Navigation

Recent Innovations about the Concept of "Universe"

Dark Energy and an Accelerating Universe?

A major clue to the open vs closed vs flat Universe seems to have been found in recent results from HST and ground "super-telescopes" observations. In fact, some cosmologists believe this to be the most important and enlightening discovery about the Universe since Hubble's observations more than 80 years ago. Some of what is presented in the next group of paragraphs has been extracted from an excellent review in the NOVA series on PBS, this November 2000 program being entitled "The Runaway Universe", from the references cited below, and from a recent book - The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos, by Mario Livio, 2000, J. Wiley and Sons. The latest volume on this subject is The Extravagant Universe, by Robert Kirschner, Princeton University Press. A quick overview of the accelerating Universe idea is given at this Web site which deals with one of the currently competing concepts, Quintessence, a special form of the dark energy that seems to be powering this acceleration.

The basic premise behind this so-called runaway Universe is summarized in this diagram prepared by the Space Telescope Institute in Baltimore, MD:

Diagram showing the history of relative expansion during the Universe's lifetime.

Following the initial inflation during the earliest moments of the Big Bang, the strong gravitational forces, during the extended time when the Universe was smaller and matter/density was higher, led to attraction forces that caused a net deceleration in expansion. But as the Universe continued to enlarge, and its density reduced the gravitational effect, the Universe then began to re-accelerate. Current speculation points to a repulsive force, probably in the form of "dark energy" analogous to that underlying Einstein's cosmological constant, that is gradually dominating the driving forces that determine relative expansion rates. Succinctly put, dark energy is a form of vacuum energy which has two defining properties: 1) in contrast to gravity, it acts as though it impresses a negative pressure (producing the aforementioned repulsive force) on all matter in the Universe and 2) it increases in proportion to the increasing size of the Universe. Its net effect, at present, has been to cause the rate of Universe expansion to accelerate during the second half of Universe history. In the next few paragraphs, these ideas are expounded at some length.

Dark Energy has so far proved difficult to detect directly and thus to pin down as to its nature. (It is sometimes referred to as "Phantom Energy".) The prime supportive evidence for this renewed expansion acceleration is being found through determination of redshifts of certain supernovae (the Latin plural; also written as supernovas). The results from analysis of more than 60 Type 1a supernova events lying between 4 and 7 billion light years from Earth (time-distance regions [intervals] in which the supernovae included in the study are still close enough to be easily observed) imply faster rates of expansion at some stage in cosmic time than heretofore calculated from deep quasar studies. They appear fainter than theory predicts for their distance from us, implying that they have moved farther than expected in the time involved, which would be explainable by an increase in expansion speeds at some stage. (As we shall see this could have occurred after expansion started or perhaps it is a "hangover" from Inflation.)

In fact, such rates indicate the Universe may 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 ~ 30% of the amount needed and most of it being dark matter [the visible radiation components of the Universe such as galaxies comprise only 5%]) to ever close it). In order to account for this apparent acceleration, a form of energy (as yet unidentified and unproved as present in space) that induces repulsion is assumed to exist.

Several ideas in the preceding paragraph are among the hottest topics of discussion in the astronomy and cosmology communities. In 1998, a conference on "Missing Energy in the Universe" focused on many relevant aspects. The announcement of evidence for an expanding Universe was judged the top scientific story of that year. 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 (either in the library or at the three Internet sites cited as links below) but a brief synopsis of each is given in the next sequence of paragraphs.

The first article, Surveying Space-Time with Supernovae (Hogan, Kirshner and Suntzeff) (Internet Site 1), 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 to a level forcing 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. Generally, the amount of mass in the Type 1a that is involved in the supernova is very similar in all such supernovae so that the energy released, in the form of detectable luminosity, is almost the same for every Type 1a event - meaning that these serve as nearly constant "standard candles" so that the decline in luminosity with distance is systematic. A Type 1a supernova therefore is an excellent means of judging distance between it and Earth observers. And, any departure in that distance from what would be predicted from the inverse square law becomes significant, as will be demonstrated below.

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, 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. In the search by astronomers, the goal is to find a supernova that is still in its early (front-end) stage of explosion and then follow its period of high luminosity over the next several months. This next image pair shows a "before" and "after" sequence in which a supernova appears in the M88 galaxy in 1999.

Supernova in Galaxy M88.

Here is another example of a more distant galaxy (bright center) with a second bright area off center that is the supernova.

A supernova within a galaxy.

The Chandra X-ray telescope has observed an expanding cloud, marked by clots, in the Constellation Hydra which has been interpreted as a Supernova 1a remnant:

A possible Type 1a supernova in a galaxy located within the Hydra constellation; Chandra X-ray Telescope image.

A Type 1a supernova occurs, on average, about once every 300 years in a galaxy but because there are many galaxies in which individual exploding 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 1/2 to 2/3 its present size.

In using Type 1a supernovae as a "standard candle", i.e., one whose luminosity or intrinsic brightness can be known with a high degree of accuracy, has been difficult, but work by several groups (the Perlmutter team; the Kirschner/Schmidt team, and others) has over the last decade achieved the ability to estimate brightness to within +/- 90% of actual value. These groups have found that Type 1a's brighten and then dim at variable rates. By analyzing brightness curves and applying theoretical corrections for the fade frequency, they have come up with a model that leads to a value for the brightness that allows it to act as an absolute magnitude event, for which distance (from Earth) can then be calculated with high accuracy. This distance should at face value indicate how far the star's radiation has traveled.

The surprise in these observations is that these Type 1a explosions were up to 25% less bright than they should be at the distances first postulated, i.e., these are fainter than expected assuming a simple slowing down of expansion owing to gravitational attraction. This relies on the postulated assumption that the Universe has continually been decelerating. Using redshift values for stars in the supernova's galaxy, the expected rate of expansion for the galaxy relative to Earth as the observing point can be determined for the continuous deceleration case. But, the diminution from the expected luminosities implies that the Type 1a star and its galaxy are really farther away (and hence appear to us as less luminous than predicted from a steadily decelerating Universe) and must have reached their more extended positions because of increase in expansion rate at some time during the Universe's enlargement. Put another way: these distant Type 1a events - early in Universe time - have redshifts less than expected and appear to be moving relatively slower than Type 1a events closer to Earth (which of course have smaller absolute redshifts). Thus, those closer in are moving relatively faster which, since these appear to us as they are much later in cosmological time, indicates that the acceleration of the Universe is in fact increasing.

As the number of Type 1a observations increase, a pattern in which they grew ever fainter relative to their redshifts emerged. Extrapolating this drop in brightness to the behavior of the entire Universe led to a conclusion that the Universe's density was headed towards the unbelievable condition in which its density would by implication become less than zero.

To recapitulate, the best explanation that prevents this contradictory density trend and is compatible with the brightness anomaly favors the idea that at some time in the past the Universe started experiencing a second episode increasing acceleration (the first being Inflation) 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.

Astronomers would like to confirm that the pattern of expansion rates pictured in the top diagram on this page (slowing - then accelerating) extends even further back in time from the 5 billion l.y. observations just described. The oldest 1a supernovae known, lying near an elliptical galaxy, at a distance of ~11.5 billion l.y., has had its redshift measured, with results consistent with the acceleration model. It is shown as a bright incandescent cloudlike object in the left image below; the right image is a time difference overlay of the scene for a two year period in which most objects cancel out leaving a bright spot that is the supernova centered within the envelope of ejected materials at the arrow point. Serendipitously, this supernova happened to lie within the scene used in calibrating the HST optics after the corrective servicing Shuttle mission in February 1997, so that its short-term changes are evident in the sequence of views obtained then. Its intrinsic brightness is greater than expected at its age and distance, consistent with the idea that it was slowing down during that time frame rather than speeding up as indicated in the younger Type 1a supernovae.

Supernova1997, occurring in deep space (10+ billion l.y.).

In the second paper, Cosmological Antigravity (L. Krauss) (Internet Site 2), the presence of a repulsive force (actually, some type of still mysterious energy) pervading all of space (including the so-called empty part that comprises the dominant portion of the Universe's volume) which offsets gravitational attraction can account for the observations and many of their ramifications. This energy is similar in important respects to the Cosmological Constant concept proposed by Albert Einstein more than 70 years ago to explain how a static Universe (the prevailing model at that time) can retain a constant size when gravity is acting to pull mutually on all matter, including the galaxies, such that matter subjected to attractive forces would ultimately collapse inwardly. In his view, there must be a repulsive gravitational force needed to stabilize this static Universe. Einstein later abandoned his idea (saying "It was my biggest mistake.") when evidence for expansion and the Big Bang became almost completely accepted. In retrospect his notion of a Cosmological Constant (which we will refer to by the arabic letter L; in equations containing it, the Greek letter Lambda Λ is used) once more has merit but at the time it was first conceived did not satisfactorily explain a Universe whose expansion characteristics were still poorly known. (Consult the University of Oregon's Astronomy course notes on the cosmological constant lambda for a summary of the meaning and history of Lambda.) Like Einstein's Cosmological Constant, this arcane repulsive energy is postulated to be real largely on the basis of necessity - the need to account for the acceleration if the above supernovae observations are sustained. The time at which this acceleration began to act is currently unknown; it may be a vestige of the original Inflation or may be a later (second) inflation whose cause for inception is still a mystery.

So, it now appears that Einstein's Cosmological Constant has been rehabilitated, but used in a different way. As Einstein conceived of its use (in his static Universe), it was a term added to his field equation for relating his geometry of spacetime to the distribution of energy and matter:

Gμv = 8πGTμv

, where G is Newton's Gravitational Constant, T is a Stress-Energy tensor, and μ and v are mass and energy terms. When his Cosmological Constant Λ is added to this fundamental equation, and multiplied by gμv, the spacetime metric tensor, the above equation now incorporates this repulsive force which he decided was needed to counter the collapsing effect of gravity:

Gμv + Λgμv = 8πGTμv

The Einstein Cosmological Constant has been revised to become a new term: ρvacgμv, the energy density of the Universe (the vacuum energy described on this page), that enters the equation on the right side:

Gμv = 8πGTμv + ρvacgμv

The reason for placing the modified ρ to the right is that it now is tied into the energy side of the equation rather than the spacetime term on the left. So, Einstein's Cosmological Constant has changed in its makeup and placement but still represents a force working to overcome gravitational effects.

A positive Λ (L) value generates long term repulsive forces that act as though they produce a negative pressure (-P). Another way to look at this is to refer to it as "antigravity". This negative acceleration means that the (Open) Universe will continue to expand forever without ultimately slowing down. L is associated with another parameter, Ω (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 (Ω = 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 perhaps small part of an infinite Universe which appears to be flat - like a tiny area on a large sphere or on a hyperbolic saddle), when an inventory of all matter and conventional energy throughout the Universe is estimated, the amounts fall far 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 [Λ] itself is finite in this model in contradistinction to the value in the Table on page 20-9). 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 (the so-called vacuum), a seeming oxymoron in that space is then not really empty; see this footnote (*) for comments on the types of vacuums and voids referred to in Cosmology. The type believed to account for all possibilities is the "false vacuum", which is characterized by some form(s) of energy. Quantum theory holds that zero energy is not possible in any part of space, even after electromagnetic, thermal, nuclear, and other forms of energy are completely removed. The energy content is distributed within a constant vacuum density (new energy is just balanced by increased spatial volume) and this energy acts to offset gravity. Quantum theory suggests several of its characteristics, which are the subject of the research field of quantum cosmology (and its subset, cosmongony, the study of cosmic origins and history). This virtual vacuum fluctuation energy is a form of "Dark Energy", i.e., neither emitting or absorbing light - which means that it does not give off any radiation that can be detected. The energy is inert in that it doesn't react per se to change matter even though it influences it by causing repulsion. Vacuum energy (also called Zero-Point Energy) is brought into play by exceedingly brief quantum fluctuations that create virtual particles and corresponding anti-particles that develop momentarily - and seemingly spontaneously - in the apparent vacuum of space and then mutually annihilate, releasing energy in a form that drives the acceleration by a repulsive force. This phenomenon, in which particles seem to be created from "nothing" (the Aristotelian concept of 'potency' - something has the potential of coming into existence - seems a good analog drawn from metaphysical thinking.), can be conceptually explained from principles of quantum physics.

The basis for the conclusion that the vacuum of the Universe (and keep in mind that the vast majority of space within the Universe can be said to be an almost perfect vacuum) is not truly empty of everything is the Heisenberg Uncertainty Principle. Thus, consider any finite portion of this vacuum; it can be totally devoid of any form of matter. But it contains at any instant of time the above Vacuum Energy, in what theory says is a very large quantity (the actual amounts are presently unknown). The Uncertainty Principle requires that statistically the probability is not zero that from this energy a particle-(antiparticle) pair can emerge at any instant and will likely last momentarily before annihilation. Thus, this vacuum unit contains virtual energy that has a finite potential to transform into a virtual particle. But, only rarely does a particle or antiparticle survive. It is still speculative, but with growing favor, that the end product of such a virtual fluctuation appearance is the form of energy now included in the concept of Dark Energy (it may comprise the entire amount of D.E. or other forms may also be present).

For more background into these ideas, visit the University of Oregon Astronomy Web site for their discussion of virtual particles.

That virtual particles exist has been verified by the Casimir effect. Consider this experimental setup:

Left: Apparatus used in producing the Casimir effect; Right: diagram depicting the 'quashing' of virtual particle wavelengths.

A pair of metal Casimir plates are place in close-spaced proximity. The entire apparatus is enclosed in a near-perfect vacuum. Virtual particles are assumed to pop in and out of the enclosed space. These have characteristic wavelengths. Those between the plates cannot function or propagate because the plate separation is less than the wavelengths of the particles. Those particles outside are not thusly restricted so that they can strike the outer plates producing a weak but measurable pressure. This results in an imbalance of forces that causes the plates to push together. Since no other sources of this attractive force can be accounted for, the conclusion is a verification that virtual particle were present and therefore exist.

Empty space is continually invaded by myriads of these individual (virtual) particles that have only fleeting existence but the process continues constantly as long as space exists and releases vast quantities of the above-mentioned as yet undetermined form of energy that powers the acceleration. 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 taking command and is causing expansion to speed up, an effect implied by the fainter supernovae observations.

-------------------------------

The following four paragraphs attributed to D.A. Stenger (1996) have been taken off the Internet and added here as supplementary information:

In General Relativity, spacetime can be empty of matter or radiation and still contain energy stored in its curvature. Uncaused, random quantum fluctuations in a flat, empty, featureless spacetime can produce local regions with positive or negative curvature. This is called the "spacetime foam" and the regions are called "bubbles of false vacuum" (these describe the infinitesimal volume of space at the outset of a Big Bang). Wherever the curvature is positive a bubble of false vacuum will, according to Einstein's equations, exponentially inflate. In 10-42 seconds the bubble will expand to the size of a proton and the energy within will be sufficient to produce all the mass of the universe.

The bubbles start out with no matter, radiation, or force fields and maximum entropy. They contain energy in their curvature, and so are a "false vacuum." As they expand, the energy within increases exponentially. This does not violate energy conservation since the false vacuum has a negative pressure so the expanding bubble does work on itself.

The forces and particles that appear are more-or-less random, governed only by symmetry principles (like the conservation principles of energy and momentum) that are also not the product of design but exactly what one has in the absence of design.

As the bubble universe expands, a kind of friction occurs in which energy is converted into particles. The temperature then drops and a series of spontaneous symmetry breaking processes occurs, as in a magnet cooled below the Curie point and a essentially random structure of the particles and forces appears. Inflation stops and we move into the more familiar Big Bang.

--------------------------------

This concept of the Universe experiencing ever decreasing density (the result of enlarging volume) of ordinary matter/dark matter with time while the repulsive energy (that part of dark energy equivalent to the Cosmological Constant or Quintessence [see below], or some similar form yet to be defined and quantified) is increasing, is presented in this simple generalized plot (Note: it is based on a Universe age of around 12 billion years [too young by about 2.7 b.y,]). The important feature is the notion of the two curves crossing sometime in the past with the Cosmological Constant energy now in greater amount than the matter curve.

Plot of densities of matter and the Cosmological Constant as a function of time.

Verifying the existence of Dark Energy in space itself and determining its nature (properties) and origin is now near the top of the list of priorities for further observations and theoretical explanations by cosmologists and astronomers. Dark Energy may be the most fundamental and extensive of all physical components of the Universe. As shown on page 20-9, it may be, in reality, the very ingredient that makes up the estimated 70% of the total mass/energy of the Universe. It, as has been shown, determines the expansion rate of the Universe and thus its ultimate fate. Despite this importance, its detection so far has proved elusive but future experiments offer promise for its belated "discovery", followed by plausible interpretation(s) of its significance.

We can make a few logical inferences about Dark Energy that may eventually be substantiated, modified, or discarded. First, Dark Energy is presumably the most fundamental physical thing" in our Universe, and probably in the "void" that exists beyond the this Universe's ever expanding outer boundary. Second, following the Einstein equivalency equation E = mc2, Dark Energy can itself be converted into Dark Matter under the proper circumstances, and this may be reversible. Third, ordinary matter bears a still undefined relation with Dark Matter/Energy in that after the Big Bang baryons and other forms of ordinary matter came into existence coincidentally with the Dark Matter/Energy that pervades the Universe. Fourth, as covered in various paragraphs on this page, the nature of Dark Energy is still undecided; it may be best connected with ideas of vaccum energy, with inflation, with quintessence, with negative pressure, or a combination of these or something yet to be conceived. Fifth, Dark Energy seems to have played a key role in forming galaxies and affecting their evolution and dispersal, as shown in this figure:

A model of the early Universe with strands of matter forming into galaxies (orange), as influenced by Dark Energy (black).

The key point made in this paper - and the heart of the new ideas emanating from the evidence for an accelerating Universe - is that there seems to be some preferred value for the Ωs of Dark Energy (L or Λ) and the other forms of matter and energy M. As discussed earlier and above, there are various estimates of these Ω values. Here are two plots of ΩΛ versus ΩM

Models of types of Universes based on Omega values for Dark Energy vs all other forms of matter and energy

The plot shows boundary conditions for Open and Closed Universes, as well as for Accelerating and Decelerating Universes. In the white field are contours that represent the confidence limits for the statistical locations of the range of Omega values that can be related to supernova 1a measurements that fall within the plot. In the plot below, the data collected by the High-z Supernova search team have been used to define a field of most likely Omega combinations. The maximum falls close to ΩΛ = 0/7 and ΩM = 0.3 values. This is in agreement with the current estimates (as percentages) for dark energy and all other forms of energy/matter.

Similar to the above graph but simplified to show a region where supernova 1a redshift data indicate the most likely Omega-Lambda; vs Omega-M values.

The presumption in this ΩΛ vs ΩM plot is: ΩΛ plus (+) ΩM = 1, which is the case for a Flat Universe, the model now most in favor.

This second paper included an elegant version of the graphs shown above. It must be entered at a large size, which will slow your downloading. So, we have placed it as an option to select by clicking on page 20-10a. Read the comments on that page which also apply to the above graphs.

Having become familiar with these new models for an accelerating Universe, it is now propitious to show the following plot.

Redshift versus cosmic distance plots using Supernova 1a data.

This figure contains data obtained by the High-z Supernova Search Team and the Supernova Cosmology Project. The upper plot shows Supernova redshift values versus distance in Megaparsecs to the SN 1a events monitored as described above. Four different sets of plots based on values for ΩΛ and ΩM are evident in the upper right where they diverge slightly from one another (at lower z values they are superimposed). This narrow spread is hard to see at this scale. The curves diverge when the log of the relative distances forms the ordinate. Some indication of the acceleration trend is evident but the plot points and their statistical spreads are not sufficiently separated to be decisive. More and improved observations, especially farther out in space, will be needed to convincingly confirm the Accelerating Universe model. A future satellite, SNAP (SuperNova Acceleration Probe), specifically dedicated to SN 1a data collection, has been proposed. Some information about SNAP is given in this figure:

The SNAP program.

The third paper, Inflation in a Low Density Universe (Bucher and Spergel) (Internet Site 3) 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. (Note: In other, similar diagrams, the parameter for the vertical axis is called energy density and the horizontal axis is labelled Higgs Field (in which the inflaton is equivalent to the [possibly now detected] Higgs boson).

Schematic diagram depicting one suggested behavior for a ball representing the very early Universe moves in a Higgs Field as it goes through the Inflation process.

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 Bang 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.

In a January 2001 paper in Scientific American, J.R. Ostriker and P.J. Steinhardt describe the Quintessential Universe in which the vacuum energy is modified so that it can interact with matter. It produces a dynamic quantum field, described by their term "quintessence, in which the presence of matter reduces the rate of acceleration below magnitudes predicted for cosmological constant expansion. In the long term future, say 30 to 50 billion years from the present, the quintessential expansion may either continue to increase or may be slowed down and even reversed by the influence of matter, even if that is then dispersed into a much greater volume. Click here for a good review of the essence of quintessence.

One of the "curiosities" still intact after the proposal in the 1990s of expansion acceleration is that it seems to be relatively recent in the cosmological time scale. In other words, this acceleration has only in the last 5 billion years or less (initial estimates of 7 billion years have now been lowered to more recent age) reasserted itself (amounting to a "second coming", following the first abrupt acceleration or Inflation in the early moments of the initial Big Bang), i.e, it has increasingly acted to counter the effects of Dark Matter gravity and may become even more dominant in the future. One might logically surmise that this repulsive force has existed all along but was less than the gravitational forces when the Universe was smaller during its first half of existence. Now, with further expansion, and greater separation of galaxies and reduced mass density, the repulsive force has started to exceed the attractive force (gravity) leading to a renewal of acceleration such that the pushing apart overtakes the earlier slowing of expansion when gravity exerted more effect on the Universe's growth.

Another consequence of this incremental expansion is that it causes the age of the Universe to increase insofar as it is determined from the Hubble constant. This offsets the anomaly first reported by investigators using the HST to refine the value of HO in which their best estimate at first was 12 billion years - a value contradicted by independent estimates of the ages of the farthest galaxies seen so far as 13-14 or more billion years and by the ages of stars, some of which give evidence of being older than 12 billion l.y. But, recalculating Hubble ages with the acceleration parameter included yields ages from 14-15 billion years, which removes the discrepancy inherant to the star age anomaly.

As the consequences of inflation-deceleration-acceleration Universe gained increasing favor, workers considering this reached the conclusion that the two principal components of the Universe - Dark Matter and Dark Energy - were the principal players in the observed expansion history. Thus the small amount of ordinary matter (less than 4%) plays an insignficant role. This diagram summarizes the current thinking on this "new" Universe:

From Riess and Turner, Scientific American, February, 2004

Thus, the decreasing Dark Matter (red) and increasing Dark Energy (yellow) curves cross about 5 billion years ago, at a redshift value when the Universe was about 4/5ths its present size. Thereafter (to the right) the Dark Energy (in terms of Energy Density) has continued to increase relative to decreasing Dark Matter (again, relative to Energy Density since it is likely that the equivalence of matter and energy [in the Einstein sense] noted for ordinary energy and matter holds for the always dominant Dark forms). .

To sum these preceding 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 sufficient attractive power to more than counterbalance the dark matter expansion, are discovered in the future, the Open model - with space increasing infinitely - is most likely to be the favored scheme. Here, the ideal of infinite space gains more clarity: if the constituents of the Universe survive forever in some form (see below), then as they move ever outward, they will always be creating new space (that which holds particles) out of the Void, (a term increasingly being used to conceptualize that seemingly truly empty vacuum into which space [defined by all volume [to some degree of multidimensionality] lying within its outer limits that contain discrete entities] can move into as it expands) in perpetuity. One consequence of the Accelerating Universe model is that it produces a density that almost exactly matches that predicted from the earlier Inflation mode. Track down the January 1999 and January 2001 issues of Scientific American to improve your insights and understanding of these complex ideas. Also, consider this general diagram that summarizes some of the above ideas.

World cone graphic illustrating the Accelerating Universe model.

Espousals of competing hypotheses such as above are a hallmark of scientific inquiry. As new evidence rolls in and specialists re-interpret existing evidence, major challenges to the prevailing theories arise; many die away but sometimes older ideas are abandoned and strikingly new concepts emerge. This has been especially true for Cosmology. With the launch of powerful, versatile new space observatories and much better ground-based telescopes, the wealth of new data over the last 20 or so years has prompted refinements of earlier models and explanations or their overthrow and advancement of better substantiated ones. This is true for Dark Energy. As of the middle of 2003, the notion of Dark Energy seemed plausible. Then, in December of 2003 a competent group of astrometers from ESA's Space Research and Technology Centre presented results of their interpretation of data from XMM-Newton, an X-ray observing spacecraft. The gist of their argument is based on studies of a group of galaxies about 10 billion light years from Earth. These appear to be giving out much more x-radiation than galaxies do today. If this is verified by further observations, the implication is that the early Universe was much denser than hitherto surmised. Also, their interpretation is that there were fewer galaxies then, and that all galaxies seem to continue growing by conversion of Dark Matter into Ordinary Matter. The higher densities during the first part of cosmological history run counter to the "concordance" model that requires Dark Energy and a lower average density. Instead, much of that fundamental component of the Universe could have been Dark Matter, one consequence being that Dark Energy had "condensed". However, this new hypothesis so far does not explain the apparent acceleration of the present Universe.

These new interpretations (and speculations) described above have also turned attention to the nature and role of gravity in the expansion/deceleration-acceleration history of the Universe. One of the aforementioned articles in the February 2004 issue of Scientific American, "Out of the Darkness" by Geori Dvali, reviews the latest thinking on gravity and its involvement in the story of this history. We review the main ideas in the paragraphs below:

The prime thesis is that there may be two kinds of gravity: the convention attractive mode (+) with which we are familiar and a repulsive form (-), possibly being the explanation for the cosmological constant or the quintessence modes described above, but in Dvali's proposals of a different nature described in this and the next paragraph. The argument is made in the article that the gravity we observe at Earth and even galaxy scales is of the + variety whereas at much larger scales the - variety takes over. Thus gravity does not behave the same when the entire Universe is considered as a unit. This repulsive gravity can be deduced, and its properties predicted, when matter and energy are examined at quantum scales - this is the so-called quantum gravity concept that many physicists feel will tie all particles and forces together in the Theory of Everything looked at several times in this Section. This implies that the standard Laws of Physics break down, or function differently, at the largest scales.

Dvaili draws upon Superstring Theory and the concept of the (Mem)Brane we were introduced to on page 20-1 to account for the dual behavior of gravity. He examines the possibility that the graviton, which provides the force particle that accounts for attractive gravity as it flows between particles, is capable of moving in various ways within and into/out of the Brane (see pages 20-1a and 20-1b). Gravity within the Brane (a flat but three-dimensional Universe) operates over galactic scales as an attracting force and follows the inverse square law. In four dimensions, gravity follows an inverse cube law whose effects do not reduce its ability to hold material and energy together when galaxies or galaxy groups define the scale. But when the postulation of multiple dimensions beyond four is introduced, these extra dimensions can accommodate the participation of gravity in a way that allows its to induce a repulsive force when the large scale Universe is considered. Gravity becomes weakened when the gravitons are able to move outside the Brane (the flat Universe). They escape into the extra dimensions, which can be of more than one proposed type: 1) curling (into closed loops) within the scales near quantum levels (Standard model); 2) infinite and curled (Randall-Sundrum model); 3) infinite and straight (Dvali model). Gravitons, according to this hypothesis, are the only particles capable of "leaking" out of Brane space into truly empty space beyond. Those (less energetic) gravitons that have low momentum (yielding long wavelengths) are the ones now escaping (some can return to the Brane but the net effect is a loss). This loss, on a grand scale, is reducing the overall ability of gravitons to decelerate the Universe, and at some time in the past the influence of the overall attractive effect of gravity, now weakened as the net strength of the supply of gravitons is diminished, has affected the Universe's behavior according to Einstein's General Relativity in the sense that the energy-density of the Universe modifies space curvature. The changing curvature is expressed as a geometry that gradually favors acceleration overcoming the restraining force of conventional gravity, which becomes unable to pull back on both matter and energy (dark and luminous). The Dvali mechanism obviates the necessity to presume the repulsive force implied by either the Cosmological Constant or Quintessence; there are in fact several other models that lead to acceleration (see Dvali's article).

There are astronomers who have questioned many of these ideas concerning Dark Matter, Dark Energy, renewed expansion, etc. As is so true of all Science, each new idea begets several variations or even alternatives. Here is an excellent example: Extremely small but significant variations in the Cosmic Background Radiation temperatures are cited as evidence for these invisible (and still undetected) forms of matter/energy. Their distribution is said to control the regions where galaxies first formed in the early, much smaller Universe. But Dr. Thomas Shanks (University of Durham) and colleagues have studied this distribution in terms of where major galactic clusters occur in the Cosmos and find a positive correlation. He cites a mechanism known as theInverse Compton Scattering Effect in which the hot gases within the clusters will energize CBR. This has the effect of shortening the wavelengths of the microwave radiation, as shown in this diagram:

The inverse Compton scattering effect on CBR, such that the deviations in WMAP temperatures may actually relate to passage of microwaves through galactic gas.

The implication is that the anomalous irregularities in the CBR are actually related to modifications imposed by interaction with galactic gases as faraway radiation passes through high concentrations of gases in galactic clusters.

We close this provocative subsection with a recent graph whose form is similar to that shown in the figure taken from Joseph Silk's Big Bang book shown near the middle of the previous page. The graph plots the relative size of the Universe against cosmic time, both parameters extending through the past, present, future.

Possible changes in Universe size throughout cosmic time.

Note the top curve. What Expansion Model in Silk's figure does it most closely resemble? Comment: Was the Abbe Lemaitre (previous page) guided by Divine Providence, by scientific prescience, or was he just plain lucky?

Multiple Universes


After these next paragraphs were written, the front cover of the May 2003 issue of Scientific American was enblazened with the title "Infinite Earths in Parallel Universes Really Exist" by Dr. Max Tegmark of the University of Pennsylvania, which is the lead article inside. It is a most profound summary of present-day thinking about whether many (infinite numbers may be possible!) Universes in infinite space could actually exist. The writer (NMS) has now read this mind-blowing ("Excedrin headache" level) article three times and I am just beginning to fathom new insights. For those reading this page, this publication is a challenge I toss to you to read at least once. I'll make no attempt to summarize the paper's contents here. But his ideas are also included in this Scientific American Web site.


(Multiverses are also tied into the concept of Membrane theory (M-theory), which will not be discussed in this subsection because of its complexity. Interested readers can turn to this Web siteS.M. von Weber which discusses Cosmic Membrane Theory as it applies to gravity.)

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). Most, if not all, 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 (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 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.

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 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.

The English cosmologist, Dr. Martin Rees has cited his own list of the irreducible number of fundamental parameters that determine the development of the Universe we can measure in his book Just Six Numbers: The Deep Forces that Shape the Universe, 2000, Basic Books. These are: 1) N; = ratio of the electric force holding atoms together to the (much weaker) force of gravity, 1036; if this number were larger then only a miniature and short-lived Universe would have formed; 2) ε = a measure of the strong nuclear force, determined from the energy released in the fusion of hydrogen to helium (differential of 0.007); if much different than this value, a different mix of chemical elements results, with carbon very scarce; 3) Ω = amount of matter/energy in the Universe (see above); it is the ratio of actual density to the critical density; if too high, the Universe collapses and if too low, expansion would be so fast that there would be insufficient time for stars and galaxies to form; 4) λ = antigravity force (Cosmological Constant); if too large, the Universe would have expanded so rapidly as not to develop as it has; 5) Q = force needed to dissemble a gravitationally stabilized cosmic structure (star; galaxy), measured as the ratio of the energy needed to overcome the gravity force to the energy bound in the rest mass of the cosmic body, given as 10-5; if Q deviates from this value, a smaller number would have prevented the ripples that gave rise to galaxies, leaving only a dispersed gas but if much larger, only black holes would exist today; 6) D = number of spatial dimensions (3) needed to sustain life in our planet and similar bodies; 2 or 4 would have doomed our existence.

Our Universe seems to follow the 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. Some intelligence, be it called God or some other analogous name or concept, had to play the pivotal role in establishing and structuring a scientific Universe.

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 - probably on the grand scale that is represented not only on Earth but in countless planets in innumerable galaxies. This situation - at the heart of the Anthropic Principle - this includes the idea that since intelligent beings (us, at least) indeed exist, the governing conditions for our Universe must have been been such as to permit life to develop - suggests that special factors must be present for a Universe like ours to succeed.

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. We have already alluded to the fact that we are 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, ~14 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 (the multiverses) 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 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

An interesting speculation rummaged through the writer's mind after producing the above paragraph. Suppose that there are 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". 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 [page 20-9]) 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. The figure below exemplifies some of these ideas:

A variant of these ideas 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 (beyond 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.

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.

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 absence 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 (for the writer at least) 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 Veneziano model of the Big Bang; from Scientific American.

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 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 B.B. 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).

And, once more, new observations and interpretation of acceleration and dark energy have been reported since the above was written, and will be added here at the end of this subsection. A A group of astronomers under study leader Steve Allen at the Institute of Astronomy at Cambridge University was successful in amplifying our knowledge of acceleration by analyzing data for 26 galaxy clusters at varying distances from about 1 billion to 8 billion l.y. away. These clusters are each surrounded by huge balls of hot gas. Three of the 26 are shown here, in images made by combining Hubble and Chandra data:

Gas envelopes around three named galaxy clusters at different distances from Earth.

From theoretical considerations, they deduce that these clusters should have fallen apart and dispersed by now, yet they are intact. They ascribe the surviving coherence of the clusters to the influence of dark matter. They have calculated ratios of the mass of the hot gas (which comprises about 85% of the luminous mass in a cluster) to the inferred mass of dark matter, using Chandra X-ray data. By assuming that the relative amount of gas is roughly the same proportionately at every cluster, these gas balls serve as another indicator of a luminous source of specifiable output, analogous to the Type 1A supernova as a "standard candle". They calculate the distance to each using the redshift z values. Their findings: the oldest clusters show deceleration but about 6 billion years ago this reversed and the younger clusters are now accelerating (are out too far; their present distance being more than expected from a non-accelerating situation). A constant dark energy (behaving very much like Einstein's Cosmological Constant) is the best fit to the apparent renewal of acceleration. Their take on the possible models for the Universe's future is summarized in this diagram:

Possible fates of the Universe.

They conclude that neither the "Big Rip" (wide dispersal of the Universe's materials as everything cools down) nor the "Big Crunch" (return of deceleration and collapse of the Universe) are likely, but the Universe will continue to expand coherently into the infinite future but observers on Earth (if we survive well into the future) will loose "sight" of the farthest objects near the cosmic horizon.

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 time, 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 rransientm, 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 they 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:

Vacuum energy graph.
From Scientific American, September 2004

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 troough. 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.

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. A bit of metaphysics or "astrophilosophy" is still inherent in the speculations. Let's touch upon several of these. 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.

If these last paragraphs touch upon the arcane and confuse you (as I freely admit to having my uncertainties), I can only suggest you read the appropriate articles (at least twice) for, hopefully, more insight than provided here.

Some Philosophical Implications

As a comment in the above ideas, from a metaphysical perspective, the argument that virtual particles are sufficient to explain creation of a Universe ex nihilo (from seemingly nothing) by postulating virtual particles that follow some mechanism converting them to "degrees of universe" has one serious flaw. These particles, even though they may not survive for any extended moment because the vast majority are quickly annihilated, still are something in the realm of existence. Therefore, they have a reality and are subject to principles of causality. So too is the existence of a vacuum which serves as the matrix for virtual particles. Thus, the age-old doctrine (traceable to the Greeks) that "only nothing can come from nothing" remains applicable - some independent agent is needed to "create" the vacuum and the potentiality of virtual particles. Of course, a conundrum arises at once: what is the origin (cause) of the agent. Theologians who cite the agent to be what is called "God" or "The Intelligent Designer" are forced to postulate a special property of being uncaused.

The question naturally arises: What is the ultimate Fate of the one Universe we know of for sure? If it is closed, as some cosmologists still maintain as an option owing to remaining uncertainties about total mass/energy, the Universe will face the Big Crunch; what happens after a plausible new singularity is reached is a subject of imaginative speculation (a singularity formed by crunching may be different and behave differently than the virtual particle singularity postulated for the earlier Big Bang). 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 current indications that the Universe's expansion appears to be accelerating, suggests that galaxies (which would gradually dissipate as the available hydrogen is used up and stars degenerate by processes already described) will finally move away from one another at the speed of light. With increasing distance from Earth (assuming it survives over the vast amount of time in the future as the Universe ages [in fact, it will not survive because the Sun will have consumed it within the next 5 billion years]; better to think abstractly about some long-lived observing point in the Cosmos), distant galaxies will become ever dimmer. Less and less of the presently monitored Universe will be observable as time proceeds. Outer galaxies will pass through an event horizon analogous to that which surrounds a black hole. As a galaxy, or its remnants, moves across that horizon, it will appear to any surviving observer as though "frozen", i.e., no further changes will be noted. Well into the future, the hydrogen fuel that makes up the stars will become exhausted as individual stars burn out and increasingly fewer new stars are formed. The remnants of such a Universe have been described as "embers". The Universe then consists of highly dispersed dark matter and dark energy, both with densities far less than in the Universe of today. When this state occurs in the future is still subject to "any body's guess". The number '50 billion years after the Big Bang' is a specific prediction based on astrophysical models. But infinity itself may be the better time and space reference.

The acceleration now being measured, caused by the gradual ascendancy of dark energy (which seems to be the basis for the Cosmological Constant) over gravity, is, from calculations, likely to progressively increase. This will cause galaxies, and the ever more common galactic remnants, to move at speeds approaching and eventually attaining that of light itself. If intelligence persists and exists long into the future, from the present laws of physics it can be predicted that thinking beings on any one location (planet) will be unable to "communicate" and monitor nearly all objects in the Universe except perhaps those still very close to the observation point. This model of the Universe has it "dying out" with a "wimper". One astronomer (Abraham Loeb) describes the galaxies as having become "ghosts". Such a distant Universe (both in time and space) may be dominated by Black Holes - the final victors in the dispersal and capture of matter and energy.

This has been a challenging page, replete with "wild" and "intriguing" facts and speculations. All that remains in our Tour through Astronomy and Cosmology is to see how Planets come to be and to weigh the possibilities of life - especially in some intelligent form(s) - throughout the Universe. That is treated on the next page, which also includes more metaphysical speculations about not only how but why Universe(s) exist in the first place, and, more particularly, musings on the role intelligent beings might play in the physical and (if it exists) spiritual aspects of the existence implied by assuming the Universe has some absolute reality. We have left largely unsaid this most fundamental of questions: Can a Universe (or multiverse) come into existence on its own or does it (them) require some external agent, and how did that agent (one version being "God") itself arise without some preceding cause? The next page will return to this conundrum, but don't expect any pat answers.


*Conceptually, a "true" vacuum contains absolutely nothing, neither matter nor energy in any form, and never will; the term "false" vacuum applies to space that can be totally empty at times but in which matter and/or energy is potentially present and can be manifested at any moment, either as short-lived particles that self-destruct and can recur repeatedly or as an event set in motion [e.g., the Big Bang]; in all likelihood, there is no "true" vacuum, namely "a complete nothingness", that has a physical or spatial location, cannot exist, despite the inclination to think of the Universe or Multiverse [see above] as expanding into that which is characterized by a real "true nothing" capable of becoming something; however, philosophers can imagine such a total nothingness as an essential component in their notion of Being.

Navigation


Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net

 

Next Previous Next Table of Contents Previous