CONTENT:CHAP26:FIGURES:FG26_001.PCT
"Pencil Beam" Survey Of Galaxy Distribution - The results of a deep "pencil-beam" survey of two small portions of the sky in opposite directions from the Earth perpendicular to the galactic plane. The graph shows the number of galaxies at different distances from us, out to a distance of about 2000 Mpc. The distinctive "picket fence" appearance seems to show voids and sheets of galaxies on scales of 100 or 200 megaparsecs, but gives no indication of any larger structure.
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CONTENT:CHAP26:FIGURES:FG26_002.PCT
Galaxy Distribution In Cube Of Space - Diagram of galaxies contained within an enormous cube, 300 Mpc on a side. Cosmologists believe that regardless of where we placed this cube in the universe its contents would look similar. (Not drawn to scale; for accurate scaling see the deep-space photos in Figures 24.18, 24.27, and 24.28.)
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CONTENT:CHAP26:FIGURES:FG26_003.PCT
Olber's Paradox - If the universe were homogeneous, isotropic, infinite in extent, and unchanging, any line of sight from the Earth should eventually run into a star, and the entire night sky should be bright. This obvious contradiction of the facts is known as Olbers's paradox.
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CONTENT:CHAP26:FIGURES:FG26_004.PCT
The Hubble Law - The Hubble law is the same no matter who makes the measurements. The top set of numbers are the distances and recessional velocities as seen by an observer on the middle of five galaxies, galaxy 3. The bottom two sets are from the points of view of observers on galaxies 2 and 1, respectively. In all cases, the same Hubble law holds.
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CONTENT:CHAP26:FIGURES:FG26_005.PCT
Balloon Analog Of Expanding Universe - The coins taped to the surface of a spherical balloon recede from one another as the balloon inflates (left to right). Similarly, galaxies recede from one another as the universe expands. As the coins recede, the distance between any two of them increases, and the rate of increase of this distance is proportional to the distance between them. Thus, our balloon expands according to Hubble's law.
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CONTENT:CHAP26:FIGURES:FG26_006.PCT
Relative Motion Of Points On Expanding Universe - The relative motion of any two points in the expanding universe (a) can be addressed as a problem in Newtonian mechanics, even though Einstein's theory of general relativity is needed to explain why it is correct to do so. If the rest of the universe is ignored-as in part (b)-then the Newtonian calculation of B's motion relative to A would give the right answer.
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CONTENT:CHAP26:FIGURES:FG26_007.PCT
Balloon Analog Of Cosmological Redshift - As the universe expands, photons of radiation are stretched in wavelength, giving rise to the cosmological redshift.
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CONTENT:CHAP26:FIGURES:FG26_008.PCT
Spacecraft Analog To Open Vs. Closed Universe -  (a) A spacecraft (arrow) leaving a planet (blue ball) with a speed greater than the escape velocity leaves on an unbound trajectory (top). The graph (bottom) shows the distance between the ship and the planet as a function of time. (b) If the launch speed is less than the escape velocity, the ship eventually drops back to the planet. Its distance from the planet first rises, then falls again.
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CONTENT:CHAP26:FIGURES:FG26_009.PCT
Three Scenarios For Future Of Universe - Distance between two galaxies as a function of time in each of the three possible universes discussed in the text: unbound, bound, and marginally bound. The point where the three curves touch represents the present time.
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CONTENT:CHAP26:FIGURES:FG26_010.PCT
Scenario For High-Density Universe - A high-density universe (a) has a beginning, an end, and a finite lifetime. The lower frames (b) illustrate its evolution, from explosion to maximum size to recollapse.
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CONTENT:CHAP26:FIGURES:FG26_011.PCT
Scenario For Oscillating Universe - An oscillating universe has neither a beginning nor an end. Each expansion-contraction phase ends in a "bounce," which becomes the "Big Bang" of the next expansion. There is currently no information on whether or not this can actually occur.
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CONTENT:CHAP26:FIGURES:FG26_012.PCT
Scenario For Low Density Universe - A low-density universe (a) expands forever from its explosive beginning. The lower frames (b) illustrate the continuing expansion of the universe in this case. The upper curve represents a universe with density less than the critical value. The lower curve represents a universe with density exactly equal to the critical value.
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CONTENT:CHAP26:FIGURES:FG26_013.PCT
The Deceleration Of The Universe - As the density of the universe increases, its deceleration increases, too. The universe contains some matter, so whatever the model, its trajectory on this graph will lie below the line for the constant-velocity, empty universe. Thus, the age of the universe is always less than 1 over the Hubble constant. The true age decreases for larger values of the present-day density.
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CONTENT:CHAP26:FIGURES:FG26_014.PCT
Hubble Diagram Showing Evidence For Deceleration - An idealized Hubble diagram, showing how we might detect evidence for a deceleration of the universe by observing a departure from the usual Hubble relationship (solid line). The dashed curves show the expected departure from the solid line for different evolving models of the universe. (The departures of the dashed curves from the solid line are exaggerated for clarity.) Uncertainties in the measured luminosities of distant galaxies (black dots) affect estimates of their distances, so this technique is too imprecise to distinguish among the various possibilities.
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CONTENT:CHAP26:FIGURES:FG26_015.PCT
Path Of Light Beam In Closed Universe - In a closed universe, a beam of light launched in one direction might return someday from the opposite direction after circling the universe, just as motion in a "straight line" upon the Earth's surface will eventually encircle the globe.
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CONTENT:CHAP26:FIGURES:FG26_016.PCT
Portrait Of A. Penzias And R. Wilson - This "sugarscoop" antenna, originally built to communicate with Earth-orbiting satellites, was used in discovering the 3K cosmic background radiation. Pictured, left to right, are Robert Wilson and Arno Penzias, who used the antenna to make the discovery.
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CONTENT:CHAP26:FIGURES:FG26_017.PCT
Black Body Curves For The Expanding Universe - Theoretically derived black-body curves for the universe (a) 1 second after the Big Bang, (b) 100,000 years after the Big Bang, (c) 10 million years after the Big Bang, and (d) at present, approximately 13 billion years after the Big Bang.
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CONTENT:CHAP26:FIGURES:FG26_018.PCT
COBE Satellite Measurement Of Background Radiation - The intensity of the cosmic background radiation, as measured by the COBE satellite, agrees very well with that expected from theory. The curve is the best fit to the data, corresponding to a temperature of 2.735K. The experimental errors in this remarkably accurate observation are smaller than the dots representing the data points.
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CONTENT:CHAP26:FIGURES:FG26_019.PCT
Doppler Shift In Microwave Background -  (a) To an observer at rest with respect to the expanding universe, the microwave background appears isotropic. (b) A moving observer measures "hot" blueshifted radiation in one direction (the direction of motion) and "cool" redshifted radiation in the opposite direction.
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CONTENT:CHAP26:FIGURES:FG26_020.PCT
COBE Satellite Map Of Microwave Background - A COBE map of the microwave sky reveals that the microwave background appears a little hotter in the direction of the constellation Leo and a little cooler in the opposite direction. The maximum temperature deviation from the average is about 0.0034 K, corresponding to a velocity of 400 km/s.
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CONTENT:CHAP26:SLIDES:SL26_001.PCT
Gravitational Lens In Cluster - This is a spectacular example of gravitational lensing where the gravitational field of a rich foreground cluster of galaxies distorts the light of far more distant galaxies to create a series of arc-like features.  Light rays passing through the cluster are deflected, much as a glass lens refracts light.  This gravitational lensing brightens and magnifies the images of the more distant galaxies.
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CONTENT:CHAP26:SLIDES:SL26_002.PCT
Primordial Spiral Galaxy - A Hubble Space Telescope view of a very distant cluster of galaxies, 12 billion light years away. The enlargement (right) shows a primordial disk-like galaxy near a bright quasar in the cluster.
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CONTENT:CHAP26:SLIDES:SL26_003.PCT
X-Ray Emission From Galaxy Cluster - an optical picture of small galaxy cluster is combined with Xray data from the ROSAT satellite to show the location of hot gas (false-colored as pink) that is contained in the center of the cluster, presumable by dark matter.
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CONTENT:CHAP26:SLIDES:SL26_004.PCT
Gravitational Lens Shows Primeval Galaxy - This Hubble Space Telescope image shows several blue, loop-shaped objects that actually are multiple images of the same galaxy. They have been duplicated by the gravitational lens of the cluster of yellow, elliptical and spiral galaxies - called 0024+1654 - near the photograph's center. The gravitational lens is produced by the cluster's tremendous gravitational field that bends light to magnify, brighten and distort the image of a more distant object.  The cluster is five billion light-years away in the constellation Pisces, and the blue-shaped galaxy is about two times farther away. 
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