CHRONOS Science Program

Abstract: Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter(CHRONOS) is a proposed next-generation ground-based gravitational-wave observatory designed to explore the sub-Hz frequency band with unprecedented sensitivity. Utilizing a cryogenic torsion-bar interferometric configuration with quantum non-demolition speed-meter readout, CHRONOS targets a frequency window that bridges space-based missions and current high-frequency ground-based detectors, opening a new frontier in gravitational-wave astronomy. The observatory will enable long-duration tracking of compact binary inspirals well before merger, significantly improving source localization, parameter estimation, and tests of general relativity. In addition to transient signals, CHRONOS is optimized to probe the stochastic gravitational-wave background (SGWB) in the sub-Hz regime, providing powerful constraints on primordial gravitational waves, inflationary tensor spectra with red or blue tilts, first-order phase transitions, cosmic strings, and other relics of high-energy physics. By connecting gravitational-wave measurements across cosmological frequency scales-from cosmic microwave background polarization to pulsar timing arrays and high-frequency interferometers-CHRONOS will contribute to a coherent reconstruction of the gravitational-wave spectrum over more than twenty orders of magnitude. Crossing critical sensitivity thresholds in the sub-Hz band, CHRONOS will establish a new pillar of gravitational-wave astronomy and cosmology, enabling transformative advances in astrophysics and fundamental physics.