The Cyclic Shell Model of the Universe.

The Shell Universe Model 2.1 describes a hypothetical cosmological framework in which a cyclically expanding and contracting region of the universe does not necessarily contain all matter of the total universe. During earlier contraction phases, a fraction of the matter could remain permanently outside the subsequent bounce region and, over multiple cycles, form a dynamic and inhomogeneous outer region. The work examines possible physical mechanisms for this matter transfer, the long-term evolution of the outer region, its possible gravitational and cosmological backreaction on later cycles, as well as the hypothesis of a terminal cycle without a renewed global collapse. Version 2.1 is explicitly not intended as a fully developed mathematical theory. Its aim is to formulate the model assumptions, open questions, possible observational signatures, and falsification criteria as clearly as possible so that independent mathematical, numerical, and observational testing becomes possible. Qualified readers are explicitly invited to critically examine, simulate, and, where appropriate, falsify the model.