By the time the Universe had cooled to a temperature of about 3 × 108 K after 1000 s, the particles had insufficient energy to undergo any more reactions. The era of primordial nucleosynthesis was at an end, and the proportion of the various light elements was fixed. The rates of reaction to form helium and the other light elements have been calculated, and the abundances predicted may be compared with the abundances of these nuclei that are observed in the Universe today. There is close agreement between theory and observation.
To answer this question, Allègre and his colleagues studied the isotopes of rare gases. These gases--including helium, argon and xenon--have the peculiarity of being chemically inert, that is, they do not react in nature with other elements. Two of them are particularly important for atmospheric studies: argon and xenon. Argon has three isotopes, of which argon 40 is created by the decay of potassium 40. Xenon has nine, of which xenon 129 has two different origins. Xenon 129 arose as the result of nucleosynthesis before Earth and solar system were formed. It was also created from the decay of radioactive iodine 129, which does not exist on Earth anymore. This form of iodine was present very early on but has died out since, and xenon 129 has grown at its expense.
Oxygen from algae
THE ISSUE OF CARBON remains critical to how life inuenced the atmosphere. Carbon burial is a key to the vital process of building up atmospheric oxygen concentrations--a prerequisite for the development of certain life-forms. In addition, global warming is taking place now as a result of humans releasing this carbon. For one billion or two billion years, algae in the oceans produced oxygen. But because this gas is highly reactive and because there were many reduced minerals in the ancient oceans--iron, for example, is easily oxidized--much of the oxygen produced by living creatures simply got used up before it could reach the atmosphere, where it would have encountered gases that would react with it.
The presence of oxygen in the atmosphere had another major bene t for an organism trying to live at or above the surface: it ltered ultraviolet radiation. Ultraviolet radiation breaks down many molecules--from DNA and oxygen to the chlorouorocarbons that are implicated in stratospheric ozone depletion. Such energy splits oxygen into the highly unstable atomic form O, which can combine back into O2 and into the very special molecule O3, or ozone. Ozone, in turn, absorbs ultraviolet radiation. It was not until oxygen was abundant enough in the atmosphere to allow the formation of ozone that life even had a chance to get a root-hold or a foothold on land. It is not a coincidence that the rapid evolution of life from prokaryotes (single-celled organisms with no nucleus) to eukaryotes (single-celled organisms with a nucleus) to metazoa (multicelled organisms) took place in the billion-year-long era of oxygen and ozone.
In the early 1990s Tyler Volk of New York University and David W. Schwartzman of Howard University proposed another Gaian solution. They noted that bacteria increase carbon dioxide content in soils by breaking down organic matter and by generating humic acids. Both activities accelerate weathering, removing carbon dioxide from the atmosphere. On this point, however, the controversy becomes acute. Some geochemists, including Kasting, now at Pennsylvania State University, and Holland, postulate that while life may account for some carbon dioxide removal after the Archean, inorganic geochemical processes can explain most of the sequestering. These researchers view life as a rather weak climatic stabilizing mechanism for the bulk of geologic time.
The final chapter in our cosmology saga concerns the history andevolution of the universe. This focuses first on the thermal historysince the Big Bang, transitioning from the radiation-dominated to thematter-dominated epoch. The is not just acatchphrase here, but defines the era when the nucleosynthetic statusquo was effectively established. The principal interactions in primordialnucleosynthesis and the roles of deuterium and lithium in probing thematter content of the universe are identified. The eras ofrecombination and last scattering also form a focus, followed by a briefforay into the gravitational perturbation physics of structureformation. Finally, the course will venture into the exotic world ofinflation, the postulate for the rapid expansion of the quantum soupthat constituted the extremely early universe. This theory has recentlygarnered excitement again with the topical evidence for propulsion ofthe universe's expansion by a mysterious form of Dark Energy.
Fred Hoyle, the atheist British astronomer who formulated the theory of stellar nucleosynthesis and originally coined the term, "Big Bang" on March 28, 1949 had this to say at the end of his career three decades later (Note: There are about 2,000 enzymes in the simplest cell, a bacterium, and 'only' about atoms in the entire universe.):
Nuclei with a mass number greater than seven did not survive in the early Universe. This is because there are no stable nuclei with a mass number of eight — notice from above that the beryllium nuclei decay spontaneously, leading ultimately to more helium-4. The reactions that by-pass this bottleneck take much longer than the few minutes that were available for nucleosynthesis at this time. (Remember, we're now talking about a time-span of around 15 minutes when the Universe had an age of between 100 and 1000 s.) Before more advanced reactions could occur, the Universe cooled too much to provide the energy necessary to initiate them.
It takes nearly 10 million years to burn through the hydrogen and then things heat up and the helium begins fusing together. Stellar nucleosynthesis continues to create heavier and heavier elements, until you end up with iron.
At high temperatures (greater than 109 K), there are a lot of high-energy photons so this reaction is favoured to go from right to left. As a result, deuterium nuclei were rapidly broken down. However, as the temperature fell below 109 K when the Universe was about 100 s old, deuterium production was favoured. Virtually all of the remaining free neutrons in the Universe were rapidly bound up in deuterium nuclei, and from then on other light nuclei formed. One of the reactions that occurred was:
The rapid outgassing of the planet liberated voluminous quantities of water from the mantle, creating the oceans and the hydrologic cycle. The acids that were probably present in the atmosphere eroded rocks, forming carbonate-rich rocks. The relative importance of such a mechanism is, however, debated. Heinrich D. Holland of Harvard University believes the amount of carbon dioxide in the atmosphere rapidly decreased during the Archean and stayed at a low level.
And,likewise, elements heavier than iron are not produced in stars, so what istheir origin?.The construction of elements heavier than Fe (iron) involves nucleosynthesis byneutron capture.
As the temperature continued to decrease, protons and neutrons were able to combine to make light nuclei. This marked the beginning of the period referred to as the era of primordial nucleosynthesis (which literally means 'making nuclei'). The first such reaction to become energetically favoured was that of a single proton and neutron combining to produce a deuterium nucleus, with the excess energy carried away by a gamma-ray photon: