From Wiki: Multiverse

**
Multiverse**

From Wikipedia, the free encyclopedia

The **
multiverse** (or **meta-universe**) is a
hypothetical
set of various possible
universes
including the universe which humans live in. Together, these universes comprise
everything that exists: the entirety of
space,
time,
matter,
energy,
the
physical
laws
and the
constants
that describe them.^{[1]}^{[2]}^{[3]}^{[4]}
The miscellaneous distinct universes within the multiverse are called the "parallel
universes", "other universes" or "alternative universes".^{[5]}^{[6]}^{[7]}

**
Contents**

- 1 History of the concept
- 2 Brief explanation
- 3 Search for evidence
- 4 Proponents and skeptics
- 5 Arguments against multiverse theories
- 6 Classification schemes
- 7 M-theory
- 8 Black-hole cosmology
- 9 Anthropic principle
- 10 Occam's razor
- 11 Modal realism
- 12 See also
- 13 References
- 14 Bibliography
- 15 External links

**
History of
the concept**

In his book, Opticks (1704), Isaac Newton suggested the idea of a multiverse :

And since
Space is divisible in infinitum, and Matter is not necessarily in all places, it
may be also allow'd that God is able to create Particles of Matter of several
Sizes and Figures, and in several Proportions to Space, and perhaps of different
Densities and Forces, and thereby to vary the Laws of Nature, and make Worlds of
several sorts in several Parts of the Universe. At least, I see nothing of
Contradiction in all this.^{[8]}

In
Dublin
in 1952,
Erwin
Schrödinger
gave a lecture in which he jocularly warned his audience that what he was about
to say might "seem lunatic". He said that when his equations seemed to describe
several different histories, these were "not alternatives, but all really happen
simultaneously".^{[9]}

The
American philosopher and psychologist
William
James
used the term "multiverse" in 1895, but in a different context.^{[10]}

**
Brief
explanation**

The structure of the multiverse, the nature of each universe within it, and the relationships among these universes vary from one multiverse hypothesis to another.

Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology, and literature, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternate universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "parallel worlds", "parallel realities", "quantum realities", "alternate realities", "alternate timelines", "alternate dimensions", and "dimensional planes".

The physics community continues to debate the multiverse hypotheses. Prominent physicists are divided in opinion about whether any other universes exist.

Some
physicists say the multiverse is not a legitimate topic of scientific inquiry.^{[11]}
Concerns have been raised about whether attempts to exempt the multiverse from
experimental verification could erode public confidence in science and
ultimately damage the study of fundamental physics.^{[12]}
Some have argued that the multiverse is a
philosophical
rather than a
scientific
hypothesis
because it cannot be
falsified.
The ability to disprove a theory by means of scientific experiment has always
been part of the accepted
scientific
method.^{[13]}
Paul
Steinhardt
has famously argued that no experiment can rule out a theory if the theory
provides for all possible outcomes.^{[14]}

In 2007,
Nobel laureate
Steven
Weinberg
suggested that if the multiverse existed, "the hope of finding a rational
explanation for the precise values of quark masses and other constants of the
standard model that we observe in our
Big Bang
is doomed, for their values would be an accident of the particular part of the
multiverse in which we live."^{[15]}

**
Search for
evidence**

Around
2010, scientists such as Stephen M. Feeney analyzed
Wilkinson
Microwave Anisotropy Probe
(WMAP) data and claimed to find evidence suggesting that our universe collided
with other (parallel) universes in the distant past.^{[16]}^{[17]}^{[18]}
However, a more thorough analysis of data from the WMAP and from the
Planck
satellite,
which has a resolution 3 times higher than WMAP, did not reveal any
statistically significant evidence of such a
bubble
universe
collision.^{[19]}^{[20]}
In addition, there was no evidence of any gravitational pull of other universes
on ours.^{[21]}^{[22]}

**
Proponents
and skeptics**

Proponents
of one or more of the multiverse hypotheses include
Brian
Greene,^{[23]}^{[24]}
Max
Tegmark,^{[25]}
Alan Guth,^{[26]}
Andrei
Linde,^{[27]}
Michio
Kaku,^{[28]}
David
Deutsch,^{[29]}
Leonard
Susskind,^{[30]}
Alexander
Vilenkin,^{[31]}
Yasunori
Nomura,^{[32]}
Raj
Pathria,^{[33]}
Laura
Mersini-Houghton,^{[34]}^{[35]}
Neil
deGrasse Tyson,^{[36]}
Lawrence
Krauss
and
Sean
Carroll.^{[37]}
Stephen
Hawking
was a proponent when he was still alive.^{[38]}

Scientists
who are generally skeptical of the multiverse hypothesis include:
Steven
Weinberg,^{[39]}
David
Gross,^{[40]}
Paul
Steinhardt,^{[41]}
^{[42]}
Anna Ijjas
^{[42]}
Abraham Loeb
^{[42]}
David
Spergel
^{[43]}
Neil Turok,^{[44]}
Viatcheslav Mukhanov,^{[45]}
Michael S.
Turner,^{[46]}
Roger
Penrose,^{[47]}
George
Ellis,^{[48]}^{[49]}
Joe Silk,^{[50]}
Carlo
Rovelli,^{[51]}
Adam Frank,^{[52]}
Marcelo
Gleiser,^{[52]}
Jim
Baggott,^{[53]}
and
Paul
Davies.^{[54]}

**
Arguments
against multiverse theories**

In his
2003 *New York Times* opinion piece, "A Brief History of the Multiverse",
the author and cosmologist
Paul
Davies
offered a variety of arguments that multiverse theories are non-scientific:^{[55]}

For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there are an infinite number of universes, credibility reaches a limit. As one slips down that slope, more and more must be accepted on faith, and less and less is open to scientific verification. Extreme multiverse explanations are therefore reminiscent of theological discussions. Indeed, invoking an infinity of unseen universes to explain the unusual features of the one we do see is just as ad hoc as invoking an unseen Creator. The multiverse theory may be dressed up in scientific language, but in essence it requires the same leap of faith.

— *Paul
Davies, *
*The New
York Times**,
"A Brief History of the Multiverse"*

George Ellis, writing in August 2011, provided a criticism of the multiverse, and pointed out that it is not a traditional scientific theory. He accepts that the multiverse is thought to exist far beyond the cosmological horizon. He emphasized that it is theorized to be so far away that it's unlikely any evidence will ever be found. Ellis also explained that some theorists do not believe the lack of empirical testability falsifiability is a major concern, but he is opposed to that line of thinking:

Many physicists who talk about the multiverse, especially advocates of the string landscape, do not care much about parallel universes per se. For them, objections to the multiverse as a concept are unimportant. Their theories live or die based on internal consistency and, one hopes, eventual laboratory testing.

Ellis says
that scientists have proposed the idea of the multiverse as a way of explaining
the nature of existence. He points out that it ultimately leaves those questions
unresolved because it is a
metaphysical
issue that cannot be resolved by empirical science. He argues that observational
testing is at the core of science and should not be abandoned:^{[56]}

As skeptical as I am, I think the contemplation of the multiverse is an excellent opportunity to reflect on the nature of science and on the ultimate nature of existence: why we are here.... In looking at this concept, we need an open mind, though not too open. It is a delicate path to tread. Parallel universes may or may not exist; the case is unproved. We are going to have to live with that uncertainty. Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is.

— *George
Ellis, *
*
Scientific American**,
"Does the Multiverse Really Exist?"*

**
Classification schemes**

Max Tegmark and Brian Greene have devised classification schemes for the various theoretical types of multiverses and universes that they might comprise.

**
Max
Tegmark's four levels**

Cosmologist
Max
Tegmark
has provided a
taxonomy
of universes beyond the familiar
observable
universe.
The four levels of Tegmark's classification are arranged such that subsequent
levels can be understood to encompass and expand upon previous levels. They are
briefly described below.^{[57]}^{[58]}

**
Level I:
An extension of our Universe**

A prediction of chaotic inflation is the existence of an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.

Accordingly, an infinite universe will contain an infinite number of Hubble
volumes, all having the same
physical
laws
and
physical
constants.
In regard to configurations such as the distribution of
matter,
almost all will differ from our Hubble volume. However, because there are
infinitely many, far beyond the
cosmological horizon,
there will eventually be Hubble volumes with similar, and even identical,
configurations. Tegmark estimates that an identical volume to ours should be
about
10^{10115}
meters away from us.^{[25]}

Given
infinite space, there would, in fact, be an infinite number of Hubble volumes
identical to ours in the universe.^{[59]}
This follows directly from the
cosmological principle,
wherein it is assumed that our Hubble volume is not special or unique.

**
Level II:
Universes with different physical constants**

Bubble
universes
– every disk represents a bubble universe. Our universe is represented by one of
the disks.

Universe 1 to Universe 6 represent bubble universes. Five of them have different
physical
constants
than our universe has.

In the
chaotic
inflation
theory, which is a variant of the
cosmic
inflation
theory, the multiverse or space as a whole is stretching and will continue doing
so forever,^{[60]}
but some regions of space stop stretching and form distinct bubbles (like gas
pockets in a loaf of rising bread). Such bubbles are embryonic level I
multiverses.

Different
bubbles may experience different
spontaneous symmetry breaking,
which results in different properties, such as different
physical
constants.^{[59]}

Level II also includes John Archibald Wheeler's oscillatory universe theory and Lee Smolin's fecund universes theory.

**
Level III:
Many-worlds interpretation of quantum mechanics**

Hugh Everett III's many-worlds interpretation (MWI) is one of several mainstream interpretations of quantum mechanics.

In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different probability. According to the MWI, each of these possible observations corresponds to a different universe. Suppose a six-sided die is thrown and that the result of the throw corresponds to a quantum mechanics observable. All six possible ways the die can fall correspond to six different universes.

Tegmark argues that a Level III multiverse does not contain more possibilities in the Hubble volume than a Level I or Level II multiverse. In effect, all the different "worlds" created by "splits" in a Level III multiverse with the same physical constants can be found in some Hubble volume in a Level I multiverse. Tegmark writes that, "The only difference between Level I and Level III is where your doppelgängers reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional Hilbert space."

Similarly,
all Level II bubble universes with different physical constants can, in effect,
be found as "worlds" created by "splits" at the moment of spontaneous symmetry
breaking in a Level III multiverse.^{[59]}
According to
Yasunori
Nomura,^{[32]}
Raphael
Bousso,
and
Leonard
Susskind,^{[30]}
this is because global spacetime appearing in the (eternally) inflating
multiverse is a redundant concept. This implies that the multiverses of Levels
I, II, and III are, in fact, the same thing. This hypothesis is referred to as
"Multiverse = Quantum Many Worlds".

Related to
the *many-worlds* idea are
Richard
Feynman's
*
multiple histories*
interpretation and
H. Dieter
Zeh's
*
many-minds*
interpretation.

**
Level IV:
Ultimate ensemble**

The
ultimate
mathematical universe hypothesis
is Tegmark's own hypothesis.^{[61]}

This level considers all universes to be equally real which can be described by different mathematical structures.

Tegmark writes:

Abstract mathematics is so general that any Theory Of Everything (TOE) which is definable in purely formal terms (independent of vague human terminology) is also a mathematical structure. For instance, a TOE involving a set of different types of entities (denoted by words, say) and relations between them (denoted by additional words) is nothing but what mathematicians call a set-theoretical model, and one can generally find a formal system that it is a model of.

He argues
that this "implies that any conceivable parallel universe theory can be
described at Level IV" and "subsumes all other ensembles, therefore brings
closure to the hierarchy of multiverses, and there cannot be, say, a Level V."^{[25]}

Jürgen Schmidhuber, however, says that the set of mathematical structures is not even well-defined and that it admits only universe representations describable by constructive mathematics—that is, computer programs.

Schmidhuber explicitly includes universe representations describable by
non-halting programs whose output bits converge after finite time, although the
convergence time itself may not be predictable by a halting program, due to the
undecidability
of the
halting
problem.^{[62]}^{[63]}^{[64]}
He also explicitly discusses the more restricted ensemble of quickly computable
universes.^{[65]}

**
Brian
Greene's nine types**

The
American
theoretical physicist
and
string
theorist
Brian
Greene
discussed nine types of multiverses:^{[66]}

**
Quilted**

The quilted multiverse works only in an infinite universe. With an infinite amount of space, every possible event will occur an infinite number of times. However, the speed of light prevents us from being aware of these other identical areas.

**
Inflationary**

The inflationary multiverse is composed of various pockets in which inflation fields collapse and form new universes.

**
Brane
**
the** **
brane multiverse
version postulates that our entire universe exists on a membrane (brane)
which floats in a higher dimension or "bulk". In this bulk, there are other
membranes with their own universes. These universes can interact with one
another, and when they collide, the violence and energy produced is more than
enough to give rise to a
big bang.
The branes float or drift near each other in the bulk, and every few trillion
years, attracted by gravity or some other force we do not understand, collide
and bang into each other. This repeated contact gives rise to multiple or "cyclic"
big bangs.
This particular hypothesis falls under the string theory umbrella as it requires
extra spatial dimensions.

**
Cyclic**

The cyclic multiverse (via the ekpyrotic scenario) has multiple branes (each a universe) that have collided, causing Big Bangs. The universes bounce back and pass through time until they are pulled back together and again collide, destroying the old contents and creating them anew.

**
Landscape**

The landscape multiverse relies on string theory's Calabi–Yau spaces. Quantum fluctuations drop the shapes to a lower energy level, creating a pocket with a set of laws different from that of the surrounding space.

**
Quantum**

The quantum multiverse creates a new universe when a diversion in events occurs, as in the many-worlds interpretation of quantum mechanics.

**
Holographic**

The holographic multiverse is derived from the theory that the surface area of a space can simulate the volume of the region.

**
Simulated**

The simulated multiverse exists on complex computer systems that simulate entire universes.

**
Ultimate**

The ultimate multiverse contains every mathematically possible universe under different laws of physics.

**
Cyclic
theories**

*
Main
article: *
*Cyclic
model*

In several theories, there is a series of infinite, self-sustaining cycles (for example, an eternity of Big Bangs, Big Crunches, and/or Big Freezes).

**
M-theory**

*
See also:
*
*
Introduction to M-theory**,
*
*
M-theory**,
*
*Brane
cosmology**,
and *
*String
theory landscape*

A
multiverse of a somewhat different kind has been envisaged within
string
theory
and its higher-dimensional extension,
M-theory.^{[67]}

These
theories require the presence of 10 or 11 spacetime dimensions respectively. The
extra 6 or 7 dimensions may either be compactified on a very small scale, or our
universe may simply be localized on a dynamical (3+1)-dimensional object, a
D3-brane.
This opens up the possibility that there are other
branes
which could support other universes.^{[68]}^{[69]}

**
Black-hole
cosmology**

*
Main
article: *
*
Black-hole cosmology*

Black-hole
cosmology
is a cosmological model in which the
observable
universe
is the interior of a
black hole
existing as one of possibly many universes inside a larger universe.^{[}^{citation
needed}^{]}
This includes the theory of
white
holes,
which are on the opposite side of
space-time.

**
Anthropic
principle**

*
Main
article: *
*
Anthropic principle*

The concept of other universes has been proposed to explain how our own universe appears to be fine-tuned for conscious life as we experience it.

If there were a large (possibly infinite) number of universes, each with possibly different physical laws (or different fundamental physical constants), then some of these universes (even if very few) would have the combination of laws and fundamental parameters that are suitable for the development of matter, astronomical structures, elemental diversity, stars, and planets that can exist long enough for life to emerge and evolve.

The weak anthropic principle could then be applied to conclude that we (as conscious beings) would only exist in one of those few universes that happened to be finely tuned, permitting the existence of life with developed consciousness. Thus, while the probability might be extremely small that any particular universe would have the requisite conditions for life (as we understand life), those conditions do not require intelligent design as an explanation for the conditions in the Universe that promote our existence in it.

An early
form of this reasoning is evident in
Arthur
Schopenhauer's
1844 work "Von der Nichtigkeit und dem Leiden des Lebens", where he argues that
our world must be the worst of all possible worlds, because if it were
significantly worse in any respect it could not continue to exist.^{[70]}

**
Occam's
razor**

Proponents
and critics disagree about how to apply
Occam's
razor.
Critics argue that to postulate an almost infinite number of unobservable
universes, just to explain our own universe, is contrary to Occam's razor.^{[71]}
However, proponents argue that in terms of
Kolmogorov
complexity
the proposed multiverse is simpler than a single idiosyncratic universe.^{[59]}

For example, multiverse proponent Max Tegmark argues:

[A]n
entire
ensemble
is often much simpler than one of its members. This principle can be stated more
formally using the notion of
algorithmic information
content. The algorithmic information content in a number is, roughly speaking,
the length of the shortest computer program that will produce that number as
output. For example, consider the
set
of all
integers.
Which is simpler, the whole set or just one number? Naively, you might think
that a single number is simpler, but the entire set can be generated by quite a
trivial computer program, whereas a single number can be hugely long. Therefore,
the whole set is actually simpler... (Similarly), the higher-level multiverses
are simpler. Going from our universe to the Level I multiverse eliminates the
need to specify
initial
conditions,
upgrading to Level II eliminates the need to specify
physical
constants,
and the Level IV multiverse eliminates the need to specify anything at all.... A
common feature of all four multiverse levels is that the simplest and arguably
most elegant theory involves parallel universes by default. To deny the
existence of those universes, one needs to complicate the theory by adding
experimentally unsupported processes and ad hoc postulates:
finite
space,
wave
function collapse
and ontological asymmetry. Our judgment therefore comes down to which we find
more wasteful and inelegant: many worlds or many words. Perhaps we will
gradually get used to the weird ways of our cosmos and find its strangeness to
be part of its charm.^{[59]}^{[72]}

— *Max
Tegmark*

**
Modal
realism**

Possible
worlds
are a way of explaining probability and hypothetical statements. Some
philosophers, such as
David
Lewis,
believe that all possible worlds exist and that they are just as real as the
world we live in (a position known as
modal
realism).^{[73]}