Gravitational waves in warm inflation

[shinobi20]

https://arxiv.org/pdf/astro-ph/0006077v2.pdf

page 5. Tensor Perturbations

"Tensor perturbations do not couple strongly to the thermal background and so gravitational waves are only generated by quantum fluctuations, as in standard supercooled inflation".

Why? Tensor perturbations are created by inflation but why don't they couple strongly to the thermal background?

 

[Chalnoth]

From what I recall, it's because twisting forces have no effect on radiation. This kind of thing can interact with solid objects, but not fluids.

 

[shinobi20]

From what I recall, it's because twisting forces have no effect on radiation. This kind of thing can interact with solid objects, but not fluids.

Why is that twisting forces have no effect on radiation? Do you have any resources that I can look into?

 

[Chalnoth]

Why is that twisting forces have no effect on radiation? Do you have any resources that I can look into?

I think it's because low-viscocity fluids don't resist shear (twisting) forces: instead of building up a shear force, they flow.