The possibility that our universe is not the only universe is one of the most polarizing theories in physics today. Speculative and controversial, this so-called multiverse proposal is suggested by the latest developments in cosmology and string theory, and is supported by some of the most regarded physicists. But are these theories rightly considered scientific? Is it possible to gather evidence that would support or refute the possibility that we inhabit a multiverse?

*The World Science Festival’s annual salon series offers in-depth conversations with leading scientists, extending the discussion of the Festival’s premiere public programs to graduate students, postdocs, faculty and well-informed members of the general public. *

Astrophysicist

Mario Livio is an internationally known astrophysicist, a best-selling author and a popular lecturer. His popular book The *Golden Ratio: The Story of Phi, the World’s Most Astonishing Number* won the Peano Prize for 2003, and the International Pythagoras Prize for 2004, as the best popular book on mathematics, while his* Is God A Mathematician? *was selected by the Washington Post as one of the best books of 2009.

Livio is a senior astrophysicist at the Space Telescope Science Institute (STScI), which conducts the scientific program of the Hubble Space Telescope, and will conduct the program for the upcoming James Webb Space Telescope. He has a regular blog, A Curious Mind, about science, art, and the links between them.

During the past decade, Livio’s research focused on supernova explosions and their use in cosmology to determine the rate of expansion of the universe, and the nature of the “dark energy” that causes the cosmic expansion to accelerate. He has also done extensive work on extrasolar planets.

Livio’s recent book, *Brilliant Blunders* (2013), was a New York Times Bestseller, and was selected by the Washington Post as one of the “Best Books” of 2013.

Theoretical Cosmologist

Andreas Albrecht is a leading theoretical cosmologist. He received his Ph.D. from the University of Pennsylvania in 1983 where, with Paul Steinhardt, he wrote one of the original papers on “new” or “slow roll” inflation. Slow roll inflation has since become the dominant theory of the early universe and has passed numerous observational tests with flying colors. Deep puzzles remain regarding the theoretical underpinnings of cosmic inflation and Albrecht is a leading figure in this research area. The discovery of cosmic acceleration (often referred to as “the dark energy”) has been transformative to the field of cosmology and Albrecht is known for his groundbreaking work on dark energy theory and phenomenology. Albrecht moved from a professorship at Imperial College to UC Davis in 1998 to build the cosmology program there. He is now physics department chair at UC Davis. He is a fellow of both the American Physical Society and the Institute of Physics.

Physicist

Raphael Bousso is recognized for discovering the general relation between the curved geometry of space-time and its information content, known as the “covariant entropy bound.” This allowed for a precise and general formulation of the holographic principle, which is believed to underlie the unification of quantum theory and Einstein’s theory of gravity. Bousso is also one of the discoverers of the landscape of string theory, which explains the small but non-vanishing value of the cosmological constant (or “dark energy”). His work has led to a novel view of cosmology, the multiverse of string theory. Bousso is currently professor of physics at the University of California, Berkeley.

Cosmologist

Alan Guth is a professor of physics at MIT, and world-renowned for his discovery of inflationary cosmology, the dominant cosmological paradigm for over two decades. His current research focuses on developing mathematical tools for quantitatively analyzing inflation’s suggestion that there are an infinite number of universes.

Cosmologist

Andrei Linde is a professor of physics at Stanford University, one of the authors of the inflationary theory and the theory of inflationary multiverse. He invented the theory of chaotic inflation, which is the most general form of inflationary cosmology. Linde also helped to develop the mechanism of vacuum stabilization in string theory, which helped to incorporate the theory of inflationary multiverse in the context of string theory. He is the author of the books *Inflation and Quantum Cosmology* and *Particle Physics and Inflationary Cosmology*. His honors include the Dirac Medal, Peter Gruber Prize, and the Fundamental Physics Prize.

Physicist

Neil Turok develops and tests fundamental theories of the cosmos. He pioneered the open inflation and cyclic universe models, as well as observational tests for dark energy and the nature of the relic perturbations using the microwave sky, both subsequently confirmed. Currently, he is developing a mathematically complete description of the Big Bang singularity and how a universe can pass through it, as well as a new, non-inflationary model of the Big Bang.

After his Ph.D. at Imperial, Turok was associate scientist at Fermilab, professor of physics at Princeton and chair of mathematical physics in Cambridge. He is now director and Niels Bohr Chair at the Perimeter Institute for Theoretical Physics in Ontario, Canada. His awards include the James Clerk Maxwell medal and the 2008 TED prize for his work in cosmology, and for founding the African Institute for Mathematical Sciences. More recently, Turok gave the 2012 CBC Massey Lectures, published as *The Universe Within: From Quantum to Cosmos*.

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