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.
