Horizonless ultracompact objects and dark matter in quadratic gravity
  We show that in quadratic gravity sufficiently light objects must be
horizonless and construct explicit analytic examples of horizonless
ultracompact objects (UCOs), which are more compact than Schwarzschild black
holes. Due to the quadratic terms, gravity becomes soft and eventually vanishes
in the high-energy limit leading to a "linearization mechanism": light objects
can be described by the linearized theory when their Schwarzschild radius is
smaller than the Compton wavelength of the new gravitational degrees of
freedom. As a result, we can analytically describe UCOs with a mass-to-radius
ratio higher than for a Schwarzschild black hole. The corresponding spacetime
is regular everywhere. We show that the Ostrogradsky instabilities can be
avoided and discuss the relation with the Higgs vacuum metastability. Due to
the lack of a horizon, light UCOs do not evaporate. Therefore, they may play
the role of dark matter. We briefly discuss their phenomenology.
