The modal framework precludes a stable zero-extension terminus on strict grounds ([§7.6.1](<7.6.1 What is Disputed.md>)). What remains strictly modal beyond that is only that the conditions sustaining expansion recur rather than originate once; that this recurrence takes a *returning, cyclic* form (and which physical mechanism realizes it) is the abductive layer. Three lines of support converge on cyclic or bouncing cosmology as the leading instantiation of that recurrence. What this slot establishes is the *consistency* of the modal prediction with the independent scientific record (convergent and, in the CMB case, contested), not a completed empirical verification.

One distinction earns its keep here. The recurrence the modal floor secures is *structured*: each cycle delivers a genuine low-entropy boundary condition through the bounce ([§7.6](<7.6 Singularity.md>), the per-cycle Past Hypothesis), not a chance dip in an equilibrium that would otherwise persist. That is what separates these models from the stochastic *eternal return* of Poincaré recurrence or Boltzmann fluctuation, and it is why the *Boltzmann-brain* problem, the objection that in a fluctuation-dominated eternal cosmos freak observers assembled by random fluctuation should vastly outnumber ordinary ones, rendering our ordered observations improbable (Andreas Albrecht, Lorenzo Sorbo, 2004); (Sean M. Carroll, 2017), does not arise on the cyclic reading. A structured bounce reseeds order as a matter of law at each pass; it does not wait on a fluctuation to supply it, and so generates no population of freak observers for ordinary ones to be swamped by.

*Independent cyclic programs.* Beyond Penrose's Conformal Cyclic Cosmology, the Steinhardt–Turok ekpyrotic model (the proposal that our universe arises from the collision of two higher-dimensional branes, the resulting cycle alternating contraction and expansion phases without ever passing through a singular state), the Brandenberger–Peter matter bounce (a non-singular cosmology in which a contracting matter-dominated phase smoothly turns over into the observed expanding phase), and Loop Quantum Cosmology (the cosmological application of Loop Quantum Gravity, in which spacetime's discreteness at the Planck scale generates a repulsive effect that replaces the Big Bang singularity with a bounce) (all discussed above), several more programs converge on cyclic or bouncing cosmology. Pre-Big-Bang string cosmology (Gasperini, M., and G. Veneziano, 1993) proposes an inflationary phase before the bang, driven by the dilaton (a scalar field native to string theory whose rolling dynamics can sustain accelerated expansion) and related to the post-bang phase by T-duality, a string-theoretic symmetry under which physics on a very small circle is equivalent to physics on a very large one. The new cyclic universe (Ijjas, A., and P. J. Steinhardt, 2019) uses a slow-contraction phase to dilute entropy and smooth inhomogeneities before the bounce, so each cycle starts nearly empty, specifically designed to evade Tolman's objection -(Richard C. Tolman, 1934) (the thermodynamic argument that on a naive cyclic cosmology each cycle would inherit the prior cycle's entropy, so cycles would have to grow indefinitely and any infinite past is precluded, the standard constraint every modern cyclic cosmology has had to address). Phantom cyclic cosmology (Baum, L., and P. H. Frampton, 2007) drives a turnaround before heat death via phantom dark energy (an equation of state w < -1, under which energy density increases with expansion rather than diluting), with entropy shed at reversal. Einstein–Cartan torsion bounce (Poplawski, N. J., 2010); (Poplawski, N. J., 2012) extends General Relativity by letting spacetime's connection carry torsion (a twist in its geometry) which couples to fermion spin and generates repulsion at extreme densities; on this picture the interior of every black hole gives birth to a new universe, realizing Cosmological Natural Selection (Smolin, L., 1992) as a fractal cycle, each black hole a bounce already underway on an adjacent branch.

Distinct theories, distinct mechanisms, same direction. The structural argument commits to none specifically; it commits to what they share.

The last variant deserves a moment: less because it is confirmed than because it reshapes the geometry of the cycle if it is right. On Poplawski's picture, every black hole is a universe being born. The cycle is no longer a single closed arc but a branching tree, every observed black hole a bounce already underway on an adjacent branch. The empirical status is open; the appeal is architectural.

*Observational probes.* The question is empirically live. CMB "Hawking point" anomalies (Daniel An, Krzysztof A. Meissner, Paweł Nurowski, Roger Penrose, 2018); (Daniel An, Krzysztof A. Meissner, Paweł Nurowski, Roger Penrose, 2020); (Meissner, K. A., 2025) consolidating the CCC case have been claimed as signatures of previous aeons; independent re-analyses find the signal weaker once ring size is marginalized over (Jow, D. L., and D. Scott, 2020). The signal is contested but the program remains active. The spectrum of primordial gravitational waves and the *non-Gaussianity* of primordial density fluctuations (the extent to which they depart from a pure Gaussian distribution) distinguish bouncing models from the standard *slow-roll inflation* picture, in which a scalar field rolls gradually down its potential and yields an approximately scale-invariant fluctuation spectrum. LiteBIRD, CMB-S4, and Planck follow-ups will discriminate within the decade. The prediction is falsifiable, and the experiments are already built.

*Cycles at every other scale.* Wherever a full arc has been observed, Nature returns its constituents to circulation: stellar lifecycles (nebula → star → supernova → nebula), galactic gas recycling, matter through black holes via accretion and Hawking evaporation. None of these is dissipation-free, and none is a literal return to a prior state: each carries entropy forward and reseeds a *different* next round through structural transformation, not informational reset. The analogy to a cosmological cycle is therefore suggestive rather than probative: these are throughput processes, and reading them as evidence for a *returning* cosmos trades on more than the word "cycle" strictly licenses.

The honest scope is narrower. What the strict modal floor forecloses is a *stable zero-extension terminus* ([§7.6.3](<7.6.3 What is Predicted.md>)); it does not foreclose an open-ended expansion that never arrives at one. A ΛCDM-style de Sitter future (asymptotic dilution toward heat death) is fully consistent with the modal preclusion, because heat death is not a zero-extension state and the approach to it never completes. The returning, cyclic form is therefore the *abductively preferred* completion of the recurrence the floor secures, given the convergence surveyed above, not a consequence forced by it. The cyclic models earn their standing as the better explanation of that recurrence, not as the only option left after the floor has spoken.


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← [7.6.3 What is Predicted](<7.6.3 What is Predicted.md>) | [7.6.5 What is Concluded](<7.6.5 What is Concluded.md>) →
