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}.
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 paper addresses a fundamental unresolved problem in inflationary cosmology: what quantum-state structure makes the one-realization classical description used in late-time cosmology physically meaningful within unitary quantum theory? We identify a structure that supports such a description, which we call Cosmological Classical Branches (CCBs). After reheating, the curvature perturbation is a collective variable rather than an independent subsystem. This shifts the classicality problem from the reduced state of the curvature perturbation itself to records of primordial information in the post-reheating matter state. When distinct matter components respond to the same primordial realization and their conditional states become distinguishable across different realizations, they carry records of the information about the curvature perturbation. A record in one component makes the realization readable, while a compatible record in a disjoint component suppresses interference accessible to operations confined to the first component, forming CCBs. Conditioning on a physical record yields a one-realization description within a CCB, in which the resolved primordial value acts as a c-number label while the matter degrees of freedom remain quantum. We test this criterion using effective models of two radiation fluids during radiation domination and of photons and cold dark matter around radiation--matter equality. We also show that a CCB typical with respect to the primordial Born measure exhibits the expected one-realization statistics: mode averages approach the primordial two-point statistics at fixed nonzero wavenumber in the large-volume limit. Overall, these results provide an operational framework at the quantum-state level connecting the primordial quantum superposition to the one-realization cosmological history assumed in standard cosmology.
Inflation is the standard scenario for the Early Universe providing
a mechanism to explain the generation of primordial density perturbations
from which all the structures we see in the universe formed by subsequent
gravitational growth (like, e.g., anisotropies of the Cosmic Microwave
Background, or the large-scale distribution of galaxies). The inflationary
seeds for structure formation are traced back to quantum fluctuations
of one or more (scalar) fields. However galaxies or cluster of galaxies
do behave as classical objects. This entails the issue of understanding
a quantum-to-classical transition in a cosmological framework. From
this point of view inflation and cosmological observations can represent
an interesting laboratory to explore some issues related to the quantum
measurement problem. This issue has been long known, still it has
been not explored in full details, and recently there has been a renowned
interest in it. The thesis will consist in a first part of review
of the most recent studies on the subject. It will then focus on some
aspects of the problem, namely how the classical to quantum transition
impacts on primordial gravitational waves which are a smoking gun
of inflation, and whether and how such a transition can leave specific
signatures in cosmological observables that go beyond the usual analyses
(such as, e.g., non-Gaussianity of the cosmological perturbations
from inflation).
Description : This work introduces a novel, model-independent diagnostic framework for quantifying quantum decoherence in primordial inflationary perturbations. The study addresses the fundamental problem of the quantum-to-classical transition in cosmology by defining a normalized persistence functional that captures the suppression of quantum coherence over time. Within an open quantum system approach, each perturbation mode is treated as an effective quantum degree of freedom interacting with an environment, leading to Gaussian suppression of off-diagonal density matrix elements. The proposed functional provides a direct and scalable measure of decoherence efficiency without relying on detailed microscopic modeling. A key feature of the framework is the introduction of a residual coherence parameter, which accounts for possible late-time deviations from complete classicalization. The work further establishes a monotonicity property for the decoherence functional and connects the persistence measure to entropy growth, offering an information-theoretic interpretation of the decoherence process. Importantly, the framework suggests a phenomenological link between residual coherence and observable non-Gaussianity in the cosmic microwave background, opening a pathway for potential observational constraints. The proposed diagnostic functional is broadly applicable across inflationary models and provides a unified tool for comparing decoherence efficiency under different environmental interactions. This work lays the foundation for future studies on quantum signatures in cosmological observables and the persistence of quantum features in the early universe. Keywords : Quantum Decoherence Inflationary Cosmology Primordial Perturbations Open Quantum Systems Quantum-to-Classical Transition Non-Gaussianity Cosmological Perturbation Theory Density Matrix Quantum Cosmology Early Universe Physics Decoherence Functional Entropy and Information Theory
According to the theory of cosmic inflation, the large scale structures observed in our Universe (galaxies, clusters of galaxies, Cosmic Background Microwave—CMB—anisotropy…) are of quantum mechanical origin. They are nothing but vacuum fluctuations, stretched to cosmological scales by the cosmic expansion and amplified by gravitational instability. At the end of inflation, these perturbations are placed in a two-mode squeezed state with the strongest squeezing ever produced in Nature (much larger than anything that can be made in the laboratory on Earth). This article studies whether astrophysical observations could unambiguously reveal this quantum origin by borrowing ideas from quantum information theory. It is argued that some of the tools needed to carry out this task have been discussed long ago by J. Bell in a, so far, largely unrecognized contribution. A detailled study of his paper and of the criticisms that have been put forward against his work is presented. Although J. Bell could not have realized it when he wrote his letter since the quantum state of cosmological perturbations was not yet fully characterized at that time, it is also shown that Cosmology and cosmic inflation represent the most interesting frameworks to apply the concepts he investigated. This confirms that cosmic inflation is not only a successful paradigm to understand the early Universe. It is also the only situation in Physics where one crucially needs General Relativity and Quantum Mechanics to derive the predictions of a theory and, where, at the same time, we have high-accuracy data to test these predictions, making inflation a playground of utmost importance to discuss foundational issues in Quantum Mechanics.
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.
A bstract Understanding how well future cosmological experiments can reconstruct the mechanism that generated primordial inhomogeneities is key to assessing the extent to which cosmology can inform fundamental physics. In this work, we apply a quantum metrology tool — the quantum Fisher information — to the squeezed quantum state describing cosmological perturbations at the end of inflation. This quantifies the ultimate precision achievable in parameter estimation, assuming ideal access to early-universe information. By comparing the quantum Fisher information to its classical counterpart — derived from measurements of the curvature perturbation power spectrum alone (homodyne measurement) — we evaluate how close current observations come to this quantum limit. Focusing on the tensor-to-scalar ratio as a case study, we find that the gap between classical and quantum Fisher information grows exponentially with the number of e -folds a mode spends outside the horizon. This suggests the existence of a highly efficient (but presently inaccessible) optimal measurement. Conversely, we show that accessing the decaying mode of inflationary perturbations is a necessary (but not sufficient) condition for exponentially improving the inference of the tensor-to-scalar ratio.