Line of research overview

Fields on Curved Backgrounds and their Analogue Models

Principal Investigator: Stefano Ansoldi

Abstract: The research line will develop within the framework of quantum fields and gravity, focusing on two main aspects: i) the theory of quantum fields on curved backgrounds and ii) their analog model counterparts. Quantum field theory on curved backgrounds is a classic subject dating back to the study of vacuum decay in presence of gravity (Coleman and De Luccia) and of Hawking radiation. Despite the technical progress of the last fifty years, we still need new conceptual tools and non-standard techniques to address long-standing problems. For a long time, however, progress had to rely on abstract approaches only, with all the intrinsic limitations that this implies. A couple of decades ago, however, it was recognized that condensed matter systems, including acoustics in flowing fluids, light in moving dielectrics, and quasiparticles in a moving superfluid, can serve as analogs for curved spacetime geometries and field propagation. For instance, a fluid flow with a supersonic region can emulate an acoustic black hole for phonons in the fluid. These systems offer experimentally accessible models of quantum field theory on curved backgrounds, as well as of phenomena expected in an emergent spacetime. Furthermore, their UV departures from standard relativistic phenomenology can serve to test the robustness of the typical phenomena in QFT in curved spacetime.

Status of project and perspectives:

This research line builds on the idea that for an efficient progress in this wide area of research, it is instrumental to merge successfully these two complementary points of view. Some of the goals that such of a synergy will achieve include:

• testing astrophysical phenomena like superradiance and cosmological particle creation;
• replicating evanescent phenomena like Hawking radiation from black holes and extend them also beyond the standard relativistic settings;
• understanding in more detail the effect of curvature on phenomena related to particle physics (like symmetry breaking/restoration) and relativistic quantum information;
• providing as toy models for emergent gravity scenarios.

In particular, in this extended framework, it becomes feasible to take full advantage from the feedback coming from phenomenology and high precision future experiments (both, in cosmology and astrophysics). In this way, conceptual ideas and results in quantum field theory on curved backgrounds can be concretely realized and better understood using appropriate condensed matter analog systems.

In the past, we developed the analog approach primarily focusing on analogue black holes. We have investigated the information loss problem by studying a simpler model based on analogue cosmological particle creation while accounting for analogue quantum gravitational degrees of freedom. Additionally, we have examined the back reaction of Hawking radiation in the analogue context, finding parallels with general relativistic phenomena despite the different equations. On the quantum field theory side, we have focused our attention on tractable models that show how, techniques that are otherwise well-established in flat spacetime, could be pathological in presence of curvature (we concentrated especially on models in early universe cosmology). While most of these results have been considered for the most part on one side of the analog correspondence only, they can be can be refined, extended and, especially, understood at a deeper conceptual level, in a more complete analog picture.

Concretely, some of our future goals include:

• Extending the investigation of the resolution of the information loss problem by performing the same calculation in a proper analogue black hole setting. We also aim to explore the possible connections between the analogue gravity resolution of the problem and proposed schemes for recovering the information within semiclassical gravity and AdS/CFT inspired frameworks.
• Collaborating with groups in quantum gravity so to explore the possibility if similar mechanism to those found in analogue gravity can lead to a resolution of the information loss problem in those frameworks.
• Considering analog models in the context of cosmology, and discuss on the analog side the difficulty in the application of some otherwise standard techniques (like the WKB approximation). In situations like, e.g., tunneling with wormhole production, it is very hard, both, conceptually and technically, to have a clear physical understanding/interpretation on the behavior of the system, and the analog picture would represent an invaluable help.
• Proposing experimental settings within which the found insights can be tested and verified also by actively collaborating with experimental groups working on cold atoms.

In all these situations, the goal of the project is to combine diverse ideas and techniques in an interdisciplinary context, finding connections with observable systems and discussing the phenomenological implications, for instance, in connection with gravitational waves, CMB physics, and black hole physics.