Applicability of entropy bounds to horizonless rolling cosmologies

Determine whether the Covariant Entropy Bound and the Cohen–Kaplan–Nelson bound can be consistently applied to rolling FLRW solutions without an apparent horizon by using the Hubble radius as the infrared cutoff, and ascertain the appropriate infrared scale for such horizonless cosmologies to establish when these bounds are valid.

Background

The paper studies scale separation in compactifications of ten- and eleven-dimensional supergravity and contrasts vacuum solutions with time-dependent rolling solutions. It then examines the implications of entropy bounds—specifically the Covariant Entropy Bound (CEB) and the stronger Cohen–Kaplan–Nelson (CKN) bound—both for vacua and for rolling cosmologies.

While for de Sitter vacua it is standard to take the Hubble radius as an infrared scale when formulating entropy bounds, the rolling solutions considered here typically lack an apparent (cosmic) horizon. The authors observe that a naive use of the Hubble radius as the IR cutoff can lead to apparent violations of the bounds for rolling solutions, raising the question of whether such an IR choice is justified in the absence of a horizon.