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5 What Is the Nature of Dark Matter and Dark Energy
Pages 78-104

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From page 78...
... Big bang nucleosynthesis, the very successful theory of the nuclear origin of the lightest elements in the periodic table, together with recent measurements of the amount of deuterium (heavy hydrogen) in the universe, indicates that only about ~ percent of the mass and energy in the universe is in the form of ordinary matter (baryons)
From page 79...
... This result implies the existence of large amounts of "dark energy" whose gravitational force is repulsive (see Box 5.11. Perhaps the biggest puzzle of all is the odd mix that makes up our universe ordinary matter, exotic dark matter, and dark energy, all in significant amounts.
From page 80...
... While the central idea of Einstein's theory is the description of gravity as a property of curved space rather than a force, one can still discuss Einstein's gravity in terms of forces. Because mass and energy are related through Einstein's most famous equation, E= mc2, it is not surprising that energy replaces mass in Einstein's version of a gravitational force.
From page 81...
... The relative abundance of these elements, particularly deuterium, is sensitive to the density of ordinary matter. The recent measurement of the primordial 81 The central idea of Einstein's theory of relativity is the curvature of space-time.
From page 82...
... or may diverge (negatively curved space) and the sum of angles in a triangle may be more than 180 degrees (positively curved space)
From page 83...
... deuterium abundance in primeval gas clouds along the line of sight to distant quasars has provided a precision measurement of the average baryon density, about ~ percent of the critical density. Secondly, the statistical properties of the fluctuations in the cosmic microwave background are sensitive to the baryon density.
From page 84...
... The vertical lines correspond to measurements of the dark matter density and the sloping triangle represents the CMB measurements that indicate a flat, critical universe (here, matter plus dark energy sum to one)
From page 85...
... The position of the first peak indicates that we live in a flat universe; the ratio of the amplitudes of the first and second peaks indicates a baryon density of around 4 percent, consistent with the big bang nucleosynthesis determination. The solid curve is the theoretical prediction for a flat universe whose composition is 4 percent ordinary matter, 29 percent cold dark matter, and 67 percent dark energy.
From page 86...
... . .U 0.5% Stars 3.5% warm/hot gas - 25-35% Cold Dark Matter Dark Energy FIGURE 5.4 Pyramid diagram of the composition of the universe and the different techniques that can be used to measure the various components.
From page 87...
... Similar . ~ ~ FIGURE 5.5 An image of the Andromeda galaxy with its rotation curve (velocity of gas clouds and stars orbiting the center of the galaxy vs.
From page 88...
... Because there is strong evidence that it is not made of ordinary matter, discovering dark matter's nature will also have deep implications for physics.
From page 89...
... FIGURE 5.6 (a) Hubble Space Telescope image of the gravitational lensing of light from distant galaxies (distorted and multiple blue images)
From page 90...
... The amount of dark matter (about 35 percent of the critical density) is significantly greater than the amount of ordinary matter inferred from big bang nucleosynthesis and from the cosmic microwave background (3 percent to 5 percent of the critical density)
From page 91...
... Putting aside the CMB and big bang nucleosynthesis measurements of the amount of ordinary matter, the implausibility of hiding 99 percent of the baryons in a form that is not detectable is daunting. For example, putting 35 percent of the critical density in white dwarfs or neutron stars would require far more star formation than the evidence supports.
From page 92...
... Further, as discussed below, the pattern of structure formation seen in the universe is not consistent with the idea that neutrinos constitute the bulk of the dark matter. In fact, the neutrino mass implied by experiment indicates that neutrinos contribute between 0.1 percent and 5 percent, about as much mass as do bright stars.
From page 93...
... as the dominant part of the dark matter. Moreover, the formation of structure with cold dark matter has been simulated on supercomputers, and the predictions agree well with a wide array of observations, including the masses and abundances of galaxies, clustering of galaxies and clusters of galaxies, the distribution of gas clouds at high redshift, and fluctuations in the cosmic microwave background radiation.
From page 94...
... The measuring techniques include the study of beta-decay spectra, the study of neutrinoless double-beta decay, and searches for oscillations between the different neutrino species. Neutrino oscillation studies using neutrinos produced by cosmic rays in Earth's atmosphere and by the Sun have already produced strong evidence for nonzero neutrino mass and indicate a minimum cosmic density of neutrinos compa.
From page 95...
... DARK ENERGY Is Expansion of the Universe Speeding Up Rather Than Slowing Down? Type la supernovae, the thermonuclear explosions of white dwarf stars slightly more massive than the Sun, have remarkably uniform peak luminosities (when corrected for the rate of decline of brightness)
From page 96...
... recently found that the expansion of the universe is speeding up, not FIGURE 5.7 Images of distant Type la supernovae. For about a month, these thermonuclear stellar explosions produce about as much light as the rest of the stars in the galaxy combined.
From page 97...
... The acceleration requires something new, as matter in any amount should cause the universe to decelerate. The nature of this new component, called dark energy in the spirit of Zwicky's naming of dark matter, is not understood.
From page 98...
... The ultimate fate of the universe depends on the nature of the dark energy. If the dark energy is truly the energy of the quantum vacuum, then the fate of the universe is continued acceleration of its expansion.
From page 99...
... Observations of distant supernovae can probe the detailed expansion history directly back to redshifts of around 2, corresponding to times from a few billion years after the big bang until the present. The current data have already provided the first strong evidence for the existence of dark energy.
From page 100...
... can be used to measure the large-scale clustering properties of matter during the last 10 billion years. The amplitude of this clustering as a function of scale and redshift depends on the amount of dark energy and its properties.
From page 101...
... The importance of complementary measurements and crosschecks is illustrated bythe current data i n Figure 5.2 . For the study of dark energy and dark matter, a new type of telescope may be called for.
From page 102...
... These puzzles may be a sign that scientists do not understand gravity, and some models of dark energy predict the existence of new, weaker-than-gravity forces that could be discovered with more sensitive tests of the equivalence principle. NEW OPPORTUNITIES As the new century begins, scientists have a first, tentative accounting of the universe: one-third matter and two-thirds dark energy, adding up to the critical density and a flat universe.
From page 103...
... What Is the Nature of Dark Energy? Two independent lines of evidence indicate the presence of a new form of energy pervading the universe that accounts for two-thirds of the critical density and is causing the expansion of the universe to speed up rather than slow down.
From page 104...
... 104 CON N ECTI N G QUARKS WITH TH E COSMOS In studying dark energy and dark matter a new kind of special-purpose telescope may prove useful. The mapping of dark matter with gravitational lensing and the search for distant supernovae both require the search of large swaths of sky.


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