[Paper Review] Semiclassical Gravity and Mesoscopic Physics

B. L. Hu|arXiv (Cornell University)|Nov 28, 1995

Cosmology and Gravitation Theories6 references6 citations

TL;DR

This paper proposes that semiclassical gravity provides a critical framework for understanding the quantum-to-classical transition in the early Universe, linking cosmological phenomena like structure formation and black hole entropy to mesoscopic physics. By analyzing quantum coherence, fluctuations, and correlations, it offers insights into Planck-scale metric dynamics, phase transitions, and decoherence-driven classicality.

ABSTRACT

Developments in theoretical cosmology in the recent decades show a close connection with particle physics, quantum gravity and unified theories. Answers or hints to many fundamental questions in cosmology like the homogeneity and isotropy of the Universe, the sources of structure formation and entropy generation, and the initial state of the Universe can be traced back to the activities of quantum fields and the dynamics of spacetime from the Grand Unification time to the Planck time at 10^{-43} sec. A closer depiction of this primordial state of the Universe requires at least a semiclassical theory of gravity and the consideration of non-equilibrium statistical processes involving quantum fields. This critical state is intermediate between the well-known classical epoch successfully described by Einstein's Theory of General Relativity and the completely unknown realm of quantum gravity. Many issues special to this stage such as the transition from quantum to classical spacetime via decoherence, cross-over behavior at the Planck scale, tunneling and particle creation, or growth of density contrast from vacuum fluctuations share some basic concerns of mesoscopic physics for condensed matter, atoms or nuclei, in the quantum/classical and the micro/macro interfaces, or the discrete/continuum and the stochastic/ deterministic transitions. We point out that underlying these issues are three main factors: quantum coherence, fluctuations and correlation. We discuss how a deeper understanding of these aspects of fields and spacetimes can help one to address some basic problems, such as Planck scale metric fluctuations, cosmological phase transition and structure formation, and the black hole entropy, end-state and information paradox.

Motivation & Objective

• To bridge theoretical cosmology with quantum field theory and quantum gravity by analyzing the semiclassical regime between quantum and classical spacetime.
• To address unresolved cosmological problems—such as initial conditions, structure formation, and entropy generation—through the lens of non-equilibrium quantum processes.
• To identify common physical principles—quantum coherence, fluctuations, and correlations—underlying both early-universe dynamics and mesoscopic systems in condensed matter or nuclear physics.
• To explore how decoherence and Planck-scale effects govern the transition from quantum to classical spacetime.
• To examine the implications of these dynamics for black hole entropy, information paradox, and cosmological phase transitions.

Proposed method

• Adopting a semiclassical gravity framework to model spacetime dynamics influenced by quantum fields during the Planck to Grand Unification era.
• Analyzing non-equilibrium statistical processes involving quantum fields to describe the evolution of the primordial Universe.
• Applying concepts from mesoscopic physics—such as decoherence, stochasticity, and discrete-continuum transitions—to gravitational systems.
• Focusing on the interplay of quantum coherence, vacuum fluctuations, and correlations as central mechanisms in spacetime emergence.
• Using effective field theory and open quantum systems formalism to describe the transition from quantum to classical behavior in spacetime.
• Examining the role of metric fluctuations at the Planck scale as a manifestation of quantum gravity effects within a semiclassical setting.

Experimental results

Research questions

• RQ1 How do quantum fluctuations in the early Universe lead to the growth of density contrast and structure formation?
• RQ2 What is the role of decoherence in the transition from quantum to classical spacetime at the Planck scale?
• RQ3 How do quantum coherence, fluctuations, and correlations jointly influence cosmological phase transitions?
• RQ4 In what way do mesoscopic physics principles help explain the emergence of classical spacetime from quantum gravity?
• RQ5 How can semiclassical gravity resolve or clarify the black hole information paradox and entropy origin?

Key findings

• Quantum coherence, fluctuations, and correlations are identified as the three fundamental factors governing the quantum-to-classical transition in early-universe spacetime.
• The semiclassical regime serves as a critical bridge between general relativity and full quantum gravity, enabling analysis of phenomena such as particle creation and tunneling.
• Planck-scale metric fluctuations are shown to be intrinsic to the quantum nature of spacetime, emerging from vacuum fluctuations in a non-equilibrium setting.
• Cosmological structure formation can be traced to the amplification of vacuum fluctuations through gravitational coupling, consistent with mesoscopic growth mechanisms.
• Decoherence processes are essential for explaining the emergence of classical spacetime, suggesting a dynamical mechanism for the classical limit of gravity.
• The black hole entropy and information paradox may find deeper understanding through the lens of non-equilibrium quantum field dynamics in curved spacetime.