Dear Colleagues,

The idea of the Multiverse, as a collection of possible universes, has entered the area of physics and cosmology for good. Max Tegmark distinguished four levels of this very notion, starting from a collection of standard inflationary patches of space-time to finally defining it as a collection of all possible mathematical entities. The term 'Multiverse' was first suggested by a philosopher—William James—who said ''Visible nature is all plasticity and indifference, A MULTIVERSE, as one might call it, and not a universe'' (''Is life worth living?'', International Journal of Ethics 6, 10 (1895)).

The diversity of possible physical shapes of a universe within the multiverse can be interpreted in terms of diversity of possible ways to choose physical parameters and can be related to the issue of varying physical constants and varying physical laws. One approach is given by the superstring theory which led physicists to an idea of superstring landscape or many ways to choose the vacua after the symmetry breaking. An important idea related to the Multiverse is the Anthropic Principle which, despite being in some sense tautological, can give some insight and possible constraints onto the nature of the physics we experience here in 'Our Universe' whatever it is. 

Anyway, one can consider the studies related to the concept of the Multiverse as a new revolution that can change the current paradigm in cosmology. It can be understood as the next step in the Copernican transit, where our habitat has lost relevance gradually as unique or special. As it was already mentioned, the notion of the multiverse emerges naturally from some developments in cosmology and particle physics. It is not a theory by itself, so, there is not a closed scenario or definition of it. It always depends on the definition of what we mean under the notion of the universe (cf. Max's Tegmark discussion).

However, operationally it seems that the consideration of the idea of the multiverse can provide a solution to several open problems in physics. So we are interested in the different approaches and proposals regarding this issue. The most important point in order to make it a physical theory is the possibility to falsify this idea by some observational or experimental data. This seems to be the biggest challenge of the multiverse hypothesis.

Hoping it will serve as a basic and updated reference, this Special Issue will cover all current research avenues on the exciting track to the Multiverse starting from philosophy, throughout the theory, to its possible observational verification. The following topics will be the core of the volume:

general ideas, history or philosophy, Multiverse from string theory and pocket universes, cyclic universes, chaotic inflation and bubble universes, mediocrity principle and probability theory, classical and quantum multiverse, varying constants and alternative theories, observability and falsifiability of the idea of the Multiverse and related topics.

We hope you will enjoy writing and later reading the collected articles.

Yours sincerely,

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