A Distant Origin For Magnified LIGO/Virgo Black Holes Implied By Binary Component Masses

TL;DR

This paper proposes that the high masses of black hole binaries detected by LIGO/Virgo are due to gravitational lensing magnification of distant events at redshift around 2, explaining the mass distribution and dispersion.

Contribution

It introduces a lensing-based explanation for the observed black hole binary mass distribution, linking high apparent masses to distant, magnified sources at z>1.

Findings

High apparent masses are consistent with lensing magnification at z~2.

Most high-mass events are likely distant, intrinsically lower-mass black holes.

The model explains the mass dispersion and absence of high-spin signals.

Abstract

The primary and secondary masses of the binary black holes (BBH) reported by LIGO/Virgo are correlated with a narrow dispersion that appears to increase in proportion to mass. The mean binary mass ratio $1.45\pm0.07$ we show is consistent with pairs drawn randomly from the mass distribution of black holes in our Galaxy. However, BBH masses are concentrated around $\simeq 30M_\odot$, whereas black holes in our Galaxy peak at $\simeq 10M_\odot$. This mass difference can be reconciled by gravitational lensing magnification which allows distant events to be detected with typically $z\simeq 2$, so the waveform is reduced in frequency by $1+z$, and hence the measured chirp masses appear 3 times larger than their intrinsic values. This redshift enhancement also accounts for the dispersion of primary and secondary masses, both of which should increase as $1+z$, thereby appearing to scale with mass, in agreement with the data. Thus the BBH component masses provide independent support for lensing, implying most high chirp mass events have intrinsic masses like the stellar mass black holes in our Galaxy, coalescing at $z>1$, with only two low mass BBH detections, of $\simeq 10M_\odot$ as expected for unlensed events in the local Universe, $z\simeq 0.1$. This lensing solution requires a rapidly declining BBH event rate below $z<1$, which together with the observed absence of BBH spin suggests most events originate within young globular clusters at $z>1$, via efficient binary capture of stellar mass black holes with randomly oriented spins.