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Foreword by Walter Cronkite  
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Internet: Changing the Way we Communicate  
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Astronomy: Exploring the Expanding Universe
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Astronomy: Exploring the Expanding 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.

Radio Telescopes (VLA) - click for details 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.Boomerang Telescope - click for details 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.

A galaxy in the constellation Cygnus - click for detailsAnd 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."

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Voyage to the Center of the Sun
New Tools, New Discoveries
At the Center of the Milky Way
The Origins of the Universe
The Hunt for Dark Matter
Shedding Light on Cosmic Voids
Visualizing the Big Picture
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