Primordial gravastar from inflation
  The dS bubbles can nucleate spontaneously during inflation, and will be
stretched by the cosmological expansion to astrophysical scale. We report on a
novel phenomenon that such a bubble might develop into a gravastar (an
ultra-compact object with a dS core) after inflation, which witnessed the
occurrence of inflation and would survive till today. It is pointed out that if
a primordial gravastar was involved in one of the LIGO/Virgo gravitational wave
(GW) events, the post-merger object could be a gravastar that will eventually
collapse into a black hole. As a result, the late-time GW ringdown waveform
will exhibit a series of "echoes" with intervals increasing with time.
