Strong lensing of gravitational waves as seen by LISA

TL;DR

This paper explores the potential of the LISA mission to detect strong gravitational lensing of gravitational waves from massive black hole mergers, which could provide insights into cosmology and gravity theories.

Contribution

It analyzes the probabilities and implications of observing multiple lensed gravitational wave events with LISA, considering various formation models and lensing effects.

Findings

Up to 4 multiple lensed events expected in 5 years for optimistic models.

Significant chances of detection even with conservative seed models.

Lensing amplification can make faint signals detectable.

Abstract

We discuss strong gravitational lensing of gravitational waves from merging of massive black hole binaries in the context of the LISA mission. Detection of multiple events would provide invaluable information on competing theories of gravity, evolution and formation of structures and, with complementary observations, constraints on H_0 and other cosmological parameters. Most of the optical depth for lensing is provided by intervening massive galactic halos, for which wave optics effects are negligible. Probabilities to observe multiple events are sizable for a broad range of formation histories. For the most optimistic models, up to 4 multiple events with a signal to noise ratio >= 8 are expected in a 5-year mission. Chances are significant even for conservative models with either light (<= 60%) or heavy (<= 40%) seeds. Due to lensing amplification, some intrinsically too faint signals are brought over threshold (<= 2 per year).