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This resulted in a decrease in the temperature of the universe.

The universe had already existed for a very small fraction of a second and was dominated by radiation.

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KW - Formation and abundances of the elements

Stellar nucleosynthesis is the process by which elements are created within stars by combining the and together from the nuclei of lighter elements. All of the atoms in the universe began as hydrogen. Fusion inside stars transforms hydrogen into helium, heat, and radiation. Heavier elements are created in different types of stars as they die or explode.

Abstract. Primordial nucleosynthesis provides a probe of theUniverse during its early evolution.

Hydrogen and helium account for nearly all the nuclear matter in today's universe. This is consistent with the standard or "" model. The process of forming the hydrogen and helium and other trace constituents is often called "". Schramm's figures for relative abundances indicate that helium is about 25% by mass and hydrogen about 73% with all other elements constituting less than 2%. Carroll & Ostlie give 23 to 24% helium. There is a window of uncertainty, but it is clear that hydrogen and helium make up 98% plus of the ordinary matter in the universe. This high percentage of helium argues strongly for the big bang model, since other models gave very small percentages of helium. Since there is no known process which significantly changes this H/He ratio, it is taken to be the ratio which existed at the time when the deuteron became stable in the expansion of the universe. This ratio is significant as a test of cosmological models since it will be affected by the time period from the time when the temperature dropped below that necessary to produce neutrons from protons to the time when the deuteron became stable, halting the of the free neutrons.

The reason for this was once again the expansion of the universe.

But in the early universe, with its high temperature and density, this was common.

The temperature drops during the universe expansion until the neutrons and protons have frozen out. The era of element building is somewhat later than the neutron/proton freeze-out, but the time interval is short enough that the decay of the free neutrons has no major impact on nucleosynthesis. Although reactions involving leptons and the neutron-proton conversions are generally slow, their reaction rates are very temperature-dependent and initially the neutron/proton ratio is the thermodynamic equilibrium value. This ratio depends on temperature since the mass energy of the neutron at rest is larger that of the proton—a higher temperature gives a higher neutron/proton ratio. For a more rapid expansion the freeze-out temperature is higher so that a more rapidly expanding universe has a greater neutron abundance and ultimately a greater 4He abundance.

In the time prior to element building, neutrinos are formed in equilibrium with their associated electrons, muons, and tau particles. The number of distinct leptons governs the number of neutrino families that can be created during the lepton era. This, in turn, governs the energy density of the universe since a larger number of distinct neutrino types increases the energy in the form of neutrinos. Because of the free expansion character of the early evolution of the universe, a larger number of neutrino types increases the rate at which the universe expands and reduces the time available for element building. Consequently, the abundances predicted by the Big Bang nucleosynthesis models constrain the number of independent neutrino families.

Chronology of the universe - Wikipedia

This started to tip the balance in favor of the proton-forming reactions.

The simplest type of atom in the universe is a hydrogen atom, which contains a single proton in the nucleus (possibly with some neutrons hanging out, as well) with electrons circling that nucleus. These protons are now believed to have formed when the incredibly high energy quark-gluon plasma of the very early universe lost enough energy that began bonding together to form protons (and other , like neutrons).

Hydrogen formed pretty much instantly and even helium (with nuclei containing 2 protons) formed in relatively short order (part of a process referred to as Big Bang nucleosynthesis).

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  • Early Universe and Big Bang Nucleosynthesis | …

    In the early universe, as a result of the rapid collisions between particles, there was a state of thermal equilibrium.

  • Big Bang Nucleosynthesis - Georgia State University

    Therefore, we can follow the evolution of the early universe through these formulas, even though we weren't there.

  • Nucleosynthesis in Supernovae and the Early Universe: …

    The first interaction to be considered was the constant annihilation and re-creation of electrons and positrons.

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Supernova Nucleosynthesis in the Early Universe : …

In the early universe there were no physical "walls" to contain these objects, but there were so many collisions happening so quickly that the collisions themselves acted like the walls of the universe.

Nucleosynthesis in the Early Universe - SAO/NASA ADS

Gravity took over and eventually these atoms were pulled together into massive clouds gas in the vastness of space. Once these clouds got large enough they were drawn together by gravity with enough force to actually cause the atomic nuclei to fuse together, in a process called . The result of this fusion process is that the two one-proton atoms have now formed a single two-proton atom. In other words, two hydrogen atoms have begun one single helium atom. The energy released during this process is what causes the sun (or any other star, for that matter) to burn.

Nucleosynthesis in the Early Universe - ResearchGate

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.

$r$-process nucleosynthesis in the early Universe …

Given the progress exploring theconstituents, structure, and recent evolution of the Universe, it is timely to review the status of Big Bang Nucleosynthesis (BBN) and to confront its predictions, and the constraints which emerge from them, with those derived from independent observations of the Universe at much later epochs in its evolution.

[0809.4242] Supernova Nucleosynthesis in the Early Universe

Following an overview of the key physics controlling element synthesis in the early Universe, the predictions of the standard models of cosmology and particle physics are presented, along with those from some non-standard models.

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