# Big-Bang Nucleosynthesis And The Baryon Density Of The Universe

By definition, in thermodynamic equilibrium the energy is distributed evenly among all components of a system.For a simple gas, this would mean that, on average, all of the myriads of particles flying around have the same kinetic energy.

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The baryon density is best from the CMB matching, but the observed confirmed matter density is mostly from interstallar mass, and then the stars and galaxies, and it accounts for maybe 3/4 of it.

See estimated photon to baryon ratio $\eta$ is on the order of 1 billion.

Current models of the universe suggest the first galaxies began forming about 100 million years after the Big Bang, marking the beginning of the end of the dark ages.

This process of star and galaxy formation gradually continued until virtually all the hydrogen and helium that make up most of the universe was once again ionized, this time by starlight, about 500 million years after the Big Bang.

The articles describes other estimates for the baryon density from the CMB.

The primordial prevalence of baryons over their antiparticles remains research area.

The ratio of baryons to photons or the baryon abundance is defined as: $n \equiv \frac \simeq 2.75 \times 10^ \; \Omegah^2$ where $N_b$ is the number density of baryons, and $N_\gamma = 4.11 \times 10^8 \; m^$ is the number density of photons.

Thus the primordial abundances of baryonic matter in the standard Big Bang nucleosynthesis scenario (BBN) is proportional to $\Omegah^2$.

Some systematic uncertainties remain in the calculations arising from the reaction cross sections.

"Though the early universe is assumed to be dominated by radiation, how do we know this?