This thesis deals with the problems of exploration and map construction of a dangerous network by mobile agents, and it introduces new general mechanisms for inter-agent communication, which could be applied to other mobile agents' problems. A dangerous network contains a harmful process called Black Hole that destroys all agents entering the node where it resides, without leaving any observable trace. The task for the agents, which are moving asynchronously, is to construct a map of the network with edges incident on the black hole unambiguously identified. Two types of communication mechanisms are considered: whiteboards and tokens. In the whiteboard model every node provides a shared memory on which agents can read and write. When communication occurs through tokens, instead, the agents have some pebbles that can be placed on and picked up from the nodes. Four different costs for comparing the efficiency of the protocols are taken into account: the number of agents required, the number of moves performed, the size of the whiteboard (or the token capacity at a node), and time. The black hole search problem is considered first in ring networks with whiteboards, and optimal exact time and move complexities are established improving all existing results. The same problem is then studied in arbitrary unknown graphs and it is solved in the token model by using a constant number of tokens in total. The protocol improves on existing results and is based on a novel technique for communicating using tokens. Finally, the new method of communicating using tokens described in the context of black hole search is generalized to propose a novel communication mechanism among the agents that could possibly be employed for any distributed algorithm by mobile agents.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22252号 / 理博第4566号 / 新制||理||1656(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 上田 佳宏, 准教授 岩室 史英, 教授 長田 哲也 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20898号 / 理博第4350号 / 新制||理||1624(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 田中 貴浩, 准教授 樽家 篤史, 教授 川合 光 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20918号 / 理博第4370号 / 新制||理||1627(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 上田 佳宏, 准教授 野上 大作, 教授 太田 耕司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Thesis advisor: Kevin Bedell / Using the AdS/CFT correspondence, it has been shown that the ratio of shear viscosity to entropy density is bounded from below in strongly coupled field theories with a gravity dual. More recently, this bound has been shown to be grossly violated in novel non-Fermi liquids and the unitary Fermi gas in the presence of superfluid fluctuations above T_c. Nevertheless, a holographic approach to such systems which break the lower bound have been strongly reliant on AdS spacetimes with massive gravitons. In this work, we propose a violation of the viscosity over entropy bound in 3+1 dimensional AdS spacetimes that support stable black hole solutions with non-zero scalar field. Such a black hole is shown to be characterized by a novel phase transition at large negative mass, where the underlying thermodynamics agrees with the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF)-like phase seen in the unitary Fermi gas near Tc and the bound is similarly broken. Such a work paves the way for a holographic description of strongly-entangled quantum fluids at high Reynolds number. / Thesis (BS) — Boston College, 2021. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Scholar of the College. / Discipline: Physics.

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京都大学 / 新制・課程博士 / 博士(理学) / 甲第25108号 / 理博第5015号 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 田中 貴浩, 准教授 久徳 浩太郎, 教授 橋本 幸士 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

In this work, we study the galaxy cluster MS0735.6+7421 that hosts the most energetic observed active galactic nucleus (AGN) outburst so far. Explaining this very energetic AGN outburst is found to be challenging. McNamara et al. 2009 grappled with this problem and proposed two possible solutions: either the black hole (BH) must be an ultra massive one (with mass $> 10^{10} \ \text{M}_\odot$), or the efficiency of the mass to energy conversion ($\epsilon$) should be higher than the generally assumed value of $\epsilon \sim 0.1$. However, the efficiency of the mass to energy conversion depends on the BH's spin {Benson and Babul 2009}; higher $\epsilon$ can be achieved with a higher spinning BH. Here, we explore the second solution in detail, and ask the question: How did the BH spin up to the very high spins in advance of the outburst? We also explore the attendant physical processes, such as star formation, during the spin-up mode and investigate the associated observational implications. Comparing our results with what is generally expected from simulations and observational studies suggests that for all intents and purposes, the existence of an ultra massive BH is the simplest solution. / Graduate