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Evolution of the Universe, Radiation Laws and Cosmic Background Radiation "In the beginning, there was nothing at all. Earth was not found, nor Heaven above, a Yawning-Gap there was, but grass nowhere." The Edda -- collection of Norse Myths dating to 1200 In this lecture period, we wish to learn: What What What What can we know about the origin of the Universe? is the evidence for the Big Bang ? role does radiation play ? are some of the fundamental laws ? Format for printing The Origin of the Universe "In the beginning".... The question of how and why the world around us came into being is perhaps the most fundamental question of all. It is the root of faith and fantasy, dogma and bewilderment, poetry and intensive scientific inquiry. While we cannot hope to resolve these questions completely, we can use the modern scientific approach to describe and discuss possible timelines, scenarios, and outcomes. Such a discussion can help to illuminate our fledgling understanding of today's universe. The activity below explores the main steps in its evolution (requires free Macromedia plug-in; http://www.macromedia.com/downloads/ ). Load Macromedia browser plug-ins (Flash and Shockwave); http://www.macromedia.com/downloads/ from NOVAonline We live in an amazing time. For the first time in human history, we can rationally speculate on the origin of the Universe and make tangible progress in reaching a description, and even perhaps a rudimentary understanding. We will try in this lecture to sketch out the modern scientific view of the origin of the Universe. The scientific evidence is now overwhelming that the Universe began with a "Big Bang" ~15 billion (15,000,000,000 or 15E9) years ago. We review the major supporting pieces of evidence for this assertion and discuss some of the consequences. First, let's look at our place in the Universe and introduce some terms, with images. Earth A planet with a rocky composition and a temperate climate at an intermediate distance from an average star. The Earth is unique in many ways. For example, it is the only body we know of where water can exist in all three phases: vapor, liquid and solid forms. Click on the image for a general tour that is offered through Windows to the Universe. Sun A medium-sized, moderately bright, middle aged star, born ~5 billion years ago from a gaseous nebula, with perhaps another 4-5 billion years to live before expanding to a "red-giant", engulfing the Earth and finally cooling to become a fading "white dwarf" star. This image of today's Sun, as seen in hydrogen emissions that show the turbulent solar atmosphere, is the most recent available from NASA. Nebulae The birthplace of stars. Our own sun formed in just such a nebula. The example shown here is the Great Orion Nebula, one of the youngest objects in the sky - thought to be less than 20,000 years old. Orion is very hot (about 20,000 K). Other spectacular examples are the Cat's Eye Nebula, and the Trifid Nebula. Star clusters are recently-born families of stars that form in such nebula and then gradually drift apart. Click on Orion and see the Horsehead Nebula, which is 1500 light years away. A "light year" is a unit of distance which represents the distance traveled by light in one year. Galaxy A collection of billions of stars, held together by gravity. Our own galaxy is known as the "Milky-Way" Galaxy and is 100,000 light years across. Galaxies are in many ways "Island Universes". Each galaxy contains billions of stars, with some having more than 1000 billion stars. This Hubble Space Telescope true-color image is the Cartwheel Galaxy, located 500 million light-years away in the constellation Sculptor. Click on it to see our nearest neighbor galaxy, Andromeda. Visible Universe The visible universe contains at least 100 billion galaxies - these are incredible numbers. This image is of very young galaxies observed by the Hubble Space Telescope at the very limit of its range. The sky is full of such strange looking galaxies in all directions (except where masked by intervening dust clouds). The universe is home to a variety of exotic objects. For example, quasars, which were first discovered in 1960, are still baffling objects. Incredibly energetic, they are found at great distances near what is thought to be the edge of the known universe (the most distant one has been estimated to be 10 billion light years away). Some quasars produce more energy than 100 large galaxies. Some scientists think that quasars may represent holes to other universes. Radiation Laws The Universe is an enormously vast system. How did all this come into being? To address this question, we first discuss radiation as it offer the means by which we perceive the universe. We start with basic concepts of radiation, since our ability to sense radiation is responsible for what little we know of the early Universe. What is radiation? Electromagnetic Radiation includes visible light, radio waves, microwaves, x-rays, gamma rays, and infra-red (heat) rays. All these forms of radiation are characterized by traveling oscillations of a combined electric and magnetic field. These electromagnetic waves differ by the wavelength of the oscillation, with shorter wavelength radiation carrying more energy than longer wavelength radiation, as we shall see. Scientists usually think of light as consisting either of innumerable "wave packets" (characterized by specific wavelengths or colors) or "photons", particles of energy that carry no mass, but travel at the speed of light (300,000 kilometers per second). Our eyes perceive just a tiny fraction of the possible wavelengths of light, centered on the wavelengths that the Sun happens to emit. All possible wavelengths make up the electromagnetic spectrum. The electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. (from NASA) To understand radiation we need to discuss a few important physical laws (laws that no Congress can repeal!). You will find that these radiation laws will reappear throughout this course. Everything emits Let's start with the statement that everything (including you, me and the chair you are sitting on) emits radiation. The type and amount of radiation emitted depends on the temperature of the object. The hotter the object the more radiation is emitted. (Our hot Sun emits lots of light). Also, the hotter the object, the more energetic (i.e. the bluer) the radiation. To see how this works, let's state (not derive) the following important physical laws. Planck's Law Planck's Law is sometimes called the "black-body" formula. It works perfectly for objects that are perfect black bodies (i.e., objects that absorb and emit radiation uniformly and with 100% efficiency). Planck's Law works very well for celestial bodies. Where E (lambda) is the amount of radiant energy emitted at a given wavelength, lambda. T is the temperature of the object, and a and b are constants. This strange-looking mathematical formula has a quite simple meaning. The spectrum of wavelengths emitted by a body at a temperature, T, has a characteristic shape that is strongly dependent on the wavelength (to the inverse fifth power). This law describes the spectral distribution of radiation emitted by a black body. We can best understand what this means with a picture. The figure shows the shapes of the wavelength-dependent radiation emitted from so-called "black-bodies" (objects that agree perfectly with Planck's law). Black-body radiation curves, showing the wavelength distribution of emitted photons at different temperatures, on a logarithmic scale. Note the different regions of the electromagnetic spectrum Very hot bodies (3000 - 20,000 K) like our Sun emit a lot of light at visible wavelengths. This is why our eyes have evolved to be sensitive to the visible wavelength range. The Sun acts like a black body near 6000K, whereas the Earth acts like a black body near 300 K (can you guess where its curve would lie?). Stefan-Boltzmann Law ("E equals sigma T to the fourth") This one is just as important as E = mc2, and as easy to remember: where E is the total energy emitted (calculated by adding up the areas under the curves of Figure 1), sigma is a constant, and T is temperature. The Stefan-Boltzmann law tells us that the amount of energy emitted by an object increases as the fourth power of its temperature. This means that a small increase in temperature results in a much bigger increase in radiation. To see how this works, take another look at the figure, and mentally add up (or integrate) the areas under the curves (remember that it is a logarithmic plot). Wein's Law Wein's Law is really just a consequence of Planck's Law. Wein's Law states that the wavelength of the peak radiance [lambda (max)] decreases linearly as the temperature increases, where c is a constant: This means that as objects get hotter, the wavelengths emitted become shorter (bluer). You can see how this works by looking at the Figure above. Notice how the peak wavelength shifts to the left as the temperature increases. Radiation laws in action The figure shows how the radiation laws work for the two celestial bodies of greatest interest to us: the Sun and the Earth. While the Sun emits in the visible end of the spectrum, the much cooler Earth emits longer infra-red wavelengths back out to space. The temperature of the Earth is controlled by the balance between the incoming visible light and the outgoing infra-red light. We will discuss this in greater detail later. Click here to see how the S-B law is used to calculate planetary temperatures. Radiation emitted from the Sun and from the Earth have different wavelengths, as predicted by the radiation laws. Is summary: Planck's law gives us the shape of the curves. Stefan-Boltzmann's Law tells us that much more energy comes from the Sun than from the Earth. Wein's law tells us that the hot Sun is much bluer than the cooler Earth. One last principle, the Doppler Effect, and we're ready to talk about the origin of the Universe in more detail. The Doppler Effect Although you may not know it, you are already familiar with the Doppler Effect. This is the phenomenon that occurs when the wail of an ambulance seems to get faster as it gets closer. But this phenomenon can occur with more than sound; an equivalent effect occurs for light, due to the wave-like property of radiation. When we observe a sound or light wave from a source at rest, the time between the arrival of wave crests at our instruments is the same as the time between crests as they leave the source. However, when the source is moving away from us, the time between arrivals of successive wave crests is increased over the time, because each crest has a little farther to go on its journey to us than the crest before. Click here to see how the speed of an object affects wavelength. The Doppler effect shifts the light to longer wavelengths (red shift) for a receding object, and to shorter wavelengths (blue shift) for an approaching object. Similarly, if the source is moving towards us, the time between arrival of wave crests is decreased because each successive wave crest has a shorter distance to go, and the wave appears to have a shorter wavelength. Think of the traveling salesman who writes a letter home each week at the same time, but as he gets farther and farther away from home, the letters are received more than once a week apart. Then, when he turns back towards home, the letters arrive at shorter intervals. The Doppler Effect for Light is calculated by: Change of wavelength Speed of source = Normal wavelength Speed of light The Doppler Shift turns out to be the most useful of all astronomical tools, since it gives us the ability to determine the velocity of celestial bodies by comparing the wavelengths of known emission features with stationary sources. Big Bang Theory The Big Bang hypothesis is the most widely accepted theory of the origin of the universe. The Big Bang theory states that the Universe began when primordial mass exploded in titanic holocaust. This fireball gradually cooled as it expanded outward, and giant clouds of swirling gas formed the celestial bodies. Entire galaxies took shape as they were propelled outward by the initial cataclysm. The universe created by the Big Bang may expand forever or it may gradually slow and eventually collapse in on itself. The Big Bang theory does not explain why the bang occurred, but predicts (with surprising accuracy) what the consequences of such an event would be. Several key pieces of evidence have been assembled that support the Big Bang theory and have laid to rest many of the competing theories. We will summarize the three most important "smoking guns" for the Big Bang theory. Smoking Gun #1 The first smoking gun is from the field of atomic physics. If the big bang occurred, the initial temperatures must have been so unimaginably high that matter could only have existed in exotic and unstable forms. As the temperature cooled in the first second, free hydrogen nuclei (atomic mass 1) were formed that could undergo fusion reactions to give heavier forms of hydrogen (atomic mass 2, 3) and helium (atomic mass 3 and 4). The lack of stable nuclei of atomic mass 5 and rapidly dropping temperatures limited the fusion reactions at this point. Thus, the theory predicts a early universe with only a mixture of ~75% hydrogen and 25% helium (by weight) and no heavier species. This ratio is exactly what is observed in stars - a major piece of evidence supporting the big bang. Smoking Gun #2 Measurements of low energy microwave radiation (see Figure) have shown that the visible universe is permeated by "cosmic background" microwave radiation, coming uniformly (or nearuniformly) from all directions and similar to what is expected from a black body at 3K. The Big Bang theory predicts that such radiation is the red-shifted remnant of the radiation released when matter and light became decoupled about 1 million years after the Big Bang. The American scientists who first made this measurement in 1965 (Penzias and Wilson) obtained the Nobel Prize. They were not looking for the cosmic background, and were at first irritated by the unexpected source of noise in their measurements. A timeline for the Big Bang model of the Universe. At ~1 million years after the Big Bang, temperatures cool sufficiently to allow hydrogen- and helium-neutral atoms to form from the plasma (charged particles). This event freed up radiation that was previously contained in thermal equilibrium with matter. Since then, radiation and matter have gone their own ways. When astronomers observe the cosmic ray background, they are looking at photons released from the big bang when radiation and matter became uncoupled. Smoking Gun #3 Measurements of the red shifts of virtually all galaxies (except a few in our immediate vicinity) show that the visible universe is expanding in all directions. Everything is moving away from everything else, much as painted spots on a balloon will all move away from each other as the balloon is blown up. The further away the galaxy, the faster it recedes. The constant of proportionality between the distance and velocity of recession is known as Hubble's constant. These measurements support the Big Bang theory, which states that the Universe must continually expand away from the "singularity" of the Big Bang. The theory leaves open the possibility that matter might at some point collapse inwards again; this will depend on exactly how much mass there is and whether gravitational attraction is strong enough to overcome the effects of the explosion. Smoking Gun #4 discovered in 1992? Measurements from the NASA Cosmic Ray Background Explorer (COBE) mission have provided additional supporting evidence for the big bang. Briefly, the COBE satellite results are showing that the cosmic ray background is not completely uniform in direction (see Figure), but that there is some clumping in preferred directions with differences in effective temperature of only one hundred millionth of a degree. This clumpiness would have been necessary for the big bang to produce galaxies, since a perfectly uniform explosion would not produce localized high densities. Summary The Radiation Laws mathematically describe the manner in which all objects emit radiation according to their temperature. The Big Bang theory is the most widely accepted theory of the creation of the Universe. Several predictions made by the theory have been verified by experiment. These include: the observed solar system hydrogen to helium ratio the cosmic background radiation the expansion determined by measurements of red shifts Suggested Readings Weinberg, S., The First Three Minutes, Basic Books, 1988 Cox, P. A., The Elements: Their Origin, Abundance, and Distribution Oxford Scientific Publications, 1989. Self Test Take the Self-Test for this lecture. All materials © the Regents of the University of Michigan unless noted otherwise.