WMAP's Introduction to Cosmology Related Topics Cosmology: The Study of the Universe Cosmology is the scientific study of the large scale properties of the universe as a whole. It endeavors to use the scientific method to understand the origin, evolution and ultimate fate of the entire Universe. Like any field of science, cosmology involves the formation of theories or hypotheses about the universe which make specific predictions for phenomena that can be tested with observations. Depending on the outcome of the observations, the theories will need to be abandoned, revised or extended to accommodate the data. The prevailing theory about the origin and evolution of our Universe is the so-called Big Bang theory. Choose from the links in the left column for discussed at length. This primer in cosmological concepts is organized as follows: The main concepts of the Big Bang theory are introduced in the first section with scant regard to actual observations. The second section discusses the classic tests of the Big Bang theory that make it so compelling as the most likely valid and accurate description of our universe. The third section discusses observations that highlight limitations of the Big Bang theory and point to a more detailed model of cosmology than the Big Bang theory alone provides.

As discussed in the first section, the Big Bang theory predicts a range of possibilities for the structure and evolution of the universe. The final section discusses what constraints we can place on the nature of our universe based on current data, and indicates how WMAP furthers our understanding of cosmology. In addition, a few related topics are discussed based on commmonly asked questions.

For purposed of citation of this portion of the site or the downloadable PDF you can use this information: WMAP Science Team, "Cosmology: The Study of the Universe," NASA's Wilkinson Microwave Anisotropy Probe,last modified June 6, 2011, or The History & Structure of the Universe (Infographic) The History & Structure of the Universe (Infographic) By Karl Tate, SPACE.com Infographics Artist | October 31, 2011 10:01pm ET MORE The Illuminated Universe Credit: Illustration: Karl Tate, based on a photo of Galaxy M74 (NASA, ESA, and Hubble Heritage Collaboration) and the engraving "Awakening of the Pilgrim" from "The Atmosphere: Popular Meteorology" by Camille Flammarion, 1888 Our journey toward understanding the nature of our universe began thousands of years ago and had its roots in religion and philosophy. Around 2,300 years ago, careful observers in the Mediterranean deduced that the Earth must be round and must orbit the sun.

With no way for these early theories to be proved correct, however, they could not stand against the more flattering notion that the Earth was at the center of everything and that the cosmos existed to support human life and destiny. When Italian astronomer Galileo Galilei invented the astronomical telescope some 1,900 years later, it was finally possible to make precise observations about the planets and stars. A science of the structure and history of the entire universe, called "cosmology," emerged. FIRST UP: What We Know Now >> It took quite a bit more than seven days to create the universe as we know it today. SPACE.com looks at the mysteries of the heavens in our eight-part series: The History & Future of the Cosmos . This is Part 4 in that series. The Whole Enchilada Credit: Hubble Space Telescope Science Institute Our current understanding of the history of the universe is visualized above, with time running from left to right. We think that immediately after its creation at the time of the Big Bang, the universe expanded dramatically – an event called inflation. Our Earth formed when the universe was around 9.2 billion years old. The expansion of the universe continues today and is accelerating.

In this series of infographics, we will first look at the structure of the universe at larger and larger scales and find out a little about how we came to our current understanding of it. In the second part of our sequence, we will begin with the Big Bang and move forward in time to see how the universe has evolved to the present day. NEXT: First Stop, Earth >> The Earth Is Round Credit: Earth image: NASA; Eratosthenes portrait: unknown artist Our first stop is the planet we call home. The knowledge that the Earth is shaped like a ball is actually quite old. About 2,500 years ago, Greek travelers reported that different constellations were visible in the sky when one went far to the north or south. Keen observers also would have noticed that during an eclipse of the moon, the shadow cast by the Earth has a round edge. A few centuries later, the scholar Eratosthenes estimated the size of the Earth by noting the difference between the lengths of shadows cast by the sun in locations a few hundred miles apart. By assuming that the sun was so far away that its rays of light were parallel, Eratosthenes could use simple geometry to calculate the circumference of the Earth.

It is not known how accurate his measurement was, but it may have been off from the true figure by no more than a few percentage points. NEXT: The Solar System >> Earth Is a Planet Credit: Karl Tate, SPACE.com; Johannes Kepler portrait: unknown artist Now we pull back to see the Earth in the context of the inner solar system. Early ideas about the movements of the sun, Earth and planets were derived from theological, astrological and philosophical notions of how God must have ordered the world. Polish astronomer Nicolaus Copernicus caused an uproar in the mid-1500s by suggesting the Earth moved around the sun and not, as leaders of Christianity taught, the sun around the Earth. For centuries the planets were thought to move because they were embedded in nested "crystal spheres" that rotated around a central point. However, it was noted in the 16th century that comets moved in such a way that would crash them through those crystal spheres. Replacing the spheres was the idea of "epicycles," circles superimposed on circles, mathematically influencing each other to result in the observed planetary motions.

Finally, in 1609, German mathematician Johannes Kepler published his theories of planetary motion, which established that bodies in our solar system move in orbits shaped like ovals rather than circles. NEXT: Planets Are Other Worlds >> The Planets Are Worlds Credit: Karl Tate, SPACE.com; Galileo Galilei: portrait by Ottavio Leoni From the earliest eras of human pre-history, the entire universe was thought to encompass only the elements visible to the naked eye: Earth, its moon and sun, five points of light that moved and were called "planets," plus a distant sphere upon which the stars and the glowing band of the Milky Way were embedded. Theories of astrology and, later, astronomy were devised to explain the movements of these celestial objects, but their true nature could only be guessed at. When in 1609 the Italian astronomer Galileo finally trained a crude telescope on the heavens, he discovered that the planets were other worlds. Several of these worlds were found to have moons of their own. With the aid of the telescope, previously unknown planets were discovered in our solar system: Uranus in 1781 and Neptune in 1846.

With the telescope it became possible to study smaller bodies such as comets and asteroids, and also the stars and nebulas on the distant celestial sphere. NEXT: A Sea of Stars >> The Stars Are Suns Credit: Diagram of local stars: Karl Tate, based on public domain data plot; Friedrich Bessel portrait: Christian Albrecht Jensen In the 17th century, the invention of the telescope by Galileo and the discovery of the laws of motion by Kepler prompted the realization that stars were just like the sun, all obeying the same laws of physics. In the 19th century, spectroscopy — the study of the wavelengths of light that are emitted by objects — made it possible to investigate the gases that stars are made of. Scientists also figured out in the 19th century how to measure the distances to stars. When an object is viewed from different vantage points, the object appears to shift relative to the more distant background. The shift is called "parallax." As the Earth orbits the sun, it provides a changing vantage point for observing the stars. Since the stars are so much more distant than objects in our own solar system, the parallax shift is very small and hard to measure.

The German mathematician and astronomer Friedrich Bessel was the first to successfully measure the parallax of the star 61 Cygni and estimated its distance from Earth to be 10.4 light-years. (Later estimates adjusted this distance to 11.4 light-years.) NEXT: The Galaxy and Dark Matter >> A Galaxy Held Together by Dark Matter Credit: Milky Way Galaxy map: Robert Hurt; Fritz Zwicky photo via University of Virginia Dept. of Astronomy The layout of our galaxy is difficult to figure out from our vantage point, which is embedded in it. By studying the shapes of distant galaxies and carefully measuring the objects that we see in our own galaxy, we have inferred that ours is a barred-spiral galaxy. A central bar-shaped core composed of stars (and harboring an extremely large black hole) is surrounded by spiraling arms, also formed of stars as well as gas and dust. We are located in a spur, or branch, that stretches between major spiral arms. The exact configuration of spiral arms is still debated by astronomers, but a recent survey found that our Milky Way galaxy has two major arms, which branch out into four arms toward the outside.

The spiral arms of our galaxy are thought to be a kind of density wave that travels around the flat disk. Material bunches up, and stars are formed along the arms. Everything in the galaxy orbits around its center, and the arms are not solid structures. Our solar system travels into and out of the spiral arms as it orbits. While studying the rotation of galaxies, it was noted that they do not rotate as we would expect them to based on the gravitational pull of the matter we can see. Swiss astronomer Fritz Zwicky suggested in 1934 that there must be a large amount of invisible, or "dark," matter present, making spiral galaxies more massive than they appear. Since that time astrophysicists have searched for this dark matter, often speculating that it might consist of exotic particles unlike anything we know on Earth. Current estimates show that our universe is mostly composed of unknown forms of dark matter and dark energy, with familiar atoms being only a tiny fraction of the total. NEXT: Galaxies Filled With Stars >> Galaxies Are Made of Stars Credit: Milky Way Galaxy map: Robert Hurt; Edwin Hubble photo via NASA The Milky Way, a faint ribbon of light that spans the sky, has been known throughout history.

Its true nature was not discovered until the 17th century, when Galileo Galilei studied the Milky Way with a telescope and determined that the ribbon was composed of a multitude of stars. Small fuzzy patches of light can be seen in the sky; these were called nebulae. By the 18th century it was speculated that the Milky Way was a huge system of stars bound together by gravity, but the nature of the nebulae remained unknown. They could have been small clouds of gas within the Milky Way, or perhaps they were external to our galaxy. It could not be proved whether or not the Milky Way constituted the entire universe. Using the newly constructed 100-inch telescope at Mount Wilson Observatory in California, American astronomer Edwin Hubble studied stars called Cepheids, which brighten and dim in a pattern related to their intrinsic brightness, making them suitable for use as a yardstick in estimating cosmic distances. In a 1925 paper, Hubble concluded that some of the nebulae were external to the Milky Way, and were giant galaxies in their own right, revealing a universe much larger than our own home galaxy.

NEXT: Universe Gets Organized >> Massive Organization Credit: Diagram: Karl Tate based on NASA illustration; Brent Tully photo via Institute for Astronomy, University of Hawaii It was first noticed in the latter half of the 19th century that there is a large group of nebulas in the constellation Virgo. Later it was discovered that these nebulae are separate galaxies external to our Milky Way. One hundred years later, astronomers speculated that the apparent alignment of these galaxies might indicate a higher level of cosmic structure, variously dubbed a "metagalaxy" or "supercluster." In 1982 astronomer R. Brent Tully published an analysis of the distances to the supercluster member galaxies, showing that they were indeed part of a larger organization. The distances were determined by noting the redshift of the spectra of light from the galaxies. (See "Time Zero: The Big Bang" later in this gallery for a fuller explanation of redshift.) NEXT: Largest Structures in Space >> The Largest Structures in Space Credit: Credit: 2dF Galaxy Redshift Survey, Anglo-Australian Observatory; Margaret Geller photo via Harvard University Dept.

of Astronomy The largest structures that we know of are the galactic filaments – also called supercluster complexes – that surround vast voids in space. The galaxies in a filament are bound together by gravity. When the first of these structures was discovered by Margaret Geller and John Huchra in 1989, it was dubbed "the Great Wall." A much larger structure, the "Sloan Great Wall," was discovered in 2003 by J. Richard Gott III and Mario Jurić. Current research into the large-scale structure of the universe utilizes data gathered by redshift surveys such as the Sloan Digital Sky Survey. These efforts use digital camera sensors to photograph regions of the sky, capturing millions of distant objects along with the data needed to map them in 3-D space. NEXT: The Farthest We Can See >> The Farthest We Can See Credit: Simulation of observable universe: Karl Tate, SPACE.com; Alan Guth photo via Brookhaven National Laboratory The observable universe is everything that we can detect. It is a sphere 93 billion light-years in diameter, centered on Earth. We cannot perceive the entire universe at once, due to the slowness of the speed of light compared with the vast scale of the universe.

As we look out into space, we see objects as they were at earlier and earlier times in history. Also, because of the accelerating expansion of the universe, distant objects are much farther away than their age would have us think. For example, the edge of the observable universe is estimated to be about 46 billion light-years away, even though the universe itself is only 13.7 billion years old. The true extent of the universe is unknown. It could be much bigger than the observable universe – perhaps even infinite in size. However, light from the most-distant regions would never be able to reach us; the space it must pass through is simply expanding too fast. Our current picture of the observable universe owes a lot to American physicist Alan Guth, who in the 1980s worked out how a universe resembling our own might have emerged from the Big Bang event which created it. Next, we will reset the clock to time zero and see how the universe evolved from its beginning to today.

NEXT: The Big Bang >> The Big Bang: 13,750,000,000 years ago Credit: Karl Tate, SPACE.com In the early 20th century, Belgian astronomer and Catholic priest Georges Lemaitre calculated that the universe is expanding. By mathematically running the expansion backward, he theorized that everything in the universe once must have been compacted into a small, dense object, which he called "the primeval atom." This atom exploded, an event that astronomer Fred Hoyle flippantly called "The Big Bang." The expansion of the universe explains why the light from distant objects is shifted toward the red end of the spectrum, a phenomenon called "redshift." Just as the Doppler effect causes sound from moving vehicles to change pitch, redshift causes light from moving stars to change color as its wavelength gets stretched by expanding space. The farther an object is from Earth, the more the intervening space has expanded, and the more the object's light will have been shifted toward red. American astronomer Edwin Hubble later proved with observations that redshift was indeed related to distance, and the correlation is now known as Hubble's law.

NEXT: Universe's First Second >> Earliest Fraction of a Second Credit: Map of Cosmic Microwave Background temperature fluctuations from Wilkinson Microwave Anisotropy Probe (WMAP) data; Alan Guth photo via Brookhaven National Laboratory Astronomers in the 1970s had a problem understanding the early universe. When they probed deep space with radio telescopes, they discovered a faint background glow of microwave radiation. Variations in the density of the microwave signal were interpreted as variations in the density of matter in the early universe. Surprisingly, the background glow of radiation was found to be uniform in every direction. This seemed unreasonable; scientists expected to find regions of space with different densities and temperatures, because these regions seemed too far apart to have evolved together. American physicist Alan Guth proposed an explanation in 1980. He theorized that in the tiny fraction of time just following the Big Bang, the universe underwent extremely rapid expansion. In a flash, its volume increased by a factor of 10^78 (the number 10 followed by 78 zeroes). Almost immediately the universe cooled slightly and the event, called "inflation," was over.

The inflationary model explains why the universe appears uniform in all directions: Everything in it evolved together before inflation. It has other staggering implications, too: The part of space that we can see must be just a tiny patch in what must be a vast universe that we can never directly detect. NEXT: The First Three Minutes >> 0.001 Second to 3 Minutes After the Big Bang Credit: Graphic: Karl Tate based on image of data plot from collision of gold ions, Brookhaven National Laboratory Relativistic Heavy Ion Collider Following inflation, the cooling but still unimaginably hot universe experienced a phase transition. Elementary particles were created from a form of matter called quark-gluon plasma. A thousandth of a second following the Big Bang, vast amounts of matter and antimatter annihilated each other (leaving behind the material that exists in the universe today). Within three minutes the temperature of the universe dropped to about a billion degrees, and atoms could begin to form, starting with the simplest elements: hydrogen and helium. The quark-gluon plasma of the early universe is still theoretical and is thought to be possible because of a theory called Quantum Chromodynamics. American physicist Murray Gell-Mann was among the first to formulate this theory.

The basic nuclear particles – protons and neutrons – are thought to be made from still more-fundamental particles called "quarks," which are never found traveling alone except under very high temperatures like those that existed just after the Big Bang. Physicists are trying to re-create on Earth the plasma that is thought to have comprised the early universe; they are using particle accelerators to smash subatomic particles together at high energy. NEXT: 3 Minutes to 379,000 Years >> 3 Minutes to 379,000 Years After the Big Bang Credit: NASA, ESA During this period, the early universe was hot and opaque. Starting at about 379,000 years after the Big Bang, the universe cooled enough so that light could separate from matter and travel freely. In short, the universe became transparent. This photo shows galaxy UDFy-38135539, one of the oldest and earliest galaxies yet found, appearing just after the Dark Age at about 480 million years after the Big Bang.

NEXT: The First Billion Years >> 150 Million to 1 Billion Years After the Big Bang Credit: Artist's conception of a quasar: NASA/ESA; Maarten Schmidt photo: California Institute of Technology In the 1960s Dutch astronomer Maarten Schmidt identified strange deep-space objects, very bright in radio wavelengths, which he termed "quasi-stellar radio sources." U.S. astrophysicist Hong-Yee Chiu named the phenomena "quasars." Quasars had been picked up in the 1950s by large Earth-bound antennas called radio telescopes. When Schmidt measured the quasars' distance by studying the redshift of their spectrum, what he found was astonishing. The objects were billions of light-years away, and therefore had to be incredibly bright to be detected on Earth. Later study showed that the mysterious quasars were active galaxies that had formed very early in the history of the universe. Gravitational collapse had caused matter to coalesce, eventually forming giant black holes with the mass of billions of suns.

A black hole sits at the center of a quasar, collecting matter and heating it to become high-temperature plasma that can be shot out into huge jets traveling close to the speed of light.that light could separate from matter and travel freely. In short, the universe became transparent. This photo shows galaxy UDFy-38135539, one of the oldest and earliest galaxies yet found, appearing just after the Dark Age at about 480 million years after the Big Bang. NEXT: The Universe, Age 9 Billion Years >> 9 Billion Years After the Big Bang Credit: Artist's conception of a young solar system: NASA/JPL-California Institute of Technology; Albert Einstein photo via United States Library of Congress The earliest stars formed when the universe was only 300 million years old. They were short-lived and supermassive, composed mostly of hydrogen and helium and containing no metals. These first stars exploded into supernovas, and successive generations were created from the remains of the earlier suns. Analysis of the spectrum of the light from our sun shows that it is rich in metals, and therefore could have been created only following many generations of stars.

The sun's power source was a mystery until German physicist Albert Einstein worked out in 1905 that matter could be converted into energy, with his famous equation E=mc^2. In 1920 British astrophysicist Sir Arthur Eddington suggested that the sun might be powered by a nuclear fusion reactor, generating heat and light energy by converting hydrogen into helium. Study of the spectrum of light from the sun and other stars led to a confirmation that nuclear fusion processes created the atomic elements from which our world is composed. NEXT: The Modern Universe >> The Universe Now Credit: NASA Scientists have put together an impressive picture of the origin, history and nature of our universe. However, we do not know everything there is to know. Many open questions remain in the fields of physics and cosmology. For example: What is the ultimate fate of the universe? SOURCES: NASA, Hubble Space Telescope Science Institute, "Space, Our Final Frontier" by John Gribbin (2001), "The Forgotten Revolution: How Science Was Born in 300 BC and Why It Had to Be Reborn" by Lucio Russo (2004), "The Inflationary Universe" by Alan H.

Guth (1997) It took quite a bit more than seven days to create the universe as we know it today. SPACE.com looks at the mysteries of the heavens in our eight-part series: The History & Future of the Cosmos . This is Part 4 in that series. Author Bio Karl Tate, SPACE.com Infographics Artist Karl's association with SPACE.com goes back to 2000, when he was hired to produce interactive Flash graphics. Starting in 2010, Karl has been TechMediaNetwork's infographics specialist across all editorial properties. Before joining SPACE.com, Karl spent 11 years at the New York headquarters of The Associated Press, creating news graphics for use around the world in newspapers and on the web. He has a degree in graphic design from Louisiana State University. To find out what his latest project is, you can follow Karl on Google+ .

previous | next The Illuminated Universe Credit: Illustration: Karl Tate, based on a photo of Galaxy M74 (NASA, ESA, and Hubble Heritage Collaboration) and the engraving "Awakening of the Pilgrim" from "The Atmosphere: Popular Meteorology" by Camille Flammarion, 1888 Our journey toward understanding the nature of our universe began thousands of years ago and had its roots in religion and philosophy. Around 2,300 years ago, careful observers in the Mediterranean deduced that the Earth must be round and must orbit the sun. With no way for these early theories to be proved correct, however, they could not stand against the more flattering notion that the Earth was at the center of everything and that the cosmos existed to support human life and destiny. When Italian astronomer Galileo Galilei invented the astronomical telescope some 1,900 years later, it was finally possible to make precise observations about the planets and stars. A science of the structure and history of the entire universe, called "cosmology," emerged. FIRST UP: What We Know Now >> It took quite a bit more than seven days to create the universe as we know it today.

SPACE.com looks at the mysteries of the heavens in our eight-part series: The History & Future of the Cosmos . This is Part 4 in that series. The Whole Enchilada Credit: Hubble Space Telescope Science Institute Our current understanding of the history of the universe is visualized above, with time running from left to right. We think that immediately after its creation at the time of the Big Bang, the universe expanded dramatically – an event called inflation. Our Earth formed when the universe was around 9.2 billion years old. The expansion of the universe continues today and is accelerating. In this series of infographics, we will first look at the structure of the universe at larger and larger scales and find out a little about how we came to our current understanding of it. In the second part of our sequence, we will begin with the Big Bang and move forward in time to see how the universe has evolved to the present day. NEXT: First Stop, Earth >> The Earth Is Round Credit: Earth image: NASA; Eratosthenes portrait: unknown artist Our first stop is the planet we call home. The knowledge that the Earth is shaped like a ball is actually quite old.

About 2,500 years ago, Greek travelers reported that different constellations were visible in the sky when one went far to the north or south. Keen observers also would have noticed that during an eclipse of the moon, the shadow cast by the Earth has a round edge. A few centuries later, the scholar Eratosthenes estimated the size of the Earth by noting the difference between the lengths of shadows cast by the sun in locations a few hundred miles apart. By assuming that the sun was so far away that its rays of light were parallel, Eratosthenes could use simple geometry to calculate the circumference of the Earth. It is not known how accurate his measurement was, but it may have been off from the true figure by no more than a few percentage points. NEXT: The Solar System >> Earth Is a Planet Credit: Karl Tate, SPACE.com; Johannes Kepler portrait: unknown artist Now we pull back to see the Earth in the context of the inner solar system. Early ideas about the movements of the sun, Earth and planets were derived from theological, astrological and philosophical notions of how God must have ordered the world.

Polish astronomer Nicolaus Copernicus caused an uproar in the mid-1500s by suggesting the Earth moved around the sun and not, as leaders of Christianity taught, the sun around the Earth. For centuries the planets were thought to move because they were embedded in nested "crystal spheres" that rotated around a central point. However, it was noted in the 16th century that comets moved in such a way that would crash them through those crystal spheres. Replacing the spheres was the idea of "epicycles," circles superimposed on circles, mathematically influencing each other to result in the observed planetary motions. Finally, in 1609, German mathematician Johannes Kepler published his theories of planetary motion, which established that bodies in our solar system move in orbits shaped like ovals rather than circles.

NEXT: Planets Are Other Worlds >> The Planets Are Worlds Credit: Karl Tate, SPACE.com; Galileo Galilei: portrait by Ottavio Leoni From the earliest eras of human pre-history, the entire universe was thought to encompass only the elements visible to the naked eye: Earth, its moon and sun, five points of light that moved and were called "planets," plus a distant sphere upon which the stars and the glowing band of the Milky Way were embedded. Theories of astrology and, later, astronomy were devised to explain the movements of these celestial objects, but their true nature could only be guessed at. When in 1609 the Italian astronomer Galileo finally trained a crude telescope on the heavens, he discovered that the planets were other worlds. Several of these worlds were found to have moons of their own. With the aid of the telescope, previously unknown planets were discovered in our solar system: Uranus in 1781 and Neptune in 1846. With the telescope it became possible to study smaller bodies such as comets and asteroids, and also the stars and nebulas on the distant celestial sphere.

NEXT: A Sea of Stars >> The Stars Are Suns Credit: Diagram of local stars: Karl Tate, based on public domain data plot; Friedrich Bessel portrait: Christian Albrecht Jensen In the 17th century, the invention of the telescope by Galileo and the discovery of the laws of motion by Kepler prompted the realization that stars were just like the sun, all obeying the same laws of physics. In the 19th century, spectroscopy — the study of the wavelengths of light that are emitted by objects — made it possible to investigate the gases that stars are made of. Scientists also figured out in the 19th century how to measure the distances to stars. When an object is viewed from different vantage points, the object appears to shift relative to the more distant background. The shift is called "parallax." As the Earth orbits the sun, it provides a changing vantage point for observing the stars. Since the stars are so much more distant than objects in our own solar system, the parallax shift is very small and hard to measure.

The German mathematician and astronomer Friedrich Bessel was the first to successfully measure the parallax of the star 61 Cygni and estimated its distance from Earth to be 10.4 light-years. (Later estimates adjusted this distance to 11.4 light-years.) NEXT: The Galaxy and Dark Matter >> A Galaxy Held Together by Dark Matter Credit: Milky Way Galaxy map: Robert Hurt; Fritz Zwicky photo via University of Virginia Dept. of Astronomy The layout of our galaxy is difficult to figure out from our vantage point, which is embedded in it. By studying the shapes of distant galaxies and carefully measuring the objects that we see in our own galaxy, we have inferred that ours is a barred-spiral galaxy. A central bar-shaped core composed of stars (and harboring an extremely large black hole) is surrounded by spiraling arms, also formed of stars as well as gas and dust. We are located in a spur, or branch, that stretches between major spiral arms. The exact configuration of spiral arms is still debated by astronomers, but a recent survey found that our Milky Way galaxy has two major arms, which branch out into four arms toward the outside.

The spiral arms of our galaxy are thought to be a kind of density wave that travels around the flat disk. Material bunches up, and stars are formed along the arms. Everything in the galaxy orbits around its center, and the arms are not solid structures. Our solar system travels into and out of the spiral arms as it orbits. While studying the rotation of galaxies, it was noted that they do not rotate as we would expect them to based on the gravitational pull of the matter we can see. Swiss astronomer Fritz Zwicky suggested in 1934 that there must be a large amount of invisible, or "dark," matter present, making spiral galaxies more massive than they appear. Since that time astrophysicists have searched for this dark matter, often speculating that it might consist of exotic particles unlike anything we know on Earth. Current estimates show that our universe is mostly composed of unknown forms of dark matter and dark energy, with familiar atoms being only a tiny fraction of the total. NEXT: Galaxies Filled With Stars >> Galaxies Are Made of Stars Credit: Milky Way Galaxy map: Robert Hurt; Edwin Hubble photo via NASA The Milky Way, a faint ribbon of light that spans the sky, has been known throughout history.

Its true nature was not discovered until the 17th century, when Galileo Galilei studied the Milky Way with a telescope and determined that the ribbon was composed of a multitude of stars. Small fuzzy patches of light can be seen in the sky; these were called nebulae. By the 18th century it was speculated that the Milky Way was a huge system of stars bound together by gravity, but the nature of the nebulae remained unknown. They could have been small clouds of gas within the Milky Way, or perhaps they were external to our galaxy. It could not be proved whether or not the Milky Way constituted the entire universe. Using the newly constructed 100-inch telescope at Mount Wilson Observatory in California, American astronomer Edwin Hubble studied stars called Cepheids, which brighten and dim in a pattern related to their intrinsic brightness, making them suitable for use as a yardstick in estimating cosmic distances. In a 1925 paper, Hubble concluded that some of the nebulae were external to the Milky Way, and were giant galaxies in their own right, revealing a universe much larger than our own home galaxy.

NEXT: Universe Gets Organized >> Massive Organization Credit: Diagram: Karl Tate based on NASA illustration; Brent Tully photo via Institute for Astronomy, University of Hawaii It was first noticed in the latter half of the 19th century that there is a large group of nebulas in the constellation Virgo. Later it was discovered that these nebulae are separate galaxies external to our Milky Way. One hundred years later, astronomers speculated that the apparent alignment of these galaxies might indicate a higher level of cosmic structure, variously dubbed a "metagalaxy" or "supercluster." In 1982 astronomer R. Brent Tully published an analysis of the distances to the supercluster member galaxies, showing that they were indeed part of a larger organization. The distances were determined by noting the redshift of the spectra of light from the galaxies. (See "Time Zero: The Big Bang" later in this gallery for a fuller explanation of redshift.) NEXT: Largest Structures in Space >> The Largest Structures in Space Credit: Credit: 2dF Galaxy Redshift Survey, Anglo-Australian Observatory; Margaret Geller photo via Harvard University Dept.

of Astronomy The largest structures that we know of are the galactic filaments – also called supercluster complexes – that surround vast voids in space. The galaxies in a filament are bound together by gravity. When the first of these structures was discovered by Margaret Geller and John Huchra in 1989, it was dubbed "the Great Wall." A much larger structure, the "Sloan Great Wall," was discovered in 2003 by J. Richard Gott III and Mario Jurić. Current research into the large-scale structure of the universe utilizes data gathered by redshift surveys such as the Sloan Digital Sky Survey. These efforts use digital camera sensors to photograph regions of the sky, capturing millions of distant objects along with the data needed to map them in 3-D space. NEXT: The Farthest We Can See >> The Farthest We Can See Credit: Simulation of observable universe: Karl Tate, SPACE.com; Alan Guth photo via Brookhaven National Laboratory The observable universe is everything that we can detect. It is a sphere 93 billion light-years in diameter, centered on Earth. We cannot perceive the entire universe at once, due to the slowness of the speed of light compared with the vast scale of the universe.

As we look out into space, we see objects as they were at earlier and earlier times in history. Also, because of the accelerating expansion of the universe, distant objects are much farther away than their age would have us think. For example, the edge of the observable universe is estimated to be about 46 billion light-years away, even though the universe itself is only 13.7 billion years old. The true extent of the universe is unknown. It could be much bigger than the observable universe – perhaps even infinite in size. However, light from the most-distant regions would never be able to reach us; the space it must pass through is simply expanding too fast. Our current picture of the observable universe owes a lot to American physicist Alan Guth, who in the 1980s worked out how a universe resembling our own might have emerged from the Big Bang event which created it. Next, we will reset the clock to time zero and see how the universe evolved from its beginning to today.

NEXT: The Big Bang >> The Big Bang: 13,750,000,000 years ago Credit: Karl Tate, SPACE.com In the early 20th century, Belgian astronomer and Catholic priest Georges Lemaitre calculated that the universe is expanding. By mathematically running the expansion backward, he theorized that everything in the universe once must have been compacted into a small, dense object, which he called "the primeval atom." This atom exploded, an event that astronomer Fred Hoyle flippantly called "The Big Bang." The expansion of the universe explains why the light from distant objects is shifted toward the red end of the spectrum, a phenomenon called "redshift." Just as the Doppler effect causes sound from moving vehicles to change pitch, redshift causes light from moving stars to change color as its wavelength gets stretched by expanding space. The farther an object is from Earth, the more the intervening space has expanded, and the more the object's light will have been shifted toward red. American astronomer Edwin Hubble later proved with observations that redshift was indeed related to distance, and the correlation is now known as Hubble's law.

NEXT: Universe's First Second >> Earliest Fraction of a Second Credit: Map of Cosmic Microwave Background temperature fluctuations from Wilkinson Microwave Anisotropy Probe (WMAP) data; Alan Guth photo via Brookhaven National Laboratory Astronomers in the 1970s had a problem understanding the early universe. When they probed deep space with radio telescopes, they discovered a faint background glow of microwave radiation. Variations in the density of the microwave signal were interpreted as variations in the density of matter in the early universe. Surprisingly, the background glow of radiation was found to be uniform in every direction. This seemed unreasonable; scientists expected to find regions of space with different densities and temperatures, because these regions seemed too far apart to have evolved together. American physicist Alan Guth proposed an explanation in 1980. He theorized that in the tiny fraction of time just following the Big Bang, the universe underwent extremely rapid expansion. In a flash, its volume increased by a factor of 10^78 (the number 10 followed by 78 zeroes). Almost immediately the universe cooled slightly and the event, called "inflation," was over.

The inflationary model explains why the universe appears uniform in all directions: Everything in it evolved together before inflation. It has other staggering implications, too: The part of space that we can see must be just a tiny patch in what must be a vast universe that we can never directly detect. NEXT: The First Three Minutes >> 0.001 Second to 3 Minutes After the Big Bang Credit: Graphic: Karl Tate based on image of data plot from collision of gold ions, Brookhaven National Laboratory Relativistic Heavy Ion Collider Following inflation, the cooling but still unimaginably hot universe experienced a phase transition. Elementary particles were created from a form of matter called quark-gluon plasma. A thousandth of a second following the Big Bang, vast amounts of matter and antimatter annihilated each other (leaving behind the material that exists in the universe today). Within three minutes the temperature of the universe dropped to about a billion degrees, and atoms could begin to form, starting with the simplest elements: hydrogen and helium. The quark-gluon plasma of the early universe is still theoretical and is thought to be possible because of a theory called Quantum Chromodynamics. American physicist Murray Gell-Mann was among the first to formulate this theory.

The basic nuclear particles – protons and neutrons – are thought to be made from still more-fundamental particles called "quarks," which are never found traveling alone except under very high temperatures like those that existed just after the Big Bang. Physicists are trying to re-create on Earth the plasma that is thought to have comprised the early universe; they are using particle accelerators to smash subatomic particles together at high energy. NEXT: 3 Minutes to 379,000 Years >> 3 Minutes to 379,000 Years After the Big Bang Credit: NASA, ESA During this period, the early universe was hot and opaque. Starting at about 379,000 years after the Big Bang, the universe cooled enough so that light could separate from matter and travel freely. In short, the universe became transparent. This photo shows galaxy UDFy-38135539, one of the oldest and earliest galaxies yet found, appearing just after the Dark Age at about 480 million years after the Big Bang.

NEXT: The First Billion Years >> 150 Million to 1 Billion Years After the Big Bang Credit: Artist's conception of a quasar: NASA/ESA; Maarten Schmidt photo: California Institute of Technology In the 1960s Dutch astronomer Maarten Schmidt identified strange deep-space objects, very bright in radio wavelengths, which he termed "quasi-stellar radio sources." U.S. astrophysicist Hong-Yee Chiu named the phenomena "quasars." Quasars had been picked up in the 1950s by large Earth-bound antennas called radio telescopes. When Schmidt measured the quasars' distance by studying the redshift of their spectrum, what he found was astonishing. The objects were billions of light-years away, and therefore had to be incredibly bright to be detected on Earth. Later study showed that the mysterious quasars were active galaxies that had formed very early in the history of the universe. Gravitational collapse had caused matter to coalesce, eventually forming giant black holes with the mass of billions of suns.

A black hole sits at the center of a quasar, collecting matter and heating it to become high-temperature plasma that can be shot out into huge jets traveling close to the speed of light.that light could separate from matter and travel freely. In short, the universe became transparent. This photo shows galaxy UDFy-38135539, one of the oldest and earliest galaxies yet found, appearing just after the Dark Age at about 480 million years after the Big Bang. NEXT: The Universe, Age 9 Billion Years >> 9 Billion Years After the Big Bang Credit: Artist's conception of a young solar system: NASA/JPL-California Institute of Technology; Albert Einstein photo via United States Library of Congress The earliest stars formed when the universe was only 300 million years old. They were short-lived and supermassive, composed mostly of hydrogen and helium and containing no metals. These first stars exploded into supernovas, and successive generations were created from the remains of the earlier suns. Analysis of the spectrum of the light from our sun shows that it is rich in metals, and therefore could have been created only following many generations of stars.

The sun's power source was a mystery until German physicist Albert Einstein worked out in 1905 that matter could be converted into energy, with his famous equation E=mc^2. In 1920 British astrophysicist Sir Arthur Eddington suggested that the sun might be powered by a nuclear fusion reactor, generating heat and light energy by converting hydrogen into helium. Study of the spectrum of light from the sun and other stars led to a confirmation that nuclear fusion processes created the atomic elements from which our world is composed. NEXT: The Modern Universe >> The Universe Now Credit: NASA Scientists have put together an impressive picture of the origin, history and nature of our universe. However, we do not know everything there is to know. Many open questions remain in the fields of physics and cosmology. For example: WMAP Formation of Universe Structures Related Topics How Did Structure Form in the Universe? Astronomers observe considerable structure in the universe, from stars to galaxies to clusters and superclusters of galaxies. The famous "Deep Field Image" taken by the Hubble Space Telescope, shown below, provides a stunning view of such structure. How did these structures form?

The Big Bang theory is widely considered to be a successful theory of cosmology, but the theory is incomplete. It does not account for the needed fluctuations to produce the structure we see. Most cosmologists believe that the galaxies that we observe today grew from the gravitational pull of small fluctuations in the nearly-uniform density of the early universe. These fluctuations leave an imprint in the cosmic microwave background radiation in the form of temperature fluctuations from point to point across the sky. The WMAP satellite measures these small fluctuations in the temperature of the cosmic microwave background radiation, which in turn reveals the early stages of structure formation. Hubble Deep Field Image: HST press release describing this image In its simplest form, the Big Bang theory assumes that matter and radiation are uniformly distributed throughout the universe and that general relativity is universally valid. While this can account for the existence of the cosmic microwave background radiation and explain the origin of the light elements , it does not explain the existence of galaxies and large-scale structure. The solution of the structure problem must be built into the framework of the Big Bang theory.

Gravitational Formation of Structure Most cosmologists believe that the galaxies that we observe today grew gravitationally out of small fluctuations in the density of the universe through the following sequence of events: When the universe was one thousandth its present size (roughly 500,000 years after the Big Bang), the density of matter in the region of space that now contains the Milky Way, our home galaxy, was perhaps 0.5% higher than in adjacent regions. Because its density was higher, this region of space expanded more slowly than surrounding regions. As a result of this slower expansion, its relative over-density grew. When the universe was one hundredth its present size (roughly 15 million years after the Big Bang), our region of space was probably 5% denser than the surrounding regions. This gradual growth continued as the universe expanded. When the universe was one fifth its present size (roughly 1.2 billion years after the Big Bang), our region of space was probably twice as dense as neighboring regions. Cosmologists speculate that the inner portions of our Galaxy (and similar galaxies) were assembled at this time. The stars in the outer regions of our Galaxy were probably assembled in the more recent past.

Some cosmologists suspect that some of the objects recently detected by the Hubble Space Telescope may be galaxies in formation. HST Images of Galaxies in Formation? HST press release describing this image Observing These Small Fluctuations Tiny variations in the density of matter in the early universe leave an imprint in the cosmic microwave background radiation in the form of temperature fluctuations from point to point across the sky. These temperature fluctuations are minute: one part of the sky might have a temperature of 2.7251 Kelvin (degrees above absolute zero), while another part might have a temperature of 2.7249 Kelvin. NASA's Cosmic Background Explorer (COBE) satellite, has detected these tiny fluctuations on large angular scales. WMAP re-measures the fluctuations with both higher angular resolution and sensitivity . The mission summary page offers a quick introduction to how WMAP achieves this sensitivity - more details are available on the technical information page. What Made These Small Fluctuations? While gravity can enhance the tiny fluctuations seen in the early universe, it can not produce these fluctuations. Cosmologists speculate about the new physics needed to produce the primordial fluctuations that formed galaxies.

Two popular ideas are: Inflation Topological Defects These different theories make very different predictions about the properties of the cosmic microwave background fluctuations . For example, the inflationary theory predicts that the largest temperature fluctuations should have an angular scale of one degree, while the defect models predict a smaller characteristic scale. WMAP, with its superb sensitivity, indicates that the inflationary model is more likely. Learn More About Structure Formation at These Sites: The Sloan Digital Sky Survey (SDSS) This group plans to map the positions of over 100 million galaxies and determine the distances to over a million galaxies and quasars. The effort will produce the largest (known) survey to date of cosmic structure in the universe. You can learn more about the details of the SDSS by visiting their home page at Fermilab. The Virgo Consortium The Virgo Consortium is an international grouping of scientists carrying out super computer simulations of the formation of galaxies, galaxy clusters, large-scale structure, and of the evolution of the intergalactic medium. Although most of the consortium members are British, there are important nodes in Canada, the United States, and Germany.

Largest Structure In Universe, Large Quasar Group, Challenges Cosmological Principle | The Huffington Post Largest Structure In Universe, Large Quasar Group, Challenges Cosmological Principle 01/11/2013 09:45 am ET | Updated Jan 12, 2013 6.3k By: Mike Wall Published: 01/11/2013 04:34 AM EST on SPACE.com Astronomers have discovered the largest known structure in the universe, a clump of active galactic cores that stretches 4 billion light-years from end to end. The structure is a large quasar group (LQG), a collection of extremely luminous galactic nuclei powered by supermassive central black holes . This particular group is so large that it challenges modern cosmological theory, researchers said. "While it is difficult to fathom the scale of this LQG, we can say quite definitely it is the largest structure ever seen in the entire universe," lead author Roger Clowes, of the University of Central Lancashire in England, said in a statement.

"This is hugely exciting, not least because it runs counter to our current understanding of the scale of the universe ." Quasars are the brightest objects in the universe. For decades, astronomers have known that they tend to assemble in huge groups, some of which are more than 600 million light-years wide. But the record-breaking quasar group, which Clowes and his team spotted in data gathered by the Sloan Digital Sky Survey, is on another scale altogether. The newfound LQC is composed of 73 quasars and spans about 1.6 billion light-years in most directions, though it is 4 billion light-years across at its widest point. To put that mind-boggling size into perspective, the disk of the Milky Way galaxy — home of Earth's solar system — is about 100,000 light-years wide. And the Milky Way is separated from its nearest galactic neighbor, Andromeda, by about 2.5 million light-years. The newly discovered LQC is so enormous, in fact, that theory predicts it shouldn't exist, researchers said.

The quasar group appears to violate a widely accepted assumption known as the cosmological principle, which holds that the universe is essentially homogeneous when viewed at a sufficiently large scale. Calculations suggest that structures larger than about 1.2 billion light-years should not exist, researchers said. "Our team has been looking at similar cases which add further weight to this challenge, and we will be continuing to investigate these fascinating phenomena," Clowes said. The new study was published today (Jan. 11) in the Monthly Notices of the Royal Astronomical Society. Follow SPACE.com senior writer Mike Wall on Twitter @michaeldwall or SPACE.com @Spacedotcom . We're also on Facebook and Google+ . ‘Supervoid’ is 1.8bn light-years across, the largest single structure in universe - study — RT America Tags Space The massive spherical blob known as the supervoid may be “the largest individual structure ever identified by humanity,” according to lead researcher for the project, István Szapudi, of the Institute for Astronomy at the University of Hawaiʻi at Mānoa.

“Using data from Hawaii’s Pan-STARRS1 (PS1) telescope located on Haleakala, Maui, and NASA’s Wide Field Survey Explorer (WISE) satellite, Szapudi’s team discovered a large supervoid, a vast region 1.8 billion light-years across, in which the density of galaxies is much lower than usual in the known universe,” a press release stated . “This void was found by combining observations taken by PS1 at optical wavelengths with observations taken by WISE at infrared wavelengths to estimate the distance to and position of each galaxy in that part of the sky.” Furthermore, the Cold Spot, discovered in 2004 and believed to be radiation left by the Big Bang, has become a mainstay in models explaining the impact of the Big Bang on the universe. The Cold Spot's size and temperature, though, were an anomaly. The massive empty region at its center, might explain the Cold Spot’s existence, the new study suggests. “This is the greatest supervoid ever discovered,” András Kovács, a co-author of the report from Eötvös Loránd University in Hungary said, according to the Guardian. “In combination of size and emptiness, our supervoid is still a very rare event.

We can only expect a few supervoids this big in the observable universe.” Kovács added that the supervoid is not a vacuum, but rather it is 20 percent less dense than other aspects of the universe. “Supervoids are not entirely empty, they’re under-dense,” he said. Despite such a rare discovery, the supervoid ultimately complicates the explanation of the Cold Spot, astronomers say. In addition, the supervoid only accounts for around 10 percent of the Cold Spot's low temperature. “The void itself I’m not so unhappy about. It’s like the Everest of voids – there has to be one that’s bigger than the rest,” said Carlos Frenk, a cosmologist at Britain’s University of Durham, according to the Guardian. “But it doesn’t explain the whole Cold Spot, which we’re still in the dark about.” The study was published online on April 20 in Monthly Notices of the Royal Astronomical Society.

“While the existence of the supervoid and its expected effect on the CMB [the cosmic microwave background, or Big Bang] do not fully explain the Cold Spot, it is very unlikely that the supervoid and the Cold Spot at the same location are a coincidence,” the press release stated. “The team will continue its work using improved data from PS1 and from the Dark Energy Survey being conducted with a telescope in Chile to study the Cold Spot and supervoid, as well as another large void located near the constellation Draco.”

Question: What is the name for the study of the structure of the universe?
Answer: Cosmology