Hidden Negative Energies in Strongly Accelerated Universes

suffer fromeither ghosts or gradient instabilities or both, see e.g.[8]. Theseareshort-timescaleinstabilitiesthataremostrapid at energies near the cut-off and therefore imme-diately disqualify the background solution, see however[9,10]. In Refs [11–14] noncanonical scalar-field mod-els were constructed that can serve as a medium with