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Reading Week 15 - What are dark matter and dark energy and...

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What are dark matter and dark energy, and how are they affecting the universe? Robert Caldwell, a cosmologist at Dartmouth College, explains.   Dark energy and  dark matter  describe proposed solutions to as yet unresolved  gravitational phenomena. So far as we know, the two are distinct.  Dark matter originates from our efforts to explain the observed mismatch between the  gravitational mass and the luminous mass of galaxies and clusters of galaxies. The  gravitational mass of an object is determined by measuring the velocity and radius of the  orbits of its satellites, just as we can measure the mass of the sun using the velocity and  radial distance of its planets. The luminous mass is determined by adding up all the light  and converting that number to a mass based on our understanding of how stars shine.  This mass-to-light comparison indicates that the energy in luminous matter contributes  less than 1 percent of the average energy density of the universe.  There is certainly more matter in our galaxy and other galaxies that we cannot see, but  other evidence indicates that there is an upper limit to the total amount of normal matter  present in the universe. By normal matter, I mean stuff made out of atoms. Recently,  NASA s Wilkinson Microwave Anisotropy Probe (WMAP) satellite made precision  measurements of the imprint of sound waves on the cosmic microwave background,  produced some 400,000 years after the big bang. Because sound propagation depends on  the properties of the medium--as anyone who has played with a helium balloon knows-- the pattern of the sound waves viewed by WMAP is an indicator of the abundance of  hydrogen and helium in the universe. (All other elements were built from these basic  building blocks.) These and other results agree with the theoretical predictions of the  primordial abundances of the light elements as a result of the nuclear processes that took  place in the first three minutes of the universe--also known as big bang nucleosynthesis. 
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