Another solution to the open vs closed Universe may have been found in recent results from HST and ground "super-telescopes" observations. A continuing sky inventory of large Type 1a supernova explosions has detected a significant number at distances of about 7 billion light years. Each such explosion lasts only a few Earth weeks; the duration affords a good estimate of its intrinsic brightness. In the survey, only supernovae whose beginnings can be fixed closely, so that their total lifetime can be specified, were included in using them as superior indicators of distance-time as another way to estimate recessional velocities and hence expansion rates. The results from analysis of 45 Type 1a events imply a faster rate than heretofore calculated from deep quasar studies. In fact, at the rate indicated the Universe must be expanding too fast for it to be slowing down enough to finally contract. This speaks of an open Universe that will expand forever and leads to the corollary that there must be insufficient total mass (perhaps only ~ 20% of the amount needed) to ever close it . The data also suggest that the rate of expansion may even be accelerating somewhat since light left the supernova 7 billion years ago.
The ideas in the preceding paragraph were the hottest topics of discussion among cosmologists during 1998; a conference on "Missing Energy in the Universe" focused on many relevant aspects. Three review papers in the January 1999 issue of Scientific American present details relevant to this growing concept of an ever expanding and accelerating Universe. The interested reader should consult these papers but a brief synopsis of each is given in the next three paragraphs.
The First, Surveying Space-Time with Supernovae (Hogan, Kirshner and Suntzeff) enlarges on the nature of Type 1a supernovae. These can be as small as ordinary stars that, after reaching the White Dwarf stage, continue to receive matter drawn from a companion star. This influx of new material onto the Dwarf increases its total mass until temperature rises for a sudden thermonuclear explosion that results in a form of supernova which lives for a short period whose duration depends on the mass. In turn, the mass determines the brightness. If a supernova can be detected almost at its onset, then the time involved in its duration indicates an energy output of a specific amount, so that this event can be used as a "standard candle" of notable reliability (+/- 12%) well suited to measuring distances. (Such supernovae are detected by taking telescope photos of small segments of the sky at different times, precisely superimposing them as computer images, compensating for differences in observation conditions, and subtracing later from earlier images [nulling out the same features that persist], and identifying residuals that have the appearance of supernovae). A supernova occurs, on average, about once every 300 years in a galaxy but because there are many galaxies in which stars can be resolved, a new one is found approximately once a month. Using this approach, astronomers have found a significant number of Type 1a supernovae lying at distances of 7 to 4 billion light years from Earth (as determined by redshifts) that actually occurred at those times in the past, when the Universe was ½ to 2/3 its present size. The surprise in these observations were that these Type 1a were up to 25% less bright than they should be at the determined distances. These implies that they were really further away and must have reached these positions because of changes in expansion rate. The best explanation favors increasing acceleration with time, so that the Open Universe model, associated with hyperbolic space expansion, is the most likely state. To explain this, if subsequent studies continue to support the conclusion, requires either the presence of some type of energy that counteracts expansion or a different Inflation model.
In the second paper, Cosmological Antigravity (L. Krauss), the presence of a repulsive force (actually, some type of still mysterious energy) which offsets gravitational attraction can account for the observations and many of their ramifications. This energy is similar in important respects to that postulated and then rejected by Einstein to explain how a static Universe can retain a constant size when gravity is acting to pull mutually on all matter, including the galaxies; Einstein abandoned his idea when evidence for expansion and the Big Bang became almost completely accepted, but in retrospect his notion of a Cosmological Constant (L) had merit but at the time did not apply to a Universe whose expansion characteristics were still poorly known. A positive L value generates long term repulsive forces such that the (Open) Universe will continue to expand forever without ultimately slowing down. L (in equations containing it, the Greek letter lamda [capital] is used) is associated with another parameter, O (Greek letter Omega). Omega is the ratio of the density of matter/energy in the Universe to the amount actually needed to produce a Flat Universe (Omega = 1); it is also the ratio of gravitational energy to the kinetic energy of all matter/energy extant. Although a Flat Universe seemingly fits many observations (but may be illusory in that we are only observing a small part of an infinite Universe which appears to be flat (like a small area on a large sphere or on hyperbolic saddle), when an inventory of all matter and conventional energy throughout the Universe is estimated, the amounts fall way short of that needed to achieve the true flat state. In fact, the value of Omega, as suggested from supernova observations, seems to be less than 1, which corresponds to expansion following hyperbolic geometery. (L itself is finite in this model in contradistinction to the value in the Table further up this page). If this holds up, it becomes necessary to account for this lower Omega, since matter/energy now identified falls way short of the required amounts. Thus enters the present day equivalent of Einstein's L, that is, some form of energy whose characteristics are only crudely known and existence is yet to be proved. This is the energy of empty space, an oxymoron in that space is then not really empty but its energy content is measurable as a vacuum density and this energy acts to offset gravity. Quantum theory suggests several of its characteristics. This virtual energy results from exceedingly brief quantum fluctuations that create virtual particles whose presence at any moment provide the repulsive force. (Thus, empty space is continually invaded by myriads of individual particles that have only fleeting existence but the process continues constantly as long as space exists). The value of L that influences this process may be constant (or could vary - as yet undetermined). In the early days of the Universe matter/energy density was very high but has continued to diminish with expansion. A few billion years ago, its value dropped below the (constant?) energy density associated with L, so that now its repulsive force is causing expansion to speed up, an effect implied by the fainter supernovae observations.
The third paper, Inflation in a Low Density Universe (Bucher and Spergel) presents an alternative to postulating an L-related repulsive energy. It reaches a similar conclusion that the Universe is Open and space is hyperbolic in its pattern of expansion. The model presented, the Open Inflationary Theory, is a variant of the Standard Inflation model of Guth and others. In the standard model, the inflation is related to the Inflaton Field (IF), which refers to particles that exist during inflation that result from quantum field oscillations (much like the virtual particles described above). Imagine a curve shaped like a broad, open U within an X-Y plot in which the vertical describes the potential energy changes and the horizontal changes in IF. At the onset of inflation, the IF moves down the curve towards a minimum. This process can take place within the infinite entity that is conceptualized to be everything (a continuum without bounds) within which one to many individual Universes can come into existence. In the Open Inflationary model, there is a warp in the curve near high potential energies in which the IF can be trapped, as though in a local trough, the so-called "false minimum" or "false vacuum". Many such "troughs" exist - each a potential Universe. When certain quantum processes occur, the IF state may "tunnel" out of this trough and proceed down the regular U surface towards the minimum. Each time this happens, a true inflation occurs and a Big Band ensues (note: Big Bangs only describe the growth of Universes, not the cause of the conditions that existed before inception. The IF, by its nature, imparts an antigravity force which leads to expansion. This process can occur anywhere within the continuum and not simultaneously, so that numerous universes (multiverses) can form and growth, much like the multitudes of bubbles in water approaching the boiling point. Most such "bubble Universes" never touch even as nearby ones expand but if two interact, they can experience tremendous energy effects. Some (most?) 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 (yield 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 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/experiments/calculations/conceptions but it does offer a way out of having to depend on the bizarre Cosmological Constant energy to account for acceleration.
To sum these last three paragraphs, recent evidence now suggests an Open Universe that tends towards the Flat type at one observational scale but may in fact be expanding infinitely in the hyperbolic space mode when envisioned at a greater scale. Both special forms of Inflation and the possible existence of a great quantity of repulsive energy may be involved. Unless huge amounts of matter/energy having attractive power over the expansion are discovered in the future, the Open model is most likely to be the favored scheme. Track down the January '99 issue of Scientific American to improve your insights and understanding of these complex ideas.
The question naturally arises: What is the ultimate Fate of the Universe? If it is closed, it will face the Big Crunch; what happens after a plausible new singularity is reached is a subject of imaginative speculation. If it is open, expansion will go on forever but over time all the present stars, and those to come, will run out of fuel and will either explode or burn out into degenerate matter. This Universe could contain a myriad of Black Holes, mostly small, but would nevertheless be totally non-luminous. But, such a Universe would "die out like an ember", continuing to expand as energy is dispersed and matter cooling and distributing randomly (obeying the Second Law of Thermodynamics - maximizing entropy), passing towards Eternity with a collective "whimper".
The precise Universe that has emerged is "the one we've got". Other Universes with different properties may have been possible. If certain fundamental properties and constants were to be only moderately different than the ones now identified and quantified, the nature and history of the alternate Universes would have been different - and certainly younger or older than the one we continue 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. In other words, there may not be an unlimited number of possible Universes that could have developed. Much depends on the initial conditions at the time of the Big Bang (or some other starting mechanism); these have not been fully specified (are not well known) even though the physics of the first minute have been worked out to rather exacting specifications. At the extremes in explaining how this Universe came to be are, on the one hand, the notions of a Creator spirit who "willed" the singularity, defined the Laws, and set up the proper initial conditions, and, on the other hand, a self-creating fluctuation in the quantum state of the pre-Universe that deterministically led to "some" Universe that worked (otherwise we wouldn't ever exist to know and ponder it: a somewhat philosophical notion known as the Anthropic Cosmological Principle).
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