2.2 The Big Bang 17 created via Baryogenesis, BBN then produced the lightest nuclei that existed in the early Universe. By far the most abundant was $^{4}$He . $^{2}$H and $^{3}$He were produced in smaller quantities and a very small amount of $^{7}$Li was also pro- duced. The abundance of these nuclei depend strongly on the baryon number to photon number ratio which has a value of η ∼ 10 − $^{10}$. Another great success of the HBB concerns the observed large-scale struc- ture, LSS , of the Universe. The theory predicts an expanding Universe, which we observe as the Hubble Flow , with structure forming under the influence of gravity according to the laws of GR. On cosmologically small scales, matter (baryonic and dark matter) is bound gravitationally into galax- ies, with there existing a hierarchy of structure, consisting of galaxies, galaxy groups, galaxy clusters, galaxy superclusters and galaxy filaments. The last of these structures form the boundaries with the voids of the Universe. The entire collection of components of the LSS is referred to as the Cosmic Web . Further evidence for a HBB model concerns the CMB. Due to the hot, dense early stages of the Universe, the HBB predicts that there should be a remnant blackbody radiation that was emitted when the Universe was young that should still be observable today. For a long time after BBN, photons were being continuously and rapidly scattered off of free electrons, due to Thomson scattering. As the temperature of the Universe continued to fall due to the expansion, there came a time when the free electrons became bound with the nuclei that was present, in a process known as Recombi- η ≡ n$_{B}$ n$_{γ}$ (2.40)