The multiverse hypothesis has been discussed by physicists and cosmologists since the 1960s, but it was in the 1980s and 1990s that it gained more mainstream attention and became a topic of debate among philosophers and theologians. Physicist Max Tegmark has been one of the most vocal proponents of the multiverse theory, and his book "Our Mathematical Universe" (2014) popularized the idea in the scientific community and beyond. Other physicists, such as Brian Greene and Stephen Hawking, have also discussed the possibility of a multiverse. However, it's important to note that the multiverse hypothesis remains a topic of active debate and is not universally accepted in the scientific community.

Stephen Hawking discussed the possibility of a multiverse in his book "The Grand Design," which was published in 2010. In the book, Hawking suggests that the laws of physics in our universe could be explained by the existence of multiple universes, each with its own set of physical laws. He argues that the multiverse hypothesis is a natural consequence of the laws of physics and that it could provide a solution to some of the fundamental questions in physics, such as the nature of dark matter and the origin of the universe. However, it's worth noting that Hawking's endorsement of the multiverse is not unequivocal, and he acknowledged that the hypothesis remains speculative and difficult to test experimentally.

Brian Greene, a theoretical physicist and string theorist, has been discussing the possibility of a multiverse for many years. He first wrote about it in his 1999 book "The Elegant Universe," where he described the concept of the "many-worlds" interpretation of quantum mechanics, which suggests that every possible outcome of a quantum measurement actually occurs in a separate universe. He has since expanded on the multiverse hypothesis in his subsequent books and public appearances. While Greene doesn't necessarily endorse any specific multiverse hypothesis, he considers it to be a plausible explanation for the fine-tuning of the universe and other puzzles in modern physics.

Max Tegmark is a cosmologist and theoretical physicist who has written extensively about the multiverse hypothesis. He has written several popular science books that discuss the multiverse, including:

1. "Our Mathematical Universe: My Quest for the Ultimate Nature of Reality" (2014) - In this book, Tegmark explores the idea that our universe is not the only one, but one of many possible mathematical structures that exist in a larger multiverse.

2. "Parallel Universes: The Search for Other Worlds" (2005) - This book is a collection of essays by leading physicists and cosmologists, edited by Tegmark, that explores the idea of parallel universes and the multiverse hypothesis.

3. "Cosmic Variance: Theories, Challenges, and Perspectives" (2007) - In this book, Tegmark and his co-authors explore some of the most challenging problems in cosmology, including the multiverse hypothesis, dark matter, and the nature of the early universe.

Tegmark is known for his advocacy of the "level IV" multiverse hypothesis, which posits that all possible mathematical structures exist in a mathematical universe, and that our universe is just one of them.

Yes, Max Tegmark has written about the multiverse hypothesis before 2006. In fact, Tegmark has been exploring the idea of a multiverse for many years, and has published several papers on the subject. For example, his paper "The Importance of Quantum Decoherence in Brain Processes" (1999) discusses the implications of the multiverse hypothesis for the nature of consciousness. Additionally, Tegmark's 2003 paper "Parallel Universes" presents an overview of the various types of multiverse hypotheses, including the "level I" to "level III" multiverses, which are based on different physical theories. His 2006 paper "Is 'the theory of everything' merely the ultimate ensemble theory?" also explores the idea that our universe may be one of many possible mathematical structures that exist in a larger multiverse.

Yes, the multiverse hypothesis gained popularity in part due to its connection with string theory, a theoretical framework in physics that seeks to unify all the fundamental forces of nature. String theory predicts the existence of extra dimensions beyond the three dimensions of space and one dimension of time that we are familiar with, and these extra dimensions could be compactified in different ways to give rise to different physical laws and constants in different universes. This has led some physicists, including Brian Greene and Max Tegmark, to propose that the multiverse hypothesis could be a natural consequence of string theory. However, it's worth noting that the multiverse hypothesis is not unique to string theory and has been proposed independently in other areas of physics and cosmology.

String theory gained mainstream popularity in the 1990s, following the publication of several books that aimed to make the theory accessible to a wider audience. One of the most influential of these books was "The Elegant Universe" by physicist Brian Greene, which was published in 1999. The book provided a clear and engaging introduction to string theory, and helped to popularize the idea that the theory could provide a unified description of all the fundamental forces of nature, including gravity. Around the same time, a number of groundbreaking papers were published that showed how string theory could be used to explain some of the puzzles in modern physics, such as the nature of black holes and the origin of the universe. Since then, string theory has continued to be an active area of research, with many physicists working to refine the theory and develop new predictions that could be tested experimentally.

Here are some other influential books on string theory aimed at a popular audience:

1. "The Fabric of the Cosmos" by Brian Greene (2004) - In this book, Greene explores some of the most challenging questions in physics, including the nature of space and time, and how string theory might provide answers to these questions.

2. "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos" by Brian Greene (2011) - This book explores the idea of a multiverse and how it relates to string theory, as well as other areas of physics and cosmology.

3. "The Road to Reality" by Roger Penrose (2004) - Although not strictly about string theory, this book provides an in-depth overview of modern physics, including the mathematical foundations of string theory, and how it fits into our current understanding of the universe.

4. "String Theory and the Fabric of Spacetime" by Gabriele Veneziano (2004) - This book provides a concise and accessible introduction to string theory, including its historical development, key concepts, and current status.

5. "The Trouble with Physics" by Lee Smolin (2006) - While not specifically about string theory, this book examines some of the problems facing theoretical physics, including the apparent lack of experimental evidence for string theory, and offers some alternative approaches to understanding the universe.