Finally, let us comment on potential testability of this model. As we pointed out, the simple model of cosmological perturbations, developed in Sec. 4 is already ruled out by cosmo- logical observations at > 5σ level, as it does not predict any deviations from scale-invariance. However, it is easy to imagine small corrections that could lead to a ∼ 4% deviation from scale-invariance, especially given that bulk temperature is so close (i.e. ∼ 20% of) the 5D Planck temperature. In the context of our model, the red tilt of the cosmological power spectrum implies that the amplitude of 5D bulk graviton propagator, which enters in Eq. (4.8), is getting stronger in the IR, suggesting gradual unfreezing of additional polarizations of graviton.
I'm not sure of what a graviton is, but wikipedia tells me that it is not detectable by any current mean. I'd be tempted to say in light of this that their theory is not testable either.
But that's the point, innit. Who's to say we don't discover magic quantum carbon nanotubes or something thing that makes it feasible to setup a graviton detector at some point in the future. And who's to say it won't be useful, either.
I'm serious; the Higgs-Boson was theorized to exist in 1964, and was proven to exist only after great expense in 2012, utilizing many cutting edge technologies.
General Relativity was theorized in 1915, but not really tested in 1959, and now I use it almost every day in the form of GPS.
At the point it we can test it, it becomes testable. The authors, note, did not claim it was testable. They simply indicated the direction research should go in order to test it.
As per Einstein, it's true that accurate measurements of his GR effects didn't come along for decades, but he was answering known physical problems. And the crude measurements of his day aligned with his predictions, they just weren't accurate yet.
We should distinguish "testable in principle" and "testable in practice". Simply because it is currently not testable in practice, does not mean that it's not testable in principle.
Saying a scientific hypothesis is untestable "in principle" is a deathknell while untestable "in practice" is not.
The graviton is the missing particule of the quantum mechanics.
It is, simply put, a 'gravity unit' exchange between every corpse in the universe from which 'emerges' gravity.
I bet you there are a lot of efforts made right now to find a way to measure them!
So, if we look at parts of space further away from us (so the light reaching us tells us about older times), we should see fewer kinds of gravitons than we see around us now?
Finally, let us comment on potential testability of this model. As we pointed out, the simple model of cosmological perturbations, developed in Sec. 4 is already ruled out by cosmo- logical observations at > 5σ level, as it does not predict any deviations from scale-invariance. However, it is easy to imagine small corrections that could lead to a ∼ 4% deviation from scale-invariance, especially given that bulk temperature is so close (i.e. ∼ 20% of) the 5D Planck temperature. In the context of our model, the red tilt of the cosmological power spectrum implies that the amplitude of 5D bulk graviton propagator, which enters in Eq. (4.8), is getting stronger in the IR, suggesting gradual unfreezing of additional polarizations of graviton.
[1]:http://arxiv.org/pdf/1309.1487v2