Testing general relativity and probing the merger history of massive black holes with LISA

TL;DR

LISA will enable precise measurements of massive black hole mergers, testing gravity theories and understanding black hole evolution, with parameter estimation affected by spin effects and noise assumptions.

Contribution

This study updates parameter estimation models for LISA, including spin effects and alternative gravity theories, revealing their impact on measurement accuracy and cosmological applications.

Findings

Spin effects minimally impact angular resolution and distance accuracy.

Spin couplings significantly weaken bounds on scalar-tensor gravity.

LISA can measure black hole masses and distances with high precision up to high redshifts.

Abstract

Observations of binary inspirals with LISA will allow us to place bounds on alternative theories of gravity and to study the merger history of massive black holes (MBH). These possibilities rely on LISA's parameter estimation accuracy. We update previous studies of parameter estimation including non-precessional spin effects. We work both in Einstein's theory and in alternative theories of gravity of the scalar-tensor and massive-graviton types. Inclusion of non-precessional spin terms in MBH binaries has little effect on the angular resolution or on distance determination accuracy, but it degrades the estimation of the chirp mass and reduced mass by between one and two orders of magnitude. The bound on the coupling parameter of scalar-tensor gravity is significantly reduced by the presence of spin couplings, while the reduction in the graviton-mass bound is milder. LISA will measure the luminosity distance of MBHs to better than ~10% out to z~4 for a (10^6+10^6) Msun binary, and out to z~2 for a (10^7+10^7) Msun binary. The chirp mass of a MBH binary can always be determined with excellent accuracy. Ignoring spin effects, the reduced mass can be measured within ~1% out to z=10 and beyond for a (10^6+10^6) Msun binary, but only out to z~2 for a (10^7+10^7) Msun binary. Present-day MBH coalescence rate calculations indicate that most detectable events should originate at z~2-6: at these redshifts LISA can be used to measure the two black hole masses and their luminosity distance with sufficient accuracy to probe the merger history of MBHs. If the low-frequency LISA noise can only be trusted down to 10^-4 Hz, parameter estimation for MBHs (and LISA's ability to perform reliable cosmological observations) will be significantly degraded.