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 $V^{(1/4)} \sim 1.04 \times 10^{16}\ \mathrm{GeV}\ (r / 0.01)^{(1/4)}$. 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: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
South Pole Observatory projections (2026) aim at $\sigma(r) \sim 0.001$ by about 2034 (moderate confidence).
Parent problems
- Special case of Did the early universe go through an inflationary phase? · open · from revision 1
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