阅读1
版本1：宇宙的两个理论，一个说物质会变化但总量不变，一个是会膨胀，最后说一个遥远的恒星的发现说明后一个理论更正确;
版本2：讲的是universe expanding 的两种理论，一个是density在变小。另一种是density不变。因为不断new creation补充变大的空间，然后发现了一种q.它表明前一种理论更可信。
解析：
big-bang model
widely held theory of the evolution of the universe. Its essential feature is the emergence of the universe from a state of extremely high temperature and density-the so-called big bang that occurred at least 10,000,000,000 years ago. Although this type of universe was proposed by Alexander Friedmann and Abbé Georges Lema?tre in the 1920s, the modern version was developed by George Gamow and colleagues in the 1940s.
The big-bang model is based on two assumptions. The first is that Albert Einstein''s general theory of relativity correctly describes the gravitational interaction of all matter. The second assumption, called the cosmological principle, states that an observer''s view of the universe depends neither on the direction in which he looks nor on his location. This principle applies only to the large-scale properties of the universe, but it does imply that the universe has no edge, so that the big-bang origin occurred not at a particular point in space but rather throughout space at the same time. These two assumptions make it possible to calculate the history of the cosmos after a certain epoch called the Planck time. Scientists have yet to determine what prevailed before Planck time.
According to the big-bang model, the universe expanded rapidly from a highly compressed primordial state, which resulted in a significant decrease in density and temperature. Soon afterward, the dominance of matter over antimatter (as observed today) may have been established by processes that also predict proton decay. During this stage many types of elementary particles may have been present. After a few seconds, the universe cooled enough to allow the formation of certain nuclei. The theory predicts that definite amounts of hydrogen, helium, and lithium were produced. Their abundances agree with what is observed today. About 1,000,000 years later the universe was sufficiently cool for atoms to form. The radiation that also filled the universe was then free to travel through space. This remnant of the early universe is the microwave background radiation (three degree background radiation) discovered in 1965 by Arno A. Penzias and Robert W. Wilson.
