Effective cosmology from one-body operators in group field theory. (7th April 2022)

• Record Type: Journal Article
• Authors: Gielen, Steffen; Marchetti, Luca; Oriti, Daniele; Polaczek, Axel
• Is Part Of: Classical and quantum gravity. Volume 39:Number 7(2022)
• Publication Date: 2022-04-07
• Subjects: quantum gravity -- cosmology -- relational dynamics; Quantum gravity -- Periodicals; Gravitation -- Periodicals; Relativity (Physics) -- Periodicals; Space and time -- Periodicals; Periodicals; 521.1
• DOI: 10.1088/1361-6382/ac5052
• Languages: English
• ISSNs: 0264-9381

Abstract

Abstract: We propose a new method for obtaining an effective Friedmann–Lemaître–Robertson–Walker (FLRW) cosmology from the quantum gravity dynamics of group field theory (GFT), based on the idea that an FLRW universe is characterised by a few macroscopic observables. Rather than relying on assuming a particular type of quantum state and computing expectation values in such a state, here we directly start from relations between macroscopic observables (defined as one-body operators) and formulate dynamics only for those observables. We apply the effective approach to constrained quantum systems (as developed by Bojowald and collaborators) to GFT, providing a systematic expansion in powers of ℏ . We obtain a kinematical phase space of expectation values and moments, which does not require an a priori choice of clock variable. We identify a gauge fixing of the system which corresponds to choosing one of the cosmological variables (with the role of extrinsic curvature) as a clock and which allows us to rewrite the effective dynamics in relational form. We show necessary and sufficient conditions for the resulting dynamics of expectation values to be compatible with those of classical FLRW cosmology and discuss the impact of quantum fluctuations.