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A Parallel universe or a multiverse is a hypothetical set

of multiple possible universes that together comprise

everything that exist including space, time, matter, and

energy.

The structure and functionality of multiverses, including

the different universes in the multiverse, depend on the

hypothetical set that is being considered. These universes are

all related to our universe. They must’ve branched out of our

universe or our universe branched out from another

universe. With these parallel universes, our wars would’ve

had different outcomes and species extinct here must survive

somewhere else.

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Many theories have been proposed to substantiate the

existence of parallel universe. Of these theories, the most

significant ones are

1) Tegmark’s classification,

2) Cyclic model,

3) Membrane theory, and

4) Anthropic principle.

The most important hypothesis among them is the Tegmark’s

classification as it is the simplest model and the most

practical one with sufficient evidence to support the theory.

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This theory was proposed by Dr. Max Tegmark in the

year 2003. He published many papers relating to the parallel

universe which helped him feature the cover page of the

“Science” magazine in December 2003.

He provided taxonomy for the multiverses beyond the

observable universe. According to his theory, the multiverses

are classified into 4 levels such that each level is an extension

of the previous level.

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Level 1: Beyond our cosmic horizon

In this level, the different parallel universes are proposed

to be identical to each other including the physical laws like

conservation of mass and energy. In particular, there are

many other inhabited planets in other universes with people

with same name, appearance and memories as someone on

planet earth. The only difference between the different

parallel universes is the initial conditions of each universe i.e.

after the inflation epoch. The initial conditions differ due to

the motion of matter after The Big Bang explosion.

Assumptions:

1. The space is considered to be infinitely expanding

in all directions

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Level 2: Universes with different physical laws

Level 2 multiverses is an infinite set of distinct level 1

multiverses with different dimensionality and different

physical constants. Each level 1 multiverse is considered to be

a bubble such that the spaces between the bubbles keep

expanding. These other domains are more than infinitely far

away in the sense that you would never get there even if you

travelled at the speed of light forever. The reason is that the

space between our Level I multiverse and its neighbours is

still undergoing inflation, which keeps stretching it out and

creating more volume faster than you can travel through it.

According to chaotic inflation theory, the places where

inflation ends gives rise to the universes and the rest of the

space keeps expanding exponentially.

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Level 3: Many worlds of Quantum Physics

In Many Worlds Interpretation (MWI), there is only one

universe that has spilt into many other universes with the

course of time. If fundamental equations of physics are

related to the mathematical theory of unitary, then the

universe keeps branching into parallel universes.

This theory was supported by Schrodinger’s Cat

experiment. He put a cat into a box and exposed it to a

radioactive substance. The cat would die only when the

radioactive substance decays. Hence during the transition

state where the radioactive substance is on verge of decaying,

the cat is simultaneously dead and alive.

In a level 3 multiverse, the events occur simultaneously

in all the parallel universes and the total probability (i.e.

equal to 1) is satisfied. But the outcome of the event may be

different in the different parallel universes.

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Level 4: Other Mathematical Structures

Every universe is said to be governed by some equations

that specify the structure and functionality of the universe. A

very interesting question arises at this stage: Why only these

equations and not others? Hence this level considers

universes governed by other real mathematical equations.

There is a mathematical structure associated to each physical

world called isomorphs and each physical entity has a

mathematical counterpart in the mathematical structure.

The various multiverses follow different fundamental

equations of physics.

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Before getting into the details of this hypothesis, let’s

first define a fractal. A fractal is an object or quantity that

displays self-similarity on all scales. The object need not

exhibit exactly the same structure at all scales, but the same

"type" of structures must appear on all scales.

The Cyclic Multiverse is considered to be a self-similar fractal

which might have formed just like a snowflake would.

nything from the curvature of space-time to the pattern of A

a snowflake can ultimately explained with the fractal

dimension.

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A multiverse is explained by a multidimensional

extension of string theory. Before relating the string theory to

the multiverse let’s get the basic concepts of string theory

clear.

String theory attempts to reconcile quantum mechanics

and general relativity. It states that the quarks and electrons

within an atom are 1-dimensional oscillating lines.

P-branes are the spatially extended mathematical

concept in string theory where p denotes the number of

dimensions.

D-branes are membranes of different dimensionality.

They have mass and charge. They emit and absorb closed

strings of a structure.

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According to M Theory, our universe and others are

created by collisions between p-branes in space with 11-26

dimensions. Objects in each universe are confined to the d-

branes of the universe, but may be able to interact with other

universes via gravity.

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The concept of other universes has been proposed to explain

how our Universe appears to be fine-tuned for conscious life.

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 life would only exist in one those few universes

that happened to be finely tuned, permitting the existence of

life with developed consciousness.

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The various hypotheses have been widely criticised by

many people due to the absence of physical evidence. The

question that comes to every mind is: when there are so many

unknowns in our solar system how can we predict what’s

outside our universe so confidently?

All these theories are based more on faith than scientific

proofs. To believe or not believe the theory is up to the person.

It is similar to many other controversial theories (existence of

god for instance) which depend on the mind-set of the

person.