Consistent Initial Conditions for Early Modified Gravity in Effective Field Theory

Understand

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.