Prisms and mirrors for gravitational waves
Modified gravity could allow matter to disperse, or even reflect, gravitational waves
Gravitational waves propagating through the universe are inevitably affected by the matter they encounter along the way, such as galaxies, giving rise to gravitational-wave lensing phenomena. In gravity theories beyond Einstein’s General Relativity, including alternative explanations for dark energy, new effects can emerge as gravitational waves propagate through the universe and encounter a clumpy matter distribution. This leaves imprints on the signal that cannot be produced in Einstein’s theory: search for them provides means to distinguish, and potentially constrain, different classes of modified gravity theories.
Going beyond usual approximations, researchers at the AEI, Humboldt University, and Imperial College London have explored the lens-induced dispersion of gravitational waves, a phenomenon by which the matter distribution can split a signal by its frequency components, analogous to a prism forming a rainbow out of white light. Unlike previously studied phenomena, this effect arises generically in theories beyond Einstein gravity. In the case of some simple scalar-tensor theories, the authors found a very intriguing result: there is a critical frequency below which gravitational waves can not propagate through matter, allowing certain objects to block and reflect distant signals. The results suggest that the Earth and Moon could play this role in some cases, yielding competitive constraints on certain theories. This result highlights the potential of gravitational-wave lensing for constraining modified gravity, a timely result because of increasing cosmological evidence that dark energy evolves dynamically.
Paper abstract
Gravitational waves (GWs) are direct probes of cosmological gravity, sensitive to space-time inhomogeneities along their propagation. The presence of massive objects breaks homogeneity and isotropy, allowing for new interactions between different GW polarizations, and opening up the intriguing opportunity to test modified gravity theories. This setup generalizes the notion of gravitational deflection and lensing, revealing novel phenomena in modified theories. Any non-minimal theory introduces effective mass terms for GWs, causing lens-induced dispersion (LID), a frequency-dependent phase shift on the waveform. We compute GW dispersion in Einstein’s general relativity (GR) for a spherical matter distribution, finding a small but non-zero phasing that may be constrained by existing and upcoming data. We then extend our analysis to scalar-tensor theories, focusing on symmetron gravity as an example of screened theory, combining cosmological deviations and consistency with local gravity tests. We find enhanced GW dispersion in a large region of the symmetron parameter space, compared to both GR and Brans-Dicke theory. We argue that dispersion, associated to an effective mass for the metric fluctuations, can in some cases prevent the propagation of GWs through some astrophysical bodies, turning them into reflectors. Applying this criterion to near-field lenses such as the Earth or the Moon, it is possible to identify regions of the symmetron parameter space with large dispersive effects. The richness and universality of dispersive phenomena in non-minimal theories open a new avenue to test theories of dynamical dark energy, relevant in light of recent observational results challenging the ΛCDM paradigm.
