Chiara Moretti - Measuring dark energy without prior-volume effects

This lecture was part of the Workshop on "Putting the Cosmic Large-scale Structure on the Map: Theory Meets Numerics" held at the ESI September 22 - 26, 2025.

The Effective Field Theory of Large-Scale Structure (EFTofLSS) has been successfully applied to Stage III and Stage IV spectroscopic galaxy surveys, providing tight cosmological constraints. However, its application to beyond-ΛCDM cosmologies—a key target of ongoing surveys—is known to suffer from prior-volume effects due to strong degeneracies between nuisance parameters and (extended) cosmological parameters. I will describe a simple yet effective method to resolve this issue, based on a re-parameterization of the EFTofLSS nuisance parameters to incorporate the contribution from the parameters impacting the amplitude of the EFTofLSS modelling components. This intuitive approach recovers posterior maxima that lie within the marginalised credible intervals, eliminating the significant shifts observed in standard analyses. I will demonstrate the robustness of the method for w0waCDM using synthetic data and an application to BOSS DR12 galaxy measurements combined with BAO and DES Y3 3x2-point function data, providing constraints on w0 and wa that are purely derived from LSS probes.