Beyond Stage IV: Quasar and Galaxy Clustering and the Fundamental Physics of the 2040s

TL;DR

Next-generation wide-field spectroscopic surveys targeting redshifts 1 to 6 could revolutionize our understanding of fundamental physics by precisely measuring neutrino masses, primordial non-Gaussianity, and structure growth, surpassing current Stage IV capabilities.

Contribution

The paper proposes a new survey concept with advanced multiplexing and precision techniques to achieve transformative cosmological constraints beyond Stage IV surveys.

Findings

Potential to measure neutrino masses with $ extless$0.015 eV precision

Detection of primordial non-Gaussianity at $f_{NL} extasciitilde 1$ level

Ability to map structure growth $f\sigma_8(z)$ across cosmic time

Abstract

Stage IV galaxy surveys (DESI, 4MOST, MOONS, Euclid) are establishing precision constraints on cosmological parameters through baryon acoustic oscillations and redshift-space distortions, yet fundamental questions on neutrino masses, inflationary physics, and the nature of gravity remain beyond their reach. We present a science case for next-generation wide-field spectroscopic surveys targeting $1 < z < 6$ with simultaneous observations of thousands of galaxies, quasars, and emission-line galaxies. Such surveys would deliver transformative advances: (i) cosmological constraints on absolute neutrino masses ($\Sigma m_\nu \lesssim 0.015\,\mathrm{eV}$), three times more stringent than Stage IV, enabling resolution of the neutrino mass hierarchy; (ii) detection of primordial non-Gaussianity at the level of $f_{\mathrm{NL}} \sim 1$, probing multi-field inflation; (iii) measurements of structure growth $f\sigma_8(z)$ spanning cosmic time to constrain dark energy and test gravitational modifications. Achieving these goals requires revolutionary advances in spectroscopic multiplexing ($\mathcal{O}(1000)$ simultaneous spectra), sub-$2\times10^{-4}(1+z)$ redshift precision at scale, and field-level inference techniques exploiting higher-order clustering statistics. We demonstrate that the proposed Wide-field Spectroscopic Telescope concept provides a technically feasible and scientifically compelling path to unlock the physics of neutrinos, inflation, and gravity that will remain inaccessible to Stage IV surveys.