COSMO In the literature: open

What is the amplitude of primordial gravitational waves?

In plain words

Inflation would have shaken space itself and left gravitational waves that imprint a twisting pattern on the polarization of the cosmic microwave background. Their strength would reveal the energy at which inflation happened.

Precise statement

Measure the tensor-to-scalar ratio r at pivot $k = 0.05\ \mathrm{Mpc}^{-1}$, which fixes the inflation energy scale $V^{(1/4)} \sim 1.04e16\ \mathrm{GeV}\ (r / 0.01)^{(1/4)}$. Current bound $r < 0.036$ (95 percent, BICEP/Keck 2021), $r < 0.038$ with ACT DR6. An answer is a detection or a bound $r < 0.001$ after delensing and foreground separation.

What would settle it

BICEP Array, South Pole Observatory, Simons Observatory and LiteBIRD B-mode measurements reaching $\sigma(r) \sim 0.001$.

Status in the literature

Unverified note

South Pole Observatory projections (2026) aim at $\sigma(r) \sim 0.001$ by about 2034 (moderate confidence).