As humans struggle to understand the universe, we are like explorers on a strange island in an unknown sea. At this point in our explorations we know that the island is made of rocks and grains of sand. We know how big it is, we know that the sea is large, and we know that there are many other islands out there in the distance. Now we ask a series of deeper questions about our larger environment. Do the islands go on forever? Does the sea go on forever? Does our world have an edge, or is it infinite? These are questions that astronomers have only just begun to answer with some confidence.
Cosmology is the study of the size and structure of the universe — in other words, the "geography" of the universe as a single system. Our subject is the universe, defined as all matter and energy in existence anywhere, observable or not. The scientific method is stretched thin in our speculation about the universe. As far as we know, our universe is unique. We cannot learn about it by direct comparisons with other universes, as we can with planets and stars and galaxies. Instead, cosmologists approach their subject by making simple and testable assumptions and by developing theories that describe the present state, origin, and fate of the universe. This speculation is informed by observations, and theories must agree with known physical laws.
Cosmology begins with some fundamental questions about the nature of time and space, mass and energy. By answering these questions, cosmologists have been led to some startling ideas about the universe as whole, ideas that lie far outside everyday experience. Cosmology is not the domain of scientists alone. For thousands of years, poets and priests and philosophers have pondered the universe and tried to understand its nature. Innate in the character of curious, restless humanity is the desire to understand our surroundings and to know where we came from.
Cosmology is as old as the first ancients who looked at the stars set against the velvet backdrop of night. The universe is described in the earliest surviving writings of the Babylonian, Egyptian, Greek, Chinese, and Indian civilizations. According to Indian legend, the universe is a giant egg containing land, water, animals, gods, and so on, all brought forth from primordial waters by the creator god Prajapati. A Tahitian tradition says that a creator, Taaroa, existed in the immensity of space before any universe existed, and that he later constructed the heavens and the rocky foundations of the Earth. A collection of Norse myths supposes that in the beginning there was nothing at all, with regions of frost to the north and fire to the south. The heat melted some of the frost, and from the drops of liquid there grew a giant, Ymer, who created all the inhabitants of the world. This early phase of mythological cosmology linked celestial phenomena to the spiritual life of people. We have found creation myths in almost all cultures; these cosmologies provide us with an enduring link with our early ancestors.
The first true phase of cosmology dates back to the birth of scientific inquiry in the sixth century B.C. on the shores of Asia Minor. With the ancient Greeks, the beginnings of philosophical cosmology marked the first application of the power of reason to the universe. Observations were not central to Greek cosmology; the telescope would not be invented for another 2000 years. Instead, Greek thinkers made progress by using bold hypotheses, logic, and abstract reasoning. Anaximander believed that the universe evolved from a state of primordial chaos to the order and structure that we see today. Others, like Aristotle, thought that the universe was perfectly ordered and unchanging. The twin themes of chaos and order find a strong echo in modern cosmology.
By the third century B.C., mathematical reasoning had become part of astronomy. Until then, most people believed that the stars lay on a two-dimensional backdrop, nestled snugly around the Earth. Euclid pulled together the theorems of geometry and laid the foundation for the idea of infinite space. Imaginary Euclidean triangles could be extended into space; distances to celestial bodies could be measured. Aristarchus used geometry to anticipate the Sun-centered cosmology of Copernicus by nearly 1800 years. The Greeks also wrestled with the uncomfortable implications of an infinite universe. Plato's colleague Archytus summed it up this way: "If I am at the extremity of the heaven of the stars, can I not stretch outward my hand, or my staff? It is absurd to suppose that I could not; and if I can, what is outside must be either body or space."
In the 17th century, with Isaac Newton, science enters the stage of physical cosmology. Newton described gravity as a force by which every particle in the universe attracts every other particle. He realized that this principle might allow a simple description of the structure of the whole universe. Newton's theory of gravity was brilliant and audacious. For the first time in human history, the mundane motions of objects on Earth had been unified with the stately orbits of heavenly bodies. Newton viewed the cosmos as infinite in extent and filled with randomly moving stars. He argued that no other assumption would make sense. If the universe were not infinite, or if the stars were all in one part of the universe, then gravity would eventually cause all matter to clump together in one place. Only in an infinite universe does each particle feel balanced gravitational forces from other particles in all directions in the sky. There is motion in Newton's cosmology, but the universe is static and its appearance is unchanging over time.
There were serious conceptual problems with Newton's cosmology. The force of gravity declines with distance but it has an infinite range. When gravitational forces acting on an infinite number of bodies spread over infinite space are added up, the amount of gravity is infinite too! Another objection to Newton's cosmology results from asking the simple question: "Why is the sky dark at night?" This concern was first raised by Thomas Digges in 1576, but it came to be associated with the German astronomer Wilhelm Olbers over 250 years later. Olbers' paradox can be described as follows. In an infinite universe filled with stars, every line of sight must eventually intercept a star. Moving out from Earth, the brightness of a star decreases as the inverse square of the distance (F ∝ R-2). However, the number of stars in any spherical shell (which is 4πR2ΔR, where ΔR is the thickness of the shell) increases with the square of the distance. The result is that the amount of light in each shell is the same, regardless of distance. The contribution of light from distant stars continues to pile up. In an infinite universe, the sum of all light from distant stars is infinite: the night sky should be ablaze with light!
The modern response to Olbers' paradox is subtle, invoking the age of the universe and the recession of distant galaxies. First, there is a distance beyond which we can see no galaxies or stars. This distance does not represent an edge to the universe, but a distance corresponding to a light travel time of 12 to 13 billion years, beyond which we see no galaxies because none had formed that long ago. In other words, the total number of photons emitted by the galaxies in their finite lifetimes is too low to create the kind of pervasive bright glow described by Olbers. A secondary effect that helps to explain Olbers' paradox is the expansion of the universe. The red shifts of receding galaxies cause the apparent energies of the photons we receive from them to be reduced from high energies (short wavelengths) to low energies (long wavelengths), because the photons are "stretched" in the expanding space. Photons received from galaxies with red shifts close to the speed of light are strongly reduced in energy.
Newton's theory of gravity left a very basic question unanswered. Just what was this force that operated across vast distances through the vacuum of space? Newton was acutely aware of this issue and went so far as to call the idea of gravity acting instantly at a large distance "an absurdity." He absolved himself of his ignorance about the cause of gravity by saying, "I have not been able to discover the cause of these properties of gravity from phenomena, so I frame no hypothesis." We did not gain a more profound understanding of the force of gravity until early in the 20th century. Modern cosmology would not be possible without the theoretical underpinning of the general theory of relativity.