The first minutes of the Universe released energy which changed to matter, forming stars and galaxies. Introduction

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1 THE COSMIC ENGINE CHAPTER 18 The Universe begins The first minutes of the Universe released energy which changed to matter, forming stars and galaxies Introduction Cosmology, the study of the Universe and its origins, is one of the most fascinating areas of study. Our knowledge about the Universe is increasing rapidly. New observatories orbiting Earth high above the atmosphere can see parts of the electromagnetic spectrum never properly observed before. The nature and structure of the Universe is yet to be fully understood. Indeed it is widely believed that the matter we can observe is only about one-tenth of what is out there. Dark matter and the even more mysterious dark energy are believed to be exerting their influence on the galaxies, but we know nothing about them. Figure 18.1 An artist s impression of the moment after the Big Bang, known as inflation 18.1 The Big Bang Outline the discovery of the expansion of the Universe by Hubble, following its earlier prediction by Friedmann Describing the origins of the Universe SECONDARY SOURCE INVESTIGATION PFA P1, 2 PHYSICS SKILLS 11.1 A, E; 12.3 A, B, C, D; 12.4 A, C, E, F; 14.1 A H Identify data sources and gather secondary information to describe the probable origins of the Universe The origins of the Universe have long been the topic of thought and conjecture. Once the enormous size of galaxies and the distances between them came to be realised in the early 20th century, astronomers had to explain how the Universe had not been contracted back into itself by the force of gravity long ago. 272 Wu Far - Physics Preliminary CH18.indd 272 8/4/09 3:19:54 PM

2 CHAPTER 18 THE UNIVERSE BEGINS In 1922, a Russian mathematician, Alexander Friedmann ( ), found a solution to the equations in Einstein s 1915 general theory of relativity, which required an expanding universe. Friedmann s solution was not at fi rst accepted by Einstein until Einstein had the opportunity to revise the calculations that confl icted with Friedmann s. When Einstein discovered his error, he published an apology to Friedmann. While an expanding universe fi tted mathematical requirements, it was yet to be observed. NOTE: Alexander Friedmann was an interesting man, having interest in mathematics, cosmology and meteorology. In 1925, in his role as the director of meteorology in Leningrad (now St Petersburg), Friedmann set a new hot-air balloon record, ascending to an altitude of 7400 m to make meteorological observations. During World War II Friedmann fl ew bombing raids and became involved in formulating equations to predict the path of bombs being dropped. Figure 18.2 Alexander Friedmann Observations of nebulae fuzzy balls of light seen through telescopes were thought to be island universes in space. Their true identity remained a mystery for many years. Edwin Hubble ( ) made careful observations of a number of very luminous yellow giant variable stars called Cepheid variables. This type of star could be seen within the nebulae known as M31 (the Andromeda galaxy) (see Fig. 18.3). By fi nding the Cepheids period of variation of their light output, Hubble was able to use the work of Henrietta Leavitt, who 15 years before found that all Cepheid variable stars have a period luminosity relationship. That is, the longer they take to vary their light output, the more luminous they are. In 1924, Hubble determined the distance to M31 to be around light years. NOTE: Distances in space are so huge that using normal units such as metres or kilometres becomes very diffi cult. The distance to the next nearest star to the Earth is about metres, or km. A light year is the distance that light will travel in a vacuum in one Earth year: m. Light travels at m s 1 in a vacuum. At this speed, it takes a little over one second to reach the Moon. Light from the Sun takes eight minutes and 20 seconds to reach the Earth. The light from the next nearest star, Proxima Centauri, takes 4.3 years to get to us. Using the Hubble Space Telescope, light that has taken nearly 13 billion years to travel across the Universe has been collected and photographed. Figure 18.3 M31, the Andromeda galaxy, now measured as being 2.5 million light years away Figure 18.4 Edwin Hubble and the 100-inch telescope that he used to make his observations on Mount Wilson in California 273 Wu Far - Physics Preliminary CH18.indd 273 8/4/09 3:19:56 PM

3 THE COSMIC ENGINE NOTE: There are other commonly used astronomical distance units, the parsec (about 3.26 light years) and the astronomical unit, or AU, the distance between the Sun and the Earth. SR Simulation: the Doppler effect Figure 18.5 The expanding Universe Figure 18.6 Hubble s data appeared similar to that shown on this graph of the speed of recession of galaxies versus their distance The expanding Universe Hubble s calculation of the enormous distance to the M31 nebula prompted further observations of the spectra of similar objects. By carefully comparing the wavelengths of spectral lines of hydrogen from the nebulae to the wavelengths of the same spectral lines observed in the laboratory, the relative speeds of the nebulae to the Earth could be found. This was done using the Doppler effect a shortening of the wavelengths (shift to the blue end of the spectrum) indicates an approaching nebulae while a lengthening (a shift to the red end of the spectrum) is due to a receding source. Hubble measured the relative speed of a small number of galaxies. He quickly realised that they are moving much faster than any known object within our own galaxy. He concluded that they were indeed separate galaxies in their own right, a long distance away from our own. He assumed that their apparent brightness was an indication of their distance. Putting the data together, he was able to show a simple relationship: in general, the further away the galaxy is, the faster it is moving away from us. Surprisingly, M31, the Andromeda galaxy that is the closest major galaxy to our Milky Way, is moving towards us. The explanation for the expanding Universe (see Fig. 18.5) is that it is the space between the galaxies that is expanding, in the same way that dots drawn on the surface of a balloon move further apart when the balloon is infl ated (see Fig. 18.9). The relationship can be summarised as v = H o D where v = velocity at which the galaxy is moving away Recession velocity v (km s 1 ) v = H o D v = H s D Galaxies 0 1 Distance to galaxy D (Megaparsecs) 2 (1 Megaparsec = 3.25 million light years) from us, H o = the Hubble constant and D is the distance to the galaxy. The value of the gradient of the graph shown in Figure 18.6 above gives the Hubble constant. Its unit is s 1. The inverse value of H o, or 1/ H o, gives the age of the Universe. Many corrections to the value of H o have been made over recent years and continue to be made. Each time the value of H o 274 Wu Far - Physics Preliminary CH18.indd 274 8/4/09 3:19:59 PM

4 CHAPTER 18 THE UNIVERSE BEGINS is revised, the estimated age of the Universe is revised. Currently, it is about 13.7 billion years. Another technique used by astronomers to measure the age of the Universe is to observe the background microwave radiation, a remnant of the original radiation released when the Universe formed. The wavelength of this radiation has been gradually increasing as the Universe expands. A computer-enhanced map of this radiation is shown in Figure Figure 18.7 The remnant radiation from the Big Bang seen now in the microwave region of the electromagnetic spectrum If the Universe is expanding, from where did it originate? The nature of the expansion means that from wherever the Universe is being observed, it seems that it is expanding away from that point. The centre of the Universe cannot be found. However, if time is wound back to the instant that everything came together, what was it that caused the Universe to form? This event was at fi rst mockingly referred to as the Big Bang, a term that has now been adopted widely by cosmologists and society in general. The Big Bang is now thought to have created the Universe. It is considered to be a massive explosion that created energy and matter, as well as spae and time. The equivalence of mass and energy Identify that Einstein described the equivalence of energy and mass In September 1905 Einstein first wrote about the equivalence of energy and mass. His work on the behaviour of charged particles in magnetic fields led him to believe that as a particle loses energy, it also loses a tiny amount of mass PFA P1 Outlines the historical development of major principles, concepts and ideas in physics 275 Wu Far - Physics Preliminary CH18.indd 275 8/4/09 3:19:59 PM

5 THE COSMIC ENGINE Figure 18.8 A light particle (a photon), not visible here, is annihilated and becomes two oppositely charged particles that leave behind a vapour trail in this cloud chamber, a way of seeing the paths left behind by the passage of gamma rays WWW> Einstein became convinced of the link between mass and energy when applying his special theory of relativity (studied in the Space Module in the HSC Course) to perfectly inelastic collisions. When relativistic speeds become involved, part of the loss of kinetic energy of the system can only be explained by a change in mass of the objects involved. Probably the most well-known scientific equation, Einstein wrote it originally as m = L/c 2, where L was a form of energy due to relativity and its effects. This form of the equation was re-written with E as the subject to make it E = mc 2. It took 40 years after the publication of Einstein s concept of the equivalence between mass and energy for the world to witness what such frightening amounts of energy could do. The atomic bombs dropped on Hiroshima and Nagasaki in 1945, ending World War II, threw the world s largest nations into an arms race that could have ended the world all using E = mc 2. This is ironic considering Einstein s often-stated anti-war views. The most precise confirmation of the change in the mass of a radioactive nucleus when it emits a gamma ray was made in It showed Einstein s E = mc 2 equation to be accurate to within four-hundred-thousandths of one per cent. USEFUL WEBSITES: Einstein explains the equivalence of energy and matter: Einstein light from the UNSW: ANSTO s information on E = mc 2 and more: The American Museum of Natural History: After the Big Bang Describe the transformation of radiation into matter which followed the Big Bang Identify that Einstein described the equivalence of energy and mass Before the Big Bang occurred, nothing existed, not even time itself. Asking an astrophysicist what happened before the Big Bang is like asking someone How is your dog? when they do not have a dog! The Big Bang was not like a normal explosion. It was a sudden expansion and coming into existence of a huge amount 276 Wu Far - Physics Preliminary CH18.indd 276 8/4/09 3:20:00 PM

6 CHAPTER 18 THE UNIVERSE BEGINS Figure 18.9 Nuclear power stations harness the energy released when matter is converted into energy of energy. The tiny fraction of a second after the Big Bang is referred to as the period of inflation (see Fig. 18.1). This very young Universe contained extremely hot energy too hot for even the most basic building blocks of matter to exist. After this time, as the Universe expanded (see Fig. 18.9) and cooled, energy began to condense into matter (according to Einstein s mass energy relationship), forming matter and anti-matter in approximately equal proportions. As the anti-matter collided with the matter, it was annihilated and converted back into energy (see Fig ). This process continued until, about a millionth of a second after coming into existence, the Universe had expanded and cooled sufficiently for matter to be able to exist. (Interestingly, slightly more matter than anti-matter was formed, so that most of the matter we detect now is not anti-matter.) By the first second after the Big Bang, the fundamental particles of matter had formed. Protons, neutrons and electrons existed. By three seconds old, the Universe contained the basic and simplest elements almost all hydrogen and some helium. However, it took another years before the Universe had cooled sufficiently for photons (i.e. light) to travel freely through space, making it transparent for the first time. Almost 25 years before the Big Bang theory was first coined, Einstein proposed that matter and energy were linked, and could be transformed from one form to the other. A number of scientists were working on this idea around the early 1900s. It was Einstein who, in 1905, published his now famous equation, E = mc 2. This made the relationship between matter and energy quantifiable. The number c in Einstein s equation is the speed of light in a vacuum, m s 1. A very small amount of matter can be transformed into a very large amount of energy. Einstein based his prediction of the possibility of the atomic bomb and nuclear energy by realising that when the nuclei of large atoms split, they lose a small amount of mass. If many atoms could be made to do this simultaneously, the small change in mass would create a very large amount of energy (see Fig. 18.8). Einstein s predictions were, of course, found to be true. Nuclear power stations and atomic weapons obtain their energy from E = mc 2. Figure Another way of showing the expansion of the Universe, with dots on the surface of an inflating balloon representing galaxies NOTE: The proposition of the Universe coming into existence in one single event was at fi rst dismissed by many astronomers, including the well-regarded British astronomer Fred Hoyle. When asked what he thought about such a theory in 1949, he jokingly referred to it as the big bang in an attempt to show how silly such an idea was. The term has remained in use to this day. Astronomers (including Fred Hoyle) today widely accept the Big Bang, as more observations (including the existence of cosmic microwave background radiation) are made that match and support the once ridiculed theory. 277 Wu Far - Physics Preliminary CH18.indd 277 8/4/09 3:20:02 PM

7 THE COSMIC ENGINE Figure An artist s image of the stages of the Universe shortly after the Big Bang 18.4 The formation of stars and galaxies Outline how the accretion of galaxies and stars occurred through: expansion and cooling of the Universe subsequent loss of particle kinetic energy gravitational attraction between particles lumpiness of the gas cloud that then allows gravitational collapse Once the Universe was years old, it is believed that it was spread throughout with matter as we know it atoms with protons and neutrons in the nucleus with electrons orbiting the nucleus. Most atoms were hydrogen, the simplest of the elements with one proton and one electron. For some reason that is still not fully understood, this matter was not distributed evenly. Regions with higher densities began to become attracted and clumped together under the influence of gravity. This could only happen once the speed of the particles had slowed sufficiently. With no direct observations of the Universe at this age, we can only imagine what occurred. The matter in the Universe was probably spread out like a three-dimensional spider s web. With tiny variations in density, gravity caused the denser regions to fall into themselves, or coalesce into large lumps. This in turn resulted in the formation of the first stars, in which the heavier elements were formed. Regions where stars were more densely spaced collected themselves into huge groups that we now observe as galaxies. Figure summarises this sequence. 278 Wu Far - Physics Preliminary CH18.indd 278 8/4/09 3:20:04 PM

8 CHAPTER 18 THE UNIVERSE BEGINS The Big Bang The very young Universe expanded and cooled, allowing matter in the form of simple elements (mostly hydrogen) to form. The matter was spread throughout the Universe but with very small variations in density. The Universe continued to expand and cool, resulting in the slowing down of the speed of the atoms. The more dense regions of matter began to coalesce under the force of attraction due to gravity. Density and pressure in the core of these clumps of matter allowed nuclear fusion to occur synthesising the heavier elements. Stars are formed. Figure The cooling of the Universe and formation of the first stars CHAPTER REVISION QUESTIONS 1. What did Friedmann predict about the Universe? Why was this prediction made? 2. Describe what observations were made by Hubble that supported Friedmann s prediction. 3. What is meant by the equivalence of mass and energy? 4. What was the Big Bang? 5. Outline the steps from the Big Bang until the formation of the fi rst stars. 6. Why did matter not simply spread out evenly through the Universe? 7. Research and identify a minimum of three other secondary sources that contain information describing the probable origins of the Universe. 8. When a gas expands, it cools. How does this relate to the early Universe? 279 Wu Far - Physics Preliminary CH18.indd 279 8/4/09 3:20:04 PM