托福阅读背景材料(332)：地质年龄判定

　　托福阅读背景知识/材料之地质年龄判断，更多托福阅读背景材料/知识尽在新东方在线托福考试频道！

　　2014年5月11日托福阅读真题再现：

　　版本1:

　　文章先讲太阳系里的东西都有相同的起源。先是说所有的东西是在一起的，然后说地球由于地表的水、火山活动和一个什么过程使得地球连最古老的石头都没有了。所以只能测定月球的陨石的成分了，结论是月球的表面和陨石的时间都是46亿年。因为月球表面没有地球的这些活动，所以可以测定。

　　后面又说宇宙的星系都在不断地拉开距离，通过星系的红移可以确定距离还有速度，发现宇宙一直在膨胀。发现宇宙在137亿年前是一个点。然后就有了宇宙大爆炸。

　　版本2：

　　讲地球和宇宙年龄的测量。先说太阳系大部分物质是同一时间形成的，然后说地球年龄难是因为谁腐蚀。接着引入一种物质，可以通过同位素测年龄。结果是和月球上的最古老的石头近似。然后说宇宙在膨胀，大爆炸。通过红移测年龄。

　　版本3：

　　天文类， 某种地球上的物质和月球上最古老的物质证明他。都始于自4.6million年前，于是证明太阳系的年龄是4.6 Million years. 另外还有种通过判断各星球一种wavelength的大小推断出他们在多少年前都是从个spot发展出来，于是判断了big bang的时间。

　　托福阅读相关词汇：

　　origin 起源

　　meteorite 陨石

　　galaxy 星系

　　expansion 膨胀

　　red shift 红移

　　wavelength 波长

　　解析：

　　天文主题文章的词汇专业性较强，需要提前对相关专题的TPO文章的生词熟悉，尽量减少生词恐惧带来的内耗。另外，出现天文理论的文章，结构通常都会比较清晰，但要着重识别对理论内容的态度倾向。

　　托福阅读相关背景：

　　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.Accelerating universe

　　The accelerating universe is the observation that the universe appears to be expanding at an increasing rate. In formal terms, this means that the cosmic scale factor has a positive second derivative,so that the velocity at which a distant galaxy is receding from us should be continuously increasing with time.In 1998, observations of type Ia supernovae also suggested that the expansion of the universe has been accelerating since around redshift of z~0.5.The 2006 Shaw Prize in Astronomy and the 2011 Nobel Prize in Physics were both awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess, who in 1998 as leaders of the Supernova Cosmology Project (Perlmutter) and the High-Z Supernova Search Team (Schmidt and Riess) discovered the accelerating expansion of the Universe through observations of distant ("High-Z") supernovae.

　　observations.

　　The simplest evidence for accelerating expansion comes from the brightness/redshift relation for distant Type-Ia supernovae; these are very bright exploding white dwarfs, whose intrinsic luminosity can be determined from the shape of the light-curve. Repeated imaging of selected areas of sky is used to discover the supernovae, and then followup observations give their peak brightness and redshift. The peak brightness is then converted into a quantity known as luminosity distance (see distance measures in cosmology for details).

　　For supernovae at redshift less than around 0.1, or light travel time less than 10 percent of the age of the universe, this gives a nearly linear redshift/distance relation due to Hubble's law. At larger distances, since the expansion rate of the universe has generally changed over time, the distance/redshift relation deviates from linearity, and this deviation depends on how the expansion rate has changed over time. The full calculation requires integration of the Friedmann equation, but the sign of the deviation can be given as follows: the redshift directly gives the cosmic scale factor at the time the supernova exploded, for example a supernova with a measured redshift z=0.5 implies the Universe was 1/(1+0.5) =2/3 of its present size when the supernova exploded. In an accelerating universe, the universe was expanding more slowly in the past than today, which means it took a longer time to expand from 2/3 to 1.0 times its present size compared to a non-accelerating universe. This results in a larger light-travel time, larger distance and fainter supernovae, which corresponds to the actual observations: when compared to nearby supernovae, supernovae at substantial redshifts 0.2 - 1.0 are observed to be fainter (more distant) than is allowed in any homogeneous non-accelerating model.

　　Corroboration

　　After the initial discovery in 1998, these observations were corroborated by several independent sources: the cosmic microwave background radiation and large scale structure,apparent size of baryon acoustic oscillations,age of the universe,as well as improved measurements of supernovae,X-ray properties of galaxy clusters and Observational H(z) Data.

　　Explanatory models

　　Models attempting to explain accelerating expansion include some form of dark energy, dark fluid or phantom energy. The most important property of dark energy is that it has negative pressure which is distributed relatively homogeneously in space. The simplest explanation for dark energy is that it is a cosmological constant or vacuum energy; this leads to the Lambda-CDM model, which has generally been known as the Standard Model of Cosmology from 2003 through the present, since it is the simplest model in good agreement with a variety of recent observations. Alternatively, some authors (e.g. Benoit-Lévy & Chardin, Hajdukovic, Villata) have argued that the universe expansion acceleration could be due to a repulsive gravitational interaction of antimatter.

　　Theories for the consequences to the universe

　　As the Universe expands, the density of radiation and ordinary and dark matter declines more quickly than the density of dark energy (see equation of state) and, eventually, dark energy dominates. Specifically, when the scale of the universe doubles, the density of matter is reduced by a factor of 8, but the density of dark energy is nearly unchanged (it is exactly constant if the dark energy is a cosmological constant).

　　Current observations indicate that the dark energy density is already greater than the mass-energy density of radiation and matter (including dark matter). In models where dark energy is a cosmological constant, the universe will expand exponentially with time from now on, coming closer and closer to a de Sitter spacetime. In this scenario the time it takes for the linear size scale of the universe to expand to double its size is approximately 11.4 billion years. Eventually all galaxies beyond our own local supercluster will redshift so far that it will become hard to detect them, and the distant universe will turn dark.

　　In other models, the density of dark energy changes with time. In quintessence models it decreases, but more slowly than the energy density in ordinary matter and radiation. In phantom energy models it increases with time, leading to a big rip.