Dark energy from instantons
  We show that in imaginary time quantum metric fluctuations of empty space
form a self-consistent de Sitter gravitational instanton that can be thought of
as describing tunneling from "nothing" into de Sitter space of real time (no
cosmological constant or scalar fields are needed). For the first time, this
mechanism is activated to give birth to a flat inflationary Universe. For the
second time, it is turned on to complete the cosmological evolution after the
energy density of matter drops below the threshold (the energy density of
instantons). A cosmological expansion with dark energy takes over after the
scale factor exceeds this threshold, which marks the birth of dark energy at a
redshift $1+z\approx 1.3$ and provides a possible solution to the "coincidence
problem". The number of gravitons which tunneled into the Universe must be of
the order of $10^{122}$ to create the observed value of the Hubble constant.
This number has nothing to do with vacuum energy, which is a possible solution
to the "old cosmological constant problem". The emptying Universe should
possibly complete its evolution by tunneling back to "nothing". After that, the
entire scenario is repeated, and it can happen endlessly.
