  This Horizon Study describes a next-generation ground-based
gravitational-wave observatory: Cosmic Explorer. With ten times the sensitivity
of Advanced LIGO, Cosmic Explorer will push gravitational-wave astronomy
towards the edge of the observable universe ($z \sim 100$). The goals of this
Horizon Study are to describe and evaluate design concepts for Cosmic Explorer;
to plan for the United States' leadership in gravitational-wave astronomy; and
to envisage the role of Cosmic Explorer in the international effort to build a
"Third-Generation" (3G) observatory network that will make discoveries
transformative across astronomy, physics, and cosmology.
