Eisemann, Lukas (2023): Quantum backreaction effects in macroscopic systems: black holes and inflation. Dissertation, LMU München: Faculty of Physics |

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**DOI**: 10.5282/edoc.32875

### Abstract

This thesis is concerned with two effects in macroscopic systems that are only revealed when taking into account the system’s quantum nature. For these effects to become strong, a high microstate degeneracy is essential, which motivates investigating their manifestation in de Sitter and black holes (BHs). The first part is concerned with the quantum description of phenomena of particle creation in a background. The common semiclassical treatment of such scenarios often allows for a nonperturbative method of analysis. We show that resolving the background as an N-particle state allows for a fully quantum perturbative analysis that produces the semiclassical nonperturbative results and allows to go beyond. In a model of two scalars as well as in scalar QED, we thus produce particle creation in time-dependent fields in terms of n -> 2 annihilation processes. Effects of backreaction in particular become dramatic within a single process in the case n ~ N, i.e., the near-classical system non-gradually transitions into a quantum state of a few highly energetic particles. We find that such “quantumization” is in general highly suppressed. By contrast, the reverse, “classicalizing” transitions, 2 -> N, may be unsuppressed because the degeneracy of the N-particle state can be sufficiently high in a consistent theory. For the case of N -> 2, such degeneracy is causing an enhancement only to the extent that the degenerate states are covered within an initial superposition. A BH described in terms of a bound state of N gravitons thus possesses a state unstable to single-process decay. We comment on the possibility of a near-classical BH generating the required level of superposition on relevant time scales. The second part is concerned with the so-called memory burden effect, which is universal to systems of high memory capacity and stops any gradual decay of the system. We study a prototype model to investigate whether the effect may be avoided by rewriting stored quantum information from one set of degrees of freedom to another one. We find that such rewriting-facilitated decay can proceed only very slowly compared to the initial stage of decay, s.t. the decay effectively remains suppressed. In both de Sitter and BHs, the effect manifests in a deviation from the semiclassical evolution of thermal particle emission and becomes strong the latest after a number of emissions on the order of the entropy. The stored quantum information responsible for the effect constitutes a quantum hair and may start to get released around the onset. If inflation ended not long before that time, the imprints of that primordial information can be observable. For BHs, in the absence of other strong effects of destabilizing kind, the effective stabilization after half-decay opens a new window for small primordial BHs as dark matter.

Item Type: | Theses (Dissertation, LMU Munich) |
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Subjects: | 500 Natural sciences and mathematics 500 Natural sciences and mathematics > 530 Physics |

Faculties: | Faculty of Physics |

Language: | English |

Date of oral examination: | 28. November 2023 |

1. Referee: | Dvali, Gia |

MD5 Checksum of the PDF-file: | 27c9d709f21dcf854c1e2e2f25489f94 |

Signature of the printed copy: | 0001/UMC 30082 |

ID Code: | 32875 |

Deposited On: | 21. Dec 2023 10:36 |

Last Modified: | 21. Dec 2023 10:36 |