An expanding universe creates particles, and the created field modes form squeezed quantum states. This note studies the reverse perspective: if the background geometry itself exists in a superposition of different expansion histories, the particles created by those histories become records that measure and decohere the background. In an exactly solvable cosmological model with one field mode above a superposed expansion background, each expansion branch produces a different amount of squeezing. The difference between these squeezed states provides a which-history record. The paper proves an exact self-measurement law: the visibility of the expansion-history superposition is determined by the overlap of the branch-dependent squeezed vacua, with the closed form given by the squeezing difference. The record–decoherence relation satisfies exact Englert duality, showing that differential particle creation is precisely the information that classicalizes the background. The multimode extension gives a product law: the background decoheres exponentially with the number of modes excited by the expansion history. Thus, perturbations do not merely become classical because of the background; they also act as records that classicalize the expansion history that created them. The paper further analyzes the boundary between coherent and classical behavior. Nonadiabatic expansion histories create distinguishable particles and write records, while histories connected by adiabatic evolution remain coherent because they create essentially no particles. The work is limited to solvable profiles, single-mode and multimode toy models, no backreaction, and comparisons performed in a common late-time basis. It does not claim a full treatment of quantum gravity or complete cosmological dynamics. All results are based on pre-registered predictions, computational verification, and reproducibility materials.
An expanding universe creates particles, and the created field modes form squeezed quantum states. This note studies the reverse perspective: if the background geometry itself exists in a superposition of different expansion histories, the particles created by those histories become records that measure and decohere the background. In an exactly solvable cosmological model with one field mode above a superposed expansion background, each expansion branch produces a different amount of squeezing. The difference between these squeezed states provides a which-history record. The paper proves an exact self-measurement law: the visibility of the expansion-history superposition is determined by the overlap of the branch-dependent squeezed vacua, with the closed form given by the squeezing difference. The record–decoherence relation satisfies exact Englert duality, showing that differential particle creation is precisely the information that classicalizes the background. The multimode extension gives a product law: the background decoheres exponentially with the number of modes excited by the expansion history. Thus, perturbations do not merely become classical because of the background; they also act as records that classicalize the expansion history that created them. The paper further analyzes the boundary between coherent and classical behavior. Nonadiabatic expansion histories create distinguishable particles and write records, while histories connected by adiabatic evolution remain coherent because they create essentially no particles. The work is limited to solvable profiles, single-mode and multimode toy models, no backreaction, and comparisons performed in a common late-time basis. It does not claim a full treatment of quantum gravity or complete cosmological dynamics. All results are based on pre-registered predictions, computational verification, and reproducibility materials.
This note develops a unified calculus for classicalization in exactly solvable quantum cosmological models. Previous studies in the series showed classical behavior emerging through different types of records: inserted registers, self-generated kinematic records, and created particles. This paper proves that these mechanisms share the same mathematical structure: classicalization is determined by the overlap of branch-conditional record states. The paper proves that independent record channels factorize exactly, showing that different sources of classicalization combine into a single multiplicative visibility law. The main new result studies entangled records. When a record mode is entangled with an additional spectator system, two effects occur simultaneously: the loss of coherence becomes faster, while the locally accessible record becomes weaker. The missing which-history information is stored in correlations with the spectator system rather than in the local record alone. Thus, decoherence can exceed the amount of information available to any individual subsystem, while the full information balance is preserved nonlocally. The paper derives the exact coefficient controlling the short-time decoherence rate of entangled records. This coefficient emerges from the variance of the relevant generator in the reduced record state, extending the variance-based structure developed throughout the framework. The resulting picture is: records are generated by the dynamics, their overlaps determine classicalization, independent records compose, and entanglement can redistribute information into nonlocal correlations. The work is limited to Gaussian states and channels, a single spectator system, and sudden squeezing profiles. All results are based on pre-registered predictions, computational verification, and reproducibility materials.
This note studies how a universe can generate its own classical records without any external environment or inserted measurement registers. In a solvable anisotropic quantum cosmology model with a Bianchi-I-lite constraint, the two kernel branches correspond to opposite relational arrows of time. The anisotropy degree of freedom evolves differently in the two branches, and this internal motion dynamically writes a record of which relational arrow the universe follows. The paper proves an exact self-decoherence law: the visibility between the two arrow branches is given by the characteristic function of the universe’s own clock-energy distribution. The decoherence rate is determined by the energy variance, and the record–visibility relation satisfies the exact Englert duality. Thus, the same mathematical structure governing information deficits, records, and revival phenomena in the companion framework appears here as a self-generated cosmological decoherence mechanism. The results establish a complete chain: constraint → relational motion → internal records → classicality. Unlike previous exact emergence-of-classicality models in the series, where matter records were introduced as fixed registers, this model generates the records dynamically from the universe’s own degrees of freedom. The universe acts as its own environment. The paper also identifies the boundaries of the result: dispersion returns because of the nonlinear spectrum, and finite-spectrum recurrences appear beyond the declared window. These effects are explicitly analyzed and distinguished from the continuum behavior. The work is limited to a solvable anisotropic minisuperspace model with one anisotropy degree of freedom and a fixed volume-momentum sector. It does not claim a result for full Bianchi cosmology or complete Einstein dynamics. All results are based on pre-registered predictions, computational verification, and reproducibility materials.
We analyze branch decoherence in inflationary quantum cosmology by computing reduced density matrices and branch-overlap factors for long-wavelength perturbations. The Hartle-Hawking no-boundary state is real in the semiclassical regime and contains both expanding and contracting WKB components, whereas the tunneling state is selected as an outgoing complex WKB branch; expanding-contracting decoherence is therefore central for the former and mainly diagnostic for the latter. Using the influence-functional formalism, we derive the noise kernel for a light spectator environment and evaluate decoherence under horizon-based and EFT-motivated coarse grainings. We then compute the single-mode branch overlap directly from the Bunch-Davies mode functions, obtaining ∣Dk(z)∣=[z2/(z2+1)]1/4 in the massless limit and ∣Dk(z)∣∼zν on superhorizon scales for massive fields, where z=−kη is the dimensionless wavenumber with η the conformal time. In the massless case, the accumulated geometric branch functional is evaluated in closed form, with a leading cutoff-sensitive phase-space term and a universal subleading contribution. The calculation provides an explicit quantitative bridge between quantum-cosmological boundary conditions, inflationary squeezing, and the emergence of effectively classical cosmological histories.
Inflation stretches quantum vacuum fluctuations into the density ripples we later see imprinted on the cosmic microwave background, yet the initial state is a pure, highly squeezed quantum state. The usual explanation for why those ripples look classical appeals to decoherence and the fading of a "decaying mode." This paper argues that the sharper statement is objectivity: the ripples become a fact for all observers, in the precise sense of quantum Darwinism, exactly when the growing perturbation is redundantly recorded across a vast environment of short-wavelength and gravitational-wave modes. The paper models a single super-horizon mode and its environment exactly. It proves that, once the mode crosses the horizon, its amplitude is recorded so redundantly that even a tiny fragment of the environment already fixes it, while the number of independent records grows without bound as the universe expands. Horizon crossing is therefore a Darwinism–encoding transition: an information measure rises from zero to one bit as the mode freezes. And the environment singles out one objective observable — the curvature amplitude — while its conjugate is only globally encoded, readable by no bounded fragment. The results are closed-form, and their exact algebraic cores are verified by proof-checking software. The record strength is not a free parameter: it is fixed by the mode's decoherence, computed from gravitational theory, and the leftover quantum coherence falls off exponentially with the redundancy. In short: the sky is classical not merely because it decohered, but because horizon crossing broadcast it — copying each primordial fluctuation until it became a fact for everyone.
Inflationary cosmology faces a fundamental gap between primordial quantum fluctuations and the classical cosmic history inferred from observations. Although the primordial quantum state is a superposition of field configurations, standard cosmology assumes one definite realization of curvature perturbations and uses classical perturbation theory to describe the resulting gravitational potentials, CMB anisotropies, and large-scale structure. Squeezing and decoherence of primordial fluctuations alone do not explain why this one-realization description is physically meaningful. We address this issue through Cosmological Classical Branches (CCBs), using the quantum-information-theoretic notion of physical records. In CCBs, distinct late-time matter components carry compatible records of the same primordial configuration. A record in one component makes that configuration readable, while a disjoint compatible record suppresses interference accessible to operations on the first. Each branch thus admits a common c-number primordial label at finite resolution, while the global quantum superposition persists. This Letter summarizes the framework developed in Ref.~\cite{Kuroda2026CCB}.
This note extends the frame-deficit dictionary into an exactly solvable loop quantum cosmology structure, studying how information deficits, records, and entropic bounds behave through a quantum cosmological bounce. The model realizes the solvable loop quantum cosmology dynamics of a massless scalar universe on a lattice, where contraction, a positive-volume bounce, and expansion form a single unitary relational evolution. The paper establishes four results. First, the bounce is exactly characterized: every wavepacket universe contracts, reaches a minimum positive volume, and re-expands, with the expected symmetric bounce behavior of the solvable loop quantum cosmology model. Second, information is preserved across the bounce: relational evolution remains unitary, the pre-bounce state is recoverable, and the frame deficit remains invariant before, during, and after the bounce. Third, a quantum superposition of universes with different bounce times is studied. Matter records decohere the question of when the bounce occurred, and the interference between different bounce histories decreases exactly according to the record overlap. Fourth, the deficit–classicality entropic bound remains valid throughout the entire bounce evolution. The result extends the frame-deficit framework from Wheeler–DeWitt quantum cosmology to a nonsingular bouncing universe, showing that the information structure survives the transition replacing the classical singularity. The paper also distinguishes established loop quantum cosmology results from the new contribution: the bounce dynamics and cosmic recall belong to the existing solvable LQC framework, while the new results concern the information dictionary, the invariant deficit structure, and the exact bounce-time decoherence law. The scope is limited to the exactly solvable isotropic lattice realization of loop quantum cosmology. No claim is made for full Einstein dynamics, anisotropic models, or a complete theory of quantum gravity. All results are based on pre-registered predictions, computational verification, and reproducibility materials.