We present observational constraints on a scalar-tensor gravity theory by \chi ^ { 2 } test for CMB anisotropy spectrum .
We compare the WMAP temperature power spectrum with the harmonic attractor model , in which the scalar field has its harmonic effective potential with curvature \beta in the Einstein conformal frame and the theory relaxes toward Einstein gravity with time .
We found that the present value of the scalar coupling , i.e .
the present level of deviation from Einstein gravity ( \alpha _ { 0 } ^ { 2 } ) , is bounded to be smaller than 5 \times 10 ^ { -4 - 7 \beta } ( 2 \sigma ) , and 10 ^ { -2 - 7 \beta } ( 4 \sigma ) for 0 < \beta < 0.45 .
This constraint is much stronger than the bound from the solar system experiments for large \beta models , i.e. , \beta > 0.2 and 0.3 in 2 \sigma and 4 \sigma limits , respectively .
Furthermore , within the framework of this model , the variation of the gravitational constant at the recombination epoch is constrained as |G ( z = z _ { rec } ) - G _ { 0 } | / G _ { 0 } < 0.05 ( 2 \sigma ) , and 0.23 ( 4 \sigma ) .