  In this paper, using and implementing a new line of sight CMB code, called
CMBAns [1], that allows us to modify H(z) for any given feature at any redshift
we study the effect of changes in the expansion history of the Universe on the
CMB power spectrum. Motivated by the detailed analytical calculations of the
effects of the changes in H(z) on ISW plateau and CMB low multipoles, we study
two phenomenological parametric form of the expansion history using WMAP data
and through MCMC analysis. Our MCMC analysis shows that the standard LCDM
cosmological model is consistent with the CMB data allowing the expansion
history of the Universe vary around this model at different redshifts. However,
our analysis also shows that a decaying dark energy model proposed in [2] has
in fact a marginally better fit than the standard cosmological constant model
to CMB data. Concordance of our studies here with the previous analysis showing
that Baryon Acoustic Oscillation (BAO) and supernovae data (SN Ia) also prefer
mildly this decaying dark energy model to LCDM, makes this finding interesting
and worth further investigation.
