Cosmic Microwave Background as a Gravity Probe

We show that cosmic microwave background (CMB) observations have a strong potentiality not only as a test of cosmic kinematics but also as a gravity probe. We compare the recent high-resolution CMB data to models with a direct coupling of dark energy to dark matter. This extra interaction violates the equivalence principle, acts as an additional scalar gravity on dark matter fluctuations, and imprints a characteristic signature on the CMB spectra. Defining the ratio β of the dark energy interaction to gravity, we that β < 0.16 (95% confidence level [c.l.]) from the current CMB data set, regardless of the potential. This implies that the effective equation of state between equivalence and tracking has been close to the pure matter equation of state within 1% and that scalar gravity is at least 40 times weaker than tensor gravity. Furthermore, we show that an ideal CMB experiment limited by cosmic variance only can put an upper bound β < 0.05 (95% c.l.), comparable to the best limits provided by local gravity experiments on the coupling to baryons.