Stochastic Gravitational Wave Background

The stochastic gravitational wave background (SGWB) refers specifically to the unresolved superposition of gravitational wave signals from cosmological sources that individually fall below detection threshold, producing a quasi-isotropic, quasi-Gaussian noise-like signal. Ground-based detectors sensitive in the 10–1000 Hz band seek the SGWB from compact binary mergers, while pulsar timing arrays at nanohertz frequencies probe the SGWB from SMBH binaries and possibly primordial sources. The distinction between the astrophysical SGWB — sourced by stellar-remnant populations — and a cosmological SGWB — sourced by processes in the early universe such as inflation, phase transitions, or cosmic strings — has profound theoretical significance. ΛCDM predicts a specific astrophysical SGWB amplitude from known merger populations, with any excess attributed to cosmological primordial processes requiring new physics.

Successive Collision Theory unifies the astrophysical and cosmological SGWB under a single framework rooted in established GR and the pre-existing matter populations. In SCT, there is no sharp distinction between an astrophysical background from stellar remnants and a primordial background from the collision epoch, because the pre-existing compact objects from the two colliding pockets were themselves products of prior stellar evolution in the eternal infinite universe. The SGWB has contributions from: (1) standard post-collision stellar evolution producing compact binaries on standard ΛCDM timescales; (2) pre-existing compact binaries from the collision epoch with shorter effective delay times; (3) the gravitational wave emission from the collision event itself, which deposited energy into the metric at all frequencies set by the pocket dimensions and collision velocity; and (4) the ongoing tensor mesh dissipation from frame orbital decay, contributing a slowly evolving low-frequency component.

The isotropy of the SGWB, often cited as evidence against an anisotropic collision origin, is naturally reproduced in SCT through the simultaneous thermalization mechanism. When the collision front swept the entire overlap volume faster than any internal signal, the gravitational wave emission from the collision event was isotropized by the same process that isotropized the CMB. Post-collision gravitational wave emission from compact binaries distributed throughout the thermalized debris is isotropic by the same statistical argument that makes the astrophysical SGWB isotropic in ΛCDM — source number counts average out angular fluctuations below the Poisson level. SCT predicts a mild anisotropy in the SGWB at large angular scales aligned with the collision axis, analogous to the CMB quadrupole alignment, at an amplitude of order the fractional angular momentum asymmetry in the collision geometry.