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Orbital eccentricity is a key tracer of the astrophysical origins of compact binaries; yet it remains absent from routine LIGO-Virgo-KAGRA analyses, in part because of the prohibitive computational cost of generating eccentric template waveforms. The complicated morphology of these waveforms due to the eccentric orbital timescale variations makes it difficult to construct their accurate and efficient surrogate models, especially for waveforms long enough to comprehensively cover the sensitivity bands of current ground-based gravitational-wave detectors. We present a novel and scalable surrogate building technique that makes surrogate modeling of long-duration eccentric binary black hole waveforms both feasible and highly efficient. The technique aims to simplify the harmonic content of intermediate eccentric waveform data pieces by modeling them in terms of an angular orbital element called the mean anomaly, instead of time. We show that this parametrization yields much more compressed surrogates than the standard time-based parametrizations. We also significantly simplify variations in waveform data pieces across the parameter space by expressing them in terms of the instantaneous orbital eccentricity and mean anomaly to ease their parametric fitting. Building on these developments, we construct InspiralESIGMASur: a <inline-formula><mml:math><mml:mrow><mml:mn>2.77</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msup><mml:mi>M</mml:mi></mml:mrow></mml:math></inline-formula> (850─1250 orbits) long nonspinning surrogate for the inspiral-only eccentric waveform model InspiralESIGMA [K. Paul , Phys. Rev. D 111, 084074 (2025)PRVDAQ2470-001010.1103/PhysRevD.111.084074]. The methods presented in this Letter make it feasible to build long-duration eccentric surrogates for current as well as future third-generation gravitational-wave detectors.

Dense neutrino systems, which display collectivity mediated by weak interaction, have both parallels and dissimilarities with kinetic systems with collective interactions mediated by other fundamental forces. We first identify analogues in fast flavor conversion of some time-honored nonlinear phenomena in plasmas and self-gravitating systems, focusing in particular on nonlinear Landau damping and collisionless equilibria. We then demonstrate the previously unexplored phenomenon of flavor-wave synchronization, which to our knowledge is a distinguishing characteristic of dense neutrino systems. The phenomena we exhibit are likely important pieces of the unsolved puzzle of neutrino oscillations in core-collapse supernovae and neutron star mergers.