  The discovery of the gravitational-wave source GW150914 with the Advanced
LIGO detectors provides the first observational evidence for the existence of
binary black-hole systems that inspiral and merge within the age of the
Universe. Such black-hole mergers have been predicted in two main types of
formation models, involving isolated binaries in galactic fields or dynamical
interactions in young and old dense stellar environments. The measured masses
robustly demonstrate that relatively "heavy" black holes ($\gtrsim 25\,
M_\odot$) can form in nature. This discovery implies relatively weak
massive-star winds and thus the formation of GW150914 in an environment with
metallicity lower than $\sim 1/2$ of the solar value. The rate of binary
black-hole mergers inferred from the observation of GW150914 is consistent with
the higher end of rate predictions ($\gtrsim 1 \, \mathrm{Gpc}^{-3} \,
\mathrm{yr}^{-1}$) from both types of formation models. The low measured
redshift ($z \sim 0.1$) of GW150914 and the low inferred metallicity of the
stellar progenitor imply either binary black-hole formation in a low-mass
galaxy in the local Universe and a prompt merger, or formation at high redshift
with a time delay between formation and merger of several Gyr. This discovery
motivates further studies of binary-black-hole formation astrophysics. It also
has implications for future detections and studies by Advanced LIGO and
Advanced Virgo, and gravitational-wave detectors in space.
