Pulsar timing arrays have reported common-spectrum, low-frequency timing residuals consistent with a nanohertz stochastic gravitational-wave background, but the precise origin of this excess power remains ambiguous, with supermassive black hole binaries, cosmic strings, phase transitions, or modified early-universe dynamics all viable within current uncertainties (Shannon et al. 2015; NANOGrav Collaboration 2023). In ΛCDM, matching both the measured amplitude and spectral slope without overproducing mergers, violating other gravitational-wave bounds, or invoking fine-tuned astrophysical or exotic new-physics populations is challenging, so interpreting the PTA nanohertz excess in a self-consistent way is still an open tension for the standard cosmological model (Sesana 2013; Ben-Dayan 2025).