The multiverse is the hypothetical set of
infinite or finite possible universes that
together comprise everything that exists:
the entirety of space, time, matter, and energy
as well as the physical laws and constants
that describe them.
The various universes within the multiverse
are sometimes called parallel universes.
The structure of the multiverse, the nature
of each universe within it and the relationships
among the various constituent universes, depend
on the specific multiverse hypothesis considered.
Multiple universes have been hypothesized
in cosmology, physics, astronomy, religion,
philosophy, transpersonal psychology and fiction,
particularly in science fiction and fantasy.
In these contexts, parallel universes are
also called "alternative universes", "quantum
universes", "interpenetrating dimensions",
"parallel dimensions", "parallel worlds",
"alternative realities", "alternative timelines",
and "dimensional planes," among others.
The term 'multiverse' was coined in 1895 by
the American philosopher and psychologist
William James in a different context.
The multiverse hypothesis is a source of disagreement
within the physics community.
Physicists disagree about whether the multiverse
exists, and whether the multiverse is a proper
subject of scientific inquiry.
Supporters of one of the multiverse hypotheses
include Stephen Hawking, Steven Weinberg,
Brian Greene, Max Tegmark, Alan Guth, Andrei
Linde, Michio Kaku, David Deutsch, Leonard
Susskind, Raj Pathria, Sean Carroll and Alex
Vilenkin.
In contrast, critics such as David Gross,
Paul Steinhardt, George Ellis and Paul Davies
have argued that the multiverse question is
philosophical rather than scientific, or even
that the multiverse hypothesis is harmful
or pseudoscientific.
Multiverse hypotheses in physics
Categories
Max Tegmark and Brian Greene have devised
classification schemes that categorize the
various theoretical types of multiverse, or
types of universe that might theoretically
comprise a multiverse ensemble.
Max Tegmark's four levels
Cosmologist Max Tegmark has provided a taxonomy
of universes beyond the familiar observable
universe.
The levels according to Tegmark's classification
are arranged such that subsequent levels can
be understood to encompass and expand upon
previous levels, and they are briefly described
below.
Level I: Beyond our cosmological horizon
A generic prediction of chaotic inflation
is 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 1010115 meters away
from us.
Given infinite space, there would, in fact,
be an infinite number of Hubble volumes identical
to ours in the universe.
This follows directly from the cosmological
principle, wherein it is assumed our Hubble
volume is not special or unique.
Level II: Universes with different physical
constants
In the chaotic inflation theory, a variant
of the cosmic inflation theory, the multiverse
as a whole is stretching and will continue
doing so forever, 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.
Linde and Vanchurin calculated the number
of these universes to be on the scale of 101010,000,000.
Different bubbles may experience different
spontaneous symmetry breaking resulting in
different properties such as different physical
constants.
This level 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's many-worlds interpretation
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 die is thrown that contains six
sides and that the numeric 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-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.
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 ensemble or mathematical universe
hypothesis is the hypothesis of Tegmark himself.
This level considers equally real all universes
that can be described by different mathematical
structures.
Tegmark writes that "abstract mathematics
is so general that any Theory Of Everything
that is definable in purely formal terms is
also a mathematical structure.
For instance, a TOE involving a set of different
types of entities and relations between them
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 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."
Jürgen Schmidhuber, however, says the "set
of mathematical structures" is not even well-defined,
and admits only universe representations describable
by constructive mathematics, that is, computer
programs.
He 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 Kurt Gödel's
limitations.
He also explicitly discusses the more restricted
ensemble of quickly computable universes.
Brian Greene's nine types
American theoretical physicist and string
theorist Brian Greene discussed nine types
of parallel universes:
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 where inflation fields collapse
and form new universes.
Brane
The brane multiverse follows from M-theory
and states that each universe is a 3-dimensional
brane that exists with many others.
Particles are bound to their respective branes
except for gravity.
Cyclic
The cyclic multiverse has multiple branes
that 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 shapes.
Quantum fluctuations drop the shapes to a
lower energy level, creating a pocket with
a different set of laws from 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
In several theories there is a series of infinite,
self-sustaining cycles.
M-theory
A multiverse of a somewhat different kind
has been envisaged within string theory and
its higher-dimensional extension, M-theory.
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-dimensional
object, a D-brane.
This opens up the possibility that there are
other branes which could support "other universes".
This is unlike the universes in the "quantum
multiverse", but both concepts can operate
at the same time.
Some scenarios postulate that our big bang
was created, along with our universe, by the
collision of two branes.
Black-hole cosmology
A 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 inside a larger universe.
Anthropic principle
The concept of other universes has been proposed
to explain how our Universe appears to be
fine-tuned for conscious life as we experience
it.
If there were a large number of universes,
each with possibly different physical laws,
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 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 to emerge
and evolve, this does not require intelligent
design per the teleological argument as the
only explanation for the conditions in the
Universe that promote our existence in it.
Search for evidence
Around 2010, scientists such as Stephen M.
Feeney analyzed Wilkinson Microwave Anisotropy
Probe data and claimed to find preliminary
evidence suggesting that our universe collided
with other universes in the distant past.
However, a more thorough analysis of data
from the WMAP and from the Planck satellite,
which has a resolution 3 times higher than
WMAP, failed to find any statistically significant
evidence of such a bubble universe collision.
In addition, there is no evidence of any gravitational
pull of other universes on ours.
Criticism
Non-scientific claims
In his 2003 NY Times opinion piece, A Brief
History of the Multiverse, author and cosmologist,
Paul Davies, offers a variety of arguments
that multiverse theories are non-scientific :
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, A Brief History of the Multiverse
Taking cosmic inflation as a popular case
in point, George Ellis, writing in August
2011, provides a balanced criticism of not
only the science, but as he suggests, the
scientific philosophy, by which multiverse
theories are generally substantiated.
He, like most cosmologists, accepts Tegmark's
level I "domains", even though they lie far
beyond the cosmological horizon.
Likewise, the multiverse of cosmic inflation
is said to exist very far away.
It would be so far away, however, that it's
very unlikely any evidence of an early interaction
will be found.
He argues that for many theorists, the lack
of empirical testability or falsifiability
is not a major concern.
“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.”
Although he believes there's little hope that
will ever be possible, he grants that the
theories on which the speculation is based,
are not without scientific merit.
He concludes that multiverse theory is a “productive
research program”:
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?
Occam's razor
Proponents and critics disagree about how
to apply Occam's razor.
Critics argue that to postulate a practically
infinite number of unobservable universes
just to explain our own seems contrary to
Occam's razor.
In contrast, proponents argue that, in terms
of Kolmogorov complexity, the proposed multiverse
is simpler than a single idiosyncratic universe.
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...,
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.
— Max Tegmark, "Parallel universes.
Not just a staple of science fiction, other
universes are a direct implication of cosmological
observations."
Scientific American 2003 May;288(5):40–51
Princeton cosmologist Paul Steinhardt used
the 2014 Annual Edge Question to voice his
opposition to multiverse theorizing:
A pervasive idea in fundamental physics and
cosmology that should be retired: the notion
that we live in a multiverse in which the
laws of physics and the properties of the
cosmos vary randomly from one patch of space
to another.
According to this view, the laws and properties
within our observable universe cannot be explained
or predicted because they are set by chance.
Different regions of space too distant to
ever be observed have different laws and properties,
according to this picture.
Over the entire multiverse, there are infinitely
many distinct patches.
Among these patches, in the words of Alan
Guth, "anything that can happen will happen—and
it will happen infinitely many times".
Hence, I refer to this concept as a Theory
of Anything.
Any observation or combination of observations
is consistent with a Theory of Anything.
No observation or combination of observations
can disprove it.
Proponents seem to revel in the fact that
the Theory cannot be falsified.
The rest of the scientific community should
be up in arms since an unfalsifiable idea
lies beyond the bounds of normal science.
Yet, except for a few voices, there has been
surprising complacency and, in some cases,
grudging acceptance of a Theory of Anything
as a logical possibility.
The scientific journals are full of papers
treating the Theory of Anything seriously.
What is going on?
— Paul Steinhardt, "Theories of Anything"
edge.com'
Steinhardt claims that multiverse theories
have gained currency mostly because too much
has been invested in theories that have failed,
e.g. inflation or string theory.
He tends to see in them an attempt to redefine
the values of science to which he objects
even more strongly:
A Theory of Anything is useless because it
does not rule out any possibility and worthless
because it submits to no do-or-die tests.
— Paul Steinhardt, "Theories of Anything"
edge.com'
Multiverse hypotheses in philosophy and logic
Modal realism
Possible worlds are a way of explaining probability,
hypothetical statements and the like, and
some philosophers such as David Lewis believe
that all possible worlds exist, and are just
as real as the actual world.
Trans-world identity
A metaphysical issue that crops up in multiverse
schema that posit infinite identical copies
of any given universe is that of the notion
that there can be identical objects in different
possible worlds.
According to the counterpart theory of David
Lewis, the objects should be regarded as similar
rather than identical.
Fictional realism
The view that because fictions exist, fictional
characters exist as well.
There are fictional entities, in the same
sense in which, setting aside philosophical
disputes, there are people, Mondays, numbers
and planets.
See also
References
Notes
Bibliography
External links
Aurélien Barrau, "Physics in the Multiverse."
"Multiple Universes?".
Review of Carr.
Davies, Paul.
"Multiverse Cosmological Models".
Mod.Phys.Lett.
A 19: 727–744.
arXiv:astro-ph/0403047.
Bibcode:2003MPLA...18.2895M. doi:10.1142/S0217732303012325. 
Quantum Mechanics, Gravity, and the Multiverse
Max Tegmark "Parallel Universes.
Not just a staple of science fiction, other
universes are a direct implication of cosmological
observations."
David Deutsch "The Structure of the Multiverse."
BBC Horizon -Parallel Universes.
Michael Price's Everett FAQ.
Jürgen Schmidhuber, "The ensemble of universes
describable by constructive mathematics."
Interview with Tufts cosmologist Alex Vilenkin
on his new book, "Many Worlds in One: The
Search for Other Universes" on the podcast
and public radio interview program ThoughtCast.
Joseph Pine II about Multiverse, Presentation
at Mobile Monday Amsterdam, 2008
Multiverse – Radio-discussion on BBC Four
with Melvyn Bragg
Multiverse theory suggested by microwave background,
BBC news website, 201103
