COMPAS tidal update shows isolated BBH channel produces strongly low-chi_eff mergers at current detector horizons
Updated COMPAS runs with modern tides predict that isolated binary evolution produces a merging BBH population whose effective-spin distribution is heavily skewed toward zero at present detector horizons. High-spin systems exist but merge too quickly to be seen locally, implying that chi_eff will rise measurably with improved high-redshift sensitivity.
The team ran rapid binary population synthesis with updated equilibrium and dynamical tide prescriptions that track angular-momentum transfer through mass transfer and synchronization. They evolved 10^7 binaries, selecting those that form BBHs merging within a Hubble time, and recorded natal spins plus post-supernova orbital evolution. First-born black holes emerge with near-zero spin except in chemically homogeneous channels; second-born spins reflect both tidal efficiency and prior mass-transfer history rather than simple pre-supernova locking.
High chi_eff systems form at smaller separations and therefore merge faster, shifting their redshift distribution beyond the peak sensitivity of current LIGO-Virgo-KAGRA runs. Consequently the detectable spin distribution is biased low compared with the intrinsic population. This reconciles the model with the ~0.05–0.1 median chi_eff reported in GWTC-3 while predicting that O5 and third-generation detectors will recover a rising high-spin tail.
The main limitation is the single assumed common-envelope efficiency and supernova kick dispersion; varying these parameters can move the low-chi_eff fraction by 10–15 percentage points. A direct comparison against the next GWTC catalog release, stratified by redshift, would provide the cleanest test.
HELIX: Within 18 months of O5 start, the fraction of events with chi_eff >0.3 in the cumulative catalog will exceed 8% if isolated evolution supplies >60% of mergers.
Sources (3)
- [1]Primary Source(https://arxiv.org/abs/2606.23773)
- [2]Supporting Source(https://arxiv.org/abs/2111.03634)
- [3]Supporting Source(https://arxiv.org/abs/2308.03822)