OBJECTIVES After you have finished this lesson you should understand and know about * the history of the Universe; * black holes and super novae; * modern theories of elementary particles and the forces in the Universe; * possible deaths of the Universe; and * the subject matter of this course. COMMENTARY In the beginning there wasÉ? What was the beginning? The best and most consistent picture according to accepted scientific facts and theories is that the universe expanded from a singular point some fifteen to twenty billion years ago. The expansion was pretty dramatic, dwarfing every explosion that there has ever been, and thus is called the Big Bang. One of basic tenets of physics is that for closed systems1 there is a law of conservation of total energy, thereby of total mass by EinsteinÕs relationship \X(E=mc2), that holds without exception. If we postulate that the observable2 universe is closed, this law must hold and the magnitude of the mass in the universe has remained constant over all time. There has been and still is some debate over the spontaneous creation of energy, which would mean that the universe is not closed, but the majority of astrophysicists agree that there is insufficient reason and evidence for accepting this view at the present time. If all the mass were concentrated at a singular point, the gravitational red shift must have been infinite. Hence from the viewpoint of the universe after the Big Bang time did not flow within this mass concentrated at this singularity. Thus time started at the instant of the Big Bang, and one must conclude that it is meaningless to ask what was before the Big Bang!! Further if all the universe were in one singular point, distance could not exist either! Thus Space-Time only started to exist at the instant of the Big Bang; in other words, the existence of space and time are subsequent to the existence of the universe. The universe does not exist within space and time, but rather space and time exist within the universe! According to quantum theory there is no such physical concept as a point in space or time. So how do we combine the above general relativistic view with the quantum mechanical one, which must clearly be significant when the universe was infinitesimally small. A universe of microscopic size would not have a definite position or size and processes taking place there would not have a definite duration. But there would be average values for these quantities in the Òsingular stateÓ that would correspond to zero space dimension and time duration. According to quantum theory there is an intrinsic probabilistic fluctuation of the values of these quantities about these average values. Following this line of reasoning one comes to the conclusion that the Big Bang is actually a Quantum Fluctuation. This is a possible explanation of how the Big Bang occurred but not, I think, why it did. Just after the Big Bang the universe was incredibly hot, so hot and small that all the forces were unified into one force (Grand Unification) and there were no distinctions between fermions and bosons, no particles, and no photons (i.e., no radiation). During the first 30 minutes the universe cooled and expanded enough so that the four fundamental forces of nature became distinguishable from each other. Material particles and photons Òcondensed outÓ of the high energy soup, but were continually transforming into each other. It took another 700,000 years before the first simple hydrogen and helium atoms became stable and not continually interacting with photons. The universe at this time became partially transparent to electromagnetic radiation, and we are still receiving light that was created then in the form of incandescent radiation. At the start of its journey some fifteen to twenty billion years ago it was very hot, about 10,000K3, but due to the doppler red shift caused by the expansion of the universe, we see this light as a background incandescent microwave radiation at 3K, very cold! That this radiation seems to be coming to us uniformly in all directions (i.e., it is isotropic and uniform) is good evidence that the Big Bang actually did occur. The expansion of the universe should be viewed as if we were on the surface of a balloon that is being blown up causing all points on the surface to fly away from each other. There would also of course be some local variations due to other comparatively minor explosions and processes. Light from the farthest points would take the longest to reach us, the origin of the 3K background radiation forming a horizon all around us with each point in this horizon being approximately the same spatial and temporal distance from us. As the universe gradually cooled down, clumps of hydrogen atoms and molecules formed; these clumps had mass and therefore attracted more hydrogen and become more massive and dense as the gravitational forces increased. The atoms at the center of these clumps became squashed together and thus hotter due to frequent collisions with each other. They then began to burn in a thermo nuclear process where hydrogen atoms fuse together to form helium, releasing huge amounts of energy. This process is called Thermo Nuclear Fusion and is the same process that takes place with a hydrogen bomb.4 The released energy heated up the burning gas still more, causing further expansion. Thus one had two competitive forces acting on the hydrogen: the force of gravity causing a contraction and the effect of heat causing an expansion. At some time these two forces became balanced and a star was born. Stars remain a constant size, burning hydrogen to helium for about ten billion years. Our sun is about five billion years old, thus is a second or third generation star, so we can expect it to continue in approximately the same form for another five billion years. After burning for ten billion years, the hydrogen is essentially used up and a star consists of mostly helium. As a star stops burning, it cools down and contracts, but the contraction heats things up again until the helium ignites. Once the helium has ignited a star expands to many times its former size to become a red giant. In turn the helium runs out and a star contracts once again. If a star is not very massive, it will never become hot enough to burn carbon; it will just shrink while at the same time ejecting, in an explosive manner called a nova, some outer residues to form planetary nebulae. For some time the shrunken core burns white hot (a white dwarf star), radiating energy into space, cooling down to a red color (a red dwarf), and finally emitting only in the infrared and microwave regions (black dwarf). If a star is massive enough, the contraction-expansion cycle continues until the element iron is reached. Iron will not fuse to form higher mass elements since, instead of releasing energy, this process would need energy to make it occur. Such stars collapse one final time, but their cores are very dense thus they rebound like a squashed rubber ball, exploding into supernovae. The high temperatures in supernovae provide enough energy for the formation of all the elements more massive than iron, which are ejected far from the center of the exploding star. The inner core of the star implodes in reaction to the explosion and an incredibly dense neutron star is formed. The electrons and protons in the imploding core react to form neutrons. Matter made out of neutrons is enormously denseÑa thimble full would drop straight through the earth. A neutron star is just tens of kilometers in diameter and rotates at very high speed due to the conservation of angular momentum (which is mass times speed times radius of the rotating body).5 Very often these neutron stars emit radiation from one area of the star only. As these beams are rotating, they sweep past us, illuminating us periodically rather than continually, thus causing us to observe pulses of radiation. For this reason such stars are known as Pulsars. If a dying star is three to five times the mass of our sun, then the implosive contraction does not stop with a neutron star, but continues until all the mass is contained in a volume that is only two or three kilometers in diameter. As the mass is concentrated in a volume of such small radius, it is possible for objects to come very close to the center of the mass. From NewtonÕs law of gravitation, which is approximately valid, the closer an object gets to the center of a mass producing gravitational force, the greater the gravitational force.6 If an object is travelling fast enough in the vicinity of a source of gravity it can, in general, exceed the escape velocity, break free from the force, and travel freely onwards. However, these imploded stars are so small and dense that even light can approach to such a short distance to the center of mass that it is trapped forever in the intense gravitational field. The distance at which this occurs is called the Schwarzchild radius, which depends on the mass. As light can never escape from inside this radius, the object is black (recall that by definition blackness is the absence of electromagnetic radiation), thus these objects are called Black Holes. According to the general theory of relativity, light can be viewed as either bending near gravitational sources or just following the curvature of space-time. In the latter view a black hole would be a region of very high curvature into which light would drop; in the former a black hole would be visualized as a very dense blob that attracts all light that comes close enough. Both views are equally valid. It is possible to calculate a Schwarzchild radius for any object, but in general they will be inside the material boundary of the object and it will be impossible for other objects to come that close without interacting via the other forces of nature (e.g., electromagnetic, weak, or strong forces). It is only when the Schwarzchild radius lies outside the material boundary of the object that one has a black hole. It should be emphasized that the black hole effect is due to large amounts of mass being located in very small volumes so that objects can come close enough to the center of the mass. If our sun suddenly changed into a black hole (i.e., if all of its mass became concentrated within a very small radius), the earthÕs orbit would not change since the force of gravity just depends on the total mass and the distance from the center of that mass. Thus the force felt by the earth would not change, but we would not see any light coming from the surface of this concentrated mass! How can one observe black holes if no electromagnetic radiation escapes from their gravitational field? The above example about the earth gives a clue. The mass of a black hole continues to produce a gravitational effect far from its center of mass so material will be observed being pulled into it. The ripping apart of the material will produce x-ray radiation, which is observable. If the original star was spinning, angular momentum must be conserved so orbital motion about its center of mass will continue and motion of nearby stars will be affected.7 Both of these two effects have been observed with the star Cygnus X-1. Another way black holes could observably effect their surroundings is through static electric and magnetic fields; these effects are, however, much harder to measure than the first two. Black holes, by their nature, eat everything close by, so their Schwarzchild radius must get bigger and bigger until there are no more objects in their range. Thus one would expect to find giant black holes where there is (or was!) a large concentration of matter, such as centers of galaxies. There are some observations of galaxies that suggest the existence of giant black holes, but it is hard to produce conclusive evidence for such phenomena. It has been suggested that Quasars are evidence for these giant black holes. Quasars are the brightest source of radiation in the sky, much brighter than super novae. The light observed from them has very large red doppler shifts, showing that it started its journey a long distance away in space and time. How do we know the red shifts are doppler shifts and not gravitational shifts? If the red shifts were due to high gravity sources, the gravitational field needed to produce such large red shifts would have to be so large that it would tear apart atoms. However, the light observed from Quasars contains atomic spectra, showing that the gravity could not be that intense at the source of the light. The intensity of light from Quasars corresponds to many million simultaneous super novae, which is a very unlikely spontaneous occurrence. A possible explanation could be that giant black holes at the center of some galaxies ripped to shreds and sucked in trillions of stars, and that Quasar light is produced by such cataclysmic events. What would the environment be like inside black holes? Can we know? Processes taking place outside the Schwarzchild radius, but close to the black hole, would be observed to be running slow compared to and viewed by the rest of the universe in lower gravity fields. Processes inside the Schwarzchild radius would have stopped from the viewpoint of outside observers; no electromagnetic radiation waves could escape, only static electric and magnetic fields. The trapping of light is just another way of describing an infinite gravitational red shift. Could the core of the black hole continue to contract beyond three kilometers, or would other forces force it to stop? What happens if the core becomes of microscopic size; shouldn't we take into account quantum effects? Yes, this is true, and Stephen Hawkings has shown that quantum effects predict some tunneling of matter out of black holes, but this is only significant for small black holes. The big problem in describing black holes is that we do not have a reliable, consistent theory combining the concepts of general relativity and quantum mechanics. At the present time the most promising conceptual framework to unify quantum theory and general relativity is super string theory. The basic entities of this theory are string-like objects that can combine in various ways. These string- like objects are not like everyday strings, but are quantum objects with average properties that one can identify with some properties of macroscopic strings. One of the strange features of this theory is that it predicts that there are 10 or maybe even 26 dimensions in the universe, but only three spatial and one time dimension became significant; the others can only be observed at very short distances. For example, if one measured distances of the order of 10-30 of a meter, one would find that there are not just three, but ten perpendicular directions in which one could travel around a given point in three dimensional spaceÑbut only for lengths on the order of 10-30 of a meter! Maybe some of these extra dimensions become important inside black holes? What picture of the universe have we come to understand during this course? In the beginning the universe occupied a microscopic singular point; time and space as we know them did not exist, but came into being when quantum fluctuations produced an epic expansion that we call the Big Bang. After the first few instances of the universe the uniformity of all energy was lost and different localized forms of energy became recognizable (i.e., elementary particles), but they continually transformed into each other. These elementary particles were not really separate entities, but formed collections that were quantum states of the universe as a whole, the quantum states being different probability wave patterns with varying localizations of mass and energy. It was these probability wave patterns that were in a constant state of flux and the process of varying between the different states equivalent to the effect of a single unified force.8 As the universe cooled down, the rate at which these universal probability wave patterns changed decreased and certain parts of the pattern become more persistent; the changes between the patterns could now be thought of in terms of four distinct forces. The balance among the various properties of the different forms of observable energy was determined in the first few instants of the life of this universeÑthe ratio of matter to anti-matter, the values of the different constants of nature, and the distribution of mass throughout the universe. As the universe cooled down more, galaxies full of stars and thus planets formed, and on at least one, organic life evolved. We have seen that time and space are aspects of each other, as are mass and energy, and also that energy and time are complementary properties, neither one totally characterizing processes and the states of physical systems; that in a finite closed universe there must be a smallest time duration and spatial length; that observations have an effect on the observed, as nothing can be observed without interaction; that all entities have both wave- and particle-like properties and are vibrating in probability wave patterns analogous to the harmonic tones of musical instrumentsÑthus the whole universe can be thought of as a giant orchestra where each instrument is constantly a source of celestial notes. In order to hear this music we must interact through the electromagnetic, strong, weak, and gravitational forces and at frequencies unheard of with normal musical sound. All entities in the universe are sharing and exchanging basic energy forms with each other, whether they are alive or were alive. It is the way of the universe that patterns keep changing and reforming in a fluid maelstrom of flux, but there are patterns and symmetries that with careful observation and description allow us to predict, participate, maybe locally guide, and interact. All these behaviors are manifested in changes in the universal probability wave pattern. How will it end? There are four basic scenarios: a. The universe will continue to expand forever, thus the Big Bang explosive force will overwhelm the force of gravity. As the universe expands it will cool down further and the temperature will slowly approach absolute zero. b. The universe will reach an equilibrium position where gravity forces balance the Big Bang explosive force. c. The universe will contract again to a singular point and gravity forces will win. d. The universe will expand for some time then contract, but then expand anew and continue to oscillate in sequential expansions and contractions, each new oscillation renewing space and time. The factor that will decide whether a, b, or c will occur is the overall strength of the gravity force, which in turn depends on the amount of mass in the universe. At the present time it is uncertain whether there is enough mass to cause the universe to stop expanding; it depends on whether the mass of the neutrino is zero or just very small and whether there is some so-far-unobserved dark matter out in distant space. Option d could never be observed, but is intuitively satisfying, at least to me, and would seem to be predicted by quantum theory, if the universe does indeed contract to a singular point again.