(Background) By virtually all accounts, the universe began in a fiery explosion some 10-20 billion years ago. Out of this maelstrom emerged all the energy that would later form galaxies, stars, and planets (depicted here in an artist's rendering). The story of the origin and fate of all these systems—and especially of the entire universe itself—comprises the subject of cosmology.
(Inset A) These three images of spiral galaxies lying at different distances from Earth capture representative views of what the universe was like at much earlier times. At top, about 10 billion years ago, the galaxy's spiral shape is hardly recognizable. At middle, about 8 billion years ago, the spiral is still vague, though starburst activity is evident in the outer regions. At bottom, some 5 billion years ago, the galaxy's spiral features are more prominent, with much star formation in its arms.
(Inset B) Elliptical galaxies also suggest evolution with time—indeed, evolution more rapid than that of spiral galaxies. These images of different galaxies (from top to bottom, 10, 8, and 5 billion years ago, respectively) suggest that many ellipticals had already taken on their eventual shapes early in the universe. In contrast to spirals, ellipticals seem to have made all their stars long, long ago.
Studying this chapter will enable you to:
State the cosmological principle and explain its significance.
Explain how the approximate age of the universe is determined and discuss the uncertainties involved.
Summarize the leading evolutionary models of the universe.
Discuss the factors that determine whether the universe will expand forever.
Explain the relationship between the future of the universe and the overall geometry of space.
Describe the cosmic microwave background radiation and explain its importance to our understanding of cosmology.
Our field of view now extends for billions of parsecs into space and billions of years back in time. We have asked and answered many questions about the structure and evolution of planets, stars, and galaxies. At last we are in a position to address the central issues of the biggest puzzle of all: How big is the universe? How long has it been around, and how long will it last? What was its origin, and what will be its fate? Is the universe a one-time event, or does it recur and renew itself, in a grand cycle of birth, death, and rebirth? How and when did matter, atoms, our Galaxy form? These are basic questions, but they are hard questions. Many cultures have asked them, in one form or another, and have developed their own cosmologies—theories about the nature, origin, and destiny of the universe—to answer them. In this and the next chapter we will see how modern scientific cosmology addresses these important issues and what it has to tell us about the universe we inhabit. After more than 10,000 years of civilization, science may be ready to provide some insight regarding the ultimate origin of all things.
