  While spacetime in the vicinity outside astrophysical black holes is believed
to be well understood, the event horizon and the interior remain elusive. Here,
we discover a degenerate infinite spectrum of novel general relativity
solutions with the same mass-energy and entropy that describe a dark energy
universe inside an astrophysical black hole. This regular cosmological black
hole is stabilized by a finite tangential pressure applied on the dual
cosmological-black hole event horizon, localized up to a quantum indeterminacy.
We recover the Bekenstein-Hawking entropy formula from the classical fluid
entropy, calculated at a Tolman temperature equal to the cosmological horizon
temperature. We further calculate its gravitational quasi-normal modes. We find
that cosmological black holes are detectable by gravitational-wave experiments
operating within the $\mu{\rm Hz}-{\rm Hz}$ range, like LISA
space-interferometer.
