What would we learn by detecting a gravitational wave signal in the cosmic microwave background anisotropy?

TL;DR

Inflation produces both density perturbations and gravitational waves; a gravitational-wave contribution to the CMB anisotropy would be detectable only if the inflaton field variation during inflation is at least of order the Planck scale, with the slow-roll relations $\delta_H^2(k)=\frac{1}{75\pi^2 m_{Pl}^6}\frac{V^3}{V'^2}$ and $r(k)=0.139\,\frac{{\cal P}_g}{\delta_H^2}$ linking the observables to $V$ and $V'$. A detectable $r$ would require a total field excursion $\Delta\phi/m_{Pl}\gtrsim 0.5$, imply $V^{1/4}\sim (2{-}4)\times 10^{16}\,\mathrm{GeV}$, challenge inflation models embedded in ordinary SM extensions and the idea that the inflaton is a generic superstring modulus, and would likely point to a hybrid-inflation realization with a GUT Higgs sector only if both the vacuum expectation values and the Higgs masses are of that order. The paper also notes that detecting such a signal would demand a revision of expectations about the inflationary potential, pushing models toward uncharted large-field regimes, while a non-detection would remain compatible with small-field or moduli-type scenarios; overall it argues that there is a substantial theoretical prejudice against the likelihood of observing gravitational waves in the CMB.

Abstract

Inflation generates gravitational waves, which may be observable in the low multipoles of the cosmic microwave background (cmb) anisotropy but only if the inflaton field variation is at least of order the Planck scale. Such a large variation would imply that the model of inflation cannot be part of an ordinary extension of the standard model, and combined with the detection of the waves it would also suggest that the inflaton field cannot be one of the superstring moduli. Another implication of observable gravitational waves would be a potential $V^{1/4}=2$ to $4\times 10^{16}\GeV$, which is orders of magnitude bigger than is expected on the basis of particle theory. It might emerge in a hybrid inflation model where most of the energy density comes from the Higgs sector of a GUT, but only if both the vacuum expectation values {\em and the masses } of the Higgs fields are of this order.