Examining the end of inflation with primordial black holes mass
  distribution and gravitational waves
  We explicitly construct a double-field inflationary model, which satisfies
the latest Planck constraints at the cosmic microwave background (CMB) scales
and produces the whole dark matter energy density as primordial black holes
(PBHs), in the mass range $10^{-17}~M_{\odot}\lesssim M_{{}_{\rm PBH}}\lesssim
10^{-13}~M_{\odot}$. The PBHs can be produced after the end of slow-roll
inflation from the bubbles of true vacuum that nucleate during the course of
inflation. Obtaining PBHs in this mass range enforces the scale of inflation to
be extremely low, $10^{-7} \lesssim H \lesssim 10^{-3} ~{\rm GeV}$, which makes
the efforts to observe gravitational waves at the CMB scales futile, although
it is high enough to allow for a successful big bang nucleosynthesis. We will
show that the shape of the mass distribution of the PBHs is dependent on how
inflation ends and the Universe settles from the metastable direction to the
true one. End of inflation can also be probed by examining the gravitational
waves spectrum. In particular, we show that if exit from the rolling metastable
direction to the true vacuum of the potential happens through a first-order
phase transition after the end of slow-roll inflation, it leaves behind a
stochastic gravitational wave background (SGWB), which is potentially
observable by the Laser Interferometer Space Antenna. Examining the mass
distribution of PBHs and possible SGWB from the end of inflation, we may be
able to gain invaluable information about the end of inflation.
