Searching for dark energy in the matter-dominated era

TL;DR

This paper investigates the potential of large-scale structure surveys in the matter-dominated era (redshifts 2-6) to detect dynamical dark energy signatures, using models inspired by scalar-tensor theories and current/future observational data.

Contribution

It demonstrates how existing and upcoming surveys can constrain dark energy's equation of state during the matter-dominated era, extending the search beyond late-time observations.

Findings

Existing CMB and LSS data constrain dark energy in the matter era.

Future surveys like DESI and intensity mapping can improve these constraints.

High-redshift follow-up surveys outperform low-redshift galaxy surveys in constraining models.

Abstract

Most efforts to detect signatures of dynamical dark energy are focused on late times, $z \lesssim 2$, where the dark energy component begins to dominate the cosmic energy density. Many theoretical models involving dynamical dark energy exhibit a 'freezing' equation of state however, where $w \to -1$ at late times, with a transition to a 'tracking' behaviour at earlier times (with $w \gg -1$ at sufficiently high redshift). In this paper, we study whether large-scale structure surveys in the post-reionisation matter-dominated regime, $2 \lesssim z \lesssim 6$, are sensitive to this behaviour, on the basis that the dark energy component should remain detectable (despite being strongly subdominant) in this redshift range given sufficiently precise observations. Using phenomenological models inspired by parameter space studies of Horndeski (generalised scalar-tensor) theories, we show how existing CMB and large-scale structure measurements constrain the dark energy equation of state in the matter-dominated era, and examine how forthcoming galaxy surveys and 21cm intensity mapping instruments can improve constraints in this regime. We also find that the combination of existing CMB and LSS constraints with DESI will already come close to offering the best possible constraints on $H_0$ using BAO/galaxy power spectrum measurements, and that either a spectroscopic follow-up of the LSST galaxy sample (e.g. along the lines of MegaMapper or SpecTel) or a Stage 2/PUMA-like intensity mapping survey, both at $z \gtrsim 2$, would offer better constraints on the class of dark energy models considered here than a comparable cosmic variance-limited galaxy survey at $z \lesssim 1.5$.