[分享]12RC JJ background 宇宙大爆炸

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Characteristics of Big Bang nucleosynthesis

There are two important characteristics of Big Bang nucleosynthesis (BBN):

            It lasted for only about seventeen minutes (during the period from 3 to about 20 minutes from the beginning of space expansion); after that, the temperature and density of the universe fell below that which is required for nuclear fusion. The brevity of BBN is important because it prevented elements heavier than beryllium from forming while at the same time allowing unburned light elements, such as deuterium, to exist.                

 

             It was widespread, encompassing the entire universeThe key parameter which allows one to calculate the effects of BBN is the number of photons per baryon. This parameter corresponds to the temperature and density of the early universe and allows one to determine the conditions under which nuclear fusion occurs. From this we can derive elemental abundances. Although the baryon per photon ratio is important in determining elemental abundances, the precise value makes little difference to the overall picture. Without major changes to the Big Bang theory itself, BBN will result in mass abundances of about 75% of H-1, about 25% helium-4, about 0.01% of deuterium, trace (on the order of 10^-10) amounts of lithium and beryllium, and no other heavy elements. That the observed abundances in the universe are consistent with these numbers is considered strong evidence for the Big Bang theory.

In this field it is customary to quote percentages by mass, so that 25% helium-4 means that helium-4 atoms account for 25% of the mass, but only about 8% of the atoms would be helium-4 atoms.

Sequence of BBN

Big Bang nucleosynthesis begins about one second after the Big Bang, when the universe has cooled down sufficiently to form stable protons and neutrons, after baryogenesis. The relative abundances of these particles follow from simple thermodynamical arguments, combined with the way that the mean temperature of the universe changes over time (if the reactions needed to reach the thermodynamically favoured equilibrium values are too slow compared to the temperature change brought about by the expansion, abundances will remain at some specific non-equilibrium value). Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons and neutrons based on the temperature at this point. This fraction is in favour of protons, because the higher mass of the neutron results in a spontaneous decay of neutrons to protons with a half-life of about 15 minutes. One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies are very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe. Another feature is that the process of nucleosynthesis is determined by conditions at the start of this phase of the life of the universe, making what happens before irrelevant.

As the universe expands it cools. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. At the time at which nucleosynthesis occurs, the temperature is high enough for the mean energy per particle to be greater than the binding energy of deuterium; therefore any deuterium that is formed is immediately destroyed (a situation known as the deuterium bottleneck). Hence, the formation of helium-4 is delayed until the universe becomes cool enough to form deuterium (at about T = 0.1 MeV), when there is a sudden burst of element formation. Shortly thereafter, at three minutes after the Big Bang, the universe becomes too cool for any nuclear fusion to occur. At this point, the elemental abundances are fixed, and only change as some of the radioactive products of BBN (such as tritium) decay.
                

 

Original JJ

第一段
                说big bang 前后同位素的形成。以前对同位素的形成有两种不同的说法，14世纪，一个叫G的学者认为它们是跟宇宙形成同步完成的，而另一个学者H却认为它们是在星体内部形成的。其中有一个细节题考的是下面哪一种是用来支持元素的同位素是根big bang同时产生的，我选的答案是宇宙中有很多氘元素(deuterium，氢的同位素).

 

第二段
                说现代的学者认为G和H都partly right，也就是说既有在大爆炸的时候形成的同位素，也有在星体内部形成的同位素。举例说用原子的东西来观察星球的物质证明了同位素在星体内形成，但有很多现象无法解释,比如说氢的同位素氘的形成但是观察到一中物质deuterium，这种物质不可能在星球内部形成。所以证明了两个观点都可能。

 

第三段
                在大爆炸开始的前几分钟，温度很高，形成了lightest的元素，氘等其他轻的同位素就形成了，很多重一点的元素不能被FORM，在大爆炸结束的时候，宇宙膨胀，温度低了，氘就开始escaped destruction。
                随着星体的形成，一些重一点的同位素像氧啊碳啊开始在星体内部形成。

 

题目1：问第三段的内容是什么？我选的是介绍现代学者的观点

题目2：大爆炸后会发生什么？我选的是重的同位素的形成

题目3：什么会weaken H的观点题目

题目4：问第三段的现代学者的观点是什么？

题目5：大爆炸PREVENT什么？答案就在最后一段说一个什么气体逃离什么