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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?

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?