在托福阅读真题中出现过关于宇宙理论的题目：宇宙大爆炸，宇宙起源的两种学说，稳定宇宙和发展宇宙，大爆炸理论。托福阅读中关于宇宙理论类的介绍对于对天文一无所知的考生来说是困难的，所以本文前程百利小编为考生带来了相关托福阅读背景知识。
托福阅读真题再现：
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;
解析托福阅读真题：
天文主题文章的词汇专业性较强，尽量减少生词恐惧带来的内耗。另外，出现理论对比的文章，结构比较清晰，但要着重识别对理论内容的态度倾向。比如这篇文章讲的就是在大爆炸理论盛行之前，有一种与之替换的稳定宇宙理论。但最后，还是大爆炸理论占了上风。
宇宙理论相关背景普及：
a. Big Bang
The Big Bang theory is the prevailing cosmological model for the early development of the universe. According to the theory, the Big Bang occurred approximately 13.82 billion years ago, which is thus considered the age of the universe. At this time, the universe was in an extremely hot and dense state and was expanding rapidly. After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, including protons, neutrons, and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed. The majority of atoms that were produced by the Big Bang are hydrogen, along with helium and traces of lithium. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae.
b. The steady state universe theory
In cosmology, the Steady State theory is a now-obsolete theory and model alternative to the Big Bang theory of the universe s origin (the standard cosmological model). In steady state views, new matter is continuously created as the universe expands, thus adhering to the perfect cosmological principle.
While the steady state model enjoyed some popularity in the first half of the 20th century, it is now rejected by the vast majority of professional cosmologists and other scientists, as the observational evidence points to a Big Bang-type cosmology and a finite age of the universe.
c. Big Bang or Steady State?
Creation of the Elements
The 1930s was more a decade of consolidation than of revolutionary advance in cosmology. And in the early 1940s, world war limited cosmological advance. But the war that temporarily absorbed scientific resources also promoted technologies that would lead to fundamental scientific advances.
Advances in nuclear physics helped transform cosmological speculations into quantitative calculations. This line of investigation, begun in the late 1940s, was at first pursued mainly by physicists, not astronomers. In the 1930s Georges Lema?tre had suggested that the universe might have originated when a primeval "cosmic egg" exploded in a spectacular fireworks, creating an expanding universe. Now physicists found plausible numbers for the cosmic abundances of different elements that would be created in an initial cosmic explosion. But the theory of an initial cosmic explosion was soon challenged by a new hypothesis—that the universe might be in a steady state after all.
In 1946 the Ukrainian-born American physicist George Gamow considered how the early stage of an expanding universe would be a superhot stew of particles, and began to calculate what amounts of various chemical elements might be created under these conditions. Gamow was joined by Ralph Alpher, a graduate student at George Washington University, where Gamow taught, and by Robert Herman, an employee at the Johns Hopkins Applied Physics Laboratory, where Gamow consulted. Both Alpher and Herman were American-born sons of émigré Russian Jews.
Gamow assumed expansion and cooling of a universe from an initial state of nearly infinite density and temperature. In that state all matter would have been protons, neutrons, and electrons merging in an ocean of high energy radiation. Gamow and Alpher called this hypothetical mixture "Ylem" (from a medieval word for matter). Alpher made detailed calculations of nuclear processes in this early universe. For his calculations he used some of the first electronic digital computers—developed during the war for computing, among other things, conditions inside a nuclear bomb blast. It seemed that elements could be built up as a particle captured neutrons one by one, in a sort of "nuclear cooking."
The contribution of this theory was not to set forth a final solution but, no less important, to set forth a grand problem—what determined the cosmic abundance of the elements? Could the observed abundances be matched by calculations that applied the laws of physics to an early extremely hot dense phase of an expanding universe? Gamow did succeed in explaining the relative abundances of hydrogen and helium. Calculations roughly agreed with observations of stars—helium accounted for about a quarter of the mass of the universe and hydrogen accounted for nearly all the rest. However, attempts to make calculations for other elements failed to get a sensible answer for any element beyond helium. It seemed that piling more neutrons onto helium would hardly ever get you stable elements. Gamow joked that his theory should nevertheless be considered a success, since it did account for 99% of the matter in the universe.
Indeed his theory was not wrong but only incomplete. Astrophysicists soon realized that if the heavier elements were not formed during the hot origin of the universe, they might be formed later on, in the interiors of stars. The theory depended on a special property of carbon, which British astronomer Fred Hoyle measured and found as predicted. Cosmology had entered the laboratory.
The Steady-State Theory
Hoyle s triumph in explaining how most elements could be created in stellar interiors fell outside the theory in which elements were created at the very start. It was interpreted as favoring a rival theory. And Hoyle did favor a rival theory, which he had played a large part in inventing and developing. In this theory the universe had always looked much as it does now. There never had been a "big bang"—a phrase that Hoyle invented in 1950, intending the nickname as pejorative.
There is a charming story, not taken seriously by all historians, about how steady state theory began. The idea came in 1947, Hoyle claimed, when he and his fellow scientists Hermann Bondi and Tommy Gold went to a movie. The three knew each other from shared research on radar during World War II. Hoyle was versatile, undisciplined and intuitive; Bondi had a sharp and orderly mathematical mind; Gold s daring physical imagination opened new perspectives. The movie was a ghost story that ended the same way it started. This got the three scientists thinking about a universe that was unchanging yet dynamic. According to Hoyle, "One tends to think of unchanging situations as being necessarily static. What the ghost-story film did sharply for all three of us was to remove this wrong notion. One can have unchanging situations that are dynamic, as for instance a smoothly flowing river." But how could the universe always look the same if it was always expanding? It did not take them long to see a possible answer—matter was continuously being created. Thus new stars and galaxies could form to fill the space left behind as the old ones moved apart. (You can read Gamow s verse about this idea here.)
以上就是关于托福阅读背景知识的介绍，考生可在平常托福考试复习中多留意托福阅读背景知识，以帮助自己应对各种背景下的托福阅读文章。
