Astrophysics & Particle Physics-APPC 2026

The stochastic gravitational-wave background (SGWB) is an isotropic, long-lived gravitational wave signal created by the superposition of many unresolved sources across the universe. Unlike specific gravitational wave events from black hole or neutron star mergers, the SGWB is a cosmic hum that holds information about the history and structure of the universe.Sources of the SGWB may be either astrophysical or cosmological. Astrophysical sources consist of populations of binary black holes, neutron star mergers, and supernovae, whose gravitational waves superimpose to form a permanent background. Cosmological sources can come from primordial gravitational waves produced during inflation, early universe phase transitions, or cosmic strings. Observations of the SGWB enable astronomers to explore physics unavailable from electromagnetic observations.Detection of the SGWB demands highly sensitive devices that can measure very tiny spacetime distortions.

Ground-based interferometers such as LIGO, Virgo, and KAGRA are sensitive to higher-frequency components, whereas pulsar timing arrays (PTAs) measure low-frequency, nanohertz gravitational waves. Upcoming space-based detectors such as LISA (Laser Interferometer Space Antenna) will probe sensitivity to the millihertz regime, filling the gap between ground-based and PTA observations.The stochastic gravitational-wave background is a prime target in current astrophysics and cosmology. By studying this faint but ubiquitous signal, scientists can reveal secrets about the origin and evolution of the universe, the distribution of compact objects, and the laws of fundamental physics, opening a window on the hidden cosmic world.