Spectral sirens cosmology from binary black holes populations with sharper mass features

Abstract

Spectral-sirens inference enables the extraction of cosmological parameters from gravitational-wave data alone, without electromagnetic counterparts or galaxy catalogs. We introduce new parametric mass functions for the binary black hole population that capture significant structure across the mass spectrum and are moderately favoured by Bayesian evidence over simpler models. Analysing the latest gravitational-wave transient catalog, GWTC-4.0, we show that powerlaws-only population models constrain the Hubble constant to $23\%$ precision, $H_0 = 53.3^{+14.0}_{-10.8} ~\rm km \,s^{-1} \,Mpc^{-1}$ at $68\%$ confidence level. This represents a $\sim 50\%$ improvement over the corresponding binary black hole-only analysis by the LIGO-Virgo-KAGRA collaboration, achieving precision comparable to their joint analyses including neutron stars and galaxy catalogs. We further test alternative cosmological models, establishing competitive constraints on modified gravitational-wave propagation, while bounds on the dark energy equation-of-state parameters remain uninformative. Projecting to future O5 observing run, we forecast substantial improvements in $H_0$ and modified propagation parameters with larger datasets at higher redshifts. Our results highlight the strong interplay between the black hole mass distribution and inferred cosmology, underscoring the need for suitable population models to fully exploit gravitational-wave data.

Figures (10)

• Figure 1: Sketches of the approximate shape of the three different source-frame primary mass distribution parametrisations considered in this work. In blue (left) and purple (center) the new models --- respectively 3sPL and 4sPL --- we introduce. In green (right), the fiducial model sPL2G for comparison.
• Figure 2: Marginal PPD for the sPL2G (green), 3sPL (blue) and 4sPL (purple) population models, from analyses inferring $H_0$ and $\Omega_m$ parameters of a FlatLCDM cosmological model. The thick lines denote the median value, while the shaded areas cover the 90% CI. For comparison, the gray curve shows the LVK analysis using the MultiPeak model.
• Figure 3: GWTC constraints on the Hubble constant $H_0$ with the three population models sPL2G (green), 3sPL (blue), 4sPL (purple) considered here. Solid (resp. dashed) curves denote marginalised posterior distribution on $H_0$ for analyses where $\Omega_m$ is inferred (resp. fixed). For comparison, the grey dashed (resp. dash-dotted) line represents the LVK result LIGOScientific:2025jau with the MultiPeak (resp. FullPop and all CBC events) model and fixed $\Omega_m$. The black and red shaded areas identify the 68% CI constraints on $H_0$ inferred from the CMB anisotropies by Planck Planck:2018vyg and in the local Universe by SH0ES Riess:2021jrx.
• Figure 4: GWTC constraints on wCDM and w0waCDM cosmological models obtained with the sPL2G (green), 3sPL (blue) and 4sPL (purple) population models. Posteriors on $\Omega_m$ are not informative and do not correlate with $w_0$ and $w_a$, hence not displayed. The solid black line denote the $\Lambda$CDM value of the beyond-$\Lambda$CDM parameters. Contours indicate 90% credible area.
• Figure 5: GWTC constraints and correlations on parameters of modified GW propagation models Xi0(a) and cM(b), obtained with the sPL2G (green), 3sPL (blue) and 4sPL (purple) population models. Solid (resp. dashed) lines posteriors denote results of analyses with inferred (resp. fixed) $H_0$. The solid black line denote the GR value of beyond-GR parameters. The gray dotted posteriors denote LVK results from LIGOScientific:2025jau with the FullPop model (see text). Contours indicate 90% credible area.