Forbidden Formation Histories: The Binary Black Hole Merger Rate Disfavors Long Delay Times

Original Abstract

The redshift evolution of the binary black hole (BBH) merger rate can be expressed as the convolution of the progenitor formation rate with the distribution of time delays between formation and merger. We show that starting with data-driven fits to the BBH merger rate as a function of redshift, deconvolving the inferred BBH merger rate into a delay time distribution and progenitor formation rate exposes physically incompatible delay time distributions. For a given evolution of the merger rate, certain delay time distributions are forbidden because their long-delay tails overpredict low redshift mergers independently of any assumption about the progenitor formation rate. Using delay-time distributions derived from the COMPAS population synthesis code in combination with the BBH merger rate inferred from GWTC-4.0, we reconstruct the physically permitted progenitor formation histories and find a steeper decline toward low redshift than the global star formation rate. We also find that the GWTC-4.0 data are in tension with formation channels that predict shallow power-law delay-time distributions ($α\gtrsim -0.7$), such as stable mass transfer. Conversely, imposing the COMPAS predictions for the delay time distribution as a prior reduces the median merger rate inferred in GWTC-4.0 by 10% at $z=1.5$, favoring a shallower merger rate evolution than the standard GWTC-4.0 inference. Additionally, we demonstrate that our method can constrain binary evolution physics by directly evaluating the compatibility of population synthesis parameters with gravitational wave observations. Our framework provides a model-independent avenue for ruling out regions of binary evolution and merger rate parameter space.