The experimental confirmation of the existence of gravitational waves, more than 100 years after they were predicted by Albert Einstein, ushered cosmology into a new era. So far, 11 events have been detected, allowing scientists to refine their knowledge of these true ripples in spacetime. If gravitational waves are products of gravity, do they also suffer, like the other elements of the cosmos, from its effects?
In the theory of general relativity, gravity is no longer a force but the deformation of the geometry of space-time. The distribution of matter-energy tells space-time how to bend, while this curvature, in turn, tells masses and energy how to move. This relationship is expressed in Einstein's equation. Thus, depending on the energy-matter content present, it is possible to determine how the space-time metric will be affected.
The type of particle—elementary, composite, particle, antiparticle—plays no role in the phenomenon; as soon as energy is present, mass being only one form of energy, space-time will be bent. Thus, even zero-mass particles are affected by gravity. This is particularly visible in the phenomenon of gravitational lensing, where the light rays emitted by a distant source follow the geodesics imprinted by a mass located between this source and the observer.
Therefore, there would be no reason to think that gravitational waves are an exception. After all, they share a number of things in common with photons:they are massless, travel at the speed of light in vacuum, and carry energy. And this last point is particularly important because, in general relativity, it is energy that affects space-time.
Like light, a gravitational wave has a wavelength. Like light, it has energy related to its wavelength and intensity/amplitude. And still like light, its wavelength is stretched by the expansion process of the Universe (redshift ).
Currently, 11 gravitational wave events have been detected by the LIGO and Virgo interferometers. Each corresponding to merger events between black holes or neutron stars. The closest of these was located 100 million light-years away; with such a long travel time, the effect of the expansion of the Universe is significant, and when the waves are measured on Earth, one finds that they have clearly been stretched during their journey by the expansion of the Universe.
This element clearly indicates that gravitational waves, moving through the Universe, are affected by the curvature and expansion of space. There is also another clue. In the 2017 neutron star merger observation, dubbed GW170817, the event emitted both gravitational waves and a gamma-ray burst. The two signals arrived practically simultaneously on Earth (less than 2 seconds difference).
Related:How do gravitational waves escape from black holes?
Traveling a path of 100 million light-years, and considering that there are more than 30 million seconds in a year, it is possible to show that the speed of light and gravity are identical at a value of 1 part per quadrillion (10 15 ). This shows that whatever delays photons traveling through the Universe experience due to the curvature of space, gravitational waves experience the same delays.
In an area strongly affected by gravity, each object must follow the trajectory traced by the curvature of space. Like, for example, around the massive galaxy in which event GW170817 was detected. The fact that photons and gravitational waves arrive simultaneously indicates that they both had to navigate the curvature of space to escape from the galaxy in question.
So, from our observations:gravitational waves are stretched by the expansion of the Universe, follow the same trajectories as photons, undergo the same expansions and time delays as other massless particles, and undergo the same energy changes when they enter and escape from strongly curved areas of space.
Ultimately, physicists believe that in the quantum description of gravity, gravitons are the mediating bosons of this interaction. If gravitational waves are subject to gravity, this means that the gravitons not only interact between Standard Model particles, but also between them, forming a graviton-graviton interaction.
When two different gravitational waves meet, in general relativity, they interfere. Indeed, Einstein's theory being non-linear, gravitational waves must interact and diffuse, they cannot simply pass through each other. In the context of a theory of quantum gravity, this would therefore indicate a graviton-graviton diffusion interaction.
