Deterministic Morphological Forecasts for Galactic Stellar Streams: A Universal Hydrodynamic Limit Signature for Upcoming LSST and Gaia Observations

The structural integrity of cold stellar streams in the Galactic halo provides a highly sensitive probe for theunderlying gravitational architecture and the presence of localized perturbers. Within the standard cosmological framework, morphological density ruptures (gaps) are modeled as the result of stochastic encounterswith invisible dark matter sub-halos. However, these impact models frequently rely on post-dictive parameter fitting, lacking the deterministic predictive capacity required for rigorous a priori falsification. In thiswork, we propose a phenomenological discriminatory framework based on a universal hydrodynamic limitof galactic dynamics. We hypothesize that the galactic vacuum behaves as a dissipative medium below acritical acceleration threshold a0 = 1.2001 × 10−10 ms−2. By integrating time-dependent orbital velocityprofiles against the background galactic potential, we derive the exact galactocentric windows where structural fatigue induces deterministic stream ruptures. We successfully calibrate this medium-induced dragmechanism against the established galactic morphologies of cold streams, anchoring our numerical densityvalidation on the raw Gaia EDR3 data of the Sloan stream (GD-1). By mapping orbital velocity profilesthrough the galactic potential, we resolve apparent spatial inconsistencies in threshold-crossing eccentrictrajectories. We provide blind, pre-registered forecasts for deterministic gap windows in five currently unresolved streams (Phlegethon, Fjörm, Slidr, Sylgr, and Ylgr) aiming at the Gaia DR4 and LSST primaryreleases. Crucially, we distinguish this model from sub-halo collision scenarios by predicting a strictlylaminar velocity dispersion (σv < 2.0kms−1) at gap boundaries, providing a definitive, timestamped nullhypothesis test for the structural necessity of dark components.