Evolution of gauge-invariant cosmological density perturbations through decoupling era

We carefully investigate the evolution of cosmological density perturbations through decoupling era in the baryon-photon universe by a gauge-invariant method. It is found that previous results on the same problem obtained by working in the synchronous gauge are inaccurate particularly on comoving scales greater than the horizon scale at equal time. Further, the anisotropy of microwave background radiation is estimated and the result is found to be in good agreement with previous works qualitatively but in disagreement by factors of a few. Also, an attempt is made to clarify physical processes which determine the evolution of both baryon and radiation density perturbations during decoupling era and the physical interpretation is discussed. (author)