Limits on Entanglement Effects in the String Landscape from Planck and BICEP/Keck Data

We consider observational limits on a proposed model of the string landscape in inflation. In this scenario, effects from the decoherence of entangled quantum states in long-wavelength modes in the universe result in modifications to the Friedmann Equation and a corresponding modification to inflationary dynamics. Previous work by Holman, Mersini-Houghton, and Takahashi suggested that such effects could provide an explanation for well-known anomalies in the Cosmic Microwave Background (CMB), such as the lack of power on large scales and the "cold spot" seen by both the WMAP and Planck satellites. In this paper, we compute limits on these entanglement effects from the Planck CMB data combined with the BICEP/Keck polarization measurement, and find no evidence for observable modulations to the power spectrum from landscape entanglement, and no sourcing of observable CMB anomalies. The originally proposed model with an exponential potential is ruled out to high significance. Assuming a Starobinsky-type $R^2$ inflation model, which is consistent with CMB constraints, data place a $2\sigma$ lower bound of $b > 6.46 \times 10^7\ {\rm GeV}$ on the Supersymmetry breaking scale associated with entanglement corrections.