The mean pairwise velocity is a powerful cosmological probe, complimentary to density-based statistics. We have developed an accurate model the pair-conservation equation derived break pairs from the BBGKY hierarchy, and demonstrated its constraining power down to small (~1 Mpc/h) separations. However, applying this model to tracers such as halos or galaxies raises a fundamental question: merging. Biased events destroy pairs, breaking the pair conservation assumption, at the core of the model. Understanding this effect is crucial to interpreting future small-scale data from spectroscopic surveys
Bravo et al. (2026) studied this question.