Space-Based Sirens: Massive Black Hole Mergers as High-Redshift Distance Indicators

This paper asks a single question and computes its answer. What luminosity-distance precision can a space-based detector obtain for massive black hole mergers between redshift one and eight, and what does that precision buy for the distance-duality and propagation tests? Working in a flat LambdaCDM background and carrying the bounce-frame density ceiling explicitly — the Aldred Limit, rho_A = rho_Pl (m/m_Pl)^2/(8 pi C), with the neutron branch at 9.72e+54 g per cubic centimetre — the calculation gives the headline values e1 = 3.6, e3 = 7.6, e8 = 14.6, rhoA = 9.72e+54. Per-event distance precision stays at the few per cent level to redshift three and degrades to roughly ten per cent by redshift eight, dominated by lensing. Even so, a handful of such events probes redshifts no other siren class reaches. Every number reported here comes from the deposited computation script, which states its assumptions in full. This study is simulated and computed from stated assumptions and published catalogue values; it is not a measurement.