Gravitational Waves in Scalar-Tensor Cosmology

Abstract

The detection of gravitational waves is anticipated to provide a powerful tool to discriminate between general relativity and alternative theories of gravity. An array of detectors could distinguish between massless tensor modes with spin two the only ones present in general relativity and modes of different spin, such as the spin zero modes of the scalar gravitational field in scalar-tensor gravity [887, 307, 308, 909]. Regarding the astrophysics related to the generation of gravitational waves, the most obvious feature is that scalar gravitational radiation can be monopole radiation to lowest order, in contrast to the quadrupole radiation predicted as the lowest order in Einstein’s theory. However, there are factors that may decrease the chance of detection of monopole radiation [254]. Here our main interest is in cosmological gravitational waves and the discussion is limited to a brief introduction to gravitational waves in scalar-tensor gravity. Since gravitational waves in the Jordan frame version of scalar-tensor theories are widely discussed in the literature (e.g., [909]), but their Einstein frame counterpart with fixed units of mass, time and length (which is the popular version in the literature) is not, we focus on the latter. After general consideration, we turn our attention to gravitational waves of cosmological origin and to effects that could potentially lead to the discovery of scalar gravitational waves.