  We propose a new class of metastable dark energy (DE) phenomenological models
in which the DE decay rate does not depend on external parameters such as the
scale factor or the curvature of the Universe. Instead, the DE decay rate is
assumed to be a constant depending only on intrinsic properties of DE and the
type of a decay channel, similar to case of the radioactive decay of unstable
particles and nuclei. As a consequence, the DE energy density becomes a
function of the proper time elapsed since its formation, presumably in the very
early Universe. Such a natural type of DE decay can profoundly affect the
expansion history of the Universe and its age. Metastable DE can decay in three
distinct ways: (i) exponentially, (ii) into dark matter, (iii) into dark
radiation. Testing metastable DE models with observational data we find that
the decay half-life must be many times larger than the age of the Universe.
Models in which dark energy decays into dark matter lead to lower values of the
Hubble parameter at large redshifts relative to $\Lambda$CDM. Consequently
these models provide a better fit to cosmological BAO data (especially data
from from high redshift quasars) than concordance ($\Lambda$CDM) cosmology.
