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Title: Black holes and the multiverse
Author: Jaume Garriga, Alexander Vilenkin, Jun Zhang
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Vacuum bubbles may nucleate and expand during the inflationary epoch in the early universe. After inflation ends, the bubbles quickly dissipate their kinetic energy; they come to rest with respect to the Hubble flow and eventually form black holes. The fate of the bubble itself depends on the resulting black hole mass. If the mass is smaller than a certain critical value, the bubble collapses to a singularity. Otherwise, the bubble interior inflates, forming a baby universe, which is connected to the exterior FRW region by a wormhole. A similar black hole formation mechanism operates for spherical domain walls nucleating during inflation. As an illustrative example, we studied the black hole mass spectrum in the domain wall scenario, assuming that domain walls interact with matter only gravitationally. Our results indicate that, depending on the model parameters, black holes produced in this scenario can have significant astrophysical effects and can even serve as dark matter or as seeds for supermassive black holes. The mechanism of black hole formation described in this paper is very generic and has important implications for the global structure of the universe. Baby universes inside super-critical black holes inflate eternally and nucleate bubbles of all vacua allowed by the underlying particle physics. The resulting multiverse has a very non-trivial spacetime structure, with a multitude of eternally inflating regions connected by wormholes.

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Cyclic multiverses
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Title: Cyclic multiverses
Author: Konrad Marosek, Mariusz P. Dabrowski, Adam Balcerzak

Starting with the idea of regularisation of singularities due to the variability of the fundamental constants in cosmology we first study the cyclic universe models. We find two models of oscillating mass density and pressure regularised by varying gravitational constant G. Then, we extend this idea onto the multiverse containing cyclic individual universes with either growing or decreasing entropy though leaving the net entropy constant. In order to get the key idea, we consider the doubleverse with the same geometrical evolution of the two "parallel" universes with their physical evolution (physical coupling constants c(t) and G(t)) being different. An interesting point is that there is a possibility to exchange the universes at the point of maximum expansion -- the fact which was already noticed in quantum cosmology. Similar scenario is also possible within the framework of Brans-Dicke theory.

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Brian Cox: 'Multiverse' makes sense

The presenter and physicist Brian Cox says he supports the idea that many universes can exist at the same time.
The idea may sound far-fetched but the "many worlds" concept is the subject of serious debate among physicists.
It is a particular interpretation of quantum mechanics - which describes the often counter-intuitive behaviour of energy and matter at small scales.

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Title: A critical note on time in the multiverse
Authors: Svend E. Rugh, Henrik Zinkernagel

In recent analyses of standard, single-universe, cosmology, it was pointed out that specific assumptions regarding the distribution and motion of matter must be made in order to set up the cosmological standard model with a global time parameter. Relying on these results, we critically examine the notion of time in the multiverse, and in particular the idea that some parts of the multiverse are older than others. By focusing on the most elaborated multiverse proposal in cosmology, the inflationary multiverse, we identify three problems for establishing a physically well-defined notion of global time; a quantum problem, a collision problem and a fractal problem. The quantum problem, and the closely related "cosmic measurement problem", may even undermine the idea that parts of the multiverse causally and temporally precede our universe.

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Eternal Inflation
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Title: Is Eternal Inflation Past-Eternal? And What if It Is?
Authors: Leonard Susskind

As a result of discussions with Bousso and Vilenkin I want to return to the question of whether the multiverse is past-eternal or if there was a beginning. Not surprisingly, given three people, there were three answers. However, the discussions have led to some common ground. The multiverse being past-eternal, or at least extremely old has content and potential phenomenological implications. I will discuss how the oldness of the multiverse is connected with recent speculations of Douglas.

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Title: Cycles in the Multiverse
Authors: Matthew C. Johnson, Jean-Luc Lehners

Eternal inflation is a seemingly generic consequence of theories that give rise to accelerated expansion of the universe and possess multiple vacuum states. Making predictions in an eternally inflating universe is notoriously difficult because one must compare infinite quantities, and a wide variety of regulating procedures yield radically different results. This is the measure problem of eternal inflation. In this paper, we analyse models of eternal inflation which allow for the possibility of cyclic bubble universes: in each bubble, standard cosmological evolution is re-played over and over again. Eternal inflation can generically arise in cyclic models that include a dark energy dominated phase. In such models, several problematic consequences of standard regulating procedures, such as the youngness and Boltzmann Brain problems, are substantially alleviated. We discuss the implications for making predictions in cyclic models, as well as some general implications for understanding the measure problem in eternal inflation.

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Neutrinos and multiverses: a new cosmology beckons

Physics poses some formidable questions that we are so far unable to answer. Why is the universe dominated by matter not antimatter? Why does our universe appear to be "fine-tuned" with just the right properties to give rise to galaxies, stars, planets, life and physicists?
The existing edifice of physics, built upon the twin foundations of general relativity and quantum mechanics, is clearly in need of renovation. We have been waiting for years for cracks to appear that might tell us how to go about it. But up to now, nature has remained stubbornly unmoved.

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Title: Observing the Multiverse with Cosmic Wakes
Authors: Matthew Kleban (NYU), Thomas S. Levi (UBC), Kris Sigurdson (UBC)

Current theories of the origin of the Universe, including string theory, predict the existence of a multiverse containing many bubble universes. These bubble universes will generically collide, and collisions with ours produce cosmic wakes that enter our Hubble volume, appear as unusually symmetric disks in the cosmic microwave background (CMB) and disturb large scale structure (LSS). There is preliminary observational evidence consistent with one or more of these disturbances on our sky. However, other sources can produce similar features in the CMB temperature map and so additional signals are needed to verify their extra-universal origin. Here we find, for the first time, the detailed three-dimensional shape and CMB temperature and polarisation signals of the cosmic wake of a bubble collision in the early universe consistent with current observations. The predicted polarisation pattern has distinctive features that when correlated with the corresponding temperature pattern are a unique and striking signal of a bubble collision. These features represent the first verifiable prediction of the multiverse paradigm and might be detected by current experiments such as Planck and future CMB polarisation missions. A detection of a bubble collision would confirm the existence of the Multiverse, provide compelling evidence for the string theory landscape, and sharpen our picture of the Universe and its origins.

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'Multiverse' theory suggested by microwave background

The idea that other universes - as well as our own - lie within "bubbles" of space and time has received a boost.
Studies of the low-temperature glow left from the Big Bang suggest that several of these "bubble universes" may have left marks on our own.
This "multiverse" idea is popular in modern physics, but experimental tests have been hard to come by.

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When the multiverse and many-worlds collide

Two of the strangest ideas in modern physics - that the cosmos constantly splits into parallel universes in which every conceivable outcome of every event happens, and the notion that our universe is part of a larger multiverse - have been unified into a single theory. This solves a bizarre but fundamental problem in cosmology and has set physics circles buzzing with excitement, as well as some bewilderment.
The problem is the observability of our universe. While most of us simply take it for granted that we should be able to observe our universe, it is a different story for cosmologists. When they apply quantum mechanics - which successfully describes the behaviour of very small objects like atoms - to the entire cosmos, the equations imply that it must exist in many different states simultaneously, a phenomenon called a superposition. Yet that is clearly not what we observe.

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