Alan Guth and Andre Linde independently proposed in 1981-2 that the Big Bang begins in a hot dense state, but as it cools through the so- called Grand Unification Theory (GUT) transition energy at 10^15 GeV and 10^-35 seconds, it gets caught in a false vacuum state. This causes the universe to exponentially grow in size in what is called a quasi-de Sitter state. The expansion ceases once the universe enters its true vacuum phase, with a release of energy that appears as a fireball phase of particle and radiation creation by 10^-33 seconds after the Big Bang. The expansion then resumes with the Friedmann expansion phase which it is now continuing to undergo, but at a greatly reduced speed. The transition occurs by the nucleation of true vacuum bubbles within the expanding matrix of the de Sitter false vacuum phase. These bubbles either merge together to form the present day uniformity of the universe (Old Inflation), or remain as separate domains that grow to sizes billions of times larger than our observable universe (New Inflation). The supermassive, scalar Higgs field is identified as the culprit whose phase transition initiates Inflation.
Inflationary Big Bang cosmology was eventually superseded by various other incarnations that presumed to improve on the defects of previous versions of Inflation. One of these is Supersymmetric Inflation in which Supersymmetric Grand Unification Theory is used as the starting point for the underlying physics. The culprit now switches from the Higgs field to another scalar field called the Inflaton field. The reason for this switch is that the quantum fluctuations in the Higgs field would be so strongly tied to matter fields that the matter in the universe would collapse into supermassive black holes. The Inflaton scalar field, on the other hand, is designed by hand to be weakly tied to matter, yet still has the mathematical properties of a scalar field which are necessary to drive inflation.
Vacuum Fluctuations in a Pre-existing Flat Spacetime R. Brout, Francois Englert and E. Gunzig carried Edward Tryon's idea further which stated that the universe was a long-lived 'vacuum fluctuation', by adding more detail to this basic model, proposing that the universe was created by a local quantum fluctuation within a pre-existing 4-D spacetime. In a cooperative process, the universe expanded in an Inflationary state which ended once the spacetime irregularities had smoothed-out sufficiently. In a later version of their proposal, Gunzig began, once again, with the notion of a pre-existing flat, empty 4-D spacetime, but embedded in this spacetime is a physical field that is normally constant at every point in spacetime. This field, like all quantum fields, is subject to fluctuations that locally produce a distortion in the otherwise flat geometry. In a self-sustaining process, gravitational energy is transferred from local curvature fluctuations into a rich sea of quantum black holes which then decay into matter and anti-matter touching off the Big Bang. A dramatic feature of this model is that the more uniform and flat the initial spacetime, like a pencil balanced on its point, the more likely it will be unstable to spontaneous curvature fluctuations, and to give birth to a 'child' expanding universe. Gunzig speculates that as our universe grows more uniform and flat in the far future, it too may become the Mother Spacetime for an instability leading to the creation of a new universe.
Vacuum Fluctuation in a Prior, Curved Spacetime D. Atkatz and H. Pagels pointed out that the concept of total energy is not rigorously definable in highly curved spacetimes. The only spacetime with zero total energy is exactly flat and empty everywhere. Atkatz and Pagels were able to carry Brout, Englert and Gunzig's ideas a step further, proposing that just as radioactive nuclei spontaneously decay by emitting particles, spacetime itself could decay from some primordial, empty state devoid of matter and energy. Through a process called quantum tunneling, this primordial state spontaneously decayed into an expanding universe wherein particle creation and inflation occurred in a 'Fireball' phase. They discovered, however, that only closed, finite universes could arise from such a tunneling event. Rather than the universe starting from a flat, empty spacetime as Brout, Englert and Gunzig had proposed, the pre-existing state may already have had some topological structure to it and may also have had a non- zero total energy by virtue of its state of curvature.
An intriguing connection between this scenario and higher-dimensional unification theories has also been proposed. Since superstring, grand unification theories require that spacetime be constructed from 10 dimensions of which 6 space-like dimensions had a compact topology, Atkatz and Pagels suggested that the pre-creation phase may have possessed a 10-D, closed geometry in which 4 spontaneously ballooned-up to become our familiar 4-D world at large. The birth of our universe, like the formation of ice from cold water, was a phase transition involving not just a change in particle attributes or forces, but may have included a spontaneous change in the very dimensionality and geometric structure of spacetime.
Dimensional Compactification A. Chodos and S. Detweiler investigated a simple 5-D cosmological model and discovered that prior to the Big Bang, the universe may have started out as a multidimensional object possessing more than 4 spacetime dimensions, but its expansion drove some of these dimensions to become small relative to the others. Our present universe is one in which 4 of the dimensions grew to large scales, however, there may exist compact dimensions to the physical manifold that have shrunk to sub-atomic size.
Something from Nothing Rather than the pre-existing state being a flat spacetime, or even a closed multidimensional one, this earliest conceivable state may have been quite literally nothing. More than a clever play on words, the physicists usage of the word Nothing carries with it an even more barren landscape than one normally imagines. Andre Vilenkin imagined a nothingness that was the complete negation of all conceivable attributes that we might attach to the particular fields within spacetime, or even to spacetime itself. It represented a state containing no fields, time, or space. The concept of dimensionality was also irrelevant, and without time it was the ultimate state of non-existence. Heinz Pagels vividly describes this condition in his book {\bf Perfect Symmetry} as,
" The nothingness 'before' the creation of the universe is the most complete void that we can imagine -- no space, time or matter existed. It is a world without place, without duration or eternity, without number...yet this unthinkable void converts itself into the plenum of existence -- a necessary consequence of physical laws. Where are these laws written into the void? It would seem that even the void is subject to law, a logic that existed prior to time and space."
The advantage of this state is that since everything of physical significance is negated, one may not logically inquire where that negated state arose since to do so would imply that either a place or time exist prior to the universe's coming into being, and neither of these concepts has any meaning. All further talk of 'prior states' is halted once and for all. But, this interpretation is not the final word.
Quantum Metric Transformations According to Jim Hartle and Stephen Hawkings, the Big Bang singularity did not occur because when the universe was small, its quantum state near the Planck Scale was dominated by metrics that had a so-called signature of (1,1,1,1) and therefore, time became an imaginary coordinate with a space-like character. The origin of the universe cannot be assigned to a particular moment, so there is no Singularity. The universe, in essence, emerged from a no-time state of pure space. Quantum fluctuations in the spacetime metric changed the signature from being purely Euclidean ( 1,1,1,1) to the familiar relativistic one (-1,1,1,1). The Euclidean spacetime tunneled into an inflationary de Sitter spacetime which started the Big Bang. For the non-specialist, a number of almost readable introductions to this rapidly expanding subject are now available including Stephen Hawking's "A Brief History of Time".
Pre-Geometric Vacuum State According to K. Akama and H. Terazawa, the origin of the Big Bang represents a local and spontaneous phase change of spacetime from a pre-geometric phase to a geometric one. The gravitational field, and therefore the metric of spacetime, is a composite of more fundamental mathematical objects. At high-energy, spacetime dissociates into its constituent sub-fields just as ordinary atoms dissolve into their component sub-atomic particles. Under these conditions, spacetime disappears entirely although the fundamental fields out of which it is composed, remain. These fields exist in another kind of manifold than physical spacetime. The dependency of how these fields interact as you change their energy, can be expressed mathematically, and it turns out that their potential energy reaches a minimum at the origin of the Big Bang. Prior to the Big Bang, pre-geometric fields interacted at energies above the Planck energy, and as they cooled, they underwent a phase transition at the Planck energy where the dynamical phase changed to favor the integration of these primordial fields into the synthesis of spacetime.
Eternal Inflation According to Andre Linde's reanalysis of Inflationary cosmology, there is a single spacetime that transcends not only the particular universe we live in that arose from a single miniscule patch, but also includes an infinite number of other mini-universes that developed from all the other patches as well. Rather than search for a specific unification theory that uniquely leads to the particles and forces in our universe, Andre Linde proposed that all possible GUTs are realized. As a consequence, in some of these patches, Inflation may have occurred leading to many other 'big' universes of which we are just one possibility. Some of the 'failed' universes may have lived only a microsecond or a few million years before disappearing back into the quantum foam of spacetime. Each of the inflated domains is vastly larger than the region within them that any one observer may perceive and will appear very uniform within these locally observable regions. However, on the largest scales, these domains are part of a complex foam in spacetime and may be interconnected by tunnels or 'wormholes' For some domains, these tunnels may have evaporated, rendering these universes utterly disconnected in spacetime from one another.
Soviet physicists L. Gurevich and V. Mostepanenko considered the possibility that there may exist an ensemble of universes in this unimaginably vaster spacetime manifesting all possible dimensionalities and physical laws. It may also be possible that the creation of new universes is a process occurring even now. According to Linde,
"We find ourselves inside a four-dimensional domain with our kind of low- energy physics, not because other kinds of mini-universes are impossible, but simply because our kind of life cannot exist in other domains...It is extremely complicated, if not impossible, to construct a theory in which only one kind of compactification can occur, leading precisely to a four- dimensional, inflationary universe, with the low-energy particle physics of our own experience....Now it seems more likely that the universe is an eternally existing, self-producing entity, that is divided into many mini- universes much larger than our observable portion, and that the laws of physics, and even the dimensionality of spacetime, may be different in each [of them].."
In the cosmology formulated by S. Blau, E. Guendelman and Alan Guth in 1987, the Planck-scale quantum foam may be constantly creating bubble universes that pinch- off from our spacetime, some of which may even expand into universes like our own. Since their expansion occurs in a region of spacetime completely disconnected from our own, they are forever beyond observation. The artificial manufacture of such universes may be possible under laboratory conditions, unfortunately, the same calculations seem to indicate that these conditions may not be easily achievable.
A similar idea of bubbling baby universes was later formulated by K. Ghoroku and Sidney Coleman in 1994. Universes can interact with each other at the quantum level via wormholes. A great many universes could be created from nothing, each disconnected at the scale we live in, but in contact at the Planck-level via a process called 'quantum wormhole interference'. These universes are considered to be defined by fields in superspace, and can be coherently coupled to each other via universal wave functions. As a result of quantum interference, and the spawning of baby universes by our own spacetime, the cosmological constant has a vanishing magnitude in all universes similar to our own as Sidney Coleman pointed out.
Stringy Cosmology M. Gasperini and G. Veneziano applied string theory to understanding the physics of the Big Bang near the Planck Era and found that Singularity might actually be avoidable at the quantum level. The curvature grows to a maximum, then starts decreasing as we pass through the Planck Time into the present expansion era. String theory suggests that the pre-Big Bang era may have been a state in which oscillations in the expansion of spacetime occurred in the current mixture of internal and external dimensionality to spacetime. These oscillations eventually settled down to the present 6 + 4 or 22 + 4 split with 4 large dimensions and up to 22 compact dimensions.
Mark Bowick and L. Wijewordhana also discovered that if you use string theory to describe the dynamics of gravity and matter fields near the Planck Era, that above a critical energy, the vibration modes of the string get excited to higher and higher energies until the entire physical system --the universe--- experiences a phase transition in which the strings evaporate.
David Meyer, in 1993, also combined string theory with work in quantum gravity to show that the appearance of time and ticking clocks may have occurred as a phase transition between disordered acausal states and one in which causal connections between states can be defined. While a theory based upon causality may appear to have in it an inherent notion of time, this may have arisen by way of a phase transition in the theory perhaps similar to the kind that occur in superconductors. These discussions parallel similar ones elsewhere which describe how our notion of spacetime may need to be radically altered if any of the currently active areas of the search for the Theory of Everything are to pan out.
For more information on these issues, see my Cosmology web site which contains many published popularizarizations about the history of the early universe and Inflationary cosmology.
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