The Bookended Cosmos: New Physics at the Ends of Time and a Standard Observable History

The new physics of the G2 framework is placed, by its structure, at the two ends of cosmic history and almost nowhere in between. At the early end, the symmetry breaking cascade drives an epoch of inflation and sets the initial conditions; at the late end, a cosmological constant fixed by the de Sitter horizon drives the present acceleration. Between these ends through primordial nucleosynthesis, recombination, the microwave background and the growth of structure the framework’s content is that of the Standard Model and standard cosmology, because the heavy states have all decayed and the dark sector is non-thermal. We draw this out as a single claim about the shape of cosmic history: new physics at the ends, a standard observable middle. It has definite consequences, several of them firm. Primordial nucleosynthesis is undisturbed, since no new species are present and nothing decays late. The effective number of relativistic species is the standard value, since the framework’s axion is non-thermal and adds no dark radiation. The inflationary epoch, being of the hilltop type driven by the breaking, produces a small tensor-to-scalar ratio and no large primordial gravitational wave background. And the late time acceleration is a true cosmological constant, with an equation of state fixed at minus one. The claim is falsifiable at every step: any new physics signal in the observable middle, a nucleosynthesis anomaly, dark radiation, a large tensor mode, an evolving dark energy, contradicts it. The framework predicts, unusually, that the observable universe is standard and places its novelty where it is hardest to see.