  The next generation of ground-based gravitational-wave detectors will observe
coalescences of black holes and neutron stars throughout the cosmos, thousands
of them with exceptional fidelity. The Science Book is the result of a 3-year
effort to study the science capabilities of networks of next generation
detectors. Such networks would make it possible to address unsolved problems in
numerous areas of physics and astronomy, from Cosmology to Beyond the Standard
Model of particle physics, and how they could provide insights into workings of
strongly gravitating systems, astrophysics of compact objects and the nature of
dense matter. It is inevitable that observatories of such depth and finesse
will make new discoveries inaccessible to other windows of observation. In
addition to laying out the rich science potential of the next generation of
detectors, this report provides specific science targets in five different
areas in physics and astronomy and the sensitivity requirements to accomplish
those science goals.
  This report is the second in a six part series of reports by the GWIC 3G
Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the
Edge of the Universe, ii) The Next Generation Global Gravitational Wave
Observatory: The Science Book (this report), iii) 3G R&D: R&D for the Next
Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave
Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based
Gravitational-wave Observatories: Synergies with Other Scientific Communities,
and vi) An Exploration of Possible Governance Models for the Future Global
Gravitational-Wave Observatory Network.
