Consistent Initial Conditions for Early Modified Gravity in Effective Field Theory

TL;DR

This paper derives and implements consistent initial conditions for early modified gravity models within the EFT framework, demonstrating their importance for accurate cosmological predictions and constraints.

Contribution

It provides a method to derive and implement theory-consistent initial conditions for Horndeski theories in EFTCAMB, improving the accuracy of cosmological perturbation calculations.

Findings

Consistent initial conditions lead to expected early-time perturbation behavior.

Deviations in CMB spectra are significant when using inconsistent initial conditions.

Using accurate initial conditions is crucial for robust constraints on early modified gravity models.

Abstract

Precise initial conditions (ICs) are crucial for accurate computation in cosmological perturbation theory. We derive the consistent ICs for Horndeski theory in the Effective Field Theory (EFT) approach, assuming constant EFT functions at early times. We implement the ICs into the public Boltzmann code \texttt{EFTCAMB}, and demonstrate that the expected early-time behavior of perturbations and Weyl potential can be obtained with theory-consistent MG ICs. We identify significant deviations when comparing Cosmic Microwave Background angular power spectra in MG models obtained with consistent MG ICs versus inconsistent GR ICs. Our findings underline the importance of using accurate, theory-consistent MG ICs to ensure robust cosmological constraints on early MG models.