Nucleosynthesis primordial

The universe is between 10 and 20 billion years old, with the best estimate of its age being 14 + 1 x 109 y old. The universe is thought to have begun with a cataclysmic explosion called the Big Bang. Since the Big Bang, the universe has been expanding with a decrease of temperature and density. 

At a time of 10-2s (T 1011 K), the density of the universe dropped to 4 x 106kg/m3. [In this photon-dominated era, the temperature T (K) was given by the relation 

These primordial nucleosynthesis reactions began with the production of deuterium by the simple radiative capture process  

Notice that the deuteron can be destroyed by the absorption of a high-energy photon in the reverse process. At this time, the deuteron survived long enough to allow the subsequent reactions 

3H and 3He are more strongly bound allowing further reactions that produce the very strongly bound a particles  

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Big efforts have been devoted in the last years to the study of light elements abundances. Definitively their importance is strongly related to cosmology as well as to stellar structure and evolution. In fact hints on the primordial nucleosynthesis can be achieved from Li, Be and B primordial abundances. Moreover these studies can be a precious tool for testing and understanding the inner stellar structure, especially for what regards the mixing processes in stellar envelopes [11-... 

Primordial nucleosynthesis really puts the Big Bang cosmology to the test. One might call it a baptism of fire. From these brief but brilliant and fertile beginnings arose a series of light nuclei that are today found everywhere in nature above all hydrogen, followed by helium, which between them amount to 98% of the total mass of atomic matter in the Universe. 

The crucial species in primordial nucleosynthesis of the light elements is helium. There are two reasons ... 

It is easy to see why the results of primordial nucleosynthesis, and in particular the final abundance of deuterium, should be so sensitive to the nucleonic density of the Universe. For this reason, deuterium, made up of one proton and one neutron, can be considered as an excellent cosmic densimeter. The disparate abundances for their part are related to specific nuclear properties of the isotopes under consideration. 

Fig. A1.3. Comparison between observed abundances and abundances predicted by the theory of primordial nucleosynthesis. The horizontal axis shows the ratio r between the number of baryons and the number of photons. The vertical axis shows the mass fraction of helium and the numerical ratios D/H, He/H and li/H. Observational data are represented by boxes with height equal to the error bar. In the case of helium and lithium, there are two boxes, indicating the divergence between different observers. Deuterium holds the key to the mystery, but it is difficult to measure. The region of agreement is shown as a shaded vertical ribbon (after Buries Tytler 1997). A higher level of deuterium would lead to a lower baryonic density, of the order of 2%. This would agree better with the lithium data, which have been remarkably finely established. This idea is supported by E. Vangioni-Flam and shared by myself. (From Tytler 1997.)...

Let us examine this situation in more detail. It is quite clear that the density of matter in clusters of galaxies is significantly higher than the density of nuclear matter as deduced from primordial nucleosynthesis (2-5% of the critical density). If we assume that these structures are representative of the Universe as a whole, then in order to make up the difference, we are forced to resort to clouds of exotic elementary particles left over from the Big Bang. The fate of the Universe then lies in the hands of non-nuclear matter of unknown but not unknowable nature (e.g. neutralinos). 

The extremely low abundance is the result of two factors, the relative fragility of the isotopes of Li, Be, and B and the high binding energy of 4He, which makes the isotopes of Li, Be, and B unstable with respect to decay/reactions that lead to 4He. For example, the nuclei 6Li, 7Li, 9Be, nB, and 10B are destroyed by stellar proton irradiations at temperatures of 2.0, 2.5, 3.5, 5.0, and 5.3 x 106 K, respectively. Thus, these nuclei cannot survive the stellar environment. (Only the rapid cooling following the Big Bang allows the survival of the products of primordial nucleosynthesis.)... 

The total baryonic contribution in the universe, including the hypothesised WHIM, sums to flb = 0.028 0.005. The corresponding baryon fraction flb/flm is 0.10 0.02. This is to be compared with primordial nucleosynthesis at z 109 fli, = 0.04 0.004. In addition the CMB peak heights at z 1000 yield a similar value fib = 0.044 0.003. Finally, Lyman alpha forest modelling at 2 3 suggests that fli, ss 0.04. There is also the indirect measurement of baryon fraction from the intracluster gas fraction of 15%. From this, combined with flm, we also find that fib 0.04. 

Terry P. Walker, Gary Steigman, David N. Schramm, Keith A. Olive, and Ho-Shik Kang, Primordial Nucleosynthesis Redux, Astro-physical foumal 376, 51-69 (1991). 

Cyburt R. H., Eields B. D., and Ohve K. A. (2002) Primordial nucleosynthesis with CMB inputs probing the early universe and light element astrophysics. Astropart. Phys. 17, 87—100. 

Armed with an understanding of the evolution of the early universe and its particle content, we may now proceed to the main subject of these lectures, primordial nucleosynthesis. 

The Big Bang. In what is generally known as the standard family of Big Bang (Friedmann) models, 7Li is the only LiBeB nuclide synthesised in observable amounts. This Li in full or in part is seen in warm very metal-poor stars, as the Spite plateau. Nonstandard Big Bang models in a wide variety of forms have been proposed. Often, the consequences for the primordial nucleosynthesis are a focus of these proposals. 

Although, in hindsight, the 1932 model of Z protons and N = A — Z) neutrons became obsolete in 1964 (as seen below), it is still the colloquial way of thinking in nuclear physics, and it served to describe the stellar and the major part of the prestellar (primordial) nucleosynthesis. Since geologists pointed out their dissatisfaction with the maximum life-time of 50 million years for our Sun, if powered by gravitational contraction, as first calculated by Lord Kelvin [William Thomson (1824-1907)] one had looked to something like radioactivity, but it was found that the only viable reaction is... 

Cosmochemistry, however, remains a fertile area of research, as there remain many outstanding problems. A comprehensive approach to cosmochemistry requires a combination of a number of topics like primordial nucleosynthesis, stellar nucleosynthesis, explosive nucleosynthesis and solar abundance. The Kodai school on Synthesis of elements in stars was organized to provide a glimpse of this exciting area of research to astrophysicists of tomorrow, motivated young students from India and abroad. The lectures are thus aimed at researchers who would like to venture deeper into this exciting arena. 

Abstract This chapter provides the necessary background from astrophysics, nuclear, and particle physics to understand the cosmic origin of the chemical elements. It reflects the year 2008 state of the art in this extremely quickly developing interdisciplinary research direction. The discussion summarizes the nucleosynthetic processes in the course of the evolution of the Universe and the galaxies contained within, including primordial nucleosynthesis, stellar evolution, and explosive nucleosynthesis in single and binary systems. 

Together with the equation for the evolution of the temperature Tg = 13.336/this sets the conditions for primordial nucleosynthesis. The strength of the standard Big Bang scenario is that only one free parameter — the ahove-introduced haryon-to-photon ratio 7/ — must be specified to determine all of the primordial ahtmdances ranging over 10 orders of magnitude. 

Thus, a fit of 77 to observed primordial abundances not only probes the conditions in the early Universe at the time of nucleosynthesis hut can also reveal the curvature of the Universe, or at least the baryonic contribution to that curvature (Schramm and Turner 1998). Historically, primordial nucleosynthesis was the first tool for determining the geometry of the Universe. With the increased accuracy in the resolution of the angular multipole expansion of the CMBR temperature fluctuations dehvered hy WMAP, the total density of the Universe (and not just the baryonic one) can be determined independently now (see Sect. 12.2.1). 

Because free neutrons are not stable, but decay with a half-life of Tm = (10.25 0.015) min, the neutron-to-proton ratio will change from 1/6 to 1/7 until the onset of primordial nucleosynthesis. 

The nuclear reaction rates (cross section) for the reactions specified in Fig. 12.4 are well determined, also at the interaction energies relevant to primordial nucleosynthesis, which are comparatively low by nuclear physics standards. Thus, once the initial conditions are determined, the evolution of the different species with time and the final abundances can be calculated with high accuracy. The only open parameter in the standard Big Bang nucleosynthesis model is 77. Since the baryon density is proportional to rj and the reaction rates are density dependent, the final abundances will also depend on the choice of 77. 

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