Four elements (earth, air, fire, and water) less pure than the quintessence made up everything below the innermost sphere of the moon. In about 250 bc, Greek astronomer Aristarchus of Samos became the first known person to assert that Earth moved around the Sun, but Aristotle's model of the universe prevailed for almost 1800 years after that assertion.

Early astronomers called the planets wandering stars because they move against the background of the stars. Astronomers noted that the planets sometimes moved ahead with respect to the stars but sometimes reversed themselves, making retrograde loops. In about ad 140, Egyptian scientist Ptolemy explained the retrograde motion as the result of a set of small circles, called epicycles, on which the planets moved. Ptolemy hypothesized that the epicycles moved on larger circles called deferents and that the combination of these motions caused the dominant forward motion and the occasional retrograde loops.

Sun-Centered Universe
The ideas of Ptolemy were accepted in an age when standards of scientific accuracy and proof had not yet been developed. Even when Polish astronomer Nicolaus Copernicus developed his model of a Sun-centered universe in the 1540s, he based his ideas on philosophy instead of new observations. Copernicus's theory was simpler and therefore more sound philosophically than the idea of an Earth-centered universe. A Sun-centered universe neatly explained why Mars appears to move backward across the sky: Because Earth is closer to the Sun, Earth moves faster than Mars. When Mars is ahead of or relatively far behind Earth, Mars appears to move across Earth's night sky in the usual west-to-east direction. As Earth overtakes Mars, Mars's motion seems to stop, then begin an east-to-west motion that stops and reverses when Earth moves far enough away again. Copernicus's model also explained the daily and yearly motion of the Sun and stars in Earth's sky. Scientists were slow to accept Copernicus's model of the universe, but followers grew in number throughout the 16th century. By the mid-17th century, most scientists in western Europe accepted the Copernican universe.

In the 16th century, Danish astronomer Tycho Brahe made the most scientific and accurate observations of the universe to that time. Brahe discovered discrepancies between astronomical predictions and the actual events, and built a set of large instruments that enabled him to record the positions of the planets and stars with unprecedented accuracy. He moved to Prague, and, after his death, his observations were taken over by German astronomer Johannes Kepler. Kepler discovered that the planets orbited around the sun in ellipses (elongated circles). This discovery was Kepler's first law, and he developed two more laws about how the speeds and periods of the planets changed. The first two laws were published in 1609 and the third was published in 1619.

The Italian scientist Galileo Galilei lived and worked during the same time period as Kepler. Galileo was the first astronomer to use a telescope to observe the sky and to recognize what he saw there. He saw that the Moon had craters, that Venus went through a full set of phases like the Moon, and that Jupiter had satellites, or moons, of its own. His discoveries, published in 1609, marked the scientific end of the cosmological systems of Ptolemy and Aristotle, though it took some time for his findings to be generally accepted.

Newton and Beyond
Later in the 17th century, British astronomer Edmond Halley presented British physicist Isaac Newton with a query about the shape of planetary orbits. In response, Newton began developing what would become his three laws of motion. He applied these laws to Kepler's laws of orbital motion and, from the relationship between the two sets of laws, developed the idea of universal gravitation. Newton's calculations were eventually expanded into his greatest book, Philosophiae Naturalis Principia Mathematica, which was published in 1687. In the Principia, Newton derived a wide range of theoretical results about planetary orbits and advanced the law of universal gravity. Newton's laws were the foundation of cosmological thought until the 20th century.

Newton's laws, however, left some questions unanswered. Beginning in the 17th century, scientists wondered why the sky was dark at night if space is indeed infinite (an idea proposed in ancient Greece and still accepted by most cosmologists today) and stars are distributed throughout that infinite space. An infinite amount of starlight should make the sky very bright at night. This cosmological question came to be called Olbers's paradox after the German astronomer Heinrich Olbers, who wrote about the paradox in the 1820s. The paradox was not solved until the 20th century.

In the 19th century, counts of the numbers of stars appearing in different directions in the sky left astronomers with the incorrect idea that the earth and sun were approximately in the center of the universe. This conclusion did not take into account the modern idea that dust in our Milky Way Galaxy prevented astronomers from seeing very far in any direction.

Discovering the Structure of the Universe
In 1917 American scientist Harlow Shapley measured the distance to several groups of stars known as globular clusters. He measured these distances by using a method developed in 1912 by American astronomer Henrietta Leavitt. Leavitt's method relates distance to variations in brightness of Cepheid variables, a class of stars that vary periodically in brightness. Shapley's distance measurements showed that the clusters were centered around a point far from the Sun. The arrangement of the clusters was presumed to reflect the overall shape of the galaxy, so Shapley realized that the Sun was not in the center of the galaxy. Just as Copernicus's observations revealed that Earth was not at the center of the universe, Shapley's observations revealed that the Sun was not at the center of the galaxy. Cosmologists now realize that Earth and the Sun do not occupy any special position in the universe.

Starting in about 1913, new large telescopes and advances in photography and spectrography allowed astronomers to begin measuring Doppler shifts of distant galaxies. A Doppler shift is observed when an object emitting radiation moves with respect to the observer of that radiation. If the object is moving toward the observer, each wave of radiation originates from a place that is a little bit closer to the observer than the previous wave's point of origin, so the distance between successive wave peaks, called wavelength, is shorter than usual. If the object is moving away from the observer, the wavelength is longer than usual. The wavelength change is proportional to the speed at which the object is moving relative to the observer. In visible light, a shift to longer wavelengths is equivalent to a shift toward the red end of the visible spectrum. Therefore, cosmologists refer to the Doppler shifts of galaxies that are moving away from the earth as redshifts. By measuring the redshifts of distant galaxies, astronomers began to understand how the universe was evolving.

In 1915 German American physicist Albert Einstein, who was working in Switzerland, advanced a theory of gravitation known as the general theory of relativity. His theory involves a four-dimensional space-time continuum that bends in the presence of massive objects. This bending causes light and other objects that are moving near these massive objects to follow a curved path, just as a golfer's ball curves on a warped putting green. In this way, Einstein explained gravity. His theory showed that Newton's theory of gravitation was a special case, valid in conditions normal to Earth but not in very strong gravitational fields or in other extreme conditions. Einstein's theory also made several predictions that were not part of Newton's theory. When these predictions were verified, Einstein's theory was accepted. Einstein's equations were very complicated, though, and it was other scientists who eventually found widely accepted solutions to Einstein's equations. Most of cosmology today is based on the set of solutions found in the 1920s by Russian mathematician Alexander Friedmann. Dutch astronomer Willem de Sitter and Belgian astronomer Georges Lemaître also developed cosmological models based on solutions to Einstein's equations.

In the early 1920s, astronomers debated about whether the spiral structures seen in the sky, called spiral nebulae, were galaxies like our own Milky Way Galaxy or smaller objects in the Milky Way. Measuring the distances to these galaxies depended on the Leavitt-Shapley method of observing Cepheid variable stars. In 1924 American astronomer Edwin Hubble was able to detect Cepheid variables in distant galaxies and show that the galaxies were beyond our own. These findings indicated that the spiral structures were probably galaxies separate from the Milky Way.

In 1929 Hubble had measured enough spectra of galaxies to realize that the galaxies' light, except for that of the few nearest galaxies, was shifted toward the red end of the visible spectrum. This shift increased the more distant the galaxies were. Cosmologists soon interpreted these redshifts as Doppler shifts, which showed that the galaxies were moving away from Earth. The Doppler shift, and therefore the speed of the galaxy, was greater for more distant galaxies. Galaxies in different directions at equivalent distances from Earth, however, had equivalent Doppler shifts. This constant relationship between distance and speed led cosmologists to believe that the universe is expanding uniformly. The uniform relationship between velocity of expansion and distance from Earth is known as Hubble's law.

MODERN COSMOLOGY
Modern cosmologists base their theories on astronomical observations, physical concepts such as quantum mechanics, and an element of imagination and philosophy. Cosmologists have moved beyond trying to find Earth's place in the universe to explaining the origins, nature, and fate of the universe.

The most widely accepted theory of the origin of the universe, called the big bang theory, proposes that a huge explosion set free all the matter and energy in the universe. Theories of the evolution and fate of the universe go on to describe a universe that has been expanding and cooling since the initial explosion, and will either keep expanding forever or eventually collapse back to its initial state, an extremely dense object that contains all of the matter in the universe. When the big bang theory was developed in the mid-20th century, some cosmologists found the idea of a sudden beginning of the universe philosophically unacceptable. They proposed the steady-state theory, which said that the universe has always looked more-or-less the same as it does now and that it does not change over time. The steady-state theory could not explain the background radiation, though, and essentially all cosmologists have abandoned it.

The Big Bang Theory
The big bang theory describes a hot explosion of energy and matter at the time the universe came into existence. This theory explains why the universe is expanding and why the universe seems so uniform in all directions and at all places.

The work of Edwin Hubble, which showed that the universe is expanding, led cosmologists to begin tracking the history of the universe. The dominant idea is that the universe would have been hotter and denser billions of years ago. In the 1940s Russian American physicist George Gamow and his students, American physicists Ralph Alpher and Robert Herman, developed the idea of a hot explosion of matter and energy at the time of the origin of the universe. (This theory of an explosion at the beginning of the universe was given the derisive name “big bang” by British astronomer Fred Hoyle in 1950.) Current calculations place the age of the universe at 10 billion to 15 billion years. Gamow and his students realized that some of the chemical elements in the universe today were forged in the hot early stage of the universe's existence. They also hypothesized that some radiation that remains from the big bang explosion may still be circulating in the universe, though this idea was forgotten for some time.

Current methods of particle physics allow the universe to be traced back to earlier than one second after the big bang explosion initiated the expansion of the universe. Cosmologists believe that they can model the universe back to 1 × 10 -43 seconds after the big bang; before that point, they would need a theory that merges the theory of gravitation and the theory of general relativity to explain the behavior of the universe. Scientists do not actually study the big bang itself, but infer its existence from the universe's expansion.
In the 1950s American astronomer William Fowler and British astronomers Fred Hoyle, Geoffrey Burbidge, and Margaret Burbidge worked out a series of calculations that showed that the lightest of the chemical elements (those of lowest atomic weight) were formed in the early universe shortly after the big bang. These light elements include hydrogen, hydrogen's isotope deuterium, and helium. Heavier elements, according to those calculations, were formed later. Scientists now know that the elements heavier than helium and lighter than iron were formed in nuclear processes in stars, and the heaviest elements (those heavier than iron) were formed in supernova explosions.

Steady-State Theory
In the 1940s British scientists Hermann Bondi, Thomas Gold, and Fred Hoyle were philosophically opposed to the requirements that the big bang theory put forth for the extreme conditions in the early universe. The big bang theory was framed in terms of what they called the cosmological principle—that the universe is homogeneous (the same in all locations) and isotropic (looks the same in all directions) on a large scale. Bondi, Gold, and Hoyle suggested an additional postulate, which they called the perfect cosmological principle. This principle stated that the universe is not only homogeneous and isotropic but also looks the same at all times. Since the universe is expanding, though, one might think that the density of the universe would decrease. Such a decrease would be a change that would not fit with the perfect cosmological principle. Bondi, Gold, and Hoyle thus suggested that matter could be continuously created out of nothing to maintain the density over time. The rate at which matter would have to be created was much too low to be observationally testable, however. They called this theory the steady-state theory.

Big Bang vs. Steady-State
The only evidence necessary for supporters of the big bang theory to prove that this theory was more acceptable than the steady state theory was to show that the universe changed over time. Just such a change was found in 1963 when Dutch-American astronomer Maarten Schmidt identified quasars while working at the Palomar Observatory in California. Quasars are bluish astronomical objects that resemble stars. Astronomers believe that quasars are the cores of certain types of galaxies. Quasars are all quite far from Earth, which means they must have originated during the early formation of the universe. They are distant from Earth in both time and space. The lack of quasars near Earth (and therefore nearer in time to Earth) shows that the universe has been evolving. This finding dealt a serious blow to steady-state cosmology.

In 1965 a piece of evidence was found that almost all scientists agree conclusively rules out the steady state theory of the universe. At that time, American physicists Arno Penzias and Robert W. Wilson, working at the Bell Telephone Laboratories in New Jersey (now part of Lucent Technologies), discovered faint isotropic radio waves. American astronomers James Peebles, David Roll, David Wilkinson, and Robert Dicke at Princeton University had predicted that just such radiation would have been emitted as a result of the hot, dense early universe that resulted from the big bang theory. These scientists were themselves preparing a radio telescope to search for this radiation. (Scientists only later recalled that Gamow and colleagues had earlier predicted such radiation.) This cosmic background radiation is now widely accepted as the proof that the big bang theory is very probable. The fact that calculations of the formation of the lightest chemical elements in the first few minutes after a big bang match observations of these elements' relative abundance in space is another pillar of big bang cosmology.

The Universe Through Time
In current cosmological models, the universe was at first both hot and dense, with temperatures exceeding billions of degrees. In the first second after the big bang, as the universe cooled, elementary particles such as quarks and electrons formed. After about 1 second, the universe had cooled enough that protons had formed out of the quarks. For the next 1000 seconds—in what is now known as the era of nucleosynthesis—hydrogen, deuterium, helium, and some lithium and beryllium formed.
Electrons began to combine with protons to make hydrogen atoms about 300,000 years after the big bang. The process continued until about one million years after the big bang, when the universe had cooled to about 3000° C (about 5000° F). Before this era, photons of light could not travel far in the universe without bouncing off electrons. The formation of hydrogen atoms, however, used up many of the free electrons and allowed light to travel quite far. The radiation that was set free at that time has cooled as the universe has expanded. Today the temperature of this background radiation is approximately 3 K (-270°C, or –450° F).

The Cosmic Background Explorer (COBE) spacecraft accurately measured the spectrum of the background radiation from 1989 to 1993. COBE measured radiation from the sky, then subtracted known sources of radiation from its measurements to reveal the background radiation. The measured background radiation fits the radiation predicted by the big bang theory so accurately that scientists consider it conclusive evidence that the big bang theory is the correct explanation for the beginning of the universe.

One of the experiments on the COBE spacecraft accurately mapped the isotropy of the universe and found small ripples that are currently thought to be the seeds from which galaxies and clusters of galaxies developed. Future spacecraft are being designed to make even more accurate observations of these ripples in the first decade of the 21st century.

A fundamental issue addressed in cosmology is the future of the universe—whether the universe will expand forever or eventually collapse. The first case (eternal expansion) is known as an open universe, and the second case (eventual collapse) is known as a closed universe. A closed universe would require sufficiently high density to cause gravity to stop the universe's expansion and begin its contraction. Such a collapse would require a deviation from Hubble's law, so observational cosmologists try to observe the distances between very distant galaxies and Earth using methods other than measurement of redshifts. The scientists can then compare these distance measurements with the galaxies' redshifts to see if Hubble's law holds or not. In the late 1990s astronomers compared the redshifts of supernovas in distant galaxies. Their findings supported an open universe.

Inflationary Theory
American scientists Alan Guth and Paul Steinhardt and Soviet-American astronomer Andreas Linde advanced an important cosmological theory, called the inflationary theory, in the 1980s. The inflationary theory deals with the behavior of the universe for only a tiny fraction of a second at the beginning of the universe. Theorists believe that the events of that fraction of a second, however, determined how the universe came to be the way it is now and how it will change in the future. The inflationary theory states that, starting only about 1 × 10 -35 second after the big bang and lasting for only about 1 × 10 -32 second, the universe expanded to 1 × 10 50 times its previous size. The numbers 1 × 10 -35 and 1 × 10 -32 are very small—a decimal point followed by 34 zeros and then a 1, and a decimal point followed by 31 zeros and then a 1, respectively. The number 1 × 10 50 is incredibly large—a 1 followed by 50 zeros. This extremely rapid inflation would explain why the universe appears so homogeneous: The universe had been compact enough to become uniform, and the expansion was rapid enough to preserve that uniformity.

The inflationary theory also explains why the universe is considered to be so close to the boundary between open and closed. The rate of expansion during the inflationary period was great enough to give the universe adequate density to put it close to the boundary between expanding forever or eventually beginning to contract. Though many theoretical cosmologists seem to favor the inflationary theory, it is not as widely accepted among observational cosmologists. Several astronomers and cosmologists performed studies in the late 1990s that seemed to show that the universe may be decidedly open, and not as close to the boundary between open and closed as the inflationary theory predicts.

COSMOLOGICAL EVIDENCE
Cosmologists use telescopes, astronomical satellites, and other instruments to study the universe. The data that these instruments provide allow scientists to evaluate current theories and to come up with ideas to better explain the universe. Modern cosmologists are continuously calculating the age, density, and rate of expansion of the universe.

The universe's density, expansion rate, and age are all related. The density of the universe determines how much the gravitational force will slow the expansion rate. The rate of expansion depends on the age and density of the universe. If cosmologists measure the rate of expansion by examining galactic redshifts and estimate the density of the universe, they can calculate an estimate of the universe's age. Cosmologists calculate the expansion rate of the universe by finding the relationship between the distance of an object from Earth and the rate at which it is moving away from Earth. This relationship is represented by Hubble's constant, H in the formula v = H × d, where v is velocity (or the speed of the object) and d is the distance between the object and Earth. If Hubble's constant is relatively large, the universe is expanding relatively rapidly. A universe that is rapidly expanding would be larger than a universe of the same age with a smaller value of Hubble's constant.

For a universe with very low density, the age of the universe would be directly related to its expansion rate. This universe would expand forever; this eternal expansion defines an open universe. If, on the other hand, the density of a universe is sufficiently high, the expansion rate is changing—slowing down as the universe ages. This universe would eventually stop expanding and begin contracting, which defines it as a closed universe. Astronomers and cosmologists have been able to estimate the density of the universe, but the density estimates cover a wide range of values. Estimates of density fall in the range for an open universe, the range for a closed universe, and near the boundary between the two. Age calculations for the higher densities are about two-thirds of those for the lower densities.

Several groups of astronomers conducted observational projects to determine Hubble's constant, the most important cosmological parameter, during the late 1990s. Notably, the American astronomers Wendy Freedman, Robert Kennicutt, and Barry Madore used the Hubble Space Telescope to observe Cepheid variable stars in distant galaxies, following the Leavitt-Shapley method. The Hubble Space Telescope can distinguish and follow such stars in galaxies much farther away from Earth than ground-based telescopes can. The researchers hope to determine Hubble's constant to within 10 percent of its actual value. Groups using distant supernovas, which are the very bright explosions of stars, are extending tests of Hubble's law to even greater distances.

Other astronomers used mainly ground-based telescopes to try to determine Hubble's constant. The American astronomer Alan Sandage and the Swiss astronomer Gustav Tammann have used a variety of methods to come up with an expansion estimate of 55 km/sec/megaparsec (about 34 mi/sec/megaparsec). A megaparsec is 1 million parsecs, and a parsec is about 3.26 light years (a light year is the distance that light could travel in a year—9.5 × 10 12 km, or 5.9 × 10 12 mi). So far, the cosmologists using the Hubble Space Telescope have found a value of about 70 km/sec/megaparsec (44 mi/sec/megaparsec) for the expansion rate of the universe. These expansion rates correspond to a universe between 8 billion and 13 billion years old.

These expansion rate and age findings still include many possible sources of uncertainty. For example, the galaxies observed are members of clusters of galaxies and move with respect to the other galaxies in those clusters, so cosmologists must find the average expansion velocity of these clusters of galaxies. Huge regions of mass of an unknown character affect the motion of galaxies, because of the attractive gravitational forces this mass produces, and make galaxies deviate from Hubble's constant. Additionally, motions of galaxies within their clusters and the rotation rates of spiral galaxies seem to indicate that much or most of the mass in the universe is of an unknown form. This so-called dark matter affects both the density and the expansion rate of the universe.

The European Space Agency's (ESA) Hipparcos satellite made accurate measurements of the distance between Earth and 100,000 different stars, and moderately accurate measurements of the distance between Earth and 1 million other stars, from 1989 to 1993. The ESA released the data to the scientific community in 1997, and the measurements soon began affecting cosmological theories. For example, the measurements changed the accepted distances to some globular clusters (clusters of stars outside the main disk of the Milky Way Galaxy) and led to revisions of calculations of the ages of these clusters. Before the Hipparcos data, some of these clusters appeared to be older than the universe (as predicted by Hubble's constant), but the revised distance measurements give the clusters an age within cosmologists' estimates of the age of the universe.