Testing the Quasar Hubble Diagram with LISA Standard Sirens

TL;DR

This paper demonstrates that LISA's gravitational wave observations of massive black hole mergers can effectively test high-redshift quasar-based deviations from standard cosmology, providing precise measurements of the universe's expansion.

Contribution

It introduces a method to use LISA standard sirens to validate quasar-based high-redshift cosmological claims and assesses the number of events needed for strong model differentiation.

Findings

4 standard sirens can distinguish models in 50% of cases

14 standard sirens can distinguish models in 95% of cases

15 events can measure H0 with 5% precision

Abstract

Quasars have recently been used as an absolute distance indicator, extending the Hubble diagram to high redshift to reveal a deviation from the expansion history predicted for the standard, $\Lambda$CDM cosmology. Here we show that the Laser Interferometer Space Antenna (LISA) will efficiently test this claim with standard sirens at high redshift, defined by the coincident gravitational wave (GW) and electromagnetic (EM) observations of the merger of massive black hole binaries (MBHBs). Assuming a fiducial $\Lambda$CDM cosmology for generating mock standard siren datasets, the evidence for the $\Lambda$CDM model with respect to an alternative model inferred from quasar data is investigated. By simulating many realizations of possible future LISA observations, we find that for $50\%$ of these realizations (median result) 4 MBHB standard siren measurements will suffice to strongly differentiate between the two models, while 14 standard sirens will yield a similar result in $95\%$ of the realizations. In addition, we investigate the measurement precision of cosmological parameters as a function of the number of observed LISA MBHB standard sirens, finding that 15 events will on average achieve a relative precision of 5\% for $H_0$, reducing to 3\% and 2\% with 25 and 40 events, respectively. Our investigation clearly highlights the potential of LISA as a cosmological probe able to accurately map the expansion of the universe at $z\gtrsim 2$, and as a tool to cross-check and cross-validate cosmological EM measurements with complementary GW observations.