  Gravitational-wave astronomy has revolutionized humanity's view of the
universe, a revolution driven by observations that no other field can make.
This white paper describes an observatory that builds on decades of investment
by the National Science Foundation and that will drive discovery for decades to
come: Cosmic Explorer. Major discoveries in astronomy are driven by three
related improvements: better sensitivity, higher precision, and opening new
observational windows. Cosmic Explorer promises all three and will deliver an
order-of-magnitude greater sensitivity than LIGO. Cosmic Explorer will push the
gravitational-wave frontier to almost the edge of the observable universe using
technologies that have been proven by LIGO during its development.
  With the unprecedented sensitivity that only a new facility can deliver,
Cosmic Explorer will make discoveries that cannot yet be anticipated,
especially since gravitational waves are both synergistic with electromagnetic
observations and can reach into regions of the universe that electromagnetic
observations cannot explore. With Cosmic Explorer, scientists can use the
universe as a laboratory to test the laws of physics and study the nature of
matter. Cosmic Explorer allows the United States to continue its leading role
in gravitational-wave science and the international network of next-generation
observatories. With its extraordinary discovery potential, Cosmic Explorer will
deliver revolutionary observations across astronomy, physics, and cosmology
including: Black Holes and Neutron Stars Throughout Cosmic Time,
Multi-Messenger Astrophysics and Dynamics of Dense Matter, New Probes of
Extreme Astrophysics, Fundamental Physics and Precision Cosmology, Dark Matter
and the Early Universe.
