This short note presents a preliminary conceptual analysis of a cosmological model in which the Universe is described as the three-dimensional surface of an expanding four-dimensional bubble. The global equilibrium and expansion of this hyperspherical geometry are shown to be determined primarily by the energy density of radiation, assumed to act as an effective surface tension. Matter, being spatially discontinuous and energetically subdominant at early epochs, contributes only as a perturbative element and does not influence the large-scale curvature or expansion law. This framework offers an operational entry point to a model whose full formulation is developed in complementary work.