Gravitational wave astronomy is very different from that of electromagnetic light. Gravitational waves are weak and difficult to see, but they also pass matter with little effect. In essence, the material universe is transparent to gravitational waves. This makes gravitational wave astronomy a powerful tool in studying the universe. But it’s still in its infancy and there’s a lot to learn about how gravitational waves behave.
Gravitational lenses. Source: Caltech
Take the gravitational lens, for example. It occurs when light is slightly deflected by a distant object such as a quasar as it approaches a galaxy or other massive object. The gravitational curvature created by the galaxy’s mass acts as a lens that can focus the quasar light and create multiple quasar images. This effect occurs because light moves through space and time. So if spacetime is distorted, so is the path of light.
However, gravitational waves are different. Gravitational waves are not photons that move through space, but waves of spacetime in spacetime. So can they be deflected like light or be lenticular? Einstein’s theory of relativity predicts that this is possible. Distortion of space changes the propagation of gravitational waves, so that they are also deflected. This effect was never seen, but a new study searched LIGO and Virgo data for it.
Animated representation of a gravitational wave. Photo credit: ESA – C. Carreau
The team looked for various effects of the gravitational lens on the fusion signals of the black hole detected by LIGO and Virgo, e.g. B. identical fusions from slightly different sources or an interference pattern in gravitational waves that would be caused by two images of the merger reaching Earth at slightly different times.
In the last data run with 36 fusion events, the team found no evidence of lenticular gravitational waves. This is not too surprising given the sample size. However, the team has shown that this is possible given the current technology. When new gravitational wave observatories like LISA are built, either lenticular gravitational waves are captured or we have to re-examine Einstein’s theory.
This type of study has tremendous potential for improving our understanding of the universe. Currently, through the gravitational lens of light, we can see objects further away than usual and measure aspects of cosmology such as the expansion of the universe. However, light can be obscured or dimmed between galaxies by diffuse gas and dust. Gravitational waves are not limited in this way so we can make precise measurements of cosmology through gravitational lenses. It could also test Einstein’s theory in new and subtle ways. General relativity has passed all tests so far, but new and interesting physics might be waiting to be discovered.
Reference: Abbott, R. et al. “Search for lens signatures in the gravitational wave observations from the first half of the third observation run by LIGO-Virgo.” arXiv preprint arXiv: 2105.06384 (2021).