[Paper Review] Cosmology with voids from the Nancy Grace Roman Space Telescope

G Verza, Giulia Degni+8 more|arXiv (Cornell University)|Oct 25, 2024

Historical and Architectural StudiesArts and Humanities3 citations

TL;DR

This paper forecasts the cosmological constraints achievable from cosmic voids in the Nancy Grace Roman Space Telescope's High Latitude Spectroscopic Survey using realistic Hα galaxy mock lightcones. It demonstrates that the joint analysis of the void size function and void-galaxy cross-correlation function—enabled by Roman's high tracer density and large volume—provides powerful, complementary constraints on ΛCDM, wCDM, and w₀waCDM models, with the VGCF tightening constraints on parameters not directly probed by the VSF due to cross-scale information transfer.

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 imes 10^4 $ voids and explore their constraining power in the framework of three different cosmological models: $Λ$CDM, $w$CDM, and $w_0 w_{ m a}$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.

Motivation & Objective

• To forecast the cosmological constraining power of cosmic voids from the Roman Space Telescope's High Latitude Spectroscopic Survey.
• To assess the complementarity between the void size function (VSF) and void-galaxy cross-correlation function (VGCF) in constraining cosmological parameters.
• To evaluate the impact of Roman’s high tracer density and large observed volume (2000 deg²) on void statistics precision.
• To quantify the improvement in constraints when combining VSF and VGCF, especially for dark energy and neutrino mass parameters.
• To lay the groundwork for future joint analyses using multi-realization mocks and diverse galaxy-halo connection models.

Proposed method

• Utilized a realistic Hα galaxy mock lightcone covering 2000 square degrees, simulating the Roman survey's expected galaxy distribution.
• Identified over 8×10⁴ voids using a spherical void-finding algorithm applied to the mock catalog.
• Modelled the void size function (VSF) using a halo model-based approach with an effective barrier parameterization.
• Fitted the void-galaxy cross-correlation function (VGCF) using a redshift-space distortion model including Alcock-Paczynski effects and bias parameters.
• Projected cosmological constraints using Markov Chain Monte Carlo (MCMC) sampling on ΛCDM, wCDM, and w₀waCDM models.
• Combined VSF and VGCF posterior distributions by assuming low cross-covariance, enabling independent constraint combination.

Experimental results

Research questions

• RQ1 How precisely can the Roman Space Telescope constrain cosmological parameters using void statistics, particularly the void size function and void-galaxy cross-correlation function?
• RQ2 What is the degree of complementarity between the void size function and void-galaxy cross-correlation function in constraining dark energy and matter parameters?
• RQ3 How does the combination of high tracer density and large survey volume in Roman enhance the constraining power of void statistics compared to existing surveys?
• RQ4 To what extent do constraints on parameters like σ₈ and h improve in the VGCF analysis even though they are not directly probed by the VGCF?
• RQ5 How do the constraints from VSF and VGCF differ across cosmological models, especially in the w₀waCDM framework where dark energy evolves?

Key findings

• The Roman survey is expected to detect more than 8×10⁴ voids over 2000 square degrees, enabling high-precision cosmological analysis.
• The void-galaxy cross-correlation function (VGCF) provides tighter constraints on Ωₘ than the void size function (VSF), even for parameters not directly probed by VGCF, due to cross-scale information transfer.
• The VSF and VGCF exhibit strong complementarity: VGCF constrains background expansion (via AP test), while VSF is sensitive to structure growth (via σ₈), leading to improved constraints on w₀ and wa in the w₀waCDM model.
• In the w₀waCDM model, the VSF provides tighter constraints than VGCF on Ωₘ and dark energy parameters, as the VSF directly probes growth while the VGCF does not.
• Joint analysis of VSF and VGCF yields significantly tighter posterior contours than either statistic alone, with the combined posterior showing reduced degeneracies, especially in the w₀–wa and Ωₘ–w₀ planes.
• The study demonstrates that Roman voids will provide a powerful, independent probe of dark energy and neutrino masses, with potential to constrain the dark energy equation of state with high precision.