Space-based detectors could spot gravitational waves from domain walls hinted by pulsar timing arrays — but only in a narrow region
This paper asks whether a cosmological source called a domain wall — a kind of large-scale structure left over from the early Universe — that fits the new Pulsar Timing Array (PTA) data could also be seen at higher frequencies by space-based detectors. The key point is that the gravitational-wave spectrum from domain walls can have a long ultraviolet (high-frequency) tail. That tail can reach the milli-hertz band probed by space interferometers such as the Laser Interferometer Space Antenna (LISA) and Taiji, so a joint detection across very different frequencies might be possible.
To study this, the authors ran a Bayesian parameter study of domain-wall signals as seen by LISA and Taiji. Their inference problem had ten parameters: two that describe the domain-wall signal and eight “nuisance” parameters for instrumental noise and unresolved astrophysical foregrounds. They simulated detector data and analyzed it in two ways. First they did LISA-only and Taiji-only runs using 49 simulated signal injections on a grid. Then they ran joint analyses that started from PTA-supported signal regions and used 65 additional injections. The study also included a simple model of astrophysical confusion, such as the unresolved Galactic population of compact white-dwarf binaries that is expected to dominate much of the LISA band. The authors used nested sampling to map posteriors and Clough–Tocher interpolation to make heat maps over the signal-parameter plane.
Their main finding is mixed. LISA and Taiji can probe an extended region of the domain-wall parameter space whose ultraviolet tail reaches the milli-hertz band, and this remains true even when realistic astrophysical foregrounds are included. But inside the parameter region that already fits the PTA data, the part that gives a genuinely informative space-based measurement is much smaller. It lies toward the high-signal edge of the PTA-supported region. In the space-based-detector-only analyses the posterior probability is strongly degenerate across the original model parameters, although one particular combination of parameters can be reconstructed with good precision when the signal is strong enough.