Complete Cosmos Chapter 24: Big Bang, Big Crunch Outline Sub-chapters

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1 Complete Cosmos Chapter 24: Big Bang, Big Crunch Theory of the Big Bang. From that cataclysmic explosion, the Universe continues to expand. But will it stop and reverse? Outline How did the Universe begin? How might it end? The Big Bang theory explains how the cosmos fired up - but not why. The story begins with a cataclysmic explosion that spawns matter, space and time. From that initial expansion, atoms form from protons, neutrons and electrons. Then come the first chemical elements - hydrogen, helium, and a little lithium. Expansion continues, temperatures drop. After 300,000 years, the Universe becomes transparent. Light and other radiation speed across space. Today, particle accelerators on Earth simulate conditions in the Big Bang - testing the theory. As the Universe expands, Big Bang radiation slowly cools. In 1965, this remnant "glow" - the cosmic microwave background - is confirmed. Nearly 30 years later, the Cosmic Background Explorer spacecraft, maps the temperature of the cosmos in minute detail. COBE finds evidence of a primitive structure - the precursor of galaxy formation. Galaxies in clusters and supercluster are held together by gravitational forces. Evidence of their evolution is provided by supercomputer simulation. But still, the early history of individual galaxies remains unexplained. Later, there is galactic cannibalism as larger galaxies consume smaller ones. The age of the Universe is a problem. Different methods give different answers. Some stars appear to be up to 15 billion years old. Yet certain measurements of the expansion rate of the Universe show it at only 13 billion years old. The satellite Hipparcos settles matters by precisely measuring the distance of stars. It finds some are farther away than previously thought. This changes the age estimates and solves the conundrum - both the puzzling stars and the Universe are younger than previously thought. The Universe continues to expand - but for how long? Will it go on forever? Or will it stop and reverse - ending in a Big Crunch? Sub-chapters The Big Bang The origin of the Universe and those crucial questions: how did it happen, why did it happen, and how will it end? To this day, much remains a mystery. The Veil Lifts The Big Bang theory in detail - from a blindingly hot beginning, energy spontaneously generates matter and anti-matter; matter dominates; a seething array of sub-atomic particles. The formation of protons, neutrons, electrons and the first atoms. Three minutes after the Big Bang, the first elements - hydrogen, helium, and a trace of lithium are created. As expansion continues, temperatures drop. After 300,000 years the Universe becomes transparent. Light speeds unimpeded across the ever expanding cosmos. On Earth, understanding conditions in the Big Bang by using a particle accelerator - an atom-smasher - to simulate the early Universe. The fleeting production of sub-atomic particles, energy converted into different forms. In 1965, a microwave antenna picks up a cosmic hiss - recognized as the left-over heat from the fireball of the Big Bang.

2 Primitive Structure The cosmic microwave background - how original ultraviolet radiation has shifted to the microwave part of the spectrum. COBE - the Cosmic Background Explorer satellite which detects minute temperature variations in the microwave background - the first primitive structures in the Universe. Development of large-scale structure as the Universe expands - how mutual gravitational attraction draws galaxies into clusters and groups. Formation of Galaxies The formation of galaxies remains a puzzle. Supercomputers simulate complex interactions as galaxies collide. And as galaxies merge, gravity orchestrates a chaotic dance. The violent evolution of a galaxy supercluster. Galaxy mergers. Such interactions are the norm - and continue today. A current hypothesis is that galaxies grow from smaller aggregations of matter - larger galaxies consuming smaller ones in merger after merger. Age of the Universe The age of the Universe depends on the speed of its expansion. If it's slow, the Big Bang happened some 15 billion years ago. If expansion is rapid, the Universe is younger - between 10 and 13 billion years old. Some measurements put the age of the Universe at only 11 billion years, yet the most ancient stars are thought to be 15 billion years old. But how can that be? Stars cannot be older than the Universe. The satellite Hipparcos helps determine the age of the Universe by accurately measuring the distance of stars. Some stars are more distant than previously thought - changing the scale of the Universe, and hence estimates of its age. It is younger than previously thought. The conundrum appears to be resolved. Explanation of parallax, a measurement method used by Hipparcos. The Big Crunch Its continued expansion will eventually lead to a dispersed, bleak and lonely Universe A dramatic alternative is The Big Crunch - where expansion stops and reverses. Background Cosmology Cosmology is the study of the origin and evolution of the Universe. Cosmology tries to explain the current structure of the Universe, how it was in the past and what might happen in the distant future. Attempts to answer these questions have inspired many models and theories - a lot of speculation based on comparatively little observational material. That is why it is so difficult to discriminate between different cosmological ideas the most informative parts of the Universe are farthest away. Objects in such regions are extremely faint and their exact nature unknown. It is hard to tell how similar they are to more familiar celestial objects closer to us.

3 Currently, cosmology favors the theory of the Big Bang. In an instant, from a speck smaller than an atom, the Universe was created. It happened between 15 and 20 thousand million years ago. From a superdense and unbelievably hot beginning, the Universe expanded - an expansion that continues today. For 21st century cosmologists, the most important problems are to determine the rate at which the Universe is expanding and how it has expanded in the past. And most fascinating of all, will the Universe continue to expand forever? Or, depending on its total amount of matter and energy, will it collapse back on itself in a Big Crunch? Another huge cosmological problem is hidden mass. There appeared to be evidence that vast quantities of dark matter lurked in the Universe, possibly 100 times as much as visible mass. But, again, recent work suggests there may be nothing there at all. Cosmology is an infant science. The Cosmic Background Radiation Important evidence for the Big Bang theory was a discovery in 1965 by Arno Penzias and Robert Wilson. They detected a weak signal coming in from every part of the Universe. Dubbed "cosmic background microwave radiation", it could only be explained as the relic of a primeval explosion - what else but the Big Bang? Some 300,000 years after the Big Bang - when matter and radiation had cooled to few thousand degrees - space became transparent and the original radiation spread out into the expanding Universe. Since then, the Universe has expanded a thousand fold. The original radiation has been diluted, redshifted and cooled to a temperature just degrees above absolute zero. But why is cosmic background microwave radiation so uniform across the sky? It puzzles astronomers because the radiation comes from every direction - from parts of the Universe that have never been in contact with each other. Cosmologists call it the "horizon problem". One explanation is that the early Universe expanded so fast that radiation from the Big Bang became "inflationary", smoothly spreading and stretching everywhere. The jury is still out. The Cosmic Background Explorer (COBE) The Cosmic Background Explorer (COBE) was the first US space mission devoted to cosmology. Launched by Delta rocket into circular Earth orbit, COBE traveled over both poles at a height of 900 kilometers. The date: November 19, 1989 COBE weighed 2.27 tons and carried three instruments. They were designed to map the cosmic microwave background radiation with extraordinary sensitivity. COBE was also to search for radiation released by the earliest galaxies soon after their birth. The overall objective was to answer basic questions about the Big Bang and how clusters of galaxies came about. Astronomers generally agree that the Universe originated in an explosion between ten and 20 thousand million years ago. Everything - space, time and matter - came into existence at the same moment. The Universe started to expand and has been doing so ever since. It also cooled quickly from its original very high temperature. Today, the remnant of the Big Bang is detected as a weak background of cosmic microwave radiation coming from all directions - at a temperature of degrees above absolute zero. The problem for COBE was that the background radiation seemed to be astonishingly smooth. This suggested that the original expanding Universe had also been smooth and uniform. Yet how could our present Universe - which is far from smooth - have evolved from such a beginning? How could clusters and superdusters of galaxies have formed? COBE's mission was to search for

4 minute differences in temperature across the background radiation - difference that would indicate a degree of non-uniformity. To the relief of theorists, it was announced - in April that COBE had made a very important discovery. The spacecraft had detected variations of just ten millionths of a degree - one part in 100,000 - in the temperature of the background radiation. These "ripples" appeared to be the largest and oldest structures in the Universe. They were the "seeds" that evolved into the galaxies and large-scale structures we see today. The Helium Problem The amount of helium in the Universe is further support for the Big Bang theory. Measurements show that helium is the second most plentiful element. Such an abundance could not have come from stars alone - where nuclear fusion converts hydrogen into helium. Scientists propose, therefore, that most helium was created in the Big Bang. A few minutes after the Big Bang, the Universe would have consisted almost entirely of hydrogen nuclei - single protons - and helium nuclei. Their ration, by mass, would have been percent hydrogen to percent helium. The fact that hydrogen and helium exist in just these proportions in the Universe today strongly supports the theory. In 1990, using the Hubble Space Telescope, scientists observed the spectral signature of helium in the light of the distant quasar UM675, 12 billion light years distant. It confirmed the profusion of helium in the early Universe. Links for Further Information A site explaining the main pillars of standard cosmology: A site with links to key topics on cosmology. A page explaining the theory of the Big Crunch. edu/~goodstein/crunch.html A page discussing the future of the Universe - with information about the Big Bang and Big Crunch. Questions and Activities for the Curious 1. How does the Steady State theory - popular in the 1950s and 1960s - differ from the Big Bang theory? 2. What is cosmic background radiation and why is to so important? 3. Describe some significant discoveries of the COBE satellite between 1989 and How did galaxies probably form in the early Universe? 5. Why is possible to estimate the age of the Universe if we know how fast it is currently expanding. 6. How did the Hipparcos satellite help solve the problem of some ancient stars seeming to be older than the Universe?

5 7. Describe the method of parallax used to measure the distances to the nearest stars. 8. Explain what is meant by the Big Crunch and how it might come about.