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!"#$%&'()*+),(-.%/0.(!"#$%&'( 123$4&2(56%,($72(8&"9":#(%;($72(<:"=2( !"#$%&&#"'!()*'+)",-)** . ' In this second week of the course, I plan to use our lecture time together so that we can discuss how scientists account for the origins of the universe. Today, the explanation most physicists and astronomers favour what has become known as the “Big Bang” Theory. This theory postulates that the universe suddenly came into existence around 13.7 billion years ago. Scientists estimate that this is the age of universe on the basis of measuring various cosmic phenomena, such as light emitted by stars, and temperature fluctuations in cosmic microwave background radiation (this is the radiation that permeates the universe and which scientists think was to produced in the first moments of its creation). As the name “Big Bang” Theory suggests, at the core of this theory is the assumption that the creation of the universe was an incredible, sudden explosive event. What caused this Big Bang? Scientists agree that the universe came into being by what they term a singularity – a remarkable extraordinary event unlike anything that has occurred before or since. The singularity of the universe’s creation is something that our current understanding of physics can’t really explain. It appears to have been created at a point or zone that had no dimension or volume. The Expansion of the Universe, public domain image, http://commons.wikimedia.org/wiki/File:Universe_expansion.png 2 ' / It defies our every-day common sense understanding of reality that something without dimension or volume could be the origin of something. Could something emerge from nothing? Mathematically, it is possible to conceive of our universe as having originated at a point at which the force of gravity acting on matter was so great that it was infinitely dense. However, scientists generally concur we cannot say anything with any certainty about the universe before its existence. Further, as to what occurred happened it came into existence, well, that is something about which we cannot construct anything like a robust scientific explanation; and as the eminent mathematician and physicist, Stephen Hawking, has pointed out, whatever events that may have occurred before the creation of the universe have had no consequences after its creation, and thus should not form part of any scientific model of the universe. Scientists have hypothesized that in the first few infinitesimal fractions of a second after the Big Bang, remarkable things occurred in a number of successive distinct phases. The first phase between the moment the Big Bang occurred and around 10-43 of a second later, is what scientists have called the Planck Epoch, in honour of the great German physicist Max Planck (1858-1947). It’s thought appropriate this earliest phase of the universe’s history be named after Plank because his work on quantum theory suggests that in this initial phase of the universe the laws of physics as we know them did not operate. The universe at this moment had zero size, was infinitely hot and the fundamental forces governing its subsequent regularities were one unified electronuclear force. Some scientists have suggested that at around 10-34 of a second after the Big Bang the universe entered a new phase, which lasted until just 10-35 of a second after the Big Bang. This phase they call the Grand Unification Epoch because the temperature of the universe was still incredibly hot, if no longer infinite. In the Grand Unification Epoch, the universe grew in size from being no bigger than that of type of sub-atomic particle we call a quark to the size of a proton, the elementary particle we find in the nuclei of atoms. The electromagnetic and strong and weak forces that now determine how the simplest particles in the universe interact with each other were still one force. However, by the end of the Grand Unification Era, the strong force that holds together quarks and another type of sub-atomic particle called gluons had into existence. Once this happened it allowed the formation of protons and neutrons, the key parts of atomic nuclei. Scientists differ about what have occurred in the universe prior to10-35 of a second after the Big Bang. Where there is greater consensus is about what happened from around 10-35 of a second. This is because scientists are able to move beyond the realm of theoretical speculation to study the effects of what happened after 10-35 of a second 3 0 ' after the Big Bang. For the effects of what happened from this time can still be observed today. From 10-35 of a second after its creation, the universe entered a phase that scientists since the early 1980s have called the inflationary epoch. During this inflationary epoch, the universe began expanding faster than the speed of light. What caused this expansion is unknown. Some scientists have suggested that it was due to some sort of unknown anti-gravitational effect; but this runs counter to how gravity is understood by modern physics. However, some astronomers have reasoned that anti-gravitational forces may exist, as their observations of certain stars have produced evidence highly suggestive that the universe is expanding at an accelerating rate. If this is so, then the most plausible explanation of this acceleration, they have argued, is that it is caused by some kind of anti-gravitational phenomena. The inflationary epoch is estimated to have ended somewhere between 10-33 and 10-32 of a second after the creation of the universe. By this time, the universe had expanded from a size possibly less than the volume of an atom to being larger than the size of galaxy. Moreover, as this expansion occurred at speeds beyond those of light, what we can now see of the universe is only a microscopic proportion of the entire universe. By the end of the inflationary period, the universe was still mostly made up of particles of light with no mass called photons, which scientists classify by the name of quarks and anti-quarks – two kinds of particle that are identical apart from possessing opposite electrical charges. What happened at this point I the early history of the universe was that every time a quark met an anti-quark, the two particles were transformed into energy – the energy known as cosmic microwave background radiation that pervades the universe, and which can be observed today. Those quarks that did not meet an anti-quark formed the matter that exists in our universe. Scientists estimate that probably one in every billion quarks did not meet with an anti-quark and survived to provide the basis of protons and neutrons – the elementary building blocks of atomic nuclei. 4 ' 1 “The quark structure of the neutron. There are two down quark in it and one up quark. The strong force is mediated by gluons (wavey). The strong force has three types of charges, the so called red, green and the blue. Note that the choice of blue for the up quark is arbitrary; the "color charge" is thought of a circulating between the three quarks.” Image: Arpad Horvath, Creative Commons licensed, http://commons.wikimedia.org/wiki/File:Quark_structure_neutron.svg David Christian amusingly describes this process by which the matter in our universe was formed in his Maps of Time as a “perverse subatomic game of musical chairs, in which quarks were the players, anti-quarks were the chairs, and the winner was the one quark in a billion that could find a particle chair.” 1 About a second after the Big Bang the universe had become so big that its temperature dropped considerably. However, the universe was still very hot, with a temperature of around 10 billion degrees Celsius – which is about a thousand times hotter than the temperature of the sun. At this early point in its history, the universe was comprised of particles such as electrons, protons and photons that interacted to produce energy. The Cosmologist Eric Chaisson has called this time the era of radiation, describing universe as a “macroscopic glowing ‘fog’ of dense, brilliant radiation”, suspended in which was a “relatively thin microscopic precipitate” of matter.2 A hundred or so seconds later, the temperature of the universe had dropped to a billion degrees, which is about how hot 1 David Christian, Maps of Time: An Introduction to Big History (Berkeley: California University Press, 2005)., pp. 24-5. 2 See especially Eric Chaisson, Epic of Evolution : Seven Ages of the Cosmos (New York, N.Y. ; Chichester: Columbia University Press, 2006). 5 2 ' it is in the core of the hottest stars. What physicists call the strong force now caused protons and neutrons to bind to each other to form simple atomic nuclei – the simplest being deuterium (a heavy form of hydrogen). Diagram of hydrogen atom Author: Webber Source: "own work" Date: 2005, public domain, http://commons.wikimedia.org/wiki/File:Hydrogen_Atom.jpg When deuterium nuclei collided with other protons and neutrons, the strong force ensured they were turned mostly into helium nuclei, but also much frequently into heavier nuclei such as those of lithium and beryllium. Scientists think that about a quarter of all protons and neutrons were turned into helium nuclei. However, helium was only produced in the first hours of the universe. Thereafter protons and neutrons became hydrogen nuclei, as the neutrons in deuterium (heavy hydrogen) lost energy and became protons. 6 ' 3 Model of the Helium atom, showing the nucleus with two protons (blue) and two neutrons (red), orbited by two electrons (waves), public domain, http://commons.wikimedia.org/wiki/File:Helium_atom_with_charge-smaller.jpg For the next million or so years the universe was mostly empty space permeated by radiation, but in which there was this thin precipitate made up mostly of hydrogen and helium atoms. As the universe expanded, the energy and matter it contained became subject to the force of gravity. As Isaac Newton, the great late seventeenth century scientist was to show, the force of gravity causes particles of matter to be attracted to each other. Moreover, as Newton further showed, the closer together particles move, the greater the gravitational attraction between them becomes. Now, had the hydrogen and helium atoms that made up most of the matter in the early universe been completely evenly distributed in empty space, the universe, as we know it today would not exist. Had these atoms been evenly distributed, all that would have happened would have been a deceleration in the rate of the universe’s expansion. There would have been no formation of stars, planets or living beings. Stars, planets and living beings were formed because the atoms within the early universe were not uniformly distributed. There were slight variations in the heat and density of matter in the early universe that resulted in the denser regions of matter becoming denser as they became subject to the force of gravity. 7 4 ' “A Volume plot of the logarithm of gas/dust density in an Enzo star and galaxy simulation. Regions of high density are white while less dense regions are more blue and also more transparent. The data used to make this image were provided by Tom Abel Ph.D. and Matthew Turk of the Kavli Institute for Particle Astrophysics and Cosmology.” Public Domain: http://commons.wikimedia.org/wiki/File:Star_formation.jpg 8 ' 5 “Located some 13 million light-years from Earth, NGC 4214 is currently forming clusters of new stars from its interstellar gas and dust. In this Hubble image, we can see a sequence of steps in the formation and evolution of stars and star clusters. The picture was created from exposures taken in several color filters with Hubble's Wide Field Planetary Camera 2.” Public Domain: http://commons.wikimedia.org/wiki/File:Fireworks_of_Star_Formation_Light_Up_a_Galaxy__GPN-2000-000877.jpg The force of gravity first acted on those atoms of hydrogen and helium that were no evenly distributed. As these atoms gravitated to each other, they formed denser clouds of gas that then became compact. As they compacted, the pressure at the centres of these masses caused temperature increases as high as 10 million degrees centigrade. At this temperature, hydrogen atoms were no longer subject to repulsion between their positively charged nuclei. They broke up to form helium atoms, each with two protons in its nucleus. This process of nuclear fusion – the same reaction that occurs when a hydrogen bomb explodes - released incredible amounts of energy. What is more, the energy released by these fusion reactions was powerful enough to resist the force of gravity and stop the atoms from collapsing into ever-denser masses. In short, the force of the nuclear reaction and that of gravity balanced each other out, thus allowing stars to form. As David Christian observes, stars can be understood as “…the result of a negotiated compromise between gravity, which crushes matter together, and the 9 67 ' explosive force of fusion reactions, which forces matter apart.”3 The oldest stars in the universe those in the centre of galaxies, or in globular clusters orbiting their centre. We can tell that they are of great age because they generally have no elements heavier than hydrogen or helium. NASA public domain image, http://commons.wikimedia.org/wiki/File:Hubble_Space_Telescope,_better_than_new.jpg In April 2002, NASA's Hubble Space Telescope (pictured above) discovered what are probably the oldest stars in our galaxy, the Milky Way, in a globular cluster called M4, some 5,600 light years away from Earth. These stars have been estimated as having formed less than 1 billion years after the Big Bang. 3 Christian, Maps of Time: An Introduction to Big History, 44. 10 ' NASA image of artist's impression of 66 the Milky Way, in public domain: http://commons.wikimedia.org/wiki/File:Milky_Way_galaxy.jpg The shapes of galaxies reflect the influence of gravity. Our own galaxy, the Milky Way, is very typical in having become by gravitational force a large spinning disk. We would do well to keep in mind that the observable universe is only a very minute portion of the entire universe. The theoretical physicist and cosmologist Alan Guth, a key figure in the development of the Big Bang Theory, has suggested that the entire universe could be as much as 1023 to 1026 bigger than the universe we can observe. Further, it is clear that the universe contains things other than stars. There are entities known as “black holes”, which are parts of space so dense that neither matter nor energy can resist their gravitational force. Cosmologist and science writer Timothy Ferris has vividly likened the density of a black hole to the earth compressed into a ball about 0.7 inches in diameter.”4 In all likelihood a massive black hole lies at the centre of our galaxy, for there is a powerful source of radio waves astronomers have detected in the area of the constellation known as Sagittarius. So powerful are these waves that they suggest this black hole has a density around 2.5 million times that of our sun. 4 Timothy Ferris, The Whole Shebang: A State-of-the-Universe(s) Report (New York Simon and Shuster, 1997), 79-80. Cited Christian, Maps of Time: An Introduction to Big History, 46. 11 6. ' NASA sponsored artistic representation of a black hole. Public domain, http://commons.wikimedia.org/wiki/File:BlackHole.jpg As to the significance of black holes in the creation of the universe, there has been much speculation, including the suggestion that they might be separate universes that we are seeing from the outside. This may seem a wild idea, but it plausibly explains why the nature of the physical forces and relative size of nuclear particles in our universe have been such as to allow the formation of stars, the range of elements that exist, and ultimately the development of complex life-forms. Stars Our focus here, however, is on stars: their creation, life and inevitable death. The life cycles of stars are largely determined by the size of the cloud of matter from which they formed. If the cloud resulting in their formation is less than around 8 percent of the size of our sun, the centre of the star will not reach the temperature required for hydrogen to be transformed by nuclear fusion into helium and progressively heavier elements. They will become what astronomers call a brown dwarf, or sometimes refer to as a “failed star” because these entities failed to reach the point of hydrogen fusion and hence are objects somewhere between being a star and a planet. The existence of “brown dwarfs” was first hypothesized in the mid-1970s, but 12 ' 6/ was not confirmed by observational evidence until 1995 using advanced imagining techniques and spectroscopic analysis. The more matter there is in a stellar cloud the greater will be the gravitational force to which it is subject. The greater the gravitational force, the faster it will compact. The faster the compaction, the denser and hotter this ball of matter will be. This in turn will determine the speed at which the star that forms will consume the material it contains. Hence the largest stars in the universe may have more matter but nuclear fusion takes place at a faster rate, and decay is much faster than in smaller stars. The life expectancy of our sun, for example, is estimated to be 5 billion years. A star ten times the size of our sun may last no more than 30 million years. The largest stars in the universe may have a life cycle of no more that several hundred thousand years. NASA / ESA sponsored artist's impression of Sirius star system. Public domain image: http://commons.wikimedia.org/wiki/File:Sirius_A_and_B_artwork.jpg Often two or more stars are created when gas clouds between more than 60 to 100 times bigger than the size of our sun have gravitationally compacted. The best-known example of a binary star system is the Sirius system (represent above), which is situated about 8.6 light years from Earth. One star, Sirius A, is the brightest start visible in Earth’s Northern Hemisphere. It is a hot white star twenty-five times brighter than our sun and around twice its mass. Its companion star, Sirius B, is a very dense white star that has ceased nuclear fusion and is gradually cooling. Sirius 13 ' 60 A, incidentally, was of great significance in ancient Egyptian and ancient Greek cultures. Its movement from just above the eastern horizon and steady westward movement at about one degree a day was used by the ancient Egyptians and Greeks to calculate when to plant crops. The best-known example of a triple star system is the Alpha Centuri system, which is made up of a pair of yellow dwarf stars some 4.4 light years from earth and a dwarf red star that is estimated to be about 4.2 light years from the sun. There are also systems containing between four to seven stars. A comparison of the sizes and colors of the stars in the Alpha Centauri system with the Sun made by David Benbennick, and licensed under Creative Commons, http://commons.wikimedia.org/wiki/File:Alpha_Centauri_relative_sizes.png Nuclear fusion within a star will gradually turn all its hydrogen into helium. As helium is a heavier element it will sink to the star’s core. As a star exhausts its available hydrogen, it will cool and gravity will cause it to collapse into a denser mass. This collapse will increase the temperature at the core of the star. If the star’s temperature becomes greater then 100 million degrees, nuclear fusion will begin again, though this time with helium being converted into heavier elements such as carbon, oxygen and nitrogen. This secondary fusion process produces much less energy and ends fairly rapidly with the heavier elements causing the core of the star to collapse into an even denser mass. When this happens, the energy produced will push the outer layers of the star out into space. If the star is large then the pressure at its core will push up the temperature again so that even heavier elements are created via nuclear fusion. Consider, for example, the star known as Betelgeuse in the constellation of Orion. This star is estimated to be about 640 light years from earth. It is a large star that has a diameter about 700 times that of our sun that has probably existed for around 8 million years. Betelgeuse is now burning helium, having exhausted its available hydrogen, and is 14 ' 61 shrinking to become denser in mass. Scientists call large stars that have reached this point in their life cycle a red supergiant, because the burning of helium causes the emission of large amounts of red light. Many scientists think that when Betelgeuse finally runs out of available fuel a supernova will occur – that is to say the star will contract at so great a speed that the resulting pressure and heat will cause a massive explosion, liberating so much energy that for several weeks the light from the explosion of the star will be as bright as a galaxy. In the year 1054 CE, many peoples throughout the Northern Hemisphere saw light from a supernova occurring within a region of our galaxy some 6,300 million light years from Earth. According to Arab astronomers, the light from the explosion could be clearly seen during the day for just over three weeks and was visible at night for around 650 days. The stellar region known as the Crab Nebular is the cloudy remains of that explosion. NSA / ESA image. Public domain, http://commons.wikimedia.org/wiki/File:Crab_Nebula.jpg 15 62 ' The death of the largest stars can be even more spectacular. They can collapse to create pressures great enough at their core to create a black hole, causing protons and electrons to form neutrons that are explosively released into space. This phenomenon generates temperatures so high that elements as heavy as uranium are formed, which are likewise explosively released into space. Most stars in our galaxy are of a size and mass that renders their explosive end unlikely. They will eventually exhaust their hydrogen and become helium burning red giants. Most will very likely then contract causing the outer layers of the star to be pushed into space away from its core, now largely consisting of carbon and oxygen. These outer layers will become ionized by ultra-violet radiation from the core, which will become what scientists term a “white dwarf.” While white dwarfs are initially very hot and appear as bright white stellar objects light they do not undergo further violent compactions and expansions, but gradually cool down becoming redder in colour. Scientists hypothesize that they eventually become cold black dwarfs. However, this remains a hypothesis because so far no signs of the existence of black dwarfs have been observed, and scientists have estimated that the time required for a white dwarf to become a black dwarf is longer than the generally accepted age of the universe (approximately 13.7 billion years). Biblio graphy Chaisson, Eric. Epic of Evolution : Seven Ages of the Cosmos. New York, N.Y. ; Chichester: Columbia University Press, 2006. Christian, David. Maps of Time: An Introduction to Big History. Berkeley: California University Press, 2005. Ferris, Timothy. The Whole Shebang: A State-of-the-Universe(S) Report. New York Simon and Shuster, 1997. 16