Are black hole spins truly near-zero?

Abstract

The fourth gravitational-wave transient catalog, GWTC-4.0, reports 153 binary black hole mergers with false-alarm rates $<1,\mathrm{yr}^{-1}$. Chirp masses are typically measured well, with the smallest fractional uncertainty being $2%$ at the $90%$ credible level. Spins, on the other hand, are poorly constrained: the median of the best-measured spin component of the population, the effective spin, is $\chi_{\rm eff}=0.04$, with a typical $90%$ credible uncertainty of $\Delta\chi_{\rm eff}=0.44$. The large majority -- $90%$ of the observed black holes -- are consistent with spin magnitudes $\chi<0.57$ and are weakly aligned with the orbits. At $90%$ credibility, the peaks of the inferred posteriors for spin magnitude are found to lie in the range $0.01$--$0.23$. We show that this ``near-zero spins'' conclusion may be prior-driven, and that uniform-in-magnitude spin priors lead to under-exploration of the moderate-to-high spin region of parameter space. Adopting a physically agnostic prior that is uniform in spin-vector configuration space (i.e., spin states uniform within a unit sphere) yields similar constraints on $\chi_{\rm eff}$, but substantially different spin-magnitude inferences than GWTC-4.0. The resulting shift in spins directly impacts tests of general relativity, constraints on near-extremal Kerr remnants, and astrophysical conclusions, including diagnostics of formation channels and hierarchical growth. In short, the data do not require vanishing spins -- the prior does, and accounting for this is essential for robust GR tests and population inferences.