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Abstract

Gravitational-wave astronomy has revolutionized humanity's view of the
universe. Investment in the field has rewarded the scientific community with
the first direct detection of a binary black hole merger and the multimessenger
observation of a neutron-star merger. Each of these was a watershed moment in
astronomy, made possible because gravitational waves reveal the cosmos in a way
that no other probe can. Since the first detection of gravitational waves in
2015, the National Science Foundation's LIGO and its partner observatory, the
European Union's Virgo, have detected over fifty binary black hole mergers and
a second neutron star merger -- a rate of discovery that has amazed even the
most optimistic scientists.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 the
gravitational-wave astronomy towards the edge of the observable universe ($z
\sim 100$). This Horizon Study presents the science objective for Cosmic
Explorer, and describes and evaluates its design concepts for. Cosmic Explorer
will continue the United States' leadership in gravitational-wave astronomy in
the international effort to build a "Third-Generation" (3G) observatory network
that will make discoveries transformative across astronomy, physics, and
cosmology.