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arXiv · 2609.35317

LISA bright sirens as a tiebreaker among cosmologies in tension

M. F. A. Medeiros·F. C. Carvalho·L. L. Sales
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Abstract

Gravitational waves from massive black hole binaries detected by LISA will deliver luminosity distances without any astrophysical calibration, offering a probe of the expansion history independent of the cosmic distance ladder. Rather than asking how tightly LISA will constrain a single fiducial cosmology, we ask which of several mutually discrepant present-day cosmologies a future LISA dataset would render most self-consistent. We build mock catalogues of LISA bright sirens under three fiducial cosmologies drawn from datasets that participate in the current $H_0$ discrepancy (Planck 2018, Union3 and Pantheon+SH0ES), with a self-consistent error budget from instrumental noise, weak lensing and host peculiar velocities. We compare the posterior from the simulated LISA data alone with that of a real-data combination of Type Ia supernovae, DESI baryon acoustic oscillations, cosmic chronometers and the compressed Planck 2018 CMB, and quantify their internal tension in the $(H_0,Ω_m)$ plane, crossing each fiducial with three SN Ia compilations. The fiducial drives the tension, from $2.08σ$ to well beyond $5σ$, while the supernova compilation changes it by at most $1.2σ$. The discriminating power is strongly asymmetric: a Pantheon+SH0ES universe is inconsistent with every background we test, at a significance far in excess of $5σ$ and still at $3$-$5σ$ for the $10$-$20$ events expected from recent population models, whereas the Planck 2018 and Union3 fiducials, which differ by only $0.24$ km/s/Mpc in $H_0$, cannot be separated by any catalogue size. LISA bright sirens are therefore a sharp arbiter of the Cepheid-calibrated scenario and, on this error budget, silent between the other two.

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M. F. A. Medeiros, F. C. Carvalho, L. L. Sales. 2026-09-28. LISA bright sirens as a tiebreaker among cosmologies in tension. https://arxiv.org/abs/2609.35317

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