The Expanding Universe Modern Cosmology The Expanding Universe For thousands of years, astronomers wrestled with basic questions about the size and age of the universe. Does the universe go on forever, or does it have an edge somewhere? Has it always existed, or did it come to being some time in the past? In 1929, Edwin Hubble, an astronomer at Caltech, made a critical discovery that soon led to scientific answers for these questions: he discovered that the universe is expanding. The ancient Greeks recognized that it was difficult to imagine what an infinite universe might look like. But they also wondered that if the universe were finite, and you stuck out your hand at the edge, where would your hand go? The Greeks' two problems with the universe represented a paradox - the universe had to be either finite or infinite, and both alternatives presented problems. After the rise of modern astronomy, another paradox began to puzzle astronomers. In the early 1800s, German astronomer Heinrich Olbers argued that the universe must be finite. If the Universe were infinite and contained stars throughout, Olbers said, then if you looked in any particular direction, your line-of-sight would eventually fall on the surface of a star.

Although the apparent size of a star in the sky becomes smaller as the distance to the star increases, the brightness of this smaller surface remains a constant. Therefore, if the Universe were infinite, the whole surface of the night sky should be as bright as a star. Obviously, there are dark areas in the sky, so the universe must be finite. But, when Isaac Newton discovered the law of gravity, he realized that gravity is always attractive. Every object in the universe attracts every other object. If the universe truly were finite, the attractive forces of all the objects in the universe should have caused the entire universe to collapse on itself. This clearly had not happened, and so astronomers were presented with a paradox. When Einstein developed his theory of gravity in the General Theory of Relativity, he thought he ran into the same problem that Newton did: his equations said that the universe should be either expanding or collapsing, yet he assumed that the universe was static. His original solution contained a constant term, called the cosmological constant, which cancelled the effects of gravity on very large scales, and led to a static universe.

After Hubble discovered that the universe was expanding, Einstein called the cosmological constant his "greatest blunder." At around the same time, larger telescopes were being built that were able to accurately measure the spectra, or the intensity of light as a function of wavelength, of faint objects. Using these new data, astronomers tried to understand the plethora of faint, nebulous objects they were observing. Between 1912 and 1922, astronomer Vesto Slipher at the Lowell Observatory in Arizona discovered that the spectra of light from many of these objects was systematically shifted to longer wavelengths, or redshifted. A short time later, other astronomers showed that these nebulous objects were distant galaxies. The Discovery of the Expanding Universe Meanwhile, other physicists and mathematicians working on Einstein's theory of gravity discovered the equations had some solutions that described an expanding universe. In these solutions, the light coming from distant objects would be redshifted as it traveled through the expanding universe. The redshift would increase with increasing distance to the object. Edwin Hubble In 1929 Edwin Hubble, working at the Carnegie Observatories in Pasadena, California, measured the redshifts of a number of distant galaxies.

He also measured their relative distances by measuring the apparent brightness of a class of variable stars called Cepheids in each galaxy. When he plotted redshift against relative distance, he found that the redshift of distant galaxies increased as a linear function of their distance. The only explanation for this observation is that the universe was expanding. Once scientists understood that the universe was expanding, they immediately realized that it would have been smaller in the past. At some point in the past, the entire universe would have been a single point. This point, later called the big bang, was the beginning of the universe as we understand it today. The expanding universe is finite in both time and space. The reason that the universe did not collapse, as Newton's and Einstein's equations said it might, is that it had been expanding from the moment of its creation. The universe is in a constant state of change. The expanding universe, a new idea based on modern physics, laid to rest the paradoxes that troubled astronomers from ancient times until the early 20th Century. Properties of the Expanding Universe The equations of the expanding universe have three possible solutions, each of which predicts a different eventual fate for the universe as a whole.

Which fate will ultimately befall the universe can be determined by measuring how fast the universe expands relative to how much matter the universe contains. The three possible types of expanding universes are called open, flat, and closed universes. If the universe were open, it would expand forever. If the universe were flat, it would also expand forever, but the expansion rate would slow to zero after an infinite amount of time. If the universe were closed, it would eventually stop expanding and recollapse on itself, possibly leading to another big bang. In all three cases, the expansion slows, and the force that causes the slowing is gravity. A simple analogy to understand these three types of universes is to consider a spaceship launched from the surface of the Earth. If the spaceship does not have enough speed to escape the Earth's gravity, it will eventually fall back to Earth. This is analogous with a closed universe that recollapses. If the spaceship is given enough speed so that it has just enough energy to escape, then at an infinite distance away from the Earth, it will come to a stop (this is the flat universe).

And lastly, if the ship is launched with more than enough energy to escape, it will always have some speed, even when it is an infinite distance away (the open universe). The Fate of the Universe For the last eighty years, astronomers have been making increasingly accurate measurements of two important cosmological parameters: Ho - the rate at which the universe expands - and w - the average density of matter in the universe. Knowledge of both of these parameters will tell which of the three models describes the universe we live in, and thus the ultimate fate of our universe. The Sloan Digital Sky Survey, with its large systematic measurement of the galaxy density in the Universe, should enable astronomers to precisely measure the density parameter w. The Heavy Elements Astronomers are not only interested in the fate of the universe; they are also interested in understanding its present physical state. One question they try to answer is why the universe is primarily composed of hydrogen and helium, and what is responsible for the relatively small concentration of the heavier elements. With the rise of nuclear physics in the 1930s and 40s, scientists started to try to explain the abundances of heavier elements by assuming they were synthesized out of primordial hydrogen in the early universe.

In the late 1940s, American physicists George Gamow, Robert Herman, and Ralph Alpher realized that long ago, the universe was much hotter and denser. They made calculations to show whether nuclear reactions that took place at those higher temperatures could have created the heavy elements. Unfortunately, with the exception of helium, they found that it was impossible to form heavier elements in any appreciable quantity. Today, we understand that heavy elements were synthesized either in the cores of stars or during supernovae, when a large dying star implodes. Gamow, Herman, and Alpher did realize, though, that if the universe were hotter and denser in the past, radiation should still be left over from the early universe. This radiation would have a well-defined spectrum (called a blackbody spectrum) that depends on its temperature. As the universe expanded, the spectrum of this light would have been redshifted to longer wavelengths, and the temperature associated with the spectrum would have decreased by a factor of over one thousand as the universe cooled. The Cosmic Microwave Background Radiation In 1963, Arno Penzias and Robert Wilson, two scientists in Holmdale, New Jersey, were working on a satellite designed to measure microwaves.

When they tested the satellite's antenna, they found mysterious microwaves coming equally from all directions. At first, they thought something was wrong with the antenna. But after checking and rechecking, they realized that they had discovered something real. What they discovered was the radiation predicted years earlier by Gamow, Herman, and Alpher. The radiation that Penzias and Wilson discovered, called the Cosmic Microwave Background Radiation, convinced most astronomers that the Big Bang theory was correct. For discovering the Cosmic Microwave Background Radiation, Penzias and Wilson were awarded the 1978 Nobel Prize in Physics. After Penzias and Wilson found the Cosmic Microwave Background Radiation, astrophysicists began to study whether they could use its properties to study what the universe was like long ago. According to Big Bang theory, the radiation contained information on how matter was distributed over ten billion years ago, when the universe was only 500,000 years old. At that time, stars and galaxies had not yet formed. The Universe consisted of a hot soup of electrons and atomic nuclei. These particles constantly collided with the photons that made up the background radiation, which then had a temperature of over 3000 C.

Soon after, the Universe expanded enough, and thus the background radiation cooled enough, so that the electrons could combine with the nuclei to form atoms. Because atoms were electrically neutral, the photons of the background radiation no longer collided with them. When the first atoms formed, the universe had slight variations in density, which grew into the density variations we see today - galaxies and clusters. These density variations should have led to slight variations in the temperature of the background radiation, and these variations should still be detectable today. Scientists realized that they had an exciting possibility: by measuring the temperature variations of the Cosmic Microwave Background Radiation over different regions of the sky, they would have a direct measurement of the density variations in the early universe, over 10 billion years ago. Density Variations in the Early Universe Map of the sky as observed by COBE. The map on the bottom shows the temperature variations of the background radiation. In 1990, a satellite called the Cosmic Microwave Background Explorer (COBE) measured background radiation temperatures over the whole sky. COBE found variations that amounted to only about 5 parts in 100,000, but revealed the density fluctuations in the early universe.

The initial density variations would be the seeds of structure that would grow over time to become the galaxies, clusters of galaxies, and superclusters of galaxies observed today by the Sloan Digital Sky Survey. With the Sloan data, along with data from COBE, astronomers will be able to reconstruct the evolution of structure in the universe over the last 10 to 15 billion years. With this information, we will have a deep understanding of the history of the universe, which will be an almost unbelievable scientific and intellectual achievement. But measuring the evolution of the density variations in the universe still does not answer the most important question: why does the universe contain these differences in density in the first place? To answer this question, astronomers and astrophysicists must understand the nature of the density variations and construct theories of the origin of the universe that predict how these variations should occur. Famous Astronomers | List of Great Scientists in Astronomy Famous Astronomers | List of Great Scientists in Astronomy By Nola Taylor Redd, Space.com Contributor | June 11, 2012 10:07am ET MORE Throughout human history, scientists have struggled to understand what they see in the universe.

Famous astronomers — many of them great scientists who mastered many fields — explained the heavens with varying degrees of accuracy. Over the centuries, a geocentric view of the universe — with Earth at the center of everything — gave way to the proper understanding we have today of an expanding universe in which our galaxy is but one of billions. On this list are some of the most famous scientists from the early days of astronomy through the modern era, and a summary of some of their achievements. Any list of famous astronomers has to include a varied collection of great scientists from the Greeks to the modern era, big thinkers who tackled many fields as well as modern astronomers who made significant discoveries and helped popularize astronomy. Credit: Karl Tate, SPACE.com The Ptolemaic geocentric model of the universe, devised by the Greek scientist Claudius Ptolemy, had everything revolving around Earth. Credit: Bartolomeu Velho, Public Domain When most people believed the world was flat, the notable Greek mathematician, astronomer and geographer Eratosthenes (276 BCE- 195 BCE) used the sun to measure the size of the round Earth.

His measurement of 24,660 miles (39,690 kilometers) was only 211 miles (340 km) off the true measurement. The ancient Greek astronomer and mathematician Claudius Ptolemy (AD 90- 168) set up a model of the solar system in which the sun, stars, and other planets revolved around Earth. Known as the Ptolemaic system, it remained in place for hundreds of years, though it turned out to be flat wrong. Persian astronomer Abd al-Rahman al-Sufi (903-986), known as Azophi to Westerners, made the first known observation of a group of stars outside of the Milky Way, the Andromeda galaxy. Nicolaus Copernicus Credit: Public Domain In 16th century Poland, astronomer Nicolaus Copernicus (1473-1543) proposed a model of the solar system that involved the Earth revolving around the sun. The model wasn't completely correct, as astronomers of the time struggled with the backwards path Mars sometimes took, but it eventually changed the way many scientists viewed the solar system.

Using detailed measurements of the path of planets kept by Danish astronomer Tycho Brahe , Johannes Kepler (1571-1630) determined that planets traveled around the sun not in circles but in ellipses. In so doing, he calculated three laws involving the motions of planets that astronomers still use in calculations today. Galileo Galilei is credited with discovering the first four moons of Jupiter. Credit: NASA Born in Italy, Galileo Galilei (1564-1642) is often credited with the creation of the optical telescope, though in truth he improved on existing models. The astronomer (also mathematician, physicist and philosopher) turned the new observational tool toward the heavens, where he discovered the four primary moons of Jupiter (now known as the Galilean moons), as well as the rings of Saturn. Though a model of the Earth circling the sun was first proposed by Copernicus, it took some time before it became widely accepted. Galileo is most widely known for defending the idea several years after Kepler had already calculated the path of planets, and Galileo wound up under house arrest at the end of his lifetime because of it.

Dutch scientist Christiaan Huygens (1629-1695) proposed the earliest theory about the nature of light, a phenomena that puzzled scientists for hundreds of years. His improvements on the telescope allowed him to make the first observations of Saturn's rings and to discover its moon, Titan. Building on the work of those who had gone before him, English astronomer Sir Isaac Newton (1643-1727) is most famous for his work on forces, specifically gravity. He calculated three laws describing the motion of forces between objects, known today as Newton's laws. Famed physicist Albert Einstein at the blackboard. Credit: NASA In the early 20th century, German physicist Albert Einstein (1879-1955) became of of the most famous scientists ever after proposing a new way of looking at the universe that went beyond current understanding. Einstein suggested that the laws of physics are the same throughout the universe, that the speed of light in a vacuum is constant, and that space and time are linked in an entity known as space-time, which is distorted by gravity.

At the same time Einstein was expanding man's view of the universe, American astronomer Edwin Hubble (1899-1953) calculated that a small blob in the sky existed outside of the Milky Way. Prior to his observations, the discussion over the size of the universe was divided as to whether or not only a single galaxy existed. Hubble went on to determine that the universe itself was expanding, a calculation which later came to be known as Hubble's law. Hubble's observations of the various galaxies allowed him to create a standard system of classification still used today. American astronomer Carl Sagan (1934-1996) may not have been a great scientists in comparison to some on this list, but he is one of the most famous astronomers. Sagan not only made important scientific studies in the fields of planetary science, he also managed to popularized astronomy more than any other individual. His charismatic teaching and boundless energy impacted people around the world as he broke down complicated subjects in a way that interested television viewers even as he educated them. Professor Stephen Hawking speaks about "Why We Should Go into Space" for the NASA Lecture Series, April 21, 2008.

Credit: NASA/Paul Alers Stephen Hawking (born 1942) has made many significant insights into the field of cosmology. He proposed that, as the universe has a beginning, it will likely also end. He also suggested that it has no boundary or border. Despite being viewed as one of the most brilliant minds since Einstein, many of Hawking's books and lectures are steered toward the general public as he seeks to educate people about the universe they live in. Other astronomers that achieved significant discoveries and are often mentioned among the greats: Italian astronomer Giovanni Cassini (1625-1712) measured how long it took the planets Jupiter and Mars to rotate, as well as discovering four moons of Saturn and the gap in the planet's rings. When NASA launched a satellite to orbit Saturn and its moons in 1997, it was fittingly dubbed Cassini.

Edmond Halley (1656-1742) was the British scientist who reviewed historical comet sightings and proposed that the comet which appeared in 1456, 1531, 1607, and 1682 were all the same, and would return in 1758. Although he died before its return, he was proven correct, and the comet was named in his honor. French astronomer Charles Messier (1730-1817) composed a database of objects known at the time as "nebulae," which included 103 objects at its final publication, though additional objects were added based on his personal notes. Many of these objects are often listed with their catalog name, such as the Andromeda Galaxy, known as M31. Messier also discovered 13 comets over the course of his lifetime. British astronomer William Herschel (1738-1822) cataloged over 2,500 deep sky objects. He also discovered Uranus and its two brightest moons, two of Saturn's moons, and the Martian ice caps.

William trained his sister, Caroline Herschel (1750-1848), in astronomy, and she became the first woman to discover a comet, identifying several over the course of her lifetime. Henrietta Swann Leavitt (1868-1921) was one of several women working as a human "computer" at Harvard college, identifying images of variable stars on photographic plates. She discovered that the brightness of a special flashing star known as a Cepheid variable was related to how often it pulsed. This relationship allowed astronomers to calculate the distances of stars and galaxies, the size of the Milky Way, and the expansion of the universe. [This entry was corrected June 12, 2012.] American astronomer Harlow Shapley (1885-1972) calculated the size of the Milky Way galaxy and general location of its center. He argued that the objects known as "nebula" lay within the galaxy, rather than outside of it, and in 1920 participated in the "Great Debate". He also incorrectly disagreed with Edwin Hubble's observations that the universe boasted galaxies other than the Milky Way.

Frank Drake with a young observer at the Lick Observatory. Credit: SETI Institute Frank Drake (born 1930) is one of the pioneers in the search for extraterrestrial intelligence. He was one of the founders of the Search for Extraterrestrial Intelligence ( SETI ) and the deviser of the Drake equation, a mathematical equation used to estimate the number of extraterrestrial civilizations in the Milky Way galaxy able to be detected. American astronomer William K. Hartmann (born 1939) put forth the most widely accepted theory on the formation of the moon in 1975. He proposed that, after a collision with a large body scooped, debris from the Earth coalesced into the moon. —Nola Taylor Redd Edwin Hubble & the Expanding Universe Edwin Hubble & the Expanding Universe Emilio Segre Visual Archives Edwin Hubble at the camera on the Hooker 100 inch telescope on Mt Wilson. Trained initially as a lawyer, Edwin Hubble moved to astronomy in 1914, working at the famous Yerkes Observatory near Chicago. He went on to have a profound influence on our understanding of the Universe.

Using the largest telescope of his time, the 100 inch Hooker telescope on Mount Wilson in California, he helped resolve one of the great debates in early twentieth century astronomy. Island Universes One of the unsolved problems in early twentieth century astronomy was the question of what were nebulae? These gaseous, fuzzy clouds were thought by some astronomers to be embryonic solar systems forming while others thought that they were "island universe" like our own Milky Way galaxy. The advent of spectroscopy and photography in that late nineteenth century when used on the latest generation of large reflecting telescopes in the early twentieth century provided astronomers with the tools to study these objects. Hubble used the 100 inch Hooker Telescope on Mount Wilson, then the largest telescope in the world, to study the Andromeda Nebula, M 31. In 1923 he identified some of the stars within it as Cepheid variables. These are periodic variable stars, that is stars that vary their brightness in a regular way.

Earlier, in 1908 Henrietta Swan Leavitt, working at the Harvard College Observatory discovered that Cepheid variables in fact obey a period-luminosity relationship; the longer the period of a Cepheid, the more intrinsically luminous it is. This allowed Hubble to observe the Cepheids over time and measure their varying brightnesses to determine their periods. He could then apply the period-luminosity relationship to calculate the distance to the stars and hence the distance to the Andromeda Nebula that they were in. By 1924 Hubble had calculated that the distance to the Andromeda Nebula was 900,000 light years. Credit: NASA APOD The Andromeda galaxy, M31. Supporting evidence came from the work of Vesto Slipher at the Lowell Observatory in Arizona. He had started measuring the Doppler shift of spectral lines from spiral nebulae in 1912. By 1925 he had shown that most exhibited redshifts in their spectral lines. He interpreted this as meaning that they were in fact moving away relative to us so their spectrum was shifted to longer (ie redder) wavelengths).

Whilst he calculated that the Andromeda spiral was moving towards us at 300 km.s-1 he soon found others moving away at 1,100 km.s-1. These speeds exceeded that of any known star in the Milky Way. Taken together the evidence eventually convinced astronomers that "spiral "nebulae" such as Andromeda were in fact separate "island universes" of billions of stars like our own Milky Way but more distant. We now call these galaxies instead of island universes. The Expanding Universe & Hubble's law Hubble, aided by Milton Humason extended the work of Slipher by using the larger Hooker telescope. He took long exposures of the spectra of faint galaxies. By measuring the amount of shift of specific spectral lines relative to those produced by reference arc lamps in the spectrograph he was able to calculate values for the galaxy velocities. A few nearby galaxies had velocities that meant they were moving towards our own Milky Way, that is their lines were blueshifted but most exhibited redshift and hence had recession velocities. The majority of galaxies therefore appeared to be moving away from our own galaxy. Hubble found that that those with a smaller image in a photograph had higher redshifts.

This is shown in the diagram below that shows the images and spectra of some of the galaxies he observed. Hubble's observations of galaxies with the redshift in their spectral lines. He inferred that galaxies were similar to each other in size so those that appeared smaller must be further away. By plotting the velocity of the galaxies against their distance he came across an interesting relationship. This is now known as Hubble's law and is shown in the following plot. Hubble's distance-velocity relationship for galaxies based on his original data. This is now known as Hubble's Law and is interpreted as evidence for an expanding Universe. If you study the above plot you will see that the more distant a galaxy is, on average, the faster it is receding from us. In fact Hubble realised he could fit a linear relationship to his data, as shown by the pale blue line of best fit. The slope of this line is a constant and is now known as the Hubble constant, H0. This relationship is expressed mathematically as: v ∝ d so: v = H0d (2.2) where H0 is Hubble's constant, v is the recession velocity and d is the distance.

Hubble's velocity-distance relationship, published in 1929, suggests that once we look beyond the gravitational effects of close galaxies within the local group, galaxies are moving away from one another. Not only are they moving away but the more distant galaxies appear to be moving away faster than closer ones. This then suggests that the Universe is expanding and indeed this is now the most widely-accepted interpretation of the data. The other key point arising from the relation is that if we go back in time galaxies must have been closer together, space was smaller. If you extrapolate back far enough the Universe must have been concentrated at a point in space. If we assume that H0 provides us with a value for the current rate of expansion then its inverse, that is 1/H0 tells us the Hubble time which is a measure of the age of a universe expanding at a constant rate. Hubble calculated a value for H0 of about 500 km. s-1. Mpc-1. (1 Mpc-1 is 1 megaparsec or about 3.26 million light years. Astronomers use the parsec as the unit of distance measure rather than the light year.

Details about the parsec can be found in the Year 12 Astrophysics topic). This value results in an age of the Universe of 2 × 109 years, that is 2 billion years. Even in Hubble's day this age proved problematic as it clashed with radiometric dating values for the age of the Earth that ranged from 3 to 5 billion years and other evidence on the age of stars. Obviously this posed a dilemma - the Universe could not be younger than the stars or planets it contained! The problem was eventually resolved in the 1950s when the recalibration of the Cepheid period-luminosity relationship provided an age for the Universe in the range of 10-20 billion years. Even today astronomers spend a lot of time trying to determine a more precise and accurate value for H0 and thus also an age for the Universe. The age actually also depends on what model is assumed for the geometry of the Universe ; flat, open or closed. Recent projects involve a range of methods and do not just rely on observations of Cepheids to calibrate their data.

Estimates based on observations of the cosmic microwave background radiation (CMBR) by the WMAP probe and other CMBR experiments currently suggest an age of 13.7 billion years ± 1% based on H0 = 71 km. s-1. Mpc-1 +4/-3. This agrees closely with the value determined by the Hubble Key Project team that used the Hubble Space Telescope to observe Cepheids in galaxies and calibrated their values with other techniques. They obtained a value of H0 = 70 km. s-1. Mpc-1 &plusmn 10% George Gamow (1904 - 1968) AIP Emilio Segre Visual Archives George Gamow George Gamow, a Ukrainian-born, US-based physicist and former student of Friedmann's, made his mark early by applying quantum theory to explain how alpha particles can be ejected from nuclei in alpha decay. Moving from the USSR in 1931 he settled in the US and continued his work on stellar evolution and beta decay. He was particularly interested in trying to solve the problem about the origin of the elements.

Hans Bethe had already shown in the 1930s how helium could be synthesised inside stars through fusion of hydrogen nuclei. He had also explained how protons and neutrons added to carbon nuclei could form heavier elements. Gamow had realised from Hubble's work that the early Universe must have been much smaller, hotter and denser than it is now. In the late 1940s with his students Ralph Alpher and Robert Herman he calculated that helium could form from the fusion of protons (that is, hydrogen nuclei) and neutrons. This nucleosynthesis would cease once the available neutrons were used up and the Universe had expanded and cooled sufficiently. They also realised that the Universe should be filled with background microwave radiation, the remnant of the original big bang now cooled to about 50 kelvin. This radiation would have the spectral characteristics of a blackbody. Gamow's theory of the nucleosynthesis of primordial helium accounted for the observed abundance of helium compared with hydrogen in the Universe whereas stellar nucleosynthesis could not. His prediction of remnant radiation was neglected by others until the 1960s but was to provide the key evidence in support of the big bang model for the Universe.

What does it mean when they say the universe is expanding?(Everyday Mysteries: Fun Science Facts from the Library of Congress) What does it mean when they say the universe is expanding? When scientists talk about the expanding universe, they mean that it has been growing ever since its beginning with the Big Bang. The galaxies outside of our own are moving away from us, and the ones that are farthest away are moving the fastest. This means that no matter what galaxy you happen to be in, all the other galaxies are moving away from you. However, the galaxies are not moving through space, they are moving in space, because space is also moving. In other words, the universe has no center; everything is moving away from everything else. If you imagine a grid of space with a galaxy every million light years or so, after enough time passes this grid will stretch out so that the galaxies are spread to every two million light years, and so on, possibly into infinity. The universe encompasses everything in existence, from the smallest atom to the largest galaxy; since forming some 13.7 billion years ago in the Big Bang, it has been expanding and may be infinite in its scope.

The part of the universe of which we have knowledge is called the observable universe, the region around Earth from which light has had time to reach us. One famous analogy to explain the expanding universe is imagining the universe like a loaf of raisin bread dough. As the bread rises and expands, the raisins move farther away from each other, but they are still stuck in the dough. In the case of the universe, there may be raisins out there that we can’t see any more because they have moved away so fast that their light has never reached Earth. Fortunately, gravity is in control of things at the local level and keeps our raisins together. Who Figured This Out? The American astronomer Edwin Hubble made the observations in 1925 and was the first to prove that the universe is expanding. He proved that there is a direct relationship between the speeds of distant galaxies and their distances from Earth. This is now known as Hubble’s Law. The Hubble Space Telescope was named after him, and the single number that describes the rate of the cosmic expansion, relating the apparent recession velocities of external galaxies to their distance, is called the Hubble Constant. So, is the Universe Infinite?

It might be easier to explain about the beginning of the universe and the Big Bang Theory, than to talk about how it will end. It is possible that the universe will last forever, or it may be crushed out of existence in a reverse of the Big Bang scenario, but that would be so far in the future that it might as well be infinite. Until recently, cosmologists (the scientists who study the universe) assumed that the rate of the universe’s expansion was slowing because of the effects of gravity. However, current research indicates that the universe may expand to eternity. But research continues and new studies of supernovae in remote galaxies and a force called dark energy may modify the possible fates of the universe. Related Web Sites Cosmological Theories Through History - The Physics of the Universe Cosmological Theories Through History Luke Mastin "Cosmos" is just another word for universe , and "cosmology" is the study of the origin, evolution and fate of the universe . Some of the best minds in history - both philosophers and scientists - have applied themselves to an understanding of just what the universe is and where it came from, suggesting in the process a bewildering variety of theories and ideas, from the Cosmic Egg to the Big Bang and beyond.

Here are some of the main ones, in approximate chronological order: Brahmanda (Cosmic Egg) Universe - The Hindu Rigveda, written in India around the 15th - 12th Century B.C., describes a cyclical or oscillating universe in which a �cosmic egg�, or Brahmanda, containing the whole universe (including the Sun, Moon, planets and all of space) expands out of a single concentrated point called a Bindu before subsequently collapsing again. The universe cycles infinitely between expansion and total collapse. Anaxagorian Universe - The 5th Century B.C. Greek philosopher Anaxagoras believed that the original state of the cosmos was a primordial mixture of all its ingredients which existed in infinitesimally small fragments of themselves. This mixture was not entirely uniform, and some ingredients were present in higher concentrations than others, as well as varying from place to place. At some point in time, this mixture was set in motion by the action of �nous� (mind), and the whirling motion shifted and separated out the ingredients, ultimately producing the cosmos of separate material objects, all with different properties, that we see today.

Atomist Universe - Later in the 5th Century B.C., the Greek philosophers Leucippus and Democritus founded the school of Atomism, which held that the universe was composed of very small, indivisible and indestructible building blocks known as atoms (from the Greek �atomos�, meaning �uncuttable�). All of reality and all the objects in the universe are composed of different arrangements of these eternal atoms and an infinite void, in which they form different combinations and shapes. themes/sciences/mainpage.htm ) Aristotelian Universe - The Greek philosopher Aristotle, in the 4th Century B.C., established a geocentric universe in which the fixed, spherical Earth is at the centre, surrounded by concentric celestial spheres of planets and stars . Although he believed the universe to be finite in size, he stressed that it exists unchanged and static throughout eternity. Aristotle definitively established the four classical elements of fire, air, earth and water, which were acted on by two forces, gravity (the tendency of earth and water to sink) and levity (the tendency of air and fire to rise). He later added a fifth element , aether, to describe the void that fills the universe above the terrestrial sphere.

Stoic Universe - The Stoic philosophers of ancient Greece (3rd Century B.C. and after) believed in a kind of island universe in which a finite cosmos is surrounded by an infinite void (not dissimilar in principle to a galaxy ). They held that the cosmos is in a constant state of flux, and pulsates in size and periodically passes through upheavals and conflagrations. In the Stoic view, the universe is like a giant living body, with its leading part being the stars and the Sun, but in which all parts are interconnected, so that what happens in one place affects what happens elsewhere. They also held a cyclical view of history, in which the world was once pure fire and would become fire again (an idea borrowed from Heraclitus). Heliocentric Universe - The 3rd Century B.C. Greek astronomer and mathematician Aristarchus of Samos was the first to present an explicit argument for a heliocentric model of the Solar System, placing the Sun, not the Earth, at the center of the known universe . He described the Earth as rotating daily on its axis and revolving annually about the Sun in a circular orbit, along with a sphere of fixed stars .

His ideas were generally rejected in favour of the geocentric theories of Aristotle and Ptolemy until they were successfully revived nearly 1800 years later by Copernicus. However, there were exceptions: Seleucus of Seleucia, who lived about a century after Aristarchus, supported his theories and used the tides to explain heliocentricity and the influence of the Moon; the Indian astonomer and mathematician Aryabhata described elliptical orbits around the Sun at the end of of the 5th Century A.D.; as did the Muslim astronomer Ja'far ibn Muhammad Abu Ma'shar al-Balkhi in the 9th Century. Ptolemaic Universe - The 2nd Century A.D. Roman-Egyptian mathematician and astronomer Ptolemy (Claudius Ptolemaeus) described a geocentric model largely based on Aristotelian ideas, in which the planets and the rest of the universe orbit about a stationary Earth in circular epicycles. In terms of longevity, it was perhaps the most successful cosmological model of all time.

Modifications to the basic Ptolemaic system were suggested by the Islamic Maragha School in the 13th, 14th and 15th Centuries including the first accurate lunar model by Ibn al-Shatir, and the rejection of a stationery Earth in favour of a rotating Earth by Ali Qushji. Abrahamic Universe - Several medieval Christian, Muslim and Jewish scholars put forward the idea of a universe which was finite in time. In the 6th Century A.D., the Christian philospher John Philoponus of Alexandria argued against the ancient Greek notion of an infinite past, and was perhaps the first commentator to argue that the universe is finite in time and therefore had a beginning. Early Muslim theologians such as Al-Kindi (9th Century) and Al-Ghazali (11th Century) offered logical arguments supporting a finite universe , as did the 10th Century Jewish philosopher Saadia Gaon.

Partially Heliocentric Universe - In the 15th and early 16th Century, Somayaji Nilakantha of the Kerala school of astronomy and mathematics in southern India developed a computational system for a partially heliocentric planetary model in which Mercury, Venus, Mars, Jupiter and Saturn orbited the Sun, which in turn orbited the Earth. This was very similar to the Tychonic system proposed by the Danish nobleman Tycho Brahe later in the 16th Century as a kind of hybrid of the Ptolemaic and Copernican models. themes/sciences/mainpage.htm ) Copernican Universe - In 1543, the Polish astronomer and polymath Nicolaus Copernicus adapted the geocentric Maragha model of Ibn al-Shatir to meet the requirements of the ancient heliocentric universe of Aristarchus. His publication of a scientific theory of heliocentrism, demonstrating that the motions of celestial objects can be explained without putting the Earth at rest in the centre of the universe , stimulated further scientific investigations and became a landmark in the history of modern science, sometimes known as the Copernican Revolution.

His Copernican Principle (that the Earth is not in a central, specially favoured position) and its implication that celestial bodies obey physical laws identical to those on Earth, first established cosmology as a science rather than a branch of metaphysics. In 1576, the English astronomer Thomas Digges popularized Copernicus� ideas and also extended them by positing the existence of a multitude of stars extending to infinity, rather than just Copernicus� narrow band of fixed stars . The Italian philosopher Giordano Bruno took the Copernican Principle a stage further in 1584 by suggesting that even the Solar System is not the centre of the universe , but rather a relatively insignificant star system among an infinite multitude of others. In 1605, Johannes Kepler made further refinements by finally abandoning the classical assumption of circular orbits in favour of elliptical orbits which could explain the strange apparent movements of the planets. Galileo's controversial support of Copernicus' heliocentric model in the early 17th Century was denounced by the Inquisition but nevertheless helped to popularize the idea.

Cartesian Vortex Universe - In the mid-17th Century, the French philosopher Ren� Descartes outlined a model of the universe with many of the characteristics of Newton�s later static, infinite universe . But, according to Descartes, the vacuum of space was not empty at all, but was filled with matter that swirled around in large and small vortices. His model involved a system of huge swirling whirlpools of ethereal or fine matter , producing what would later be called gravitational effects. Static (or Newtonian) Universe - In 1687, Sir Isaac Newton published his �Principia�, which described, among other things, a static, steady state, infinite universe which even Einstein , in the early 20th Century, took as a given (at least until events proved otherwise). In Newton�s universe , matter on the large scale is uniformly distributed, and the universe is gravitationally balanced but essentially unstable. Hierarchical Universe and the Nebular Hypothesis - Although still generally based on a Newtonian static universe , the matter in a hierarchical universe is clustered on ever larger scales of hierarchy, and is endlessly being recycled.

It was first proposed in 1734 by the Swedish scientist and philosopher Emanuel Swedenborg, and developed further (independently) by Thomas Wright (1750), Immanuel Kant (1755) and Johann Heinrich Lambert (1761), and a similar model was proposed in 1796 by the Frenchman Pierre-Simon Laplace. Einsteinian Universe - The model of the universe assumed by Albert Einstein in his groundbreaking theory of gravity in the early 20th Century was not dissimilar to Newton�s in that it was a static, dynamically stable universe which was neither expanding or contracting. However, he had to add in a � cosmological constant � to his general relativity equations to counteract the dynamical effects of gravity which would otherwise have caused the universe to collapse in on itself (although he later abandoned that part of his theory when Edwin Hubble definitively showed in 1929 that the universe was not in fact static).

(Source: HETDEX: ) Big Bang Model of the Universe - After Hubble �s demonstration of the continuously expanding universe in 1929 (and especially after the discovery of cosmic microwave background radiation by Arno Penzias and Robert Wilson in 1965), some version of the Big Bang theory has generally been the mainsteam scientific view. The theory describes the universe as originating in an infinitely tiny, infinitely dense point (or singularity ) between 13 and 14 billion years ago, from where it has been expanding ever since. The essential statement of the theory is usually attributed to the Belgian Roman Catholic priest and physicist Georges Lema�tre in 1927 (even before Hubble �s corroborating evidence), although a similar theory had been proposed, although not pursued, 1922 by the Russian Alexander Friedmann in 1922. Friedmann actually developed two models of an expanding universe based on Einstein �s general relativity equations, one with positive curvature or spherical space, and one with negative curvature or hyperbolic space. Oscillating Universe - This was Einstein �s favoured model after he rejected his own original model in the 1930s.

The oscillating universe followed from Alexander Friedmann �s model of an expanding universe based on the general relativity equations for a universe with positive curvature (spherical space), which results in the universe expanding for a time and then contracting due to the pull of its gravity , in a perpetual cycle of Big Bang followed by Big Crunch . Time is thus endless and beginningless, and the beginning-of-time paradox is avoided. Steady State Universe - This non-standard cosmology (i.e. opposed to the standard Big Bang model) has occurred in various versions since the Big Bang theory was generally adopted by the scientific community. A popular variant of the steady state universe was proposed in 1948 by the English astronomer Fred Hoyle and the and Austrians Thomas Gold and Hermann Bondi. It predicted a universe that expanded but did not change its density , with matter being inserted into the universe as it expanded in order to maintain a constant density . Despite its drawbacks, this was quite a popular idea until the discovery of the cosmic microwave background radiation in 1965 which supported the Big Bang model.

(Source: Wikipedia: ) Inflationary (or Inflating) Universe - In 1980, the American physicist Alan Guth proposed a model of the universe based on the Big Bang , but incorporating a short, early period of exponential cosmic inflation in order to solve the horizon and flatness problems of the standard Big Bang model. Another variation of the inflationary universe is the cyclic model developed by Paul Steinhardt and Neil Turok in 2002 using state-of-the-art M-theory, superstring theory and brane cosmology, which involves an inflationary universe expanding and contracting in cycles. Multiverse - The Russian-American physicist Andrei Linde developed the inflationary universe idea further in 1983 with his chaotic inflation theory (or eternal inflation ), which sees our universe as just one of many �bubbles� that grew as part of a multiverse owing to a vacuum that had not decayed to its ground state. The American physicists Hugh Everett III and Bryce DeWitt had initially developed and popularized their �many worlds� formulation of the multiverse in the 1960s and 1970s.

Alternative versions have also been developed where our observable universe is just one tiny organized part of an infinitely big cosmos which is largely in a state of chaos, or where our organized universe is just one temporary episode in an infinite sequence of largely chaotic and unorganized arrangements. StarChild: Cosmology Wow! Recent observations have shown the universe to be about 13.7 billion years old! Like early astronomers from around the world, the ancient Greeks struggled to understand the universe. Thales, often called the father of Greek science and mathematics, asked questions about the universe that were not based on the actions of gods or demons. It is said that Thales provided the bridge between the world of myth and the world of reason. He used the astronomical records of the Babylonians and Egyptians to accurately predict a solar eclipse in the sixth century BC. Thales believed the Earth was flat and floated on water like a log. Aristotle, who lived from 384 to 322 BC, believed the Earth was round. He thought Earth was the center of the universe and that the Sun, Moon, planets, and all the fixed stars revolved around it. Aristotle's ideas were widely accepted by the Greeks of his time.

The exception, a century later, was Aristarchus, one of the earliest believers in a heliocentric or sun-centered universe. In the 100s BC, Hipparchus, the most important Greek astronomer of his time, calculated the comparative brightness of as many as 1,000 different stars. He also calculated the Moon's distance from the Earth. The first astronomer to make truly scientific maps of the heavens, Claudius Ptolemaeus (better known as Ptolemy of Alexandria), came along 300 years later. Like most astronomers before him, he believed the Sun, Moon, and other planets circled the Earth. He thought that each space body moved in a small circle (an epicycle) that was itself orbiting Earth. This explained why planets sometimes appeared to travel backward in the sky. The Earth-centered view of the universe was widely accepted for about 1500 years. It was not seriously challenged until 1543, when the Polish monk Nicolaus Copernicus suggested that the Sun was at the center of the universe. Because the Church taught that the Earth was central, Copernicus' theory was regarded as heresy.

Perhaps this is why he did not want it published until after his death. Copernicus' published theory, On the Revolution of the Celestial Spheres, met with great hostility from the Church. The two events most responsible for eventual acceptance of Copernicus' views were Tycho Brahe's precise observations of the sky and Galileo's use of the telescope . One night in 1572, Danish astronomer Tycho Brahe saw what he thought was a brilliant new star in the constellation Cassiopeia. (We now know he was observing a supernova.) In 1604, a second supernova was observed. These discoveries caused scientists to seriously question Ptolemy's theory that all stars were contained in an outermost sphere of the universe that never changed. In 1609, Italian scientist Galileo Galilei heard about the invention of a spyglass. He made one for himself and turned it on the heavens. One of his first discoveries was of four moons circling the planet Jupiter. Galileo's telescope revealed a miniature version of Copernicus' solar system , with the moons moving around the planet in simple, circular orbits. Galileo's discoveries forever changed the face of astronomy.

The beginnings of modern science can be attributed to Galileo and to the British genius Isaac Newton. Newton was born in the same year that Galileo died. Isaac Newton took known facts and used mathematics to explain them. He developed mathematical laws that explained how objects move on Earth as well as in space. Newton explained the movement of orbiting planets as the result of motion along a straight line combined with the gravitational pull of the Sun. His laws are all based on the idea that nothing is naturally at rest. He reasoned that all heavenly bodies are constantly moving, with no limits on space and time. In 1917, Albert Einstein proposed a description of the universe based on his Theory of General Relativity. Einstein's theory inspired many other scientists, including Willem de Sitter in Holland and Alexandr Friedmann in Russia. In fact, much of today's cosmology is based on Freedman's solutions to the mathematical equations included in Einstein's Theory. Friedmann built on the General Relativity equations to develop models that helped explain the evolution of the universe. A major breakthrough in our understanding of the universe took place in the 1920's thanks to American astronomer Edwin Hubble.

For centuries, astronomers believed that the Milky Way made up the entire universe. Hubble was among the first to show that the fuzzy patches in the sky seen through telescopes were other galaxies , not distant parts of the Milky Way. By looking at the spectra of these galaxies, he concluded they were speeding away from us - that the universe was expanding! Big Bang Georges Lemaitre, a Belgian astrophysicist and Catholic priest, came to be known as the "Father of the Big Bang". Lemaitre proposed that the universe began as a single primordial atom of energy , something hot and dense that exploded, causing space to expand outward. In the late 1940s, George Gamow, a Russian-American physicist , conceived of the Big Bang theory as we know it today. He and his colleagues proposed that if a big bang had occurred, it would have left an afterglow, traces of background radiation that would still be present. In 1965, physicists Arno Penzias and Robert Wilson started to search for faint radio signals (actually microwaves ) from the outskirts of the Milky Way Galaxy. While conducting their investigations, they actually found the afterglow predicted by Gamow.

It was important evidence that the universe began with a hot big bang. More recently, NASA's COBE satellite measured this radiation in great detail. All of the measurements were consistent with the Big Bang theory. In 1979, particle physicist Alan Guth performed calculations that led to the idea of "cosmic inflation", a brief period of rapid expansion in the early universe. Inflation solves many problems with the simple, original Big Bang. It explains why the universe is so big and so smooth, why at least four different forces act in it today, and where the large amounts of matter that make up the universe came from. Steady State The idea that the universe had a specific beginning did not appeal to all scientists. The Steady State Theory, developed in 1948, concluded the universe had no beginning and no end. This theory describes an expanding universe that stays in perfect balance like a pool kept full to overflowing by a trickle from a faucet. The "faucet" of the universe would be the continuous creation of matter from energy. Arguments against the Steady State Theory include the discovery of background cosmic heat radiation.

The fact that the amount of helium observed in the universe exactly fits what was predicted by the Big Bang and the discovery that galaxies were more crowded together in the past further discredit the Steady State theory. Today's Views Today cosmologists are concerned with the ultimate fate of the universe. Will it expand forever, expand to a certain size and stop, or will it stop and begin to collapse? Data suggesting that the universe is expanding at an accelerating rate were published in 1998. For more than ten years astronomers studied the expansion of the universe by measuring the redshift and brightness of distant supernovae. By 1998, enough information had been gathered to lead scientists to the startling discovery that the expansion of the universe is not slowing but accelerating. The supernova data combined with information from other cosmological studies strongly suggest that the universe is filled with an unidentified form of energy (currently being called "dark energy" since we know nothing about it) that is causing the expansion of the universe to accelerate. If these observations and analyses turn out to be correct, the universe would be expected to continue to expand forever. A Question How do the Big Bang and Steady State theories differ in their explanations of how the universe was created?

Astronomy - The Origins of the Universe The Origins of the Universe By observing galaxies formed billions of years ago, astronomers have been able to paint an increasingly detailed picture of how the universe evolved. According to the widely accepted Big Bang theory, our universe was born in an explosive moment approximately fifteen billion years ago. All of the universe's matter and energy-even the fabric of space itself-was compressed into an infinitesimally small volume and then began expanding at an incredible rate. Within minutes, the universe had grown to the size of the solar system and cooled enough so that equal numbers of protons, neutrons, and the simplest atomic nuclei had formed. After several hundred thousand years of expansion and cooling, neutral atoms-atoms with equal numbers of protons and electrons-were able to form and separate out as distinct entities. Still later, immense gas clouds coalesced to form primitive galaxies, and, from them, stars. Our own solar system formed relatively recently-about five billion years ago-when the universe was two-thirds its present size. In April 2000, an international team of cosmologists supported in part by NSF released the first detailed images of the universe in its infancy.

The images reveal the structure that existed in the universe when it was a tiny fraction of its current age and one thousand times smaller and hotter than today. The project, dubbed BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) captured the images using an extremely sensitive telescope suspended from a balloon that circumnavigated the Antarctic in late 1998. The BOOMERANG images were the first to bring into sharp focus the faint glow of microwave radiation, called the cosmic microwave background, that filled the embryonic universe soon after the Big Bang. Analysis of the images already has shed light on the nature of matter and energy, and indicates that space is "flat." The roots of the Big Bang theory reach back to 1929, the year Edwin Hubble and his assistant Milton Humason discovered that the universe is expanding. Between 1912 and 1928, astronomer Vesto Slipher used a technique called photographic spectroscopy-the measurement of light spread out into bands by using prisms or diffraction gratings-to examine a number of diffuse, fuzzy patches.

Eventually, Hubble used these measurements, referred to as spectra, to show that the patches were actually separate galaxies. Slipher, who did his work at Lowell Observatory in Flagstaff, Arizona, found that in the vast majority of his measurements the spectral lines appeared at longer, or redder, wavelengths. From this he inferred that the galaxies exhibiting such "red shifts" were moving away from Earth, a conclusion he based on the Doppler effect. This effect, discovered by Austrian mathematician and physicist Christian Doppler in 1842, arises from the relative motion between a source and an observer. This relative motion affects wavelengths and frequencies. Shifts in frequency are what make ambulance sirens and train whistles sound higher-pitched as they approach and lower-pitched as they move away. Hubble took these findings and eventually determined the distances to many of Slipher's galaxies. What he found was amazing: The galaxies were definitely moving away from Earth, but, the more distant the galaxy, the faster it retreated. Furthermore, Hubble and Humason discovered that the ratio of a galaxy's speed (as inferred from the amount of red shift) to its distance seemed to be about the same for all of the galaxies they observed.

Because velocity appeared proportional to distance, Hubble reasoned, all that remained was to calculate that ratiothe ratio now referred to as the Hubble Constant. And what is the value of the Hubble Constant? After 70 years of increasingly precise measurements of extragalactic velocities and distances, astronomers are at last closing in on this elusive number. Wendy Freedman is one of the scientists working to define the Hubble Constant. As head of an international team at the Carnegie Observatories in Pasadena, California, Freedman surveys the heavens using the Hubble Space Telescope to measure distances to other galaxies. With grants from NSF, she is building on the legacy of Henrietta Leavitt, who discovered in the early 1900s that the absolute brightness of Cepheid variable stars is related to the time it takes the stars to pulsate (its period). Scientists can measure the period of a Cepheid in a distant galaxy and measure its apparent brightness. Since they know the period, they know what the absolute brightness should be. The distance from Earth to the Cepheid variable star is inferred from the difference between absolute and apparent brightness. Freedman and her colleagues are using this method to determine distances to other galaxies.

With these Cepheid distances, Freedman's group calibrates other distance-determination methods to reach even more far-flung galaxies. This information, in turn, enables them to estimate the Hubble Constant. Researchers closing in on a definitive value for the Hubble Constant are doing so in the midst of other exciting developments within astronomy. In 1998, two independent teams of astronomers, both with NSF support, concluded that the expansion of the universe is accelerating. Their unexpected findings electrified the scientific community with the suggestion that some unknown force was driving the universe to expand at an ever increasing rate. Earlier evidence has supported the possibility that the gravitational attraction among galaxies would eventually slow the universe's growth. In its annual survey of the news, Science magazine named the accelerating universe as the science discovery of the year in 1998. Jeremy Mould, director of Mount Stromlo and Siding Spring Observatories in Canberra, Australia, has studied another aspect of the expansion of the universe. Scientists generally assume that everything in the universe is moving uniformly away from everything else at a rate given by the Hubble constant. Mould is interested in departures from this uniform Hubble flow. These motions are known as peculiar velocities of galaxies.

Starting in 1992, Mould and his colleague John Huchra of the Harvard Smithsonian Center for Astrophysics used an NSF grant to study peculiar velocities of galaxies by creating a model of the universe and its velocity that had, among other things, galaxy clusters. These galaxies in clusters were accelerated by the gravitational field of all the galaxies in the locality. All other things being equal, a high-density universe produces large changes in velocity. This means that measurements of peculiar velocities of galaxies can be used to map the distribution of matter in the universe. Mould and Huchra's model has seeded major efforts to collect measurements of the actual density of the universe so as to map its mass distribution directly. In the modular universewhere stars are organized into galaxies, galaxies into clusters, clusters into superclustersstudies of galaxies, such as those conducted by Mould, give us clues to the organization of larger structures. To appreciate Mould's contribution to our understanding of these organizing principles, consider that a rich galaxy cluster can contain thousands of galaxies, and each galaxy can contain tens of billions to hundreds of billions of stars. Astronomers now estimate that there are tens of billions of galaxies in the observable universe.

Large, diffuse groupings of galaxies emerging from the empty grandeur of the universe show us how the universe is put together-and perhaps even how it all came to be. Only one of those extragalactic islands of starsthe Andromeda Galaxyis faintly visible to the naked eye from the northern hemisphere, while two small satellite galaxies of the Milky Waythe Large and Small Magellanic Cloudscan be seen from Earth's southern hemisphere. Telescopes augmented with various technologies have enabled astronomersnotably NSF grantee Gregory Bothun of the University of Oregonto discover galaxies that, because of their extreme diffuseness, went undetected until the 1980s. These "low-surface-brightness" galaxies effectively are masked by the noise of the night sky, making their detection a painstaking process. More than 1,000 of these very diffuse galaxies have been discovered in the past decade, but this is only the beginning. "Remarkably, these galaxies may be as numerous as all other galaxies combined," says Bothun.

"In other words, up to 50 percent of the general galaxy population of the universe has been missed, and this has important implications with respect to where matter is located in the universe."

Question: Which astronomer proposed the theory of the expanding universe?
Answer: Edwin Hubble