Dr. N.L. Kachhara
The Big Bang Theory is currently the most accepted cosmological theory of the Universe. According to the Big Bang Theory, everything in the universe, all forms of matter and energy, and even space-time itself, came into being in a single event, a gravitational singularity; as space expanded over time, the matter and energy cooled sufficiently to allow the stable condensation of elementary particles into primordial nuclei and atoms. With further cooling, the atoms aggregated, forming large structures like stars, planets, galaxies, and clusters and super clusters of galaxies.
In the past few decades, an extraordinary claim has captivated cosmologists: the expanding universe we see around us may not be the only one; billions of other universes may be out there too. There is not one universe, but a multiverse. The multiverse (or meta-universe) is the hypothetical set of finite or infinite possible universes, including the universe we live in. The various universes within the multiverse are called "parallel universes" or "alternative universes."
"Gravitational waves rippling through the aftermath of the Big Bang, physicists suggest, point to the existence of the multiverse. This is because these gravitational waves point to a particularly prolifickind of "inflation" of the early universe: an exponential expansion of the dimensions of space to many times the size of our own universe in the first fraction of a second of the Big Bang, some 13.82 billion years ago."
According to Guth, the existence of a multiverse can explain a lot of things that now confuse cosmologists about our universe. For example, there is the 1998 discovery that galaxies in our universe seem to be spreading apart at an accelerating rate, when their mutual attraction should be slowing them down. This discovery is generally thought to imply the existence of a "dark energy" that counteracts gravity on cosmic scales. However, its nature is a profound mystery as yet. Guth says "There is tremendous mismatch between what we calculate the dark energy ought to be and what we observe". This discrepancy has been a great source of embarrassment to physicists. A multiverse could offer solution to this problem. Of all possible universe spawned by inflation, our universe might just happen to be one of the few universes in which the dark energy is relatively lame. In others antigravity force might conform to physicists' expectations and be strong enough to rip all matter apart
A multiverse might also explain the number of dimensions predicted by modern "superstring" theory. String theory describes subatomic particles as being composed of tiny strings of energy, but this requires the existence of 11 dimensions instead of the four we can actually observe. Maybe this just describes all possible universes, instead of our own.
Andrei Linde developed the theory of "eternal chaotic inflation" giving rise to multiple universes (including ours), separated by unimaginable gulfs of distance, stretching perhaps to infinity. In this model, our universe is full of stars and planets and extends to encompass many more dimensions that are devoid of such mundane things as atoms or photons of light. This means that, spread across space at distances far larger than the roughly 92 billion light-year-width of the universe we can observe, other universes reside, some with many more dimensions and different physical properties and trajectories. That is, what we had previously called "the universe" can be divided into extremely large regions that may have different laws of physics each. Of these, one part may be suitable for life and other parts unsuitable.
"The multiverse may even help explain one of the more vexing paradoxes about our world, sometimes called the "anthropic" principle: the fact that we are here to observe it. To cosmologists, our universe looks disturbingly fine-tuned for life. Without this the perfect alignment of the physical constants - everything from the strength of the force attracting electrons to atoms to the relative weakness of gravity - planets and suns, biochemistry, and life itself would be impossible."
"What is the evidence for these theories of cosmic inflation, eternal chaotic inflation and multiple universes? It is challenging considering that cosmic inflation began and ended within the tiniest fraction of the first second of our universe's existence. Eternal chaotic inflation, by definition, cannot be seen, since other universes are disconnected permanently from our universe. Yet lines of corroborative evidence (beyond the elegance of equations) have convinced many cosmologists to such a degree that cosmic inflation and eternal cosmic inflation has become, in essence, the "standard model" of cosmology. As for cosmic inflation, it seems to solve several separate enigmas in the origin and structure of the universe (including the horizon problem). Moreover, cosmic inflation makes interesting predictions, especially about the cosmic microwave background radiation, a remnant of the Big Bang - predictions that have been confirmed and reconfirmed by increasingly precise data from satellites."
Max Tegmark goes further. He envisions four kinds of multiverses that may exist, labeling them Levels:
Level I: Beyond our Cosmological Horizon. "Space in our universe goes on far beyond that which we can see, and perhaps goes on forever - which would mean that infinitely many regions exist in our own pocket universe, regions like our observable universe, where the laws of physics are the same."
Level II: Universes with different physical constants. "Infinitely many other regions exist in the same space-time as that of our universe, but they are disconnected permanently from our pocket universe and within each of them the laws of physics are different."
Level III: Many-worlds interpretation of quantum mechanics.
Level IV: Ultimate ensemble. "This level considers all universes to be equally real which can be described by different mathematical structures."
Tegmark's Level I is accepted by almost all cosmologists, and Level II has become the "standard model" of cosmology. His Level III is speculative and controversial and Level IV seeks the deep truths of existence (i.e. reality is mathematics).
Not every cosmologist is a full convert to the multiverse. Paul 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 them an attempt to redefine the values of science, to which he objects even more strongly. Physicist Paul Davies says that although there are good reasons for supposing that what we see may not be all that exists, the hypothesis falls far short of being a complete theory of existence. A multiverse is often presented as solving the mysteries of existence by assuming that if there are an infinite number of universes, then "everything is out there somewhere, so that is the end of the story." Davies said there are about 10 different basic assumptions of physical laws that are required to get the multiverse theory to work. He dismissed the idea that "any universe you like is out there somewhere. He thinks that such an idea is just ridiculous, and it explains nothing." He feels that an infinite number of universes do not make sense. Something is amiss.
