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.
It is known that the weak force has a CP asymmetry that could contribute to a larger number of particles over antiparticles.
To match the otherwise expected equality, roughly, of baryons and photons, it would have to account for the roughly 1 billion to 1 preference for particles over antiparticles.
Deuterium is observed as a Lyman-$\alpha$ feature in the absorption spectra of high-redshift quasars.
A recent analysis gives $\Omega(^2$H$)h^2 = 0.020 \pm 0.001 \quad (1 \sigma \; $CL$)$ more precisely than any other determination.Even cold hydrogen gives off light in the form of radio waves with a specific wavelength of 21 centimeters. An easy to read article on Galaxy Cosmos Redshift 7 is Science Magazine's "Astronomers spot first-generation stars, made from big bang", by Daniel Clery (Jun. The baryon to photon ratio is estimated and measured. The photon density is calculated in various ways, but one way is from the CMB temperature of 2.73K.The baryon to photon ratio is estimated in a more complex way, and is related to the ratio of baryon density to the critical density (for a flat universe), and the Hubble constant.Studies of Big Bang Nucleosynthesis - the formation of atomic nuclei like those of helium, deuterium or lithium in the early universe - have led to some of the most impressive predictions of the big bang models.A brief overview can be found in the spotlight text Big Bang Nucleosynthesis: Cooking up the first light elements - here, we take a closer look at the physics behind the predictions.Is this ratio derived from first principles -if so, how- or, if it results from a small favoritism for matter over antimatter in the early universe, then from what observations is the baryon/photon ratio calculated -and how?Though the early universe is assumed to be dominated by radiation, how do we know this?The He and other baryons produced in Big Bang nucleosynthesis (BBN) is also covered in that reference, and others.The abundance of $^4H_e$ is not too sensitive to $\eta$ but the other light elements up to $L_i$ are.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.