Recent developments in cosmology and particle physics, such as the string landscape picture, have led to the remarkable realization that our universe - rather than being unique - could be just one of many universes. The multiverse proposal helps to explain the origin of the universe and some of its observational features. Since the physical constants can be different in other universes, the fine-tunings which appear necessary for the emergence of life may also be explained. Nevertheless, many physicists remain uncomfortable with the multiverse proposal, since it is highly speculative and perhaps untestable. In this volume, a number of active and eminent researchers in the field - mainly cosmologists and particle physicists but also some philosophers - address these issues and describe recent developments. The articles represent the full spectrum of views, providing for the first time an overview of the subject. They are written at different academic levels, engaging lay-readers and researchers alike.

Table of Contents

Part I Overviews

1. Introduction and overview

2. Living in the multiverse

3. Enlightenment, knowledge, ignorance, temptation

Part II Cosmology and astrophysics

4. Cosmology and the multiverse

5. the Anthropic Principle revisited

6. Cosmology from the top down

7. The multiverse hierarchy

8. The inflationary multiverse

9. A model of anthropic reasoning: the dark to ordinary matter ratio

10. Anthropic predictions: the case of the cosmological constant

11. The definition and classification of universes

12. M/string theory and anthropic reasoning

13. The anthropic principle, dark energy and the LHC

Part Ill Particle physics and quantum theory

14. Quarks, electrons and atoms in closely related universes

15. The fine-tuning problems of particle physics and anthropic mechanisms

16. The anthropic landscape of strinq theory

17. Cosmology and the many worlds interpretation of quantum mechanics

18. Anthropic reasoning and quantum cosmology

19. Micro-anthropic principle for quantum theory

Part IV More general philosophical issues

20. Scientific alternatives to the anthropic principle

21. Making predictions in a multiverse: conundrums, dangers, coincidences

22. Multiverses: description, uniqueness and testing

23. Predictions and tests of multiverse theories

24. Observation selection theory and cosmological fine-tuning

25. are anthropic arguments, involving multiverses and beyond, legitimate?

26. the multiverse hypothesis: a theistic perspective

27. Living in a simulated universe

28. Universes galore: where will it all end?

About the Author

Bernard Carr is a Professor at Queen Mary, University of London. He received the Adams prize in 1984, one of the UK's most prestigious mathematical awards. He is a member of several professional societies, most notably the Royal Astronomical Society. He has worked in the UK, USA, Japan and Canada, and has published nearly 200 research papers and dozens of popular science articles. He regularly appears in the media.