https://www.forbes.com/sites/startswitha...0de1f52f3f
EXCERPT: . . . Like light, gravitational waves have a wavelength. Like light, they carry an energy that's defined by their wavelength and intensity/amplitude. And, like light, its wavelength gets stretched as the Universe expands. This last part allows us to move from the realm of the theoretical into the realm of the observational.
We have observed a number of different gravitational waves thanks to LIGO: 11 at last count. These all correspond to merging, massive, compact objects, where even the closest one was over 100 million light years distant. With light-travel-times (or gravitational-wave-travel-times) this large, the expansion of the Universe is important, and when we measure the waves that passed through Earth, we can see that they were definitively stretched by the effects of the Universe's expansion.
This tells us, unambiguously, that gravitational waves, as they travel through the Universe, are affected by the warping, curvature, and stretching of space. [...] This carries with it an implication that's quite profound, although it might not be intuitive. At some level, we fully expect that there is a quantum theory of gravity governing the Universe, and that the graviton is the particle responsible for the gravitational interaction.
If gravitational waves experience gravity, that means that gravitons don't just interact with the energy-carrying particles of the Standard Model, but there is a graviton-graviton interaction as well.
Two different gravitational waves, in Einstein's relativity, should interfere when they meet. But they can't simply pass right through one another; General Relativity itself is a nonlinear theory, meaning that the gravitational waves must interact and scatter off of each other at some level. This tells us there's a subtle application to quantum gravity: there's a chance of having a graviton-graviton scattering interaction.
Gravitons, the particles responsible for the gravitational force, don't only mediate interactions between the particles of the Standard Model. There's a chance that they can collide with one another, and what possibly happens when they do is a puzzle that only quantum gravity will be able to solve.
Although it might seem counterintuitive that gravitation would affect gravitational waves, this is one of those wonderful times where theory and observation line up perfectly. They demonstrate that gravitational waves must follow the curved paths set by the presence of mass and energy in the Universe; that they see their wavelengths stretch as the Universe expands; that they obey the rules of time dilation; that they follow the same paths that photons do, minus the interactions with matter....
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