Scalar gravitational waves from scalar-tensor gravity: Production and response of interferometers

Scalar-tensor gravity admits the existence of scalar modes of gravitational waves (SGWs). The mechanism of production and the response of interferometers to these scalar components of gravitational waves can be studied in three different gauges in the massless case: the transverse-traceless (TT) gauge, the so-called "Shibata, Nakao and Nakamura" (SNN) gauge, and the local Lorentz gauge. The response of interferometers to massless SGWs is invariant in these different gauges. Our work generalizes previous results which, in the study of the coupling between interferometers and massless SGWs, started from the assumption that the wavelength of the SGW is much larger than the distance between the test masses. Furthermore, considering situations motivated by string-dilaton gravity, the effect of a small mass term on the response of the interferometer is taken into account. In this case (massive SGW), we have a longitudinal effect, the response of an arm of an interferometer, which is aligned in the wave propagation direction is computed. The value of the longitudinal response function for non-relativistic massive SGW at high frequencies is very high: this fact opens the doors to the interesting possibility of detection of "massive" part of the signal, if advanced projects will achieve high sensitivities. Finally, by using previous results and the geometry of the system, the generalized coupling between interferometers (like VIRGO or LIGO) and massless SGWs is studied. The total frequency response function to massless SGWs incoming from arbitrary directions is studied.