Advances in Metaverse Investigation: Streams of Research and Future Agenda

: The metaverse has increasingly attracted the attention of academics and practitioners, who attempt to better understand its theoretical foundations and business application areas. This paper provides an overarching picture of what has already been studied and investigated in metaverse academic investigation. It adopts a systematic literature review and a bibliometric analysis. The study designs a thematic map of the metaverse research. It proposes four streams of research (metaverse technologies, metaverse areas of application, marketing and consumer behaviour and sustainability) for future investigation, which academics and practitioners should explore. It also contributes towards a systematic advancement of knowledge in the ﬁeld, provides some preliminary theoretical contributions by shedding light on future research avenues, and offers insights for business.


Introduction
The metaverse, like many innovations, is shrouded in mysticism and scepticism [1]. If many believe it will be revolutionary and fully transform how people work, shop, socialise, and play, others are sceptical, and see it as a fad. However, whether or not we think of the metaverse as a technological revolution, it is undeniable that the massive diffusion of this technology will impact on nearly all aspects of life and business in the next decade, allowing interaction in virtual and augmented spaces and a blend of both [2].
Companies in this industry are eager to explore ways to capitalise on the metaverse to create virtual experiences for their consumers. Consumers are increasingly opting for metaverse products and services, and participating in virtual worlds [3]. Tech giants such as Nvidia, Roblox, Decentraland, The Sandbox, the Chinese Baidu and the Korean Netmarble strive to create metaverse platforms as virtual worlds in which people interact, game, work and do business.
The metaverse challenge has even attracted the attention of policymakers. The European Commission, for example, 2020 introduced the Virtual and Augmented Reality Industrial Coalition, a platform for structured dialogue between the European VR/AR ecosystem and policymakers [4].
The metaverse has been defined as a three-dimensional space representation based on virtual and augmented reality, where people can use personal avatars to work, play and synchronously communicate with each other [5][6][7]. It is also increasingly attracting the interest of academics who have already tried to conceptualise the phenomenon by proposing frameworks and research agendas [3,[8][9][10].
Although metaverse is a term that has been gaining interest in the academic world since 2021, the term was coined in 1992 by Neal Stephenson in his novel Snow Crash. In 2021, Facebook CEO Mark Zuckerberg announced the decision to rebrand the company, changing its name to "Meta".
However, most of this primary research remains independent, and the academic community feels strongly the need for new theoretical frameworks that deepen the metaverse [3,11]. After the identification of the articles, the research adopted criteria for inclusion and exclusion. Firstly, the 518 articles were screened, considering English-language articles. Since it is an emerging topic, book reviews, editorials, and papers from conference proceedings were also included in the research. After the screening, a sample of 490 papers was obtained.
Afterwards, the full text of these papers was reviewed, to assess eligible articles. As a result, 342 articles were excluded because their subject matter was not closely related to the topic of the metaverse. In the end, 148 eligible documents were identified, having been published in 112 sources among journals, books, and conference proceedings.
The relevant data of the 148 documents in the final sample were saved and organised in a Microsoft Excel spreadsheet to include all the essential paper information such as After the identification of the articles, the research adopted criteria for inclusion and exclusion. Firstly, the 518 articles were screened, considering English-language articles. Since it is an emerging topic, book reviews, editorials, and papers from conference proceedings were also included in the research. After the screening, a sample of 490 papers was obtained.
Afterwards, the full text of these papers was reviewed, to assess eligible articles. As a result, 342 articles were excluded because their subject matter was not closely related to the topic of the metaverse. In the end, 148 eligible documents were identified, having been published in 112 sources among journals, books, and conference proceedings.
The relevant data of the 148 documents in the final sample were saved and organised in a Microsoft Excel spreadsheet to include all the essential paper information such as paper title, author names, affiliations, abstract, keywords and references. Subsequently, adopting the bibliometrics analysis method [25], the R-Tool 'Biblioshiny for Bibliometrix' was used to perform a comprehensive bibliometric analysis. Bibliometrix is a recent R-package that facilitates a more complete bibliometric analysis, employing specific tools for both bibliometric and scientometric quantitative research [25].
The qualitative analysis of the above-identified 148 documents was implemented through a thematic analysis which assisted in identifying and discussing the themes in the Virtual Worlds 2022, 1 106 literature. Thematic analysis is an approach broadly used in bibliometrics to explore the conceptual structure of a research domain [26,27].
Using such an approach allows us to classify and organise documents by homogenous factors, so ideas and arguments can be clustered together to form a theme. A thematic map based on co-word network analysis and clustering was also generated to identify four topological regions, according to density and centrality dimensions. Density measures the strength of internal ties among all keywords describing the research theme and is a measure of the theme's development. Centrality measures the strength of external ties to other themes: it can be understood as a measure of the importance of a theme in the development of the entire research field analysed [28].
This result was obtained from a semi-automatic algorithm by reviewing the keywords of all references analysed in this study (apart from the author's keywords) to capture deeper variations [25].

Findings
The bibliometric analysis provided information on the 148 documents, allowing us to highlight the topic's significance (Table 1). Two sides of the same coin: accessibility practices and neurodivergent users' experience of extended reality [114] 2022
How did you feel during the navigation? Influence of emotions on browsing time and interaction frequency in immersive virtual environments [115] 2022 Behaviour and Information Technology Marmaridis I., Griffith S.
Metaverse services: Extensible learning with mediated teleporting into 3D environments [116] 2009 Lecture Notes in Business Information Processing Experiences complementing classroom teaching with distance seminars in metaverses and videos [117] 2014 Ownership in the virtual world and the implications for long-term user innovation success [170] 2018 Technovation

Publication Trend
The research on the metaverse has been increasing since 2019, as shown in Figure 2. Although the first relevant paper was published in early 2007, it is only since 2019 that publications have begun to increase significantly, with a peak in 2021, which is also the year in which Mark Zuckerberg announced the change of name from Facebook to Meta. Publication trend renders the metaverse a young research field.

Most Relevant Sources
When looking at the sources overview, the analysis revealed 112 sources among journals, books, and conference proceedings, covering different fields, including marketing, management, economics, tourism and hospitality, engineering, communication, and technology.

Publication Trend
The research on the metaverse has been increasing since 2019, as shown in Figure 2. Although the first relevant paper was published in early 2007, it is only since 2019 that publications have begun to increase significantly, with a peak in 2021, which is also the year in which Mark Zuckerberg announced the change of name from Facebook to Meta. Publication trend renders the metaverse a young research field.   When looking at the sources overview, the analysis revealed 112 sources among journals, books, and conference proceedings, covering different fields, including marketing, management, economics, tourism and hospitality, engineering, communication, and technology. Figure 3 shows the sources that have published most documents on the topic. With seven documents published, the journal Sustainabilty published more research than any other journal. The series of conference proceedings Lecture notes in business information processing ranks second with four publications, followed by the proceedings of 2022 8th International Conference of the immersive learning research network, with four publications.

Seminal Papers
Interesting findings emerged considering the most globally cited documents that allow us to identify the seminal articles according to the timeliness, utility, and quality expressed by the scientific community'. Figure 4 shows the number of author citations for each article, identifying seminal works. Two of the most popular bibliometric indicators used to determine an article's quality are the number of citations and the studies cited in an article. With 84 global citations, the article 'Making real money in virtual worlds: MMORPGs and emerging business opportunities, challenges and ethical implications in metaverses' [124] is the most cited, followed by 'A content service deployment plan for metaverse museum exhibitions-Centering on the combination of beacons and HMDs' [7] with

Seminal Papers
Interesting findings emerged considering the most globally cited documents that allow us to identify the seminal articles according to the timeliness, utility, and quality expressed by the scientific community'. Figure 4 shows the number of author citations for each article, identifying seminal works. Two of the most popular bibliometric indicators used to determine an article's quality are the number of citations and the studies cited in an article. With 84 global citations, the article 'Making real money in virtual worlds: MMORPGs and emerging business opportunities, challenges and ethical implications in metaverses' [124] is the most cited, followed by 'A content service deployment plan for metaverse museum exhibitions-Centering on the combination of beacons and HMDs' [7] with 49 citations and 'Web GIS in practice V: 3-D interactive and real-time mapping in Second Life' [44], with 40 citations. Concerning the country's scientific production, Figure 5 shows that publications on the topic in the USA amount to 44. The UK ranks second, with 36 publications, followed by South Korea with 29 documents and Italy with 19. The countries in blue on the map have published research on the topic. The more intense the colour, the greater the scientific production.   [7,44,45,70,71,75,109,124,131,170].

Themes and Streams of Research
Concerning the country's scientific production, Figure 5 shows that publications on the topic in the USA amount to 44. The UK ranks second, with 36 publications, followed by South Korea with 29 documents and Italy with 19. The countries in blue on the map have published research on the topic. The more intense the colour, the greater the scientific production. Concerning the country's scientific production, Figure 5 shows that publications on the topic in the USA amount to 44. The UK ranks second, with 36 publications, followed by South Korea with 29 documents and Italy with 19. The countries in blue on the map have published research on the topic. The more intense the colour, the greater the scientific production.

Themes and Streams of Research
Results can be summarised in various research themes, mapped in two-dimensional spaces: relevance degree (centrality) and development degree (density). The map classified the metaverse themes of research into four groups: motor themes, niche themes, emerging themes and basic themes ( Figure 6). Motor themes and emerging themes were the most extensively populated areas.
Virtual Worlds 2022, 1, FOR PEER REVIEW 15 Results can be summarised in various research themes, mapped in two-dimensional spaces: relevance degree (centrality) and development degree (density). The map classified the metaverse themes of research into four groups: motor themes, niche themes, emerging themes and basic themes ( Figure 6). Motor themes and emerging themes were the most extensively populated areas. Motor themes (upper-right quadrant, strong centrality and high density) identify well-developed and relevant areas of academic investigation that can drive future research. As the study's main result, metaverses have been investigated as "virtual reality" and "virtual worlds". The importance of adopting the metaverse as a virtual world useful for "e-learning", "computer-aided instruction" and "human" perspective, represents another relevant aspect of the academic analysis.
Basic themes (lower-right quadrant, low centrality, high density) cover topics relevant to the research, but general. They, nonetheless, are potential areas of rapid development through further studies to address existing research gaps and to provide more comprehensive insights. They include "virtual reality technology" (as in the precedent quadrant) and "emotion" as a synthesis of marketing and consumer behaviour.
Emerging themes (lower-left quadrant, low density and low centrality), mainly representing themes such as "COVID-19", and "key technology" such as "5g mobile communication system"; in addition, "sustainability", "sustainable development" and "economic and social effects" have been reported in this area.
Niche themes (upper-left quadrant, high centrality, low density) contain very specialised topics that could be important for future research, such as "education", "medical" and "medical education".
Last but not least, the metaverse can be realised by exploiting augmented reality and other emerging technologies, as the intersection of the two axes shows. Table 2 summarises the research themes and the corresponding number of documents. Motor themes (upper-right quadrant, strong centrality and high density) identify well-developed and relevant areas of academic investigation that can drive future research. As the study's main result, metaverses have been investigated as "virtual reality" and "virtual worlds". The importance of adopting the metaverse as a virtual world useful for "e-learning", "computer-aided instruction" and "human" perspective, represents another relevant aspect of the academic analysis.
Basic themes (lower-right quadrant, low centrality, high density) cover topics relevant to the research, but general. They, nonetheless, are potential areas of rapid development through further studies to address existing research gaps and to provide more comprehensive insights. They include "virtual reality technology" (as in the precedent quadrant) and "emotion" as a synthesis of marketing and consumer behaviour.
Emerging themes (lower-left quadrant, low density and low centrality), mainly representing themes such as "COVID-19", and "key technology" such as "5g mobile communication system"; in addition, "sustainability", "sustainable development" and "economic and social effects" have been reported in this area.
Niche themes (upper-left quadrant, high centrality, low density) contain very specialised topics that could be important for future research, such as "education", "medical" and "medical education".
Last but not least, the metaverse can be realised by exploiting augmented reality and other emerging technologies, as the intersection of the two axes shows. Table 2 summarises the research themes and the corresponding number of documents. Looking at the similarities, and combining convergent relevant themes, the analysis of the findings offers four relevant streams as a lens for interpreting the academic metaverse investigations (Table 3): key technologies, metaverse applications, marketing and consumer behaviour, and sustainable development.

Metaverse Technologies
The paper underlines the metaverse as a combination of different technologies [10] identified in the detected literature [11,16,106,171]. If, on the one hand, the literature has focused on existing and consolidated technologies such as virtual reality and virtual worlds that make up "motor themes" and represent the condition for the existence of the metaverse, on the other hand, authors are beginning to explore new and emerging technologies such as 5g mobile communication, blockchain, digital twins, and the internet of things that make up "emerging themes", and represent the conditions for the future development of the metaverse. The metaverse world can exploit several opportunities in the coming years. The access point for the metaverse requires multisensory interactions with several technologies: virtual environments, digital objects and people immersive technologies, virtual reality (VR), augmented reality (AR), mixed reality (MR) and extended reality (XR) [172]. These technologies support the creation of the metaverse and facilitate immersive experiences in the digital world. Virtual reality technology provides users with a connected experience in the metaverse. On the other hand, augmented reality expands the use of virtual reality by overlaying digital information onto the physical environment. XR is an extended reality, a term used to include VR, AR, and MR. XR is used for virtual commerce or v-commerce to create computer-mediated indirect experiences [125].
Blockchain technology plays a crucial role, and provides a decentralised infrastructure for the metaverse, which develops robust use cases for its ecosystem [173]. It ensures data quality, privacy and security in the metaverse, and provides a complete economic system, allowing virtual goods to become physical objects [90].
Artificial intelligence (AI) plays different roles in automating the metaverse ecosystem, from traditional machine learning algorithms to advanced deep learning networks. By merging with other keys technologies, AI works behind the scenes to create secure, scalable, and realistic virtual worlds on a reliable and always-on platform, guaranteeing the reliability of infrastructure, facilitating user interaction and supporting the content creation process [174].
The internet of things (IoTs) is a technology that acts as a medium connecting the real world with the internet via computing devices for sensing and communication. They allow the sharing and receiving of information replicating the physical world in the metaverse more efficiently. The combination of immersive technologies and internet-of-things is described as XR-IoT (XRI). XRI creates a hyper-connected environment and improves the relationships between humans and objects and human-to-human relationships [175].
Lastly, digital twins-which establish a virtual twin of a real-world object by utilising real-world data to predict the expected behaviour of the real-world object [176] -are used in the metaverse to mirror the real world onto the virtual world [172]. The essence of this process is to enable the collection of data that help create simulations to model how a given product, process, or service would perform in the real world, providing trial solutions to unsolved issues.

Metaverse Areas of Application
The immersive characteristics of the metaverse play a key role in triggering companies in various fields to innovate their business model [104,177]. Due to the integration of different technologies, the possible applications of the metaverse range from the industrial field to several areas. Consumer goods, healthcare, hospitality, education and retail, gaming, and other forms of entertainment (including art shows and concerts), represent other significant groups of metaverse applications. Scholars have investigated in depth the different areas of application of the metaverse. Some fields already constitute consolidated research topics (e.g., e-learning), while others are still emerging (e.g., tourism) or are reducing their relevance (e.g., . As a 'motor theme', many authors emphasise the potential of the metaverse for technology-enhanced education and learning, due to the possibilities of the immersion experience, collaboration, and interaction 73,71]. Among the first application areas there are laboratory simulations (e.g., safety training), procedural skills development (e.g., surgery), AR-based training, language learning and management systems [7,63,100,178,179]. In the manufacturing sector, the metaverse can help train employees on safety precautions, foster participation in the simulation of risk scenarios, and facilitate the development of products and processes [10].
More 'emerging themes' are related to consumer goods and services. For example, fashion labels and retail chains are using the metaverse extensively. They exploit the metaverse technologies to create new products (e.g., 2D and 3D design and rendering); virtual events; collaboration and partnership, and enhance the customer experience and journey (e.g., virtual shopping, virtual try-on, digital twin stores and showrooms [180,181].
In the hospitality and tourism field, companies are capitalising on the metaverse to create tourism experiences, such as virtual flights, hotel experiences, destinations visits, and tours, but also to enhance real-life customer experience and attract new visitors [3,182,183].
A very specialised application of the metaverse is that related to the healthcare sector, which constitutes a 'niche theme' for scholars. The best example of metaverse applications is the prevention and treatment of clinical conditions, education, training, and research. The metaverse has emerged as a vital technology for empowering medical students' skills and knowledge. In addition, different technology, such as digital twins, can be adopted to monitor patients' health conditions directly at home, connecting real life with the virtual world [184,185].

Marketing and Consumer Behaviours
Marketing in the Metaverse is a field of study that has attracted a lot of the attention of scholars, to the point of being present both in 'motor themes' and 'basic themes'. According to several authors, the metaverse is likely to have a significant and direct impact on both customers and businesses, changing customer relationships, advertising, communication [11], customer decision-making processes [16], customer experiences and service co-creation processes [3].
Among the 'basic themes', particular attention has been paid to experiences mediated by the metaverse. Two dimensions can describe the metaverse experience: interactivity (high or low) and type of motive (hedonic or functional) [3]. According to these dimensions, different immersive experiences can be created, such as virtual shopping and retailing [132], augmented physical experiences, teaching and learning experiences, virtual classrooms [78], virtual exhibitions [7], and virtual smart cities and tourism destinations [182].
On the other hand, everything related to consumer behaviour constitutes a 'motor theme'. According to some authors, metaverse dynamics changed the traditional consumer journey, which begins with awareness and ends in purchase and loyalty and is intended more as a stream than a process marked by predetermined phases [3]. In this stream of engagement, individuals can interact with the metaverse-scape, have immersive experiences and experience virtual consumption of products and services [16]. In addition, observable changes in consumer behaviours can be directly associated with changes in the metaverse [3,16,18]. Changes may affect customers' attention toward products and services, buying behaviour, perceived value and value propositions, choice preferences, consumption patterns, and decision-making processes [3].

Sustainable Development
Sustainability, sustainable development, and the economic and social effects of the metaverse represent 'emerging themes' for scholars [41,106,125]. According to the authors, the metaverse affects them in different and multiple ways regarding resource management, governance, quality of life, social interaction, cultural heritage conservation and preservation. A recent and challenging line of studies is smart urbanism and the potential contributions of the metaverse to smart cities. In the metaverse, the virtual dimension of smart cities focuses on environmental, economic, and social sustainability goals [41,186].
The social and economic effects of this extended reality have also been a concern for scholars [41]. If, on the one hand, the metaverse helps to decrease demand for physical infrastructure, minimises waste, enhances accountability and transparency, and ensures more equitable access to services, on the other hand, different challenges emerge. In particular, concerns about energy consumption and carbon emissions of compute-intensive transactions have come to the fore, despite substituting physical goods with digital ones and replacing a real-world presence with virtual interactions.
The focus has also been on the metaverse's ethical, moral, human, and cultural influence on the quality of human social interactions and its prospective scope in reconstructing the quality of life. This is due to the absence and confusion of the corresponding moral norms, which conflict with the ethical norms of the whole society [10].

Conclusions
This paper contributes to the academic debate on the metaverse by analysing, discussing, and synthesising the existing literature on the topic. This research provides an overarching picture of what has already been studied. It identifies four streams of research: metaverse technologies, metaverse areas of application, and marketing and consumer behaviours and sustainability.
The research designs consolidated research streams and opens up emerging but undeveloped streams for future research that academics and practitioners should explore. It also contributes towards a systematic advancement of knowledge in the field of metaverse investigation, and provides some preliminary theoretical contributions, shedding light on future research avenues. Moreover, the paper offers preliminary insights into business opportunities and challenges.
Firstly, the paper confirms the metaverse as a combination of different existing and emerging technologies, such as VR, AR, MR, digital twins, blockchain, and AI. Metaverse technologies connect interoperable virtual worlds, allowing users to switch worlds cross-platform while maintaining their digital properties, purchases, and identities [10]. They open new scenarios for immersive experiences and provide new business spaces and challenges.
Secondly, the paper sees the metaverse as composed of several digital spaces and experiences currently being created by companies to offer more realistic and immersive digital experiences, enhance simulations and training, and foster remote working, teaching and learning. Among the different applications, the use of the metaverse in healthcare, education and training, and tourism, will be of particular importance in the future.
Thirdly, researchers are called to examine the metaverse's transformative impact on marketing and consumer behaviours, which deserve particular interest on the part of academics and practitioners. In particular, scholars should explore ways to promote user purchasing decisions, assess and evaluate consumer attitudes and behaviours in the metaverse, and create an experience in the metaverse-scape. In this sense, new forms of communication and marketing models are challenging practitioners. Because the metaverse is interactive and involves embodied users through avatars, the original interactivity models might need to be updated by including, for example, the second or embodied self-perspective [11].
Finally, the paper suggests sustainability and sustainable development as an emerging stream of research in which to invest for the future of firms and cities. It would be fascinating for scholars to be able to understand whether or not the metaverse opens up new dimensions of sustainability, or if it is just a fad linked to the greater opportunities for publication in this sector.
Furthermore, although the metaverse could have potential benefits for different sectors, the paper also underlines its dark side, which scholars should consider in future studies. Governance, ethics, information security issues, and data privacy are the biggest concerns and threats to the rise of the metaverse that need to be addressed in future research.
This study has limitations to be considered in future research. Firstly, the systematic literature review only includes English articles, as the metaverse is an emerging research topic, especially in Asian countries; other languages should also have been included to deepen the understanding of the topic. Secondly, this research was conducted on the database Scopus of Elsevier. Researchers could use a combination of different databases and keywords to search for new contributions and insights. Thirdly, although the paper adopts a systematic literature review, this methodology reveals the subjectivity in the social sciences. As this is a relatively young field of research, a further academic investigation is needed to overcome the study's limitations and outline new scenarios and directions for future research.