Science at the Frontier (1992) / Chapter Skim
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4 Astrophysics: Looking Farther in Space and Time
4 Astrophysics: Looking Farther in Space and Time
Pages 66-93
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From page 66... ...
Beyond, that is, not only the farthest reach of optical telescopes (that are limited by the diameter of their mirrors and the sensitivity of their detecting devices) but also outside the relatively narrow range of the electromagnetic spectrum where visible-light rays reside, with wavelengths spanning from about 3000 to 9000 angstroms (1 A = 10-8 cm)
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From page 67... ...
Ever since the 1930s soon after modern theories about the universe's evolution, now embraced as the Big Bang paradigm, were first proposed scientists have been speculating about this "missing mass or missing light problem," explained Margaret Geller from the Harvard-Smithsonian Center for Astrophysics. The Big Bang model has been generally accepted for about three decades as the reigning view of the creation of the universe.
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From page 68... ...
Although Newton was working from observations made by the Danish astronomer Tycho Brahe and calculations earlier completed by Brahe's German assistant, lohannes Kepler, he extrapolated this work on celestial bodies to any two objects with mass, such as the apocryphal apple falling on his head. The attractive force between any two bodies, Newton demonstrated, increased as the product of their masses and decreased as the square of the distance between them.
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From page 69... ...
A mass-poor space curves out like a saddle whose edges go off to infinity, fated never to meet. The Big Bang thus would become the Big Chill.
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From page 70... ...
The dark matter provides vital information, said Edmund Bertschinger of the Massachusetts Institute of Technology, "for the reigning cosmological paradigm of gravitational instability." Had there not been matter of a certain mass several hundred thousand years after the Big Bang, the universe would have been smooth rather than "clumpy," and the interactions between aggregations of particles necessary to form structures would not have taken place.
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From page 71... ...
In mapping specific regions of the celestial sky, astronomers in the 1970s began to report that many galaxies and clusters appeared to be strung out along lengthy curved chains separated by vast regions of galaxyless space called voids. Hints of such structures also emerged when Marc Davis and John Huchra at the Harvard-Smithsonian Center for Astrophysics completed the first comprehensive redshift survey of the heavens in 1981, which suggested a "frothiness" to the universe's structure, a pattern that dramatically came into focus when Geller and Huchra extended the redshift survey, starting in 1985.
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From page 72... ...
But cosmologists look at the universe from many different perspectives, and many astrophysicists interested in galaxy formation and structure energetically search the skies in hopes of finding a galaxy at its initial stage of creation. Djorgovski goes so far as to call the search for a new galaxy aborning "the holy grail of modern cosmology.
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From page 73... ...
The Big Bang paradigm allows cosmologists to establish points of reference in order to measure distances in deep space. As the universe ages, the horizon as viewed from Earth expands because light that was emitted earlier in time will have traveled the necessary greater distance to reach our view.
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From page 74... ...
Although the astronomical data currently available favor a Big Bang cosmology, predictions based on the Big Bang model are continually examined. Perhaps the most convincing single test was accomplished in 1965, when radio astronomers Arno Penzias and Rob
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From page 75... ...
Before it, classical gravitational physics can say or prove nothing, leaving all speculation to the metaphysicians. Big Bang theory encompasses a series of events that occurred thereafter, which conform to two continuing constraints: first, the laws of physics, which are believed to be universal, and second, data from observations that are continually probing farther in space, and therefore further back in time toward the event itself.
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From page 76... ...
From the earliest moment of this scenario, the universe was expanding outward. The use of the term Big Bang unfortunately suggests the metaphor of an explosion on Earth, where mass particles hurtle outward from a central source, soon to be counteracted by the comparatively massive effects of the planet's gravity.
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From page 77... ...
As an example, spectrographic studies of the Sun and other nearby stars indicate that 70 percent of the Sun is hydrogen and 28 percent helium, proportions of the two lightest elements that provide corroboration for the Big Bang model. Most stars reflect this proportion, and characteristic spectrographic signatures have also been developed for the major types of galaxies and clusters.
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From page 78... ...
If the source and observer are stationary, or are moving at equal velocities in the same direction, the waves will arrive at the receiving observer precisely as they were propagated from the source. The Big Bang universe, however, does not meet this condition: the source of light is almost invariably moving away from the receiver.
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From page 79... ...
Thus early in the 20th century, astronomy with the advent of spectrographs that allowed the measurement of redshifts stood on the brink of a major breakthrough. The astronomer who took the dramatic step which, boosted by relativity theory, quickly undermined then-current views of a static or stationary universe and provided the first strong observational evidence for the Big Bang paradigm was Edwin Hubble.
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From page 80... ...
Depending on the method used to calculate the constant, the time since the Big Bang can generally vary between 10 billion and 20 billion years. Reducing this uncertainty in the measurement of the Hubble constant is one of the primary goals of cosmology.
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From page 81... ...
Explained Bechtold: "The high luminosities of quasars enable astronomers to study the universe at great distances and hence at epochs when the universe was about 20 percent of its current age. This was a period when many important events took place," not the least of which was "the rapid turn-on of quasars themselves." Quasar surveys of the sky indicate that the number of quasars per volume of space increases with the distance from the Milky Way, which suggests that whatever caused them was more common in the distant past, possibly during the era of galaxy formation.
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From page 82... ...
absorption line, generated by hydrogen at 1215 angstroms. Analysis suggests that these data can provide vital clues, said Bechtold, to "the formation and rapid early evolution of galaxies, and the beginnings of the collapse of large structures such as clusters of galaxies and superclusters." Big Bang theory and numerous observations suggest that stars are the cauldron in which heavier elements are formed, after first hydrogen and then helium begin to aggregate through gravity.
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From page 83... ...
_, _ O , ~ ~ _ , While definitive confirmation of the full implications of the Lyman-alpha forest is in the future, Bechtold explained that the Lymanalpha absorption lines possess an indisputable value for cosmology today. "Because they are so numerous, they can serve as tracers of large-scale structure very well, in fact, in some ways better than galaxies." Their distribution throughout the universe is "much more uniform than galaxies in the present day," and thus they provide a possible precursor or link in the causal chain that might explain the 1 200 1 000 800 an ~ 600 o 400 200 o 11 ~ 1 ^1!
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From page 84... ...
Knowing the angle of arrival of the photon relative to the location of the light-emitting source, as well as the distance to the mass, one can calculate the size of the mass responsible for altering the photon's path. In 1915 Einstein had predicted just such a deflection of light by the Sun's gravitational field.
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From page 85... ...
It was a quasar seen twice two images, nearby, of the same quasar a sort of cosmic mirage." But the confirmation of Einstein, as Zwicky had predicted, was to vindicate their potential value as a sort of telescope of distant objects, a further source of information about the average large-scale properties of the universe, as well as an indicator of the presence of inhomogeneities in the universe, particularly those arising from dark matter. Tyson said that one of the reasons they seemed to offer a glimpse into the dark matter question was expressed in a seminal paper by Harvard University's William Press and lames Gunn of Princeton that ruled out a universe closed by dark matter purely in the form of black holes.
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From page 86... ...
Djorgovski supplied an analogy: "Twenty-fourth magnitude is almost exactly what a 100-watt light bulb would look like from a million kilometers away or an outhouse light bulb on the moon." Although the images Tyson and others have produced photographically at this magnitude were extremely faint, they still revealed 10,000 dim galaxies on each plate covering half a degree on the sky. Astronomers for the last five decades have been straining their eyes on such photographs, and Tyson was doing the same when he began to realize that "my data were useless." In 1977, he had been counting these faint smudges for a while, and noticed that "as the week went on, I was counting more and more galaxies on each plate.
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From page 87... ...
The blue curtain is a much more efficient probe of foreground dark matter because the curtain is far enough away to enhance the chances of foreground tensing and rich enough to provide a canvas- rather than a point of light coming from a quasar to observe the distortion. After subtracting the gravitational effects of those luminous galaxies in the foreground that they could observe, Tyson's team could nonetheless still observe fairly dramatic distortion in the blue curtain galaxy images.
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From page 88... ...
Characteristic distortions in the previously too faint images have become observable as the deep CCD optical imaging surveys and analyses continue. A portrait of dark matter is emerging that allows other theories about the evolution of the universe to be tested galaxy cluster formation, for one.
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From page 89... ...
photons Effect t of / ~ / ~> ~ gravitational Earth / lens ~,~ a = 4GM no FIGURE 4.3 A diagram showing the bending of a light path by the mass in a gravitational lens and the apparent change in the position of the source and the resulting distortion of its image. (Courtesy of J.A.
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From page 90... ...
Results from Tyson's work may aid astronomers in their quest to understand how galaxies formed. In order to create the lumps that eventually developed into galaxies, most theories of galaxy formation start with the premise that the Big Bang somehow sent a series of waves rippling through the newly born sea of particles, both large and small fluctuations in the density of gas.
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From page 91... ...
or the lar~-~l~ ~r~locity field." 1 ~ ~ } ~ ~ (~ ~ Galaxy formation can be seen as two distinct types of events, said Djorgovski: assembling the mass (which is the business of the dark matter and gravity) and "converting the primordial "hydrogen]
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From page 92... ...
These events are thought to happen in the era around a redshift of 2, perhaps a little greater, "but we don't know exactly where or when," said Djorgovski, suggesting that the process probably evolves over a considerable period of time. But it will be the emission lines in its spectra that confirm a star in the process of forming, should
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From page 93... ...
The Big Bang. Freeman, New York.
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