Scientific cosmology is the study of the entire universe, its origin, evolution, composition, and structure. Cosmology is today in the midst of a golden age, during which the basic story of the evolution of the universe is coming into clear focus. But this recent progress rests on foundations laid in the late nineteenth and early twentieth centuries, during which the first telescopes were built that allowed measurement of the distances to objects outside our home galaxy, the Milky Way. It was during this same period that the two great physical theories required for understanding the larger universe were constructed, relativity and quantum mechanics.
The ancient Egyptians and Mesopotamians pictured the earth as flat, with water above and below. This same picture is reflected in the opening words of Genesis, in which on the second day God separates the waters with a firmament and on the third day creates dry land. The first great cosmological revolution occurred when this flat earth was replaced by the Greek picture of a spherical earth surrounded by heavenly spheres carrying the moon, sun, planets, and fixed stars, with the whole system rotating about the earth every day.
In the second century AD, Claudius Ptolemy developed a detailed mathematical treatment of the motions of the planets. This spherical geocentric picture prevailed in Europe and the Middle East for more than a millennium. In the second great cosmological revolution, the geocentric universe was overthrown in the seventeenth century by Galileo's telescope and Newton's mechanics. But Newton's laws were applied just to the solar system; the true size and nature of the universe were a deep mystery. A third cosmological revolution today is constructing the first picture of the structure and history of the larger universe that may actually be true, since it is cross-checked by a wealth of new data.1
In the early years of the seventeenth century, Dutch opticians learned to make crude spyglasses. Galileo Galilei, then a young professor of mathematics in Italy, improved these early telescopes and turned his new instruments to the sky. He reported in 1610 what he had seen: that the moon has mountains, that Jupiter has four moons of its own, and that the Milky Way is made of countless stars. Shortly afterward, he discovered that Venus went through phases like the moon, except that when Venus is crescent it is large (because it is nearer to earth than to the sun) but when it is full it is much smaller (because it is then farther than the sun). The phases of Venus disproved the Ptolemaic system, in which Venus is always between the earth and the sun. Jupiter's moons strengthened the case for Copernicus's system in which the planets - including the earth with its moon - all go around the sun.
Galileo's contemporary Johannes Kepler made key discoveries about planetary motion, including that the planetary orbits are ellipses. All this was subsequently explained by Newton's mechanics and his theory of universal gravitation. Newton also invented the reflecting telescope. But improvements in telescopes were slow, and it was not until 1838 that astronomers could measure the distance to even nearby stars.
To go farther required really large telescopes, a goal embraced by American astronomers and philanthropists. John Brashear was a self-taught telescope builder who raised funds from captains of industry such as Andrew Carnegie. James Lick, who struck it rich in San Francisco real estate after the Gold Rush, was persuaded to build a 36-inch refracting telescope in the first mountain top observatory, near San Jose, California. Astronomer George Ellery Hale then persuaded Charles Tyson Yerkes, the Chicago street car magnate, to finance the University of Chicago's new observatory in Wisconsin with a 40-inch refracting telescope, still the world's largest.
Hale constantly strove to build larger telescopes to see better and farther. He persuaded Andrew Carnegie to finance a 60-inch reflecting telescope at his new Mt. Wilson Observatory near Pasadena, California. Then he convinced Los Angeles hardware and oil millionaire and amateur astronomer John Hooker to provide the funds to build a 100-inch telescope there. Hale's last telescope was the 200 inch (5 meter) reflector on Mt. Palomar in Southern California, financed by the Rockefeller Foundation.2 It was the largest telescope in the world from 1948 until 1993-96, when the twin 10 meter telescopes were finished on top of Mauna Kea, on the big island of Hawaii, with funds from oil billionaire W. M. Keck. Meanwhile, charge coupled device (CCD) detectors had improved the efficiency with which astronomers could capture light by a factor of ten compared to the best photographic plates.
Astronomers had long wondered about the nature of the "spiral nebulae" - faint clouds of light that were clearly not individual stars. The largest such object was discovered in the constellation Andromeda by the great Persian astronomer Al Sufi in the 10th century. The philosopher Immanuel Kant had speculated that the spiral nebulae were distant island universes like our own Milky Way. By 1802 William Herschel had found over 2500 nebulae in the northern sky, and then his son John Herschel added additional southern nebulae. Larger telescopes allowed astronomers to discover ever more spiral nebulae, but did not establish their nature. In the early years of the 20th century many astronomers, including Harlow Shapley, thought that they were probably clouds of gas in the Milky Way, but some astronomers made observations suggesting that Kant's island universe hypothesis was right. It was astrophotography and the new generation of large telescopes that would determine the answer.
Henrietta Leavitt, working at the Harvard College Observatory, studied Cepheid variable stars that were all about the same distance from earth. She showed in 1912 that the brighter ones had longer periods - that is, they took longer to go through the cycle from bright to dark and then bright again. This meant that wherever such stars were seen, their luminosities could be determined by measuring their periods. Their observed brightness could then be used to measure their distances.
Harlow Shapley used Cepheid variables and the Mt. Wilson 60 inch telescope in 1917 to measure the size of the Milky Way and show that the sun is located far from its center. Shapley then accepted the directorship of the Harvard College Observatory, but in doing so he lost access to the huge Mt. Wilson telescopes.
Using Hale's new 100-inch telescope there, Edwin Hubble was able to measure the periods of many Cepheid variable stars in spiral nebulae. Hubble showed in 1925 that the largest spiral nebula, the great Andromeda galaxy, lies far outside the Milky Way. Cecilia Payne-Gaposhkin, who had discovered in her doctoral research that the stars are mostly made of hydrogen and helium, was in Shapley's office when he received a letter from Hubble reporting preliminary results. He held it out to her and said, "Here is the letter that has destroyed my universe." 3
All the information we have about the distant universe comes to us in the form of light. In 1676, the Danish astronomer Ole Roemer estimated the speed of light by measuring how much later he saw an eclipse of one of Jupiter's moons when Jupiter was on the opposite side of its orbit from the earth compared to when it is nearer. Newton showed that a prism spreads a beam of white light into colors from blue to red. In 1800 the astronomer William Herschel showed that invisible light beyond the red end of the spectrum of light from a prism is radiant heat - infrared radiation. Shortly afterward, ultraviolet light was also discovered. Newton had thought that light was made of particles. But in 1803 the English polymath Thomas Young, who subsequently also decoded Egyptian hieroglyphics, proved by an ingenious experiment that light is a wave phenomenon.
The modern era of astrophysics began in 1814 when the German optician Joseph von Fraunhofer discovered that the spectrum of sunlight has many dark and bright lines. The German chemist Robert Bunsen and physicist Gustav Kirchoff were able to identify the characteristic spectra of a number of chemical elements, and astronomers showed that the spectra of many of these same elements are found in starlight.
In 1864, the theoretical physicist James Clark Maxwell showed that electricity and magnetism are intimately connected. He deduced that light is an electromagnetic phenomenon, and his calculation of the speed of light agreed with the best measurements then available. The American physicist Albert A. Michelson improved on methods developed by French physicists, and by 1879 he had made a highly accurate measurement of the speed of light. As Michelson and others improved their measurements, the results have continued to agree with Maxwell's theory.
Maxwell had followed tradition by assuming that light is an undulation in an underlying medium called the luminiferous aether. Michelson set out to detect the effect of the earth's motion through the ether. His ingenious experiment with Edward Morley made essential use of the wave nature of light. As Michelson later explained it to his children, "Two beams of light race against each other, like two swimmers, one struggling upsteam and back, while the other, covering the same distance, just crosses the river and returns. The second swimmer will always win, if there is any current in the river." 4 But the sensitive experiment revealed no evidence of any such current.
In 1905 Einstein published four amazing papers that set the agenda for physics for much of the rest of the 20th century. Two of these papers concerned special relativity. In the first of these, Einstein dispensed with any need for a luminiferous aether. Although this paper contained no references at all, Einstein later acknowledged the importance of Michelson's experimental work in leading to relativity.5 Einstein's second 1905 relativity paper derived his famous formula connecting energy and mass, E=mc2.
Another of Einstein's 1905 papers was the first convincing proof of the existence of atoms. His fourth paper was on the implications of the quantum nature of light for the photoelectric effect. The American experimental physicist Robert A. Millikan made major contributions on both topics through his oil-drop measurement of the quantum of electric charge and his subsequent experiments confirming Einstein's predictions regarding the photoelectric effect.
Einstein's greatest achievement was to create in 1915 our modern theory of space, time, and gravity, the general theory of relativity. This conceptual breakthrough provided the essential framework for cosmology. However, when he applied his theory to the entire universe, Einstein discovered that the universe could not be static - it must be contracting or expanding. In the absence of any evidence for this, Einstein reluctantly introduced what he called the cosmological constant, effectively a repulsion of space by space, to offset the attraction of matter.
It was in 1917, the very year that Einstein introduced the cosmological constant, that the American astronomer Vesto Slipher published the first observational evidence that the universe is actually expanding.6 Slipher had determined the speeds of 25 spiral nebulae by measuring the wavelength shifts of the characteristic lines in their spectra, and 21 of them were flying away at unexpectedly large speeds. The key to interpreting these redshifts was to the measure the distances to these spiral nebulae. Hubble was able to do this with the large Mt. Wilson telescopes, and in 1929 he discovered that the velocities of distant galaxies are proportional to their distances - which implies that the universe is expanding. This has since been confirmed by many thousands of observations. Einstein said that he never would have introduced the cosmological constant if he had known of Hubble's expansion.
In the past few decades, new kinds of telescopes on the ground and in space have led to the discovery of violent phenomena in the universe - including the discovery in 1965 of the cosmic background radiation from the Big Bang itself.
As Cepheid variable stars allowed the measurement the distances to nearby galaxies nearly a century ago, Type 1a supernovae are so bright and sufficiently similar to each other that they have allowed astronomers to measure the distances to extremely distant galaxies. In 1997, two teams independently reached the conclusion that about five billion years ago the universe started expanding more and more rapidly, after previously slowing its expansion for about eight billion years. That means that Einstein's cosmological constant - or a generalization of it called "dark energy" - is actually the main constituent of the universe! Everything we can see - all the stars, gas, dust, planets - only makes up about half a percent of the universe. Numerous observations have shown that the vast majority of the matter in the universe is invisible "dark matter," a mysterious substance that is not even made of atoms or their component particles.
The Double Dark theory based on dark matter and dark energy has successfully predicted the distribution of the hot and cold regions in the heat radiation from the Big Bang and the distribution of galaxies both nearby and at great distances. The pioneering research of Einstein, Hubble, and their contemporaries had made it possible to ask the basic cosmological questions - but they would surely have been surprised at the answers! Building on their work, a new generation has succeeded in creating the first picture of the history of the universe that might actually be true. The new big questions probe dark matter and dark energy theories.