Emergent spacetime physics from quantum gravity

• Applicant: Daniele Oriti, Ph.D.
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 433545361

The key question we want to address in this project is: “What is the effective macroscopic physics of fundamental quantum gravity models?”Quantum gravity is first and foremost a physical problem. Existing semi-classical models of early cosmology and black holes physics are fundamentally incomplete. For example, current cosmological scenarios (inflation, bouncing cosmologies, emergent universe scenarios) make assumptions about the initial state of the universe at the big bang or at high energy scales, or postulate new physics resolving the big bang singularity, that they cannot fully control. Only a theory of quantum gravity can justify these assumptions, modify such cosmological scenarios, or suggest new ones. Similarly, models of quantum black holes produce insights as well as puzzles that are unsolvable within semi-classical physics. Quantum gravity may also help to explain the indisputable successes of semi-classical physics in accounting for observations. This requires to connect solidly to GR and QFT, showing how their continuum structures (manifold, metric, matter fields) emerge from a fundamental theory.Accordingly, the main general objective of this project is the extraction of an approximate description of geometry and matter from quantum gravity models, identifying their observational signature, in the early universe and around black holes.The issues faced by quantum gravity approaches in this context are analogous to those faced routinely by condensed matter theorists (the extraction of macroscopic dynamics from the atomic description of a system). Inspired by this analogy, we explored the idea of spacetime as a condensate of quantum gravity (specifically, group field theory) building blocks; this early work is the basis of the present project.The project is constituted by two main directions, corresponding to the two physical contexts where we want to obtain observational consequences of fundamental quantum gravity models.Direction I: QG and fundamental cosmology The focus question of the first research direction of this project is:“What happened in the very early universe, beyond the regime described by semi-classical physics, and what are the physical signatures of its quantum gravity origin?”The main goal of this direction is to extract a detailed effective cosmological dynamics from microscopic quantum gravity models, capturing the physics of the early universe, and putting them in contact with observationsDirection II: QG and quantum black hole physicsThe focus question of the second research direction of this project is:“What is the quantum nature of black holes, responsible for their thermodynamic and information-processing properties? Can it be probed by gravitational waves observations?”The main goal of this direction is to use fundamental quantum gravity models to describe the microstructure and dynamics of black holes, how they store and process information and their thermodynamic properties.

• DFG Programme: Research Grants