The Cosmic MeV Neutrino Background as a Laboratory for Black Hole
  Formation
  Calculations of the cosmic rate of core collapses, and the associated
neutrino flux, commonly assume that a fixed fraction of massive stars collapse
to black holes. We argue that recent results suggest that this fraction instead
increases with redshift. With relatively more stars vanishing as "unnovae" in
the distant universe, the detectability of the cosmic MeV neutrino background
is improved due to their hotter neutrino spectrum, and expectations for
supernova surveys are reduced. We conclude that neutrino detectors, after the
flux from normal SNe is isolated via either improved modeling or the next
Galactic SN, can probe the conditions and history of black hole formation.
