Chapter 26: Cosmology: How the Universe Works

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Cosmology is the study of the structure and evolution of the universe. To predict the future of the universe (and to understand its past), we must know things are, and where things are . "Where things are" means knowing the distances to things (planets, stars, galaxies, etc.). ranging is a method to find distances to our (nearest, farthest?) neighbors in our solar system by seeing how long (sound, radio waves, sunlight?) take(s) to travel to an object and back again. is the application of geometry to the stars. It requires a telescope, was first successfully done in the mid-1800s, and only works at best for about 10,000-100,000 of the (nearest, farthest?) stars. The Herzsprung-Russell diagram method works for more distant stars in our own and very nearby galaxies. The " brightness" of a star is how bright the star actually is. Comparisons are made as if from (the) (same, different?) distance. The " brightness" is how it appears from earth. Comparisons are made from (the) (same, different?) distance. Absolute brightness, apparent brightness, and distance are interrelated by geometry. If you know any two, you can calculate the third. The Hertsprung-Russell diagram allows us to know the (apparent, absolute?) brightness of a star by observing its . Thus, knowing absolute brightness and observing the apparent brightness, we can calculate the distance to the star. "Where things are going" means knowing how fast and in what direction the stars and galaxies are moving relative to us and each other. The (phase, Doppler, Einstein?) shift is used to measure motion. If a source of sound waves is moving toward you, the frequency of sound is (lower, higher?) than it would otherwise be. If the source is moving away, the frequency is (lower, higher?) . Light is enough of a wave that it exhibits a Doppler shift. If the source of light is moving away from you, the shift in light frequency (color) is called a (red, blue?) shift. If the light from galaxies is viewed, one observes discrete spectra of the elements, but the frequencies are virtually all (blue, red?) shifted. This is taken as evidence that virtually all of the galaxies are moving (toward, away) to/from us. Thus, the "Cosmological Red Shift" is evidence that the universe is (expanding, contracting, staying the same?) . The expansion of the universe is understood to be an expansion of space-time itself, not of matter exploding within a pre-existing space! In such an expansion, the further away a galaxy is from us, the (slower, faster?) it moves away from us. The size (how large the frequency shift is) of the red shift thus becomes a way of estimating how far away a galaxy is. The bigger the red shift of its light, the (closer, farther?) (away) is the galaxy. The "point of beginning" of the expansion is an event in spacetime called the Before and precisely at the Big Bang, space and time have no meaning. Space and time begin to have meaning (i.e., can be measured) at events just after the Big Bang. Just after the Big Bang, matter is incredibly (hot, cold?) , incredibly (dense, rarefied?) , and consists of elementary particles (those that have no size or structure, such as electrons and quarks). As spacetime expands, the matter begins to (cool down, heat up?) and become (more, less?) dense. As the matter becomes cooler and less dense, structures begin to form and remain stable: first nucleons, then nuclei, then atoms, then molecules, then stars, galaxies and eventually life. From estimates of the rate of expansion from the red shift, we estimate that the Big Bang occurred about 15 (million, billion, trillion?) years ago. Whatever is ultimately true about the origin of the universe must explain what has been observed: it must explain why there is a cosmological red shift? it must explain why the to hydrogen ratio (mass) is about 25 percent to 75 per cent? it must explain why there is a universal background of (starlight, microwave, beta?) radiation. it must be consistent with independent measurements of the ages of the sun and earth. it must explain the origin of galaxies (i.e., how do you get "clumping" in matter that is expanding and generally getting less dense?) At present, the Big Bang Model is the only model that provides an explanation of these things. The model predicts that the universe may expand forever, or may eventually contract depending on its present density. The overall average density is not yet known precisely enought to know for sure, but the present evidence favors that it will expand forever.

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