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Title: Is the Multiverse Hypothesis capable of explaining the Fine Tuning of Nature Laws and Constants? The Case of Cellular Automata
Authors: Francisco José Soler Gil, Manuel Alfonseca

The objective of this paper is analysing to which extent the multiverse hypothesis provides a real explanation of the peculiarities of the laws and constants in our universe. First we argue in favour of the thesis that all multiverses except Tegmark's <<mathematical multiverse>> are too small to explain the fine tuning, so that they merely shift the problem up one level. But the <<mathematical multiverse>> is surely too large. To prove this assessment, we have performed a number of experiments with cellular automata of complex behaviour, which can be considered as universes in the mathematical multiverse. The analogy between what happens in some automata (in particular Conway's <<Game of Life>>) and the real world is very strong. But if the results of our experiments can be extrapolated to our universe, we should expect to inhabit -- in the context of the multiverse -- a world in which at least some of the laws and constants of nature should show a certain time dependence. Actually, the probability of our existence in a world such as ours would be mathematically equal to zero. In consequence, the results presented in this paper can be considered as an inkling that the hypothesis of the multiverse, whatever its type, does not offer an adequate explanation for the peculiarities of the physical laws in our world.

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Title: The Multiverse Interpretation of Quantum Mechanics
Authors: Raphael Bousso, Leonard Susskind

We argue that the many-worlds of quantum mechanics and the many worlds of the multiverse are the same thing, and that the multiverse is necessary to give exact operational meaning to probabilistic predictions from quantum mechanics.
Decoherence - the modern version of wave-function collapse - is subjective in that it depends on the choice of a set of unmonitored degrees of freedom, the "environment". In fact decoherence is absent in the complete description of any region larger than the future light-cone of a measurement event. However, if one restricts to the causal diamond - the largest region that can be causally probed - then the boundary of the diamond acts as a one-way membrane and thus provides a preferred choice of environment. We argue that the global multiverse is a representation of the many-worlds (all possible decoherent causal diamond histories) in a single geometry.
We propose that it must be possible in principle to verify quantum-mechanical predictions exactly. This requires not only the existence of exact observables but two additional postulates: a single observer within the universe can access infinitely many identical experiments; and the outcome of each experiment must be completely definite. In causal diamonds with finite surface area, holographic entropy bounds imply that no exact observables exist, and both postulates fail: experiments cannot be repeated infinitely many times; and decoherence is not completely irreversible, so outcomes are not definite. We argue that our postulates can be satisfied in "hats" (supersymmetric multiverse regions with vanishing cosmological constant). We propose a complementarity principle that relates the approximate observables associated with finite causal diamonds to exact observables in the hat.

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Title: Statistical Evidence Against Simple Forms of Wavefunction Collapse
Authors: Don N. Page

If the initial quantum state of the universe is a multiverse superposition over many different sets of values of the effective coupling "constants" of physics, and if this quantum state collapses to an eigenstate of the set of coupling "constants" with a probability purely proportional to the absolute square of the amplitude (with no additional factor for something like life or consciousness), then one should not expect that the coupling 'constants' would be so biophilic as they are observed to be. Therefore, the observed biophilic values (apparent fine tuning) of the coupling "constants" is statistical evidence against such simple forms of wavefunction collapse.

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Title: First Observational Tests of Eternal Inflation
Authors: Stephen M. Feeney (UCL), Matthew C. Johnson (Perimeter Institute), Daniel J. Mortlock (Imperial College London), Hiranya V. Peiris (UCL)

The eternal inflation scenario predicts that our observable universe resides inside a single bubble embedded in a vast multiverse, the majority of which is still undergoing super-accelerated expansion. Many of the theories giving rise to eternal inflation predict that we have causal access to collisions with other bubble universes, opening up the possibility that observational cosmology can probe the dynamics of eternal inflation. We present the first observational search for the effects of bubble collisions, using cosmic microwave background data from the WMAP satellite. Using a modular algorithm that is designed to avoid a posteriori selection effects, we find four features on the CMB sky that are consistent with being bubble collisions. If this evidence is corroborated by upcoming data from the Planck satellite, we will be able to gain insight into the possible existence of the multiverse.

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Title: Boundary definition of a multiverse measure
Authors: Raphael Bousso, Ben Freivogel, Stefan Leichenauer, Vladimir Rosenhaus

We propose to regulate the infinities of eternal inflation by relating a late time cut-off in the bulk to a short distance cut-off on the future boundary. The light-cone time of an event is defined in terms of the volume of its future light-cone on the boundary. We seek an intrinsic definition of boundary volumes that makes no reference to bulk structures. This requires taming the fractal geometry of the future boundary, and lifting the ambiguity of the conformal factor. We propose to work in the conformal frame in which the boundary Ricci scalar is constant. We explore this proposal in the FRW approximation for bubble universes. Remarkably, we find that the future boundary becomes a round three-sphere, with smooth metric on all scales. Our cut-off yields the same relative probabilities as a previous proposal that defined boundary volumes by projection into the bulk along timelike geodesics. Moreover, it is equivalent to an ensemble of causal patches defined without reference to bulk geodesics. It thus yields a holographically motivated and phenomenologically successful measure for eternal inflation.

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A measure for the multiverse

When cosmologist George Ellis turned 70 last year, his friends held a party to celebrate. There were speeches and drinks and canapés aplenty to honour the theorist from the University of Cape Town, South Africa, who is regarded as one of the world's leading experts on general relativity. But there the similarity to most parties ends.
For a start, Ellis's celebration at the University of Oxford lasted for three days and the guest list was made up entirely of physicists, astronomers and philosophers of science. They had gathered to debate what Ellis considers the most dangerous idea in science: the suggestion that our universe is but a tiny part of an unimaginably large and diverse multiverse.

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Looking for Life in the Multiverse

*  Multiple other universes - each with its own laws of physics - may have emerged from the same primordial vacuum that gave rise to ours.
* Assuming they exist, many of those universes may contain intricate structures and perhaps even some forms of life.
* These findings suggest that our universe may not be as "finely tuned" for the emergence of life as previously thought.

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Title: How many universes are in the multiverse?
Authors: Andrei Linde, Vitaly Vanchurin

We argue that the total number of distinguishable locally Friedmann "universes" generated by eternal inflation is proportional to the exponent of the entropy of inflationary perturbations and is limited by e^{e^{3 N}}, where N is the number of e-folds of slow-roll post-eternal inflation. For simplest models of chaotic inflation, N is approximately equal to de Sitter entropy at the end of eternal inflation; it can be exponentially large. However, not all of these universes can be observed by a local observer. We show that in the presence of a cosmological constant \Lambda an observable entropy of the cosmological perturbations, as well as the entropy of usual matter, is bounded by |\Lambda|^{-¾}. In the context of the string theory landscape, the overall number of different universes is expected to be exponentially greater than the total number of vacua in the landscape. We discuss the possibility that the strongest constraint on the number of distinguishable universes may be related not to the properties of the multiverse but to the properties of observers.

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Lee Smolin, author of the bestselling science book 'The Trouble with Physics' and a founding member and research physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, writes exclusively in the June issue of Physics World explaining why theories of cosmology that suggest that our universe is just one of many - the so-called multiverse - and thus perpetuate the notion that time does not exist are flawed.
Smolin explains how theories describing a myriad of possible universes, with less or more dimensions and different kinds of particles and forces, have become increasingly popular in the last few years. However, through his work with the Brazilian philosopher Roberto Mangabeira Unger, Smolin believes that, despite there being good reasons for the conclusion that we live in a timeless multiverse, those theories, and the concomitant assumption that time is not a fundamental concept, are "profoundly mistaken".

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Brian Greene spent a good part of the last decade extolling the virtues of string theory. He dreamed that one day it would provide physicists with a theory of everything that would describe our universe - ours and ours alone. His bestselling book The Elegant Universe eloquently captured the quest for this ultimate theory.

"But the fly in the ointment was that string theory allowed for, in principle, many universes" - Brian Greene, theoretical physicist at Columbia University in New York.

In other words, string theory seems equally capable of describing universes very different from ours. Greene hoped that something in the theory would eventually rule out most of the possibilities and single out one of these universes as the real one: ours.

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