From quantum theory and other considerations, there may be a large number (conceivably an infinite number) of individual Universes, also called parallel Universes, none of which we can at present be aware of since they lie beyond the outer limits of our Universe - the edge of which we have yet to make contact with (the Cosmic Horizon). In some models, most, if not all, such "bubble Universes" never touch even as nearby ones expand but if two interact, they can experience tremendous energy effects; other models find ways to either eventually interact physically or at least contact in some way. Each Universe is considered to form like a bubble in boiling liquid, with the bubble expanding. Many bubbles form at different times and grow until dissipated; the analogy is limited in the sense that, unlike bubbles in boiling water in a teapot, there is not confining structure (the teapot) but the water is infinite, the bubbles form and disappear infinitely, and the water itself is really just empty "void" (see below).
Some (most?) Universes do not survive Inflation to expand as does the Earth's Universe. During the first stage of Inflation protogalaxies in a given Universe are "in touch", but at speeds of Inflation greater than light speed, they may lose contact. With the end of Inflation, some galaxies re-establish contact but there are today parts of our Universe that are too far away from each other (in opposing directions from Earth) to have received light from one another in the time elapsed since the Big Bang. In this bubble model, Omega begins at zero (0), then after leaving the false vacuum trough it rises to 1 during full Inflation (yields a flat Universe) and then decays to values less than 1, giving rise to hyperbolic expansion (post-Inflation), one outgrowth of which is the acceleration noted as expansion is traced back in time. This acceleration is a consequence of the IF (Inflation Force) being maximum at the bubble boundary, diminishing inward towards the center. Needless to say, this Open Inflationary model is speculative and in the future may not stand up when tested by further observations and calculations but it does offer a way out of having to depend on the bizarre Cosmological Constant energy to account for acceleration.
The precise Universe of which we are aware that has emerged is "the one we've got". Most of its controlling properties are well known, although numbers describing some, e.g., total mass, remain to be fixed in detail. Other Universes, making up a collection of independent Universes existing in what is called the Multiverse (Multiple Universes) with different properties may have been possible. This term encompasses all such theoretical (but possible in the light of quantum Cosmology) parallel Universes. If certain fundamental properties and constants were to be only moderately different than the ones now identified and quantified, the nature and history of these alternate Universes could prove difficult to observe. And, in fact, significant changes in certain prime parameters from the values they actually have might even have denied the Universe a successful existence.
For our Universe, more than 30 such parameters (see previous page) have been cited as critical variables whose values can tolerate little or no significant differences from what they are. Among these are: the four Universal Constants (speed of light; gravitational constant; Planck's constant; Boltzmann's constant); constants associated with the four fundamental forces (Gravitational; Strong; Weak; Electromagnetic); rate of expansion; particle/antiparticle interactions; neutron/proton abundance ratios; H/He and H/Deuterium ratios; balance between nuclear and electron forces; temperature and density variations during the first minute; total number and density of neutrinos; mechanisms of star and galaxy formation; element synthesis, and others.
Our Universe seems to follow the previously mentioned Goldilocks dictum: not too hot, not too cold, just right. If chance alone (quantum control) were the governing determinant, the odds are enormously against all of the above factors - those that allow our Universe to work smoothly in its large scale operations - being just right. Most ordinary people, and some scientists, believe that there must be a superintelligence, be it called God, the Intelligent Designer or some other analogous name or concept - had to play the pivotal role in establishing and structuring a scientific Universe. However, the Evolutionary Principle of Natural Selection has been proposed as sufficient to explain the origin and development of the Universe. (The essence of Darwinian evolution applicable to living things is carried over into the changes applicable to inanimate things.)
Thus, the conditions that operate in our Universe are "fine-tuned". None can vary by too much from their narrow, critical specific values or the Universe would have expanded too fast or too slow, or might not ever have organized into the present assemblage of matter. The fact that it did is remarkable, but had things been much different, the conditions favoring life would not have occurred, presumably at least not on the grand scale that is represented not only on Earth but in countless planets in innumerable galaxies. This situation lies at the heart of the Anthropic Principle. The so-called weak version of this principle includes the idea that since intelligent beings (us, at the least) indeed exist, the governing conditions for our Universe must have been been such as to permit life to develop or, put another way, because we are here contemplating our existence, conditions (the scientific ones that we now have verified) had to be right for our Universe to succeed to the stage of evolving knowing creatures (note the similarity to the question "If no one is in the forest to hear when a tree fell, did it really make a sound"). The strong version states that life has depended on the right combination of physical constants, parameters, and conditions to allow life to begin and flourish and that these were pre-ordained by some independent "mind" such as is attributable to a Creator.
Now, let us expand the challenging, mind-boggling thesis, mentioned earlier on this page, that ours may not be the only Universe. The key word is "Multiverse" which connotes the astounding notion that, theoretically, quantum cosmology allows for the possibility that many Universes may actually exist. Multiverse can have two connotations: First, there could be multiple Universes existing simultaneously within a void continuum, but these are not in contact so that their existence cannot (presently) be confirmed by any known methodology. Or, second, there is only one Universe at the moment but it is just one of many (infinite) that have "come and gone"; it started with a Big Bang but eventually collapses on itself and restarts with another Bang; this is the Cyclic Universe model.
We have already alluded to the fact that scientific exploration is limited by the speed of light to detecting only those boundaries of our own Universe out to distances in the expanding space which contain galaxies, etc. that have transmitted radiation to us within the time frame of the Big Bang, i.e., are no older than the apparent age of the Universe (best estimate, ~13.7 b.y.). Paradoxically, there likely are parts of this Universe that are presently beyond this time horizon and won't be detected until later (for example, in a billion years astronomers will then see that part of the Universe that involves this additional time interval).
If the Universe began as many cosmologists now believe - from a sudden, extremely favorable and positive fluctuation of a quantum state in the so-called "false vacuum" in which a virtual particle instantaneously sprang into existence - containing within it all the potential energy needed to produce the matter and radiation now making up the Universe - then there is a real theoretical likelihood that the process could be repeated again and again, many times (perhaps infinitely). Thus, Universes could be springing forth from the endless (infinite in both space and time) false vacuum that nevertheless contains the "quantum capability" of energy fluctuations bringing individual Universes into existence. Each one establishes its own space which enlarges after its own time-clock is started. Because of the horizon limitations, none of the Universes are (can be) in contact with any of the others. In fact, they all may be drawing apart from one another as the Multiverse continuum itself expands at (or possibly greater than) the speed of light. They all exist independently from one another. We have no known way of communicating with other Universes (for that matter, we are very limited in communications within our own Universe) and, unless some future "breakthrough" in theory and practice that facilitates this communication happens, we will never advance beyond developing plausible models that hint that multi-universes are the norm. To add to the enormity of these concepts, there may be a number (perhaps to infinity) of individual Multiverses which in turn never contact each other. (All this is easy to state verbally but next to impossible to prove - at least for now.) More information about this aspect of multiverses, as the theory of chaotic inflation applies to it, is found at this Wikipedia web site.
Suppose that there are one or more Multiverses, each expanding and creating its own space as it enlarges. But, what then can be said about the "void" between non-touching individual multiverses? Could it qualify as a form of intermultiverse space? This would seem to be reasonable as just being a matter of scale: Since we treat intergalactic space as part of our Universe's space (the galaxies serving as reference points), so perhaps could we consider intermultiverse space to be analogous. Then, all of "superspace" would be expanding, accounting for why we will never be able to find and communicate between "verses". The fancy word now fashionable for that "void" into which each Universe enters and expands is Hyperspace (the term can have alternate meanings; but it usually connotes the notion of higher dimensions of space beyond the customary 4-dimensions). There could be an infinity of such Multiverses and no real boundary to superspace. Between each Multiverse, the "void" would not be a true vacuum but a "false" vacuum containing the potential (perhaps by means of virtual particle fluctuations) for another Multiverse to flash into existence. Multiverses are a provocative idea but frustrating in that there appears to be no scientific way of ever proving their existence. A Multiverse itself - a collection of parallel Universes - may be dimensionally infinite.
For more about Multiverses, refer to this Wikipedia review on the Internet. A more theoretical treatment on the Net examine the Many worlds. Parallel Universes are a popular topic in both scientific and non-scientific fiction. Somewhat more philosophical treatments are on the Web at sites hosted by Discover Magazine and Paul Davies. For those who like to read books, these three by Dr. Michio Kaku are recommended: Hyperspace; Parallel Worlds; Beyond Einstein - these also cover string theory, Kaku's specialty.
The idea of a single Universe that repeats itself - the Cyclic Universe - has been around for at least 80 years. Albert Einstein was once of the first to espouse this, based on his reasoning at the time that gravity would eventually stop expansion, draw all matter together again in a new singularity (the Big Crunch), and reinitiate a new Big Bang; this could happen many times into endless eternity. The concept was abandoned after others showed his idea violated thermodynamics (specifically, there were entropy problems). Since then, other proposals for repetitive Universes have been put forth. They are too detailed and complicated to consider in this Tutorial but several are described at this Wikipedia website.
One of the current champions of Cyclic Universes is Sir Roger Penrose of Great Britain. In his model the beginning of each Universe is the customary Big Bang. Expansion continues for many billions of years. Matter cools, draws farther apart, and is sucked into Black Holes. Over time the Black Holes themselves evaporate into radiation so that the final products are photons. The net effect is that mass is destroyed. One aspect of massless particles is that they have to travel at light speed. From the particles viewpoint time seems to stand still and space contracts to nothingness. To these particles the Universe would appear as though infinitely small. An infinitely small Universe acts as though it were a singularity. Like the traditional Big Bang singularity this one will explode, intiating a new Universe which repeats the process. However, some remnant of the previous Universe(s) is found within highly uniform regions dispersed in the Cosmic Background Radiation; discovery of these regions (claimed by Penrose and colleague Vahe Gurzadyan) is confirmatory evidence. There is no limit to the number of times this scenario happens. Needless to say, most cosmologists are skeptical about this model.
After having read these last few paragraphs, some of you may think that the Multiverse idea, in either of its modes, is a preposterous, wild speculation of overly imaginative minds, in effect an excursion into a dreamworld of unbridled unreality. Isn't one Universe really enough!! But, to the trained scientific mind, the concepts underlying Multiverses and Hyperspace are plausible and scientifically sound. Multiverses may exist and do not violate any fundamental laws or principles of Science. Or they may not exist and Science is confined only to this (our) Universe. At the present time, the nature of Multiverses can only be conceptualized in the mind and through theoretical calculations. Their existence cannot be directly determined experimentally since, in some models at least, we can never contact them (other models invoke collisions between Universes or travel to one another through wormholes). But as the human mind becomes more sophisticated, and new knowledge accrues, attempts to better understand the Multiverse possibilities and to verify them will continue and probably increase. There is a metaphysical side to this speculation: If Multiverses indeed exist, and if they were created by an Intelligent Designer, WHY? Wouldn't one be sufficient? Doesn't multiple Universes tend to argue against such a (wasteful) designer? These ideas are explored on page 20-12 and more fully on page 20-12b.
Just when the writer (NMS) had thought that enough has been said on this and the preceding page about the concept of "dark" as applied to the Universe, he discovered (on March 17, 2010) on the Internet reference to the latest findings about "Dark Flow", about which he had never heard before. Although still in the early stages of development, this new concept is causing a stir among cosmologists, some of whom remain skeptical about its reality. We will attempt to summarize its essential ideas in the next several paragraphs.
The discovery of Dark Flow was announced in 2008 by a team of astrophysicists, at Goddard Space Flight Center and several universities, led by Dr. Alexander Kashlinsky. They were investigating the behavior of galaxies in a 20 degree segment of the celestial sphere as these move relative to the Cosmic Microwave Background radiation. To their surprise, the motion calculated for about 1500 galaxy clusters located outward from Earth to about 2.5 billion light years away (in the initial study galaxies out to 6 billion l.y. were included) was all in the same direction and at speeds greater than expected. The galaxies "appear" to be moving toward the southern constellations of Hydra, Vela and Centaurus. When first announced, these galaxies were traveling relative to the CMB at speeds up to 2 million mph (more than 3 million kph). Now, that speed has been reduced to "between 1 and 2 million mph". This motion is different from the outward expansion of the universe (which is accelerated by the force called dark energy). The movement is not random as predicted in expansion models that consider large-scale motions, but unidirectional; thus it moves in a preferred direction.
The galaxies are all sources of high energy X-rays. By observing the interaction of the X-rays with the cosmic microwave background (CMB), which is leftover radiation from the Big Bang, scientists can study the movement of clusters. The X-rays scatter photons in the CMB, shifting its temperature in an effect known as the kinematic Sunyaev-Zel'dovich (SZ) effect. The temperature increases in the CMB are small, but consistently greater than the average values. Those temperature anomalies can be used to calculate velocities.
The Kashlinsky group's work shows that these temperature fluctuations are shifted in the direction of a number of galaxy clusters. This implies that the clusters are moving with reference to the rest of the universe, a motion that is therefore independent of its expansion. The result is significant because it cannot be accounted for by the known distribution of matter; something outside of the visible Universe seems to be pulling on the clusters.
The speeds associated with these preferred movements are greater than those calculated for standard expansion models, by small but consistently higher amounts. If this is valid, and confirmed by further observations, the question is why? Three explanations are under consideration. First, the motion could be as as yet undetermined consequence of the action of Dark Energy. Second, it could in some way be an expression of residual motion tied to Inflation. Third, it could be the result of attraction forces (gravity) resulting from huge amounts of matter beyond the observable Universe; this matter resides either in our own Universe or in a second (or more) Universe (if the Multiverse model is real).
These next comments are copied from "PhysicsWorld.com":
According to Kashlinsky, the answer to the source of Dark Flow might be found in the Universe's infancy. "What we believe is happening is that we are measuring the influence of the parts of space�time that were there before the inflation of the Universe."
The theory of inflation suggests that the early universe was like cosmic foam made up of many bubbles. One of these bubbles underwent a huge and rapid expansion, when the Universe was just 10-36 seconds old, to become the universe that we see today. It is our neighbouring bubbles that Kashlinsky holds responsible for these migrating galaxy clusters.
"Inflation says that if you go to sufficiently large scales you should see structure that is outside this bubble, outside of our Universe. If we live in such a world these structures would cause this motion and we suspect that this is what we are seeing," Kashlinky said.
As of early 2010, the notion of Dark Flow remains intriguing but still quite controversial. More measurements, particularly in different parts of the sky, are needed before it gains acceptance. But if it is finally proved beyond doubt, the implications are highly significant, especially the surmise that there may be another Universe.
The Big Bang remains the favored explanation for the inauguration of our Universe. As stated on page 20-1, the Big Bang could have originated from a moment of instability of a virtual particle. Such particles could have been ubiquitous in Hyperspace. So multiple Big Bangs are plausible.
A variant of these ideas involving Multiverses has Universes related to Black Holes as starting points for new Universes. The question then becomes one of origin of the Holes themselves. To be a Black Hole, a nearly infinite amount of matter must be somehow concentrated in a space that can range from Supergiants through a few tens of kilometers for Holes that are residual to star destruction processes (at the Neutron star stage) to essentially point (dimensionless) sizes resulting from other causes. Our own Universe contains countless Black Holes of varying sizes. Those of point size may be equivalent to singularities and could have arisen by some (as yet unknown) process in the vacuum. Under the right circumstances, a Hole may explode, initiating a new Big Bang.
Another concept is an offspring of the Inflation process. As this early phase proceeds, a segment containing a false vacuum would develop a bubble-like bulge that is connected by a neck-like extension known as a "wormhole". This can break from the parent expansion to drift free as a "Daughter" Universe that undergoes its own development, which includes its own inflation and expansion. There might even be a single "Mother" Universe (probably not ours) from which countless Universes are spawned. This "Super Universe" has always been and serves as the master control for the perpetual series of quantum fluctuations that lead to multiverses. Each remains "out of touch" with other Universes, never interconnecting or physically contacting in the Super Universe within which space only has meaning (to the finite, limited human mind) to the inhabitants of each Daughter Universe. Time in any such offshoot Universe will, in principle, be measured from its explosive conception, but time throughout the super system may be infinite.
As stated above and the previous page, the quantum cosmological idea of myriads of virtual particles emerging from the vacuum has been seized upon as an explanation of the origin of the Universe purely on the basis of a theory that does not require a "Creator" or "Intelligent Designer". In one view, virtual particles normally will encounter virtual antiparticles and will annihilate before any Universe can start. But, statistically (Quantum Physics relies heavily on probabilities) one can argue that on rare occasions, these particles (not matter in the usual sense but some still unknown energy state capable of transforming into the condition associated with a singularity) will not be destroyed but can organize into a singularity, which then reaches a critical state that leads to a Big Bang.
Drawing on some of the above ideas, the Russian cosmologist, Andre Linde, now a professor at Stanford University, has put forth still another model he calls chaotic inflation which leads, in principle, to many Universes. It relates, in part, to the idea of Primoridal Chaos mentioned on page page 20-1. He envisions an infinite void with within which the energy associated with the false vacuum prevails. Fluctuations involving virtual particles are continuously occuring throughout. Each is associated with some set of scaler properties which may or may not produce the fundamental parameters needed to originate and control a Big Bang. A huge number of different combinations of these properties/parameters are possible and individual events are likely to utilize conditions that vary considerably. As a result, the vast majority of these fluctuations fail to achieve the appropriate conditions that allow a Big Bang and instead are wiped out by annihilation leading to a zero end result. But, a small fraction produce the prerequisite set of parameters that favor a successful Big Bang. Of those, some - probably most - will have parameters that cause their Universes to expand too rapidly or too slowly to shape and evolve into ones similar to ours in which life can develop. So, only a few survive.
In mid-September, 2002, Professor Linde and his wife, Professor Renata Kallosh, announced that they have experimented mathematically with their earlier models and have devised still another scenario that requires the observed Universe to eventually cease acceleration and then contract to a Big Crunch, perhaps as soon as the next 10 to 20 billion years. Although Linde has been an advocate of Inflation, he notes that the recent second acceleration models run into conflicts with some of the mathematical aspects of Supersymmetry and Superstrings (discussed on page 20-1). As these two cosmologists explored the consequences of Dark (repulsive) energy, they found reasons from their calculations to consider that this energy, whose values must necessarily be greater than zero, could in time revert to negative, or less than zero, values. If this hypothesis holds up, the acceleration will slow down and convert to deceleration leading to a final collapse. They also point out that we may be seeing only a fraction of our full Universe; parts not seen may offer evidence of this potential contraction in future time. They also consider this new model to allow many Multiverses that end up contracting (crunching) to singularities that re-explode. Thus, the notion of individual multiverses coming and going, with an infinite number of repeats, is one possible consequence of their proposed model.
In 2006, another imaginative variant was proposed by Paul Steinhardt of Princeton University and Neil Turok of Cambridge University: its essence is that our Universe expands and then contracts over an average time of 300 billion years; its contraction leads to the crunch that explodes in a Big Bang; this process continues repeatedly so the present Universe is just one of many before and a number (perhaps to infinity) in the future; the governing factor is the evolution of Lambda, Einstein's Cosmological Constant, during the successions. The ideas are intriquing enough to warrant a summary of their hypothesis from the 'Space.com' web site for May 8, 2006. This is reproduced (with style editing) on page 20-10a under the title of "The Cyclic Universe".
Thus, there could be a range of Multiverses (in terms of size and properties) and ultimate fates of each that come and go over time, with one or some that have just the right parameters to form persistent galaxies and stars that synthesize the elements responsible for the variety of features, including organic matter, that facilitate a functional Universe comparable to ours. These surviving Universes are not necessarily identical to each other but show variations in size, composition, structural features and age, and even components without counterpart to ours, which result in different histories (including presence of life). Such Universes come and go (some may even experience Big Crunch multicycling). Suffice to say, the Chaotic Inflation concept, along with other Multiverse models, is presently impossible to test and verify but the Quantum Physics that "allows" these conceptions requires that we seriously consider these alternatives.
A model that has gained many adherents in cosmological circles is that of Quantum Tunneling, first proposed by Alex Vilenkin and then added to by collaboration with Andre Linde. In their view, the "nothing" that could have existed prior to appearance of a Universe is truly nothing. In Quantum Physics it is possible for subatomic particles to escape the nucleus or overcome repulsion barriers and then reappear in locations (and states) they do not customarily occupy. Or, on a grander scale, Quantum principles hold there is a finite probability that moving subatomic particles ordinarily stopped by thick physical barriers (such as a concrete wall) can reappear on the other side; the likelihood is small but real, given enough time. This has been called "tunnelling". Extrapolating this to the inception of a Universe, from "empty space", tunnelling permits an entry of a virtual particle across the quantum barrier that prior to that moment prevented its emergence. The Heisenberg Uncertainty Principle states that a particle's position is indeterminant at any instant. But, statistically, it can cross this barrier (perhaps some form of energy), through the tunnel, become a physical entity containing all the energy needed to make a Universe that separates into the negative energy of repulsion (causing expansion) and the positive energy (including all cosmic mass that develops). This can repeat, leading to Multiverses.
Of course, not every Universe would be successful beyond its early moments. Inflation may fail, the fundamental constants may be "off" so that expansion is abnormal, or other conditions could prevent a Universe from experiencing a proper development. Some Universes may be short-lived; others expand so slowly that factors promoting galaxy and star development inhibit attainment of a situation favoring life. But many astronomers and cosmologists now believe that there is a natural inclination - in a sense, a purpose - for Universes to "try" to develop in a manner that points to occurrence of Life as a "goal". A good espousal of this seeming teleological control of Universe formation is the book by Lee Smolin, The Life of the Universe, Oxford Press, 1997. Dr. Smolin presents the thesis that Universes are evolutionary - "natural selection on a cosmic scale". For an adventure of the mind, read this book.
Again, after most of this page was written, another very intriguing and provocative article appeared in the May 2004 issue of Scientific American. Entitled "The Myth of the Beginning of Time", it was written by Gabriele Veneziano, the originator of String Theory. He uses that theory to help explain the moment of the Big Bang but he also belongs to the group of cosmologists who now believe that time is eternal (no begining - no end) and an infinite Cosmos has had repeated "creations" of individual Universes of which there must be multitudes. The subject matter is intense and rather mind-boggling, so no attempt is made here to summarize it. The reader who really wants to know current thinking about Multiverses and the openness of time should track this article down (several readings recommended). There is one figure in the article that summarizes much of his model, that is copied and reproduced here to provoke you into having a "go" at it.
The essence of this idea is that various forms of matter exist, each composed of strings with appropriate frequencies. This matter exists for all time throughout an infinite void defining the Cosmos. The matter is not uniformly dispersed so that periodically it clumps into discrete concentrations that form Black Holes of various sizes. A Black Hole while expanding spatially also increases in density towards its center until it reaches a threshold that results in a rebound, amounting to an explosion analogous to the Big Bang of our Universe (this situation is repeated many times throughout the infinite Cosmos). For a while thereafter the BB explosion goes through decreasing expansion rate (but the model does not yet treat the recently "discovered" apparent increase in our Universe's expansion). Veneziano prefers this model but does describe an alternative, related to the Brane concept, which goes by the name "Ekpyrotic Scenario" (read the paper for details).
The September 2004 issue of Scientific American, devoted entirely to Albert Einstein and the implications of his enormous contributions (mentioned briefly in the Preface to this Section) contains a paper entitled "The String Theory Landscape" by R. Bousso and J. Polchinski. It is far too challenging a read to summarize well here - you are encouraged to check it out yourself. But a few key ideas from it are stated here without much elaboration. The paper points out that String Theory may succeed in providing the fundamental knowledge needed to integrate gravity with quantum theory and also has the ability to provide a believable model for the Universe. But "Universe" has the meaning not just our known Universe but includes all possible Universes that arise from actions caused by vacuum energy changes in the universal Universe (This is my phrase, meant to convey the idea that there is some kind of infinite [not spatially bounded] state [not quite a 'void'] in which many finite Universes, some like ours - others much different and many are transient, can have existences of varying duration.) String theory extends Kaluza-Klein theory (see page 20-1) to at least 6 added dimensions, none apparently large enough to presently be detected. These have a great variety of topologies (shape-determinant). The geometry chosen tends to adjust the associated vacuum energy to a minimum. For a quantity Bousso and Polchinski term "six-dimensional manifold", which is a string theory term, when a variable such as the overall size of the manifold is allowed to vary in a quantum event, the change of vacuum energy with size follows this general curve:
In this graph, the vacuum energy can decrease to states that place it above (+), at (0), or below (-) the abscissa. The energy change will strive to reach a minimum (one of the troughs) but which one depends on whether there is sufficient energy to climb out of a trough over a peak to the next trough. These minima are states that specify whether the vacuum energy fluctuation ends up in a forming Universe in a positive state (Universe will expand), negative state (it crunches or collapses), or, if near zero in a stable state.
When the topology of String Theory space is considered, a "landscape" of "mountains" and "valleys" (this holds for varying two dimensions but more dimensions may be involved and can't be shown in a 2D surface representation. But, to keep it simple, the 2D case (for which "landscape" is appropo) is considered to contain a huge number of mountain peaks and valley troughs. Entry into a trough may result in an unstable condition and failure to produce a Universe but there are some valleys in which stability (vacuum energy near zero) leads to long survival of the Universe that ensues. Empty space really has virtual particles and energy densities - it is not a true void. Early calculations came up with huge energy densities - very puzzling but proved outlandish as string theory improved and reconciled with quantum mechanics. But, a new determination of an energy density quantity called Λp, which is one Planck mass per cubic Planck length, showed a very large number of possible values with those near Λp = 10-120 being closest to maximum stability. Various events continually occur in the boundless Universe but most end up in states of instability. However, those near 10-120 can survive and grow as expanding, long-lived bubble Universes.Within any of these, new fluctuations may cause new Universes; a large number of Universes can keep popping up for short to long times everywhere in the (super)Universe that has infinite dimensions. Various surviving Universes (within this infinity and mostly unconnected and not interacting) will each have their set of specific parameters dictating their behavior. Most of these will be "inhospitable" for development of life; a few will have parameter values that permit life to develop and evolve. Our Universe, since we know it exists, must have had a vacuum energy density between Λp = 10-180 and Λp = 10-120, almost but not quite zero. In this model, at any given moment there can be countless bubble Universes undergoing various fates (growth, collapse, short-term existence, or almost instantaneous failure). But, if the void within its infinity is itself expanding, we should never be able to "see" or contact other bubble Universes.
One aspect of energy density needs further elaboration: There is a parameter called critical energy density. This has a value such that it just controls an efficient expansion of the Universe. Too low and the Universe expands too rapidly; too high and it collapses too early. At Planck time (10-43 sec), calculations suggest that the difference between Λcrit and the actual Λp was only 1 part in 1060. This condition at the outset of the BB was crucial to establishing a flat Universe (one whose geometry allows the sum of the angles in a triangle to be exactly 180°)
So, what is the future of the Universe, as can be predicted from what we now know about the timeline of the past Universe. The renewed cosmological expansion described above seems to rule out the Big Crunch - the model that was based on gradual deceleration, increased influence of gravity, eventual reversal of direction of galactic expansion, gradual collapse of matter inward, with all matter then "crunching" into some small volume that finally ends up as a singularity (with a probable new Big Bang; this expansion-contraction could be repeated many times). Instead, the accelerating expansion now observed predicts that matter will forever move "outward", probably for trillions of years. Matter draws apart and gradually cools as the energy of the Universe is dispersed. Thermally, absolute zero is approached for the evolving widespread matter - this condition has been called the "Big Freeze". Galaxies and stars eventually burn out (Red Dwarf stars are the last to go). This model implies that there may have only been one Universe.
From the above on this page, and in preceding pages, one can safely conclude two things: 1) a great deal more new knowledge of the Universe has accrued in the last 20 years, and 2) there is still a great fermenting of this knowledge into a gamut of hypotheses to explain the observations that are still "on the table" as theoretical cosmologists offer their variations of plausible explanations. If these last paragraphs touch upon the arcane or occult and confuse you (as I freely admit to having my uncertainties), I can only suggest you read the appropriate above-cited articles (at least twice) for, hopefully, more insight than provided here. A bit of metaphysics or "astrophilosophy" is still inherent in the speculations. Let's go off the deep end and plunge into the realm of the abstract.