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Chapter 16 on Cosmology 45039_16_p1-31 10/20/04 8:12 AM Page 1 16 Cosmology Chapter Outline 16.1 Epochs of the Big Bang Evidence from Observational 16.2 Observation of Radiation from the Astronomy Primordial Fireball Hubble and Company Observe Galaxies 16.3 Spectrum Emitted by a Receding What the Hubble Telesope Found New Blackbody Evidence from Exploding Stars 16.4 Ripples in the Microwave Bath Big Bang Nucleosynthesis ⍀ What Caused the CMB Ripples? Critical Density, m, and Dark Matter The Horizon Problem 16.6 Freidmann Models and the Age of Inflation the Universe Sound Waves in the Early Universe 16.7 Will the Universe Expand Forever? 16.5 Other Evidence for the Expanding Universe 16.8 Problems and Perspectives One of the most exciting and rapidly developing areas in all of physics is cosmology, the study of the origin, content, form, and time evolution of the Universe. Initial cosmological speculations of a homogeneous, eternal, static Universe with constant average separation of clumps of matter on a very large scale were rather staid and dull. Now, however, proof has accumulated of a sur- prising expanding Universe with a distinct origin in time 14 billion years ago! In the past 50 years interesting and important refinements in the expansion rate (recession rate of all galaxies from each other) have been confirmed: a very rapid early expansion (inflation) filling the Universe with an extremely uniform distribution of radiation and matter, a decelerating period of expan- sion dominated by gravitational attraction in which galaxies had time to form yet still moved apart from each other, and finally, a preposterous application of the accelerator to the cosmic car about 5 billion years ago so that galaxies are currently accelerating away from each other again. In this chapter we ex- plore the experimental evidence for these ideas. We also speculate on the causes of a slower or faster expansion rate. These include familiar forms of matter and energy as well as unusual forms like dark cold matter and dark en- ergy, which produces gravitational repulsion. 1 45039_16_p1-31 10/20/04 8:12 AM Page 2 2 CHAPTER 16 COSMOLOGY 16.1 EPOCHS OF THE BIG BANG In this chapter we describe one of the most fascinating theories in all of science—the Big Bang theory of the creation of the Universe—and the ex- perimental evidence that supports it. This theory of cosmology states that the Universe had a beginning and erupted from an extremely dense, pointlike sin- gularity about 14 billion years ago.1 Such an extreme of energy occurred in the first few instants after the Big Bang that it is believed that all four interac- tions of physics were unified and that all matter melted down into an undiffer- entiated “quark-gluon primordial soup.” Figure 16.1 shows the evolution of the four fundamental forces from the Big Bang to the present. During the first 10Ϫ43 s (the ultrahot epoch during which T Ϸ 1032 K), it is presumed that the strong, electroweak, and gravitational forces were joined to form a completely unified force. In the first 10Ϫ35 s fol- lowing the Big Bang (the hot epoch, T Ϸ 1029 K), gravity broke free of this uni- fication while the strong and electroweak forces remained joined. This was a period when particle energies were so great (Ͼ1016 GeV) that very massive par- ticles such as those predicted by Supersymmetry (see Section 15.12) as well as quarks, leptons, and their antiparticles existed. Then, after 10Ϫ35 s, the Uni- verse rapidly expanded and cooled (the warm epoch, T ϭ 10 29 to 1015 K) and the strong and electroweak forces parted company. As the Universe continued to cool, the electroweak force split into the weak force and the electromagnetic force about 10Ϫ10 s after the Big Bang. After a few more minutes, protons and neutrons condensed out of the cooling quark-gluon plasma. During the first half-hour of creation, the temperature was probably about 1010 K and the Universe could be viewed as a cosmic thermonuclear bomb fus- ing protons and neutrons into deuterium and then helium, producing most of Protons and Nuclei can form Quarks and leptons neutrons can form Atoms can form Gravitational Gravitation Strong force Weak force Strong and electroweak Unified Two Electroweak Three Electromagnetic force force forces forces Four forces 10–40 10–30 10–20 10–10 100 1010 1020 Big Bang Age of the Universe (s) Present age of Universe Figure 16.1 A brief history of the Universe from the Big Bang to the present. The four forces became distinguishable during the first nanosecond. Following this, all the quarks combined to form particles that interact via the nuclear force. However, the lep- tons remained separate and to this day exist as individual, observable particles. 1Data from the Wilkinson Microwave Anisotropy Probe in 2003 pinpointed the age of the Universe at 13.7 Ϯ 0.2 billion years (“WMAP Spacecraft Maps the Entire Cosmic Microwave Sky with Unprecedented Precision,” Physics Today, April 2003, 56(4): pp. 21–24.) 45039_16_p1-31 10/20/04 8:12 AM Page 3 16.2 OBSERVATION OF RADIATION FROM THE PRIMORDIAL FIREBALL 3 H atoms + protons, deuterium and helium nuclei – electrons photons – + – + + – + – – + + + – – – + + – + – + – + – – R + + + + Earth – – – – + – + + + + Figure 16.2 George Gamow, – ––+ 1904–1968, Russian-American physicist. Gamow and two of his students, Ralph Alpher and Expanding shell Robert Herman, were the first of charged (a) particles (b) to take the first half-hour of the Universe seriously. In a mostly Figure 16.3 Radiation- and matter-dominated stages in the evolution of the Universe. overlooked paper published in Ͼ Ͻ (a) The radiation-dominated stage (T 3000 K, age 400,000 years) is an expanding 1948, they made truly remark- Universe filled with protons, electrons, neutrons, photons, and neutrinos. Charged par- able cosmological predictions. ticles continually scatter photons, ensuring thermal equilibrium of radiation and mat- They correctly calculated the Ͻ Ͼ ter. (b) The matter-dominated state (T 3000 K, age 400,000 years). Hydrogen abundances of hydrogen and atoms form, making the Universe transparent to photons. Photons emitted from an ex- helium after the first half-hour ≈ panding ion shell at R 14 billion lightyears are currently seen on Earth enormously (75% H and 25% He) and pre- Doppler-shifted to the red. dicted that radiation from the Big Bang should still be pre- sent, with an apparent tempera- the helium nuclei that now exist. (See Fig. 16.2.) Until about 400,000 years ture of about 5 K. In Gamow’s after the Big Bang, the Universe was dominated by radiation. Energetic pho- own words, the Universe’s sup- ply of hydrogen and helium tons prevented matter from forming clumps, or even single hydrogen and he- were created very quickly, “in lium atoms, because photon-atom collisions would instantly ionize any atoms less time than it takes to cook a that happened to form. In addition, photons experienced continuous Comp- dish of duck and roast pota- ton scattering from vast numbers of free electrons, resulting in a universe that toes.” The comment is charac- was opaque to electromagnetic radiation. By the time the Universe was about teristic of this interesting physi- 400,000 years old—it had expanded to one-thousandth of its current size and cist, who is known as much for cooled to about 3000 K—electrons could bind to protons and helium nuclei his explanation of alpha decay to form atoms. Because of the drastic reduction in free charged particles, far and his theories of cosmology fewer photons were absorbed or scattered, and the Universe suddenly became as for his delightful popular transparent to photons. Radiation no longer dominated the Universe, and books, his cartoons, and his clumps of neutral matter steadily grew—first atoms, then molecules, gas wonderful sense of humor. A classic Gamow story holds that, clouds, stars, and finally galaxies. The state of affairs for radiation- and matter- having coauthored a paper with dominated periods is shown in Figure 16.3. Alpher, he made Hans Bethe an honorary author so that the credits would read “Alpher, 16.2 OBSERVATION OF RADIATION FROM THE Bethe, and Gamow” (to resem- PRIMORDIAL FIREBALL ble the Greek letters ␣, ␤, and ␥). (AIP Emilio Segre Visual In 1965, Arno A. Penzias and Robert W. Wilson of Bell Laboratories were Archives) testing a sensitive microwave receiver and made an amazing discovery. A pesky 45039_16_p1-31 10/20/04 8:12 AM Page 4 4 CHAPTER 16 COSMOLOGY Figure 16.4 Robert W. Wilson (left) and Arno A. Penzias with the Bell Telephone Laboratories horn-reflector antenna. (AIP Emilio Segre Visual Archives/Physics Today Collection) signal that produced a faint background hiss was interfering with their satellite communications experiments. Despite their valiant efforts, the signal remained. Ultimately it became clear that they were detecting microwave back- ground radiation (at a wavelength of 7.35 cm) that represented some of the leftover electromagnetic “glow” from the Big Bang. The microwave horn that served as their receiving antenna is shown in Figure 16.4. The intensity of the detected signal remained unchanged as the antenna was pointed in different directions. The fact that the radiation had equal strengths in all directions suggested that the entire Universe was the source of this radiation. Booting a flock of pigeons from the 20-foot horn and cooling the microwave detector both failed to remove the “spurious” signal. Through a casual conversation, Penzias and Wilson discovered that a group at Princeton had predicted the residual radiation from the Big Bang and were planning an experiment to confirm their theory. The excitement in the scien- tific community was high when Penzias and Wilson announced that they had already observed an excess microwave background of a 3-K blackbody source, which was the predicted temperature of the cooled, residual Big Bang radia- tion at the present time.
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