Our universe as we know it has certain laws of nature, everything that we observe follows them. However, the idea of “another world” with its own laws completely different from our universe is not new. In fact, we can see this concepts’ origins in some of the (our) world’s oldest religion. The Puranas of Hindu mythology have mentioned infinite universes each with its own gods!Fantasy has been using this concept in various places, right from heaven and hell to Olympus and even Valhalla.This “another world” can also be called a parallel universe. A specific group of parallel universes is called a multiverse (although this term can also be used to explain different possible universes of one reality). In both fantasy and science fiction, these parallel universes are assumed to coexist with ours, allowing the protagonist to enter a fantasy reality from author’s reality. One such example is The Chronicles of Narnia by C. S Lewis, where our world coexists with Narnia. It also points out that it is not necessary that the time in both the worlds should match, as the hours spent in Narnia were equivalent to few minutes in our world. Our world (as I put it) is a subset of a multiverse and this multiverse can exist in various forms. Though these forms are unconfirmed, they exist in different levels according to Max Tegmark.Level I: In this level, there is an infinite number of Hubble volumes (observable universes very similar to ours) that exist in space.Level II: It consists of the concept of bubble universe. The total universe (multiverse) keeps on expanding and stretching forever but at some regions, it stops stretching and forms bubbles. These bubbles are like our own universe (Hubble volumes) with different effective physical constants, dimensionality and particle content.Level III: This is formed on the basis that the ‘wave function’ at quantum level which is governed by a probabilistic mathematical concept known as the Schroedinger equation. The wave function considers all possible states in which a subatomic particle can exist. A quantum event can have a range of possibilities and each possibility gives a new “branch” of the universe. All this happens in an infinite-dimensional Hilbert space. Tegmark writes that, “The only difference between Level I and Level III is where your doppelgangers reside.”Level IV: This level says that there are different mathematical structures and according to them there are different fundamental equations of physics. Thus, universes in this level are considered real if a mathematical structure can describe it.Other than these levels, there are nine types of multiverses discussed by Brian Greene. One of the popular types is the Brane (Membrane) multiverse. Very basically membrane theory is related to string theory. It is the idea of a possible way of explaining how the 4 forces ( gravity, electromagnetism, strong and weak forces) in the universe might be unified. One key feature of Membrane theory is related to something called Membrane (brane) cosmology. Our universe basically is part of a ‘bulk’ of branes (ie an infinite kind of membrane bulk). Gravity is the difficult one to unify into the others. The thought is that gravity is a force that ‘leaks’ through all the bulk and so appears quite weak in our universe.In this theory, it is thought that the Big Bang is the result of two branes touching each other producing the laws of nature and constants we see around us. In this way, there are few more membranes that coexist along with the universe that we live in. Other membranes exist with different physical and geometrical values.