The Importance of Priors on LIGO-Virgo Parameter Estimation: the Case of
  Primordial Black Holes
  The black holes detected by current and future interferometers can have
diverse origins. Their expected mass and spin distributions depend on the
specifics of the formation mechanisms. When a physically motivated prior
distribution is used in a Bayesian inference, the parameters estimated from the
gravitational-wave data can change significantly, potentially affecting the
physical interpretation of certain gravitational-wave events and their
implications on theoretical models. As a case study we analyze primordial black
holes, which might be formed in the early universe and could comprise at least
a fraction of the dark matter. If accretion is not efficient during their
cosmic history, primordial black holes are expected to be almost non-spinning.
If accretion is efficient, massive binaries tend to be symmetrical and highly
spinning. We show that incorporating these priors can significantly change the
inferred mass ratio and effective spin of some binary black hole events,
especially those identified as high-mass, asymmetrical, or spinning by a
standard analysis using agnostic priors. For several events, the Bayes factors
are only mildly affected by the new priors, implying that it is hard to
distinguish whether merger events detected so far are of primordial or
astrophysical origin. In particular, if binaries identified by LIGO/Virgo as
strongly asymmetrical (including GW190412) are of primordial origin, their mass
ratio inferred from the data can be closer to unity. For GW190412, the latter
property is strongly affected by the inclusion of higher harmonics in the
waveform model.
