In a recently proposed Higgs-Seesaw model the observed scale of dark energy results from a metastable false vacuum energy associated with mixing of the standard model Higgs particle and a scalar associated with new physics at the GUT or Planck scale. Here we address the issue of how to ensure metastability of this state over cosmological time. We consider new tree-level operators, the presence of a thermal bath of hidden sector particles, and quantum corrections to the effective potential. We find that in the thermal scenario many additional light degrees of freedom are typically required unless coupling constants are somewhat fine-tuned. However quantum corrections arising from as few as one additional light scalar field can provide the requisite support. We also briefly consider implications of late-time vacuum decay for the perdurance of observed structures in the universe in this model.
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