[Paper Review] Cosmic voids: a novel probe to shed light on our Universe

Alice Pisani, Elena Massara+27 more|arXiv (Cornell University)|Mar 12, 2019

Galaxies: Formation, Evolution, Phenomena100 references44 citations

TL;DR

The paper advocates using cosmic voids as a powerful, complementary probe to constrain dark energy, modified gravity, neutrino masses, and galaxy evolution, highlighting observational observables and survey requirements to maximize void science.

ABSTRACT

Cosmic voids, the less dense patches of the Universe, are promising laboratories to extract cosmological information. Thanks to their unique low density character, voids are extremely sensitive to diffuse components such as neutrinos and dark energy, and represent ideal environments to study modifications of gravity, where the effects of such modifications are expected to be more prominent. Robust void-related observables, including for example redshift-space distortions (RSD) and weak lensing around voids, are a promising way to chase and test new physics. Cosmological analysis of the large-scale structure of the Universe predominantly relies on the high density regions. Current and upcoming surveys are designed to optimize the extraction of cosmological information from these zones, but leave voids under-exploited. A dense, large area spectroscopic survey with imaging capabilities is ideal to exploit the power of voids fully. Besides helping illuminate the nature of dark energy, modified gravity, and neutrinos, this survey will give access to a detailed map of under-dense regions, providing an unprecedented opportunity to observe and study a so far under-explored galaxy population.

Motivation & Objective

• Argue that cosmic voids are a sensitive, orthogonal probe for cosmology and galaxy formation.
• Identify void-related observables (e.g., redshift-space distortions, weak lensing) that constrain new physics.
• Outline survey design requirements to maximize void science for Astro2020.
• Highlight theoretical and simulation advances needed to exploit voids in upcoming data.

Proposed method

• Describe how voids trace structure growth and enable growth rate f(z) measurements via redshift-space distortions.
• Explain the Alcock-Paczyński test with void stacks as a probe of dark energy and geometry.
• Discuss void lensing and density/velocity profiles as tests of modified gravity.
• Address neutrino mass sensitivity through void statistics and their bias with respect to CDM.
• Propose survey strategies (volume vs. tracer density) to optimize void detection and analysis.

Experimental results

Research questions

• RQ1 How can redshift-space distortions around voids constrain the growth rate f/b and test General Relativity?
• RQ2 To what extent can voids provide competitive or superior constraints on Omega_m and dark energy parameters via the Alcock-Paczyński test?
• RQ3 How do neutrino masses and galaxy bias affect void observables, and how can voids inform neutrino mass constraints?
• RQ4 What survey design (volume, tracer density, spectroscopy vs. imaging) optimizes void science for the next decade?
• RQ5 What theoretical models and simulations are required to robustly interpret void statistics in current and future data?

Key findings

• Void dynamics in under-dense regions offer robust probes of structure growth and gravity.
• Void-based Alcock-Paczyński measurements can yield high-precision constraints on cosmology, with potential 1% accuracy on void shape in AP analyses.
• Current void RSD analyses already provide competitive constraints on Omega_m and f/b, e.g., 10% and 12% respectively from SDSS BOSS data.
• Neutrinos and dark energy leave distinct signatures in void size function, density and velocity profiles, and void clustering that can be exploited with future surveys.
• Void observations are complementary to high-density analyses and can map under-dense galaxy populations and their evolution.
• A combination of large volume and high tracer density surveys (e.g., extensions of WFIRST, DESI, SPHEREx, Euclid) will maximize void science.