## 1 Introduction  
There is not a unique definition of a metaverse; a broad informal description could be that it is a whole of many virtual reality worlds where users can engage with a huge variety of activities, can socialize and meet people from around the world, can be represented with customizable avatars, and can benefit from the absence of limitations due do locations and traveling. The first appearance of the term \"metaverse\" was in 1992 in the dystopic novel _Snow Crash_[1]. At the present time, the metaverse is a system still under construction, but it has been a topic being developed in the scientific literature for many years; the first relevant works can be found in the early 2000s. The authors of [2, 3] provide insights into definitions and a history of works. Several studies have been done on the metaverse, sometimes without mentioning it explicitly, by addressing the requirements, architecture, enabling technologies, usability, user experience, acceptability, and factors related to motivation for the use of VR, privacy issues, legal and ethical issues, the future of society, and other topics. A short summary of such studies is presented below. Studies have been done about technological aspects of the metaverse. The work presented in [4] is an early deep analysis of the transition from independent virtual worlds to a network of integrated virtual worlds (called \"metaverse\"). The paper, discussing the current status of technologies and requirements, presents a road map for such a transition that is composed by four key elements: realism, ubiquity, interoperability, and scalability. The research in [5] proposes a framework for supporting a visual construction of the metaverse that is composed of visualizations, graphics, and interaction techniques. A taxonomy of interaction technologies is presented, as well as two different pipelines for designing scenes in the metaverse. On the other hand, ref. [6] is a large work where a taxonomy of the metaverse has been proposed; that study notes that hardware, software, and contents are the essentials blocks for realizing the metaverse and that user interaction, implementation, and application are the three main approaches which are suitable for it. The paper also describes social influence factors and open challenges such as potential increases in inequities, legal aspects, and devices limitations. The authors of [7] first mention current developments made by some of the big tech companies, e.g., Meta aims to realize the metaverse for social purposes; Apple and Microsoft have applications for connecting users to virtual spaces, aiming to improve productivity; video-gaming and development companies such as NVIDIA and Epic Games have specific frameworks for designing and developing software for building metaverse applications. Then, scalability, accessibility, security/privacy, and legal issues are proposed and analyzed as the main requirements of the metaverse. The paper also mentions the Workrooms platform from Meta and the AltspaceVR platform from Microsoft as prototypes of the metaverse; both are virtual worlds across the internet where people can join, be present, and interact. The authors of [3] presented a systematic review analyzing and cataloguing the topics and components related to the metaverse, as well as connections with the social sciences and neurosciences. That work reported the expansion of relevant works since 2022; moreover, a set of heterogeneous definitions of the metaverse was reported, as well as enabling technologies. Several works have been done on users and related issues, focusing in particular on questions of privacy, identity, and security. The research in [2] firstly provides a detailed report on the history, definition, and main features of the metaverse. Then, it focuses on an analysis of the metaverse in education and on its requirements and components. A framework based on enabling technologies and on a pipeline for learning is proposed, and all related components are described.