  The backreaction of nonlinear inhomogeneities to the cosmic expansion is
re-analyzed in the framework of general relativity. Apparent discrepancies
regarding the effect of the nonlinear backreaction, which exist among the
results of previous works in different gauges, are resolved. By defining the
spatially averaged matter energy density as a conserved quantity in the large
comoving volume, it is shown that the nonlinear backreaction neither
accelerates nor decelerates the cosmic expansion in a matter-dominated
universe. The present result in the Newtonian gauge is consistent with the
previous results obtained in the comoving synchronous gauge. Although our work
does not give a complete proof, it strongly suggests the following no-go
theorem: No cosmic acceleration occurs as a result of the nonlinear
backreaction via averaging.
