Cosmic Purity Lost: Perturbative and Resummed Late-Time Inflationary Decoherence (2403.12240v3)

Abstract: We compute the rate with which unobserved fields decohere other fields to which they couple, both in flat space and in de Sitter space, for spectator scalar fields prepared in their standard adiabatic vacuum. The process is very efficient in de Sitter space once the modes in question pass outside the Hubble scale, displaying the tell-tale phenomenon of secular growth that indicates the breakdown of perturbative methods on a time scale parameterically long compared with the Hubble time. We show how to match the perturbative evolution valid at early times onto a late-time Lindblad evolution whose domain of validity extends to much later times, thereby allowing a reliable resummation of the perturbative result beyond the perturbative regime. Super-Hubble modes turn out to be dominantly decohered by unobserved modes that are themselves also super-Hubble. Although our calculation is done for spectator fields, if applied to curvature perturbations during inflation our observations here could close a potential loophole in recent calculations of the late-time purity of the observable primordial fluctuations.

• The paper demonstrates that standard perturbative methods break down for super-Hubble modes, necessitating a resummed Lindblad approach.
• It applies analyses in both flat and de Sitter spaces to quantify the decoherence of spectator scalar fields during inflation.
• The study highlights that super-Hubble interactions drive decoherence, refining models of the quantum-to-classical transition in cosmology.

Essay on "Cosmic Purity Lost: Perturbative and Resummed Late-Time Inflationary Decoherence"

The paper "Cosmic Purity Lost: Perturbative and Resummed Late-Time Inflationary Decoherence," is a comprehensive paper focused on understanding the decoherence process of quantum-generated density fluctuations during inflation, particularly looking at how these fluctuations evolve and potentially lose their quantum coherence over time. The authors approach this problem through both perturbative and resummed methodologies, offering insights relevant for cosmologists and physicists interested in the quantum aspects of cosmological perturbations.

Overview and Key Findings

The paper investigates the rate at which spectator scalar fields, when prepared in their adiabatic vacuum, can decohere due to interactions with unobserved environmental fields, both in flat space and de Sitter space. A key finding is that in de Sitter space, once modes exit the Hubble scale, the decoherence process exhibits secular growth—a hallmark indicator that suggests perturbative methods break down over a long timescale compared to the Hubble time. This necessitates the transition to resummed methods to continue the analysis in a valid regime, and this is elegantly handled via a matching to a late-time Lindblad evolution. This approach allows a reliable resummation that extends the perturbative calculations well beyond their initial domain of validity.

Implications in Cosmology

One practical implication lies in the understanding of how quantum fluctuations may have wielded influence over the structure formation in the universe. The authors suggest that super-Hubble modes are primarily decohered by interactions with other super-Hubble modes, challenging assumptions that sub-Hubble environmental modes might significantly affect this process. The notion that super-Hubble interactions dominate reaffirms the need for precision in handling the assumptions in cosmological inflation models.

Moreover, the insights derived here could effectively close potential loopholes in recent calculations regarding the late-time purity of observable primordial fluctuations, without relying heavily on untested approximations. The findings underscore the importance of utilizing Open Effective Field Theory (Open EFT) frameworks to predict late-time regime behaviors accurately, setting a precedent for future studies focusing on quantum cosmological phenomena.

Speculative Future Directions

Moving forward, the methodologies and insights from this paper may spur further research into a more detailed quantum-classical transition of cosmological structures. The ability to account for late-time decoherence accurately opens up potential pathways for testing whether primordial fluctuations are quantum or classical—a question that remains pivotal in cosmology. Additionally, the implications of these results might extend beyond scalar fields, prompting speculation on their applicability to other cases like tensor perturbations or more complex field interactions during inflation.

In conclusion, "Cosmic Purity Lost: Perturbative and Resummed Late-Time Inflationary Decoherence" makes significant strides in clarifying the quantum decoherence of cosmological fluctuations. By resolving the perturbative breakdown through Lindblad resummation and demonstrating the predominance of super-Hubble mode interactions, the paper provides a robust foundation in both theoretical and observational cosmology contexts.