Entanglement in Cosmology: From Field Extraction to Black Hole Information.

Research demonstrates entanglement generation in expanding universes links to gravitational particle production, encoding spacetime information. Entanglement harvesting protocols offer a pathway to extract entanglement from fields, while studies of black hole spacetimes connect Bekenstein-Hawking entropy to entanglement and propose resolutions to the information paradox via unitarity preservation.

The universe’s expansion and the enigmatic nature of black holes are linked by a subtle quantum property: entanglement. Recent theoretical work explores how this connection, where two or more particles become linked and share the same fate regardless of distance, manifests within cosmological models and extreme gravitational environments. Researchers Alessio Belfiglio (DSFTA University of Siena), Orlando Luongo (Istituto Nazionale di Fisica Nucleare – INFN), and Stefano Mancini (School of Science and Technology, University of Camerino) detail these investigations in their article, “Quantum entanglement in cosmology”. They examine entanglement generation during cosmic expansion, linking it to the production of particles by gravity, and propose methods for extracting entanglement from quantum fields. Furthermore, the study investigates the potential connection between black hole entropy and entanglement, offering a novel perspective on resolving the long-standing black hole information paradox.

Entanglement and the Expanding Universe: Linking Quantum Fields to Gravitational Dynamics

The expansion of the universe generates entanglement between quantum fields, establishing a direct relationship between this quantum phenomenon and the production of particles associated with gravity. This particle production effectively transfers energy and momentum from the expanding spacetime itself to the quantum fields, influencing both the quantity and the specific characteristics – the ‘mode dependence’ – of the resulting entanglement. Consequently, the properties of this entanglement encode information about the underlying cosmological expansion, potentially offering a novel method for probing the dynamics of spacetime and revealing fundamental connections between gravity and quantum mechanics.

Investigations currently employ both momentum-space and position-space approaches to characterise this entanglement. Momentum-space analyses focus on identifying correlations between different modes – essentially different wavelengths or frequencies – of the quantum field. Position-space approaches, conversely, examine entanglement within specific gravitational backgrounds, such as those surrounding black holes. This allows for detailed comparisons and validation of theoretical predictions. The interpretation of black hole entropy – a measure of disorder – as arising from entanglement entropy within discrete field theories aligns with the ‘area law’, which posits that entropy is proportional to the surface area of the black hole’s event horizon – the boundary beyond which nothing can escape.

Scientists are exploring the ‘entanglement harvesting’ protocol, a theoretical method for extracting entanglement from quantum fields using local interactions with external detectors. While experimentally demanding, accurate modelling of realistic detector interactions is crucial for developing viable extraction setups and potentially unlocking applications in quantum technologies. This modelling focuses on precisely characterising how detectors interact with the quantum fields to maximise entanglement yield.

Researchers are also addressing the long-standing ‘black hole information paradox’ with a novel ‘fine-grained entropy’ formula for calculating the entropy of ‘Hawking radiation’ – the thermal radiation emitted by black holes. This formula allows for the preservation of ‘unitarity’ – a fundamental principle in quantum mechanics ensuring information is neither created nor destroyed – during black hole evaporation. This suggests information isn’t truly lost when a black hole evaporates, but rather encoded in a subtle and complex manner within the emitted radiation. By carefully accounting for the quantum correlations within Hawking radiation, this approach offers a potential resolution to a longstanding problem in theoretical physics.

Current research continues to investigate the fundamental connection between entanglement, particle creation, and the emergence of classical behaviour within cosmological models, seeking to understand how quantum phenomena give rise to the classical universe we observe.