, and $w_0 w_{\rm a}$CDM. This work confirms the strong complementarity of different void statistics and showcases the constraining power to be expected from Roman voids thanks to the combination of its high tracer density and large observed volume.

Figures (14)

• Figure 1: Histograms showing the number density of VIDE voids as a function of the void effective radius, $R_{\rm eff}$. Each color corresponds to a different redshift bin, as listed in the legend.
• Figure 2: 2D marginalized posterior distributions for the moving barrier parameters in the true cosmology, Eq. \ref{['eq:moving_b']}, for each redshift bin, as labeled in the legend. The shaded area shows the 68%CL, while the outer contours corresponds to 95%CL. The crosses show the maximum of the posterior distributions.
• Figure 3: From left to right: VSF from the Roman reference HLSS-like mock zhai_2021, in each of the three redshift bins considered. Upper panels: black dots with error bars show the measured VSF from the post-processed void catalog as described in Section \ref{['subsec:vsf_cat']} with $\delta_{\rm v,g}=-0.7$, the error bars are Poissonian; blue solid lines show the best-fit moving barrier calibrated in the true cosmology as described in Section \ref{['subsec:vsf_methodology']}; blue shaded areas show 68% CL. Lower panel: blue solid lines show the relative values of the best VSF theoretical model with respect to measurements in $\sigma$ units; blue shaded areas show the 68% CL; gray hatched areas show the $\pm 1\sigma$ interval.
• Figure 4: VSF: posterior distributions of the cosmological parameters explored in the optimistic scenario, for the three cosmological models explored: $\Lambda$CDM (blue), $w$CDM (orange), $w_0 w_{\rm a}$CDM (green). The filled internal region shows the 68% CL, the outer line shows the 95% CL. Black dashed lines show the true values, crosses correspond to the maximum of the posterior distribution, the numbers in the panels along the diagonal list the parameter values corresponding to the maximum of the likelihood.
• Figure 5: VSF: posterior distributions of the cosmological parameters explored in the pessimistic scenario, for the three explored cosmological models. The plot is organized as Figure \ref{['fig:results_vsf_optimistic']}.