Can gravitational waves gain energy in an expanding FRW spacetime?

In summary, the author of the paper discusses the violation of Huygens' principle in non-conformally invariant FRW spacetimes, where gravitational waves can gain energy from the cosmological expansion. It is uncertain if this applies to the expanding FRW spacetime of the standard model of cosmology, and if it has been observed or experimentally proven. It is theorized that these waves would continue to gain energy as long as spacetime is expanding. A related paper on this topic is linked.
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TL;DR Summary
Can gravitational waves gain energy in an expanding FRW spacetime?
I was reading this paper (*Green's functions for gravitational waves in FRW spacetimes:* [https://arxiv.org/abs/gr-qc/9309025](https://arxiv.org/abs/gr-qc/9309025)) and I had a specific question about one statement in the paper that I would like to ask:

At page 6, the author says that gravitational waves would gain energy from spacetime expansion:

*"It was also shown that for all non-conformally invariant FRW spacetimes, in which R=/=0, the Green’s function violates Huygens’ principle. This is the classical analogue of particle creation in a varying gravitational field, as the gravitational waves scatter off the background curvature and gain energy from the cosmological expansion"*

Would this apply to the expanding FRW spacetime of the standard model of cosmology (i.e. our universe)? Does this mean that gravitational waves can gain energy if the universe is expanding? Has this been observed or experimentally proven? Or is it only a theoretical prediction?

And if the waves do indeed gain energy with cosmic expansion, would these waves keep gaining energy as long as spacetime is expanding?
 
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I'm unsure about the original paper. I looked up the papers which cite it on INSPIRE HEP (linked from the arxiv page), and found this paper:
https://arxiv.org/abs/2004.13554

They do a more detailed analysis which includes some inhomogeneities, and find that the gravitational waves decay with expansion.
 
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1. What are gravitational waves?

Gravitational waves are ripples in the fabric of space-time caused by the acceleration of massive objects. They were predicted by Einstein's theory of general relativity and have been indirectly observed through the detection of binary black hole mergers.

2. Can gravitational waves gain energy?

Yes, gravitational waves can gain energy. This is because they are a form of energy themselves and can transfer energy to other objects as they pass through them. However, their energy is very small and difficult to detect.

3. How do gravitational waves behave in an expanding FRW spacetime?

In an expanding FRW (Friedmann-Robertson-Walker) spacetime, gravitational waves behave similarly to other forms of energy. As the universe expands, the wavelength of the gravitational waves also increases, causing them to become less energetic and harder to detect.

4. Can gravitational waves gain energy in an expanding FRW spacetime?

Yes, gravitational waves can gain energy in an expanding FRW spacetime. This is because the expansion of the universe causes the wavelength of the waves to increase, which in turn increases their energy.

5. How does the expansion of the universe affect the energy of gravitational waves?

The expansion of the universe causes the wavelength of gravitational waves to increase, which in turn decreases their energy. This is known as redshifting and is similar to the effect observed in light from distant galaxies. However, gravitational waves can still gain energy through interactions with other objects, even in an expanding universe.

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