Baryon Physics and Tight Coupling Approximation in Boltzmann Codes
  We provide two derivations of the baryonic equations that can be
straightforwardly implemented in existing Einstein--Boltzmann solvers. One of
the derivations begins with an action principle, while the other exploits the
conservation of the stress-energy tensor. While our result is manifestly
covariant and satisfies the Bianchi identities, we point out that this is not
the case for the implementation of the seminal work by Ma and Bertschinger and
in the existing Boltzmann codes. We also study the tight coupling approximation
up to the second order without choosing any gauge using the covariant full
baryon equations. We implement the improved baryon equations in a Boltzmann
code and investigate the change in the estimate of cosmological parameters by
performing an MCMC analysis. With the covariantly correct baryon equations of
motion, we find 1% deviation for the best fit values of the cosmological
parameters that should be taken into account. While in this paper, we study the
Lambda-CDM model only, our baryon equations can be easily implemented in other
models and various modified gravity theories.
