SCINTILLATION IN SCALAR-TENSOR THEORIES OF GRAVITY

We study the scintillation produced by time-varying gravitational fiel ds within scalar-tensor theories of gravity. The problem is treated in the geometrical optics approximation for a very distant light source emitting quasi plane monochromatic electromagnetic waves. We obtain a general formula giving the time dependence of the photon flux received by a freely falling observer. In the weak-field approximation, we sho w that the contribution to the scintillation effect due to the focusin g of the light beam by a gravitational wave is of first order in the a mplitude of the scalar perturbation. Thus scalar-tensor theories contr ast with general relativity, which predicts that the only first-order effect is due to the spectral shift. Moreover, we find that the scinti llation effects caused by the scalar field have a local character: the y depend only on the value of the perturbation at the observer. This e ffect provides in principle a mean to detect the presence of a long ra nge scalar field in the Universe, but its smallness constitutes a trem endous challenge for detection.