Big bang nucleosynthesis (or primordial nucleosynthesis) happened from ten seconds to twenty minutes and spanned over the entire universe. This era is notably known for the formation of atomic nuclei via nuclear reactions. Deuterons formed as protons diffused with neutrons and released photons. These deuterons reacted with other deuterons, protons and neutrons to form helium–3, helium–4, hydrogen and lithium–7 nuclei. BBN also led to an important conjecture that protons and neutrons were almost equal at the end of hadron epoch and there were no hot photons to interact with these particles so if all of protons and neutrons fused to form deuteron and helium then we will be left with no hydrogen. So people started arguing that there must be a plenty of photons in the universe, that is, nearly a billion photons per particle of matter. This idea came from the famous Alpher–Bethe–Gamow (\alpha\beta\gamma) paper . It is important in a sense that it was the very first theoretical prediction of background cosmic radiations nearly more than a decade before their accidental discovery by Robert Wilson and Arno Penzias in 1965 . Once the BBN stopped, these relic radiations permeated all over the universe.

As the BBN ended, universe entered into the photon epoch which lasted for the next 380,000 years. This is the radiation–dominated era and the then universe was a hot, dense plasma which mostly contained atomic nuclei, electrons and photons. But due to scale dependence of the inverse fourth power of the size of the universe, radiation diluted quickly, temperature cooled which stopped the interaction of photons with nuclei. Photons started permeating all over the space uniformly, making universe transparent and created the cosmic microwave background radiation (CMBR) spectrum. It is the start of large–scale structure (LSS) formation in the universe but before going into the details of LSS, let’s visit CMBR briefly.

Figure 2.1: Evolution of the contents of the universe with the scale factor (Credits: 2011 Pearson Education. Inc.)

CMBR provides us with the most direct evidence coming directly from the universe to study the early era in its evolution. It also makes it evident that matter–radiation crossover took place at around a redshift of 1,100, nearly 50,000 years after the big bang, the last instance when the universe was opaque. This boundary between opaque and transparent universe is usually called the surface of last scattering. The then temperature of CMBR was about 3,000 kelvin and it had a blackbody spectrum . But as the universe expanded, it only red–shifted the photons quite homogeneously and lowered their temperature. Present temperature of CMBR is found to be around 2.725 kelvin and it depicts almost the same spectrum as for a blackbody but it is not exactly the same. However, from these minute temperature fluctuation, we can deduce the level of inhomogeneities present in the far past. In the last decade of twentieth century, Cosmic Background Explorer[1] (COBE) measured the intensity of CMBR in the infrared region over a full range of wavelengths to see if there are any anisotropies and it estimated its temperature to be nearly 2.725 kelvin with an uncertainty of 0.002. It affirmed the isotropy on the largest scales and hence the homogeneity too.

Figure 2.2: Most precisely measure blackbody spectrum of the comic microwave background radiations as measured by FIRAS, COBE (Credits: Quantum Doughnut)

When photons started permeating all over the universe and made a ground for the present relic radiations, there came a first moment of relief after the hot big bang. These non–interacting photons made the electrons to be able to exist calmly, so they started to combine with protons to form neutral atoms. This is termed as recombination. With the passage of time, this recombination was completed and it ceased scattering of particles in the universe. Moreover, it allowed photons to propagate freely in the Universe. This epoch is called decoupling because after this, photons didn’t scatter again as they did in recombination era. This decoupling was occurred on the surface of spherical shell which is called the surface of last scattering or the last scattering surface (LSS) and the radius of this shell is the distance traveled by each photon since its last scattering in recombination era.

Soon after the recombination and decoupling, a period of long and fierce silence started. It is called the dark ages, that is, the time between the formation of the first star in the universe and the recombination. The name compels someone to think about an era which was truly devoid of light but it is not much really true since the CMBR and the photons released by neutral hydrogen atoms, the famous 21 centimetre radiation[2] were present then. However, it is important to note that in that era electromagnetic radiation spectral line of neutral hydrogen atoms was in the infrared region and the with the passage of time, CMBR also redshifted to infrared from visible range, which made the universe dark apart from some statistical anomalies[3].

Dark ages ended with the formation of first star in the universe nearly 150 million years after the big bang. It is the start of reionization epoch which lasted for one billion years. Over time, dark matter started collapsing in halo–like structures and gravity pulled ordinary matter into these to from galaxies. And the large-scale structure formation started in the universe. It is the domination of matter over radiation (as shown in figure 2.1) which lasted for the next ten billion years.

Study of this era would not be completed without discussing hypothetical dark matter whose energy density, at this time, drove the universe to follow the bottom–up structure formation, that is, small galaxies formed and merge into each other to form clusters .

In the next blog post, we will discuss the large-scale structure formation, will try to seek the answer, why the second most abundant content[4], dark matter, is necessary to have mighty structures in the universe, as of our own galaxy Milky Way.

Previous Blog Posts of this Series

  1. Evolution History of Universe (A Story from Zero to Ten Seconds)


Ryden, B. S., & Barbara. (2003). Introduction to cosmology. Cambridge University Press. Retrieved from
Penzias, A. A., & Wilson, R. W. (1965). A Measurement of Excess Antenna Temperature at 4080 Mc/s. The Astrophysical Journal, 142, 419–419.
Wright, E. L., Meyer, S. S., Bennett, C. L., Boggess, N. W., Cheng, E. S., Hauser, M. G., … Wilkinson, D. T. (1992). Interpretation of the cosmic microwave background radiation anisotropy detected by the COBE Differential Microwave Radiometer. The Astrophysical Journal, 396, L13–L13.
Alpher, R. A., Bethe, H., & Gamow, G. (1948). The Origin of Chemical Elements. Physical Review, 73(7), 803–804.

  1. NASA has a dedicated website for COBE, which can be accessed at:
  2. It is the only available probe to the dark ages.
  3. We will discuss these anomalies in some other blog post dedicated to dark ages and CMBR.
  4. The most abundant content of the universe is dark energy, which we will discuss in very detail in the last series of blogs on the evolution history of universe.