Achieving a cosmological reach: from Advanced LIGO to the next generation of terrestrial gravitational wave detectors

Following a decade of unprecedented success through LIGO and Virgo's observations of compact binary coalescences, gravitational wave astronomy is now recognized as a key tool in our continued efforts to understand the Universe and our place within it. Far from resting on their laurels though, the gravitational wave community is forging ahead with major plans for the future. The proposed "ultimate terrestrial gravitational wave detector facility" Cosmic Explorer recently received a boost with significant funding from the NSF to proceed with a conceptual design. This paper surveys the current state-of-the-art ground-based gravitational wave detector facilities, and their planned near-term upgrades. After motivating the next-generation Cosmic Explorer concept with a discussion of the key science targets, this paper describes some of the unique technical challenges it faces, including a focus on the ongoing optical design of Cosmic Explorer's 40 km-scale laser interferometers.