Monday, January 16, 2012


Hubble Deep Field
The Hubble Deep Field project used the Hubble Space Telescope to peer deep into space—farther than had ever been possible before. In this image, the telescope focused on one tiny area of the sky for ten days. Every object visible is a far-distant galaxy containing hundreds of billions of stars, revealing the awesome magnitude of the universe.

Cosmology, study of the universe as a whole, including its distant past and its future. Cosmologists study the universe observationally—by looking at the universe—and theoretically—by using physical laws and theories to predict how the universe should behave. Cosmology is a branch of astronomy, but the observational and theoretical techniques used by cosmologists involve a wide range of other sciences, such as physics and chemistry. Cosmology is distinguished from cosmogony, which used to mean the study of the origin of the universe but now usually refers only to the study of the origin of the solar system.
Humans have been examining and wondering about the sky for many millennia. As scientific discoveries have been made, ideas about the origin of the universe have changed and are still changing.
Ancient Cosmologies
As far back as 1100 bc, Mesopotamian astronomers drew constellations, or formations of stars perceived to form shapes. Some of today’s constellation names date back to that time. Mesopotamian and Babylonian cultures mapped the motion of the planets across the sky by observing how they moved against the background of stars.
Until the 16th century, most people (including early astronomers) considered Earth to be at the center of the universe. Greek philosopher Aristotle proposed a cosmology in about 350 bc that held for thousands of years. Aristotle theorized that the Sun, the Moon, and the planets all revolved around Earth on a set of celestial spheres. These celestial spheres were made of the quintessence—a perfect, unchanging, transparent element. According to Aristotle, the outermost sphere was made of the stars, which appear to be fixed in position. Early astronomers called the stars “fixed stars” to differentiate between stars and planets. The spheres inside the sphere of the fixed stars held the planets, which astronomers called the “wandering stars.” The Sun and Moon occupied the two innermost spheres. 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 Sámos became the first known person to assert that Earth moved around the Sun, but Aristotle’s model of the universe prevailed for almost 1,800 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, Greek 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
Celestial Models by Ptolemy and Copernicus
Currently, most people consider it obvious that the sun is at the center of the solar system, but the sun-centered (heliocentric) concept was slow to evolve. In the 2nd century ad, Claudius Ptolemy proposed a model of the universe with the earth at the center (geocentric). His model (shown left) depicts the earth as stationary with the planets, moon, and sun moving around it in small, circular orbits called epicycles. Ptolemy’s system was accepted by astronomers and religious thinkers alike for several hundred years. It was not until the 16th century that Nicolaus Copernicus developed a model for the universe in which the sun was at the center instead of the earth. The new model was rejected by the church, but it gradually gained popular acceptance because it provided better explanations for observed phenomena. Ironically, Copernicus’ initial measurements were no more accurate than Ptolemy’s, they just made more sense.

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, published in 1543, he based his ideas on philosophy instead of new observations. Copernicus’s theory was simpler and therefore more sound scientifically 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) with the Sun a bit off-center at one focus. This discovery was Kepler’s first law, and he developed two more laws about how the speeds and periods of the planets changed (see Kepler’s Laws). The first two laws were published in 1609 and the third was published in 1619.
Galileo’s Telescopes
Italian astronomer Galileo made major discoveries about celestial objects in our solar system with newly-invented telescopes in the early 17th century. His discoveries helped turn cosmology into a science based on observation, rather than philosophy. These telescopes are now in the Museo della Scienza in Florence, Italy.

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 1610, 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. Newton responded with his three laws of motion (see Mechanics: Newton’s Three Laws of Motion). Newton also developed the idea of universal gravitation, realizing that the same force that makes an apple fall to Earth also keeps the Moon constantly falling toward Earth, although in the Moon’s case Earth continually moves out of the way, resulting in the Moon orbiting the planet. 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 Earth and the 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
Hubble Photo of Galaxy M100
In 1924 American astronomer Edwin Hubble showed that fuzzy patches in the sky called “spiral nebulas” were in fact galaxies like our Milky Way. The orbiting telescope named after him, the Hubble Space Telescope, took this picture of a distant galaxy called M100 in 1995.

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 spectroscopy, the study of the particular colors making up a beam of light, allowed astronomers to observe and begin measuring a reddening of the light from distant galaxies. These redshifts are similar to those caused by the Doppler effect. The Doppler effect 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 shifts in the color of light coming from galaxies that are moving away from Earth as redshifts. The faster a galaxy is moving away, the more red its light will appear. 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 other 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 all shifted toward the red end of the visible spectrum. This shift increased the more distant the galaxies were. Cosmologists soon interpreted these redshifts as akin to Doppler shifts, which meant that the galaxies were moving away from Earth. The redshift, 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 redshifts. 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. The redshifts are not true Doppler shifts but rather result from the expansion of space, which carries the galaxies along with it.
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 current “standard model” of the origin of the universe, called the big bang theory, proposes that a major event, not unlike a huge explosion, set free all the matter and energy in the universe and started its expansion. Theories of the evolution and fate of the universe go on to describe a universe that has been expanding and cooling since the big bang. Early versions of the theory held that the universe would 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
George Gamow
Russian-born American physicist George Gamow developed the theory that the universe began as a hot explosion of matter and energy, or a “big bang.” Gamow also made important contributions to other fields of physics, including radioactivity and nuclear physics.

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. Recent versions of the theory also explain 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 originally derisive name “big bang” by British astronomer Fred Hoyle in 1950.) Current calculations place the age of the universe at about 13.7 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 a tiny fraction of a second—1 × 10-43 seconds—after the big bang explosion initiated the expansion of the universe. To understand the behavior of the universe before that point cosmologists would need a theory that merges quantum mechanics and general relativity. 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 ordinary 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
Sir Fred Hoyle
English astronomer and mathematician Sir Fred Hoyle predicted the existence of quasars. Hoyle also coined the term big bang as a disparaging reference to the theory that the universe originated billions of years ago from a hot explosion of matter and energy. Hoyle proposed an alternate version of the steady-state theory explanation, which suggested that the density of the expanding universe remained constant as new matter was slowly created.

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
Quasars are distant astronomical objects, most of which emit very strong radio waves. This false-color radio map of a quasar was taken by the Very Large Array radio telescope in New Mexico. All the quasars that astronomers have found have been very distant. Their light has taken billions of years to reach the earth, so studying quasars is like studying what the universe looked like billions of years ago. The fact that no quasars exist closer to the earth is evidence that the universe has changed over time.

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. As seen from Earth, 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.
Discovery of Cosmic Background Radiation
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 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 recently predicted that just such radiation would have been emitted as a result of the hot, dense early universe predicted by 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 proof of the big bang theory. The existence of cosmic background radiation is the third pillar of modern cosmology. The other two pillars are: (1) the uniform expansion of the universe and (2) the match between calculations of the amounts of the lightest chemical elements that would be formed in the first few minutes after a big bang and observations of these elements’ actual relative abundance in space.
In current cosmological models, the universe was at first both extremely hot and incredibly dense, with temperatures exceeding billions of degrees. In the first second after the big bang, as the universe expanded and cooled, elementary particles such as quarks and electrons formed.
After about one second, the universe had cooled enough that protons had formed out of the quarks. For the next 1,000 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 1 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 found small irregularities, or ripples, in the background radiation that are thought to be the clumps of matter in the early universe—the seeds from which galaxies and clusters of galaxies developed. These ripples were studied in more detail in limited regions of the sky by a variety of ground-based and balloon-based experiments. A more recent spacecraft, NASA's Wilkinson Microwave Anisotropy Probe (WMAP), was designed to make even more accurate observations of these ripples across the entire sky, as COBE did. In 2003 WMAP’s results confirmed and extended the intermediate experiments, providing a full-sky map of the ripples.
Inflationary Theory
In the 1980s American scientists Alan Guth and Paul Steinhardt and Soviet American astronomer Andreas Linde advanced an important cosmological theory called the inflationary theory. This 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 × 1050 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 × 1050 is incredibly large—a 1 followed by 50 zeros. This extremely rapid inflation would explain why the universe appears so homogeneous: In its earliest moments, the universe had been compact enough to become uniform, and the expansion was rapid enough to preserve that uniformity over the portion of the universe observable to us.
The Future of the Universe
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 eventually 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. Surprisingly, distant supernovas were slightly fainter than had been expected. This result was tentatively interpreted as an acceleration of the expansion of the universe. Astronomers were so surprised by the suggestion that the universe might be accelerating its expansion that they attempted to find other explanations for the relative dimness of distant supernovas, such as absorption by dust. By a few years into the 21st century, however, these other conceivable explanations had been ruled out, and the accelerating universe concept became widely accepted. The search continues to discover more and more distant supernovas.
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’s matter 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 measure of the distance scale in a universe that is rapidly expanding would be larger than a measure of the distance scale in 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 until the Wilkinson Microwave Anisotropy Probe (WMAP) results were released the density estimates covered a wide range of values. Some estimates of density fell in the range for an open universe, others in the range for a closed universe, and still others near the boundary between the two. Age calculations for the higher densities are about two-thirds of those for the lower densities.
Estimates of the age, density, and expansion rate of the universe include many possible sources of uncertainty. For example, many galaxies orbit each other as members of clusters of galaxies. The velocity of any one galaxy in the cluster as seen from Earth varies over time as it circles the cluster, moving toward Earth through part of its orbit and away through the remainder. Cosmologists, therefore, must find the average expansion velocity of the entire cluster. Recent studies drawing on data collected by the Hubble Key Project, the Hipparcos satellite, and WMAP have helped reduce the uncertainty of estimates for age, density, and expansion rate.
Hubble Key Project
Several groups of astronomers conducted observational projects to determine Hubble's constant, the most important cosmological parameter, during the late 1990s. Notably, the Hubble Key Project, carried out by 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. Their final value was a Hubble constant of 72 kilometers per second per megaparsec (45 miles per second per megaparsec). A parsec is about 3.26 light-years (a light-year is the distance that light could travel in a year—9.5 × 1012 km, or 5.9 × 1012 mi). The units of the Hubble constant mean that for each million parsecs (megaparsec) of distance between two objects, the space between them expands by 72 kilometers every second. Their result was accurate to within about 10 percent. It corresponds to an age of the universe of 12 billion to 14 billion years, depending on the rate of deceleration.
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.
Wilkinson Microwave Anisotropy Probe
In 2003 astronomers released results from the Wilkinson Microwave Anisotropy Probe (WMAP) that thoroughly confirmed existing ideas of cosmology and also produced several revelations about the nature of the universe. The probe studied the distribution of the ripples in the cosmic background radiation. A major conclusion from WMAP data linked with other observations is that the universe follows Euclidean geometry—that is, given any line in the universe, one and only one parallel line may be drawn through any point not on the original line. Such a universe is known as 'flat,' although it extends infinitely in all directions. If the universe is flat, it must be at the critical density that marks the boundary between an open and closed universe.
WMAP results also confirmed that the density of baryons—the elementary particles that make up regular matter—account for only 4 percent of the critical density. The probe further showed that another 23 percent of the universe consists of dark matter, a mysterious substance that does not shine in any part of the spectrum. The gravity of dark matter, however, is detectable. It binds clusters of galaxies together and causes the outer portions of galaxies to rotate faster than they would otherwise. Astronomers do not know the composition of dark matter, but they can theorize what it might be like. A slowly moving, cold dark matter, for example, could consist of not-yet-discovered particles that have names such as axions and weakly interacting massive particles (WIMPs). A rapidly moving, hot dark matter could be made up of particles called neutrinos, but measurements of neutrino mass indicate that they are too lightweight to account for much of the dark matter.
Since normal matter and dark matter account for only 27 percent of the material necessary for the universe to be at the critical density, the remaining 73 percent of the universe must be composed of a still more mysterious substance that astronomers have named “dark energy.” The composition of dark energy is not known, but its effect on the universe is detectable. Dark energy exerts a negative pressure that acts as antigravity, accelerating the universe's expansion. The effect of dark energy was smaller in the past, allowing gravity to slow the universe's expansion, but on the largest scale the repulsive force of dark energy now overwhelms the attractive force of gravity.
WMAP results also showed that the universe is 13.7 billion years old, with an uncertainty of only 0.2 billion years, and that the cosmic background radiation was set free 389,000 years after the big bang, a value uncertain by only 8,000 years. WMAP estimated a value for the Hubble constant of 71 kilometers per second per megaparsec (44 miles per second per magaparsec), in near agreement with the value predicted by the Hubble Key Project. WMAP’s wide-ranging results will be refined as the spacecraft makes additional observations. Observations made by the European Space Agency's Planck spacecraft, scheduled for launch in 2007, will be even more precise.

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