A New Probe of Dark Matter and High-Energy Universe Using Microlensing
  We propose the existence of ultracompact minihalos as a new type of massive
compact halo object (MACHO) and suggest an observational test to discover them.
These new MACHOs are a powerful probe into the nature of dark matter and
physics in the high energy Universe. Non-Gaussian energy-density fluctuations
produced at phase transitions (e.g., QCD) or by features in the inflaton
potential can trigger primordial black hole (PBH) formation if their amplitudes
are delta > 30%. We show that a PBH accumulates over time a sufficiently
massive and compact minihalo to be able to modify or dominate its microlensing
magnification light curve. Perturbations of amplitude 0.03% < delta < 30% are
too small to form PBHs, but can nonetheless seed the growth of ultracompact
minihalos. Thus, the likelihood of ultracompact minihalos as MACHOs is greater
than that of PBHs. In addition, depending on their mass, they may be sites of
formation of the first PopIII stars. Ultracompact minihalos and PBHs produce a
microlensing light curve that can be distinguished from that of a "point-like"
object if high-quality photometric data are taken for a sufficiently long time
after the peak of the magnification event. This enables them to be detected
below the stellar-lensing "background" toward both the Magellanic Clouds and
the Galactic bulge.
