Nonsingular bounce
A bounce replaces a classical singular transition with contraction reaching a minimum scale before re-expansion. In effective loop quantum cosmology one often encounters H² = (8πG/3)ρ(1−ρ/ρ_c), which forces H to vanish at a critical density. The central questions are whether the effective description remains reliable and whether perturbations pass consistently through the bounce.
Ekpyrotic and cyclic histories
Ekpyrotic scenarios use a slowly contracting phase with a stiff effective equation of state to suppress anisotropy and generate a smooth background. Cyclic models attempt repeated phases of contraction and expansion. They must control entropy production, instabilities and the transition between phases while still explaining the observed primordial perturbations.
Quantum cosmology
Quantum cosmology asks whether the origin problem should be described by a wave function and boundary condition rather than by an earlier classical epoch. In minisuperspace, a Wheeler–DeWitt equation takes a schematic form such as [−d²/da² + U(a)]Ψ(a)=0. No-boundary and tunnelling proposals impose different conditions on Ψ.
How to compare fairly
Each model should be forced through the same sequence: define degrees of freedom; evolve the background; calculate perturbations; reach a hot radiation-dominated state; predict observables; compare with data; identify assumptions that cannot yet be tested. A model is not stronger merely because it avoids the word “beginning.”
Why the question remains open
Current observations constrain classes of early-universe physics but do not uniquely reconstruct the pre-hot-Big-Bang history. The most useful competition is therefore between explicit predictive models, not between broad labels.