Probing nonlinear structure formation beyond $\Lambda$CDM with the LSS bootstrap: a joint power spectrum and bispectrum analysis

TL;DR

This paper introduces a model-independent framework called the LSS bootstrap to constrain deviations from $\Lambda$CDM$ using joint power spectrum and bispectrum analysis, demonstrating its effectiveness with real and simulated data.

Contribution

It presents the first MCMC constraints on the LSS bootstrap parameters, extending the PyBird code for a joint analysis of galaxy clustering data to probe physics beyond $\Lambda$CDM$.

Findings

Achieved 7% constraints on $\varepsilon_f$ and 57% on $\varepsilon_{d_{ ext{ extgamma}}}$ with BOSS data.

Revealed 1% precision on $\varepsilon_f$ and 25% on $\varepsilon_{d_{ ext{ extgamma}}}$ with larger-volume PT Challenge simulations.

Showed the complementary roles of $\varepsilon_f$ and $\varepsilon_{d_{ ext{ extgamma}}}$ in distinguishing background and nonlinear structure effects.

Abstract

We present the first MCMC-derived constraints on the parameters of the Large Scale Structure (LSS) bootstrap, a model-independent framework that captures deviations from $\Lambda$CDM using symmetry arguments alone. Focusing on modifications to the linear growth rate and to the quadratic perturbation-theory kernel -- quantified by the fractional parameters $\varepsilon_f$ and $\varepsilon_{d_{\gamma}}$, respectively -- we carry out a joint analysis of the one-loop galaxy power spectrum and the tree-level bispectrum multipoles within the EFTofLSS, employing the \texttt{PyBird} code extended to implement the bootstrap parametrization. We apply this analysis pipeline to two datasets: the BOSS DR12 LRG sample and the large-volume ``PT Challenge'' simulations. For BOSS, combining the power spectrum with the bispectrum monopole yields $\sim 7\%$ constraints on $\varepsilon_f$ and $\sim 57\%$ constraints on $\varepsilon_{d_{\gamma}}$. For the PT Challenge, whose survey volume is about 100 times larger, we reach $\sim 1\%$ precision on $\varepsilon_f$ and $\sim 25\%$ on $\varepsilon_{d_{\gamma}}$, including the bispectrum quadrupole in the analysis. Our results underscore the complementary roles of $\varepsilon_f$ and $\varepsilon_{d_{\gamma}}$ in separating changes to the background expansion from those affecting nonlinear structure formation, and they show that the LSS bootstrap offers a competitive, model-agnostic method for probing physics beyond $\Lambda$CDM with existing and upcoming galaxy surveys.