The latest BICEP experiment analysis, using data through 2018, further constrains the magnitude of any primordial gravitational waves (strictly speaking the "scalar to tensor ratio").
This further narrows the parameters space allowed for cosmological inflation theories and the data are consistent with the non-existence of primordial gravitational waves. It also continues to be consistent with a universe that isn't quite scale free.
The abstract of the preprint states:
The likelihood analysis yields the constraintat 95% confidence.
Running maximum likelihood search on simulations we obtain unbiased results and find that
These are the strongest constraints to date on primordial gravitational waves.
The body text notes that:
The BKP analysis yielded a 95% confidence constraint r0.05 < 0.12, which BK14 improved to r0.05 < 0.09, and BK15 improved to r0.05 < 0.07. The BK18 result described in this letter, r0.05 < 0.036, represents a fractional improvement equivalent to the two previous steps combined. The BK18 simulations have a median 95% upper limit of r0.05 < 0.019.
The distributions of maximum likelihood r values in simulations where the true value of r is zero gave σ(r0.05) = 0.020 for BK15 which is reduced to σ(r0.05) = 0.009 for BK18. . . .
The system is projected to reach σ(r) ∼ 0.003 within five years with delensing in conjunction with SPT3G.