William Shakespeare, in Hamlet, wrote: “I could be bounded in a nutshell and count myself king of infinite space”. It is subtle in the dialogue, the human comprehension of the universe. The discernment of this ‘infinite space’ started to begin when man looked at the bluish vast scary dome for the first time. This is the start of cosmology—a science which is regarded as the scientific study of the universe on the basis of theoretical predictions, dubbed in the elegant mathematical equations, and observations, coming directly from the far–off patches of the universe. Cosmology, being very different and distinct from other branches of science, is very special in its essence. It urges to know about the ultimate cause of creation and to seek how things work at the most fundamental level. Let us have a brief look upon how our cosmos ultimately evolves to its present state.

The story of physical cosmology is centered around the exploration of the hot big bang model, which is the widely accepted model of how universe first came into being and then evolved. According to this model, universe started from a “mighty bang” which caused the infinitely dense singularity, containing all of the matter, energy and spacetime, to expand and cool as the time elapsed. The term big bang was coined in 1949 by Fred Hoyle . Big bang happened nearly 13.8 billion years ago and since then universe passed through a series of stages which have been shown visually in the figure 1.1. Cosmologists categorized the evolution of the universe into following three stages:

  1. very early universe,
  2. early universe,
  3. large–scale structure formation.

These stages are further sub–categorized into different epochs based on theory, observational data and the experimental evidence .

Figure 1.1: Evolution history of the universe (Credits: Contemporary Physics Education Project 2016)

The era which started just after the bang and lasted up till one Planck second (10^{-43} seconds), is known as Planck era. Einstein’s general relativity usually applies from this scale onwards. Neither there is any implementable theory nor any observational probe to know what was before the Planck time. It is thought that during this epoch, all fundamental forces were unified into one, also known as the force of quantum gravity. But at the end of this epoch, the grand unified force, the unification of electroweak and strong nuclear force, separated from gravity—the very first occasion in the history of universe when the symmetry of forces broke down.

Began from the 10^{-43} seconds, grand unification epoch ended around 10^{-35} seconds. It is the first known era to which we have ‘somehow’ applicable theories. Many cardinal events took place during this era as it triggered cosmic inflation, proposed by Alan Guth in early 1980s . Also the electroweak force separated from strong nuclear force at the end of this epoch. It is the start of electroweak era which lasted until 10^{-12} seconds. Inflation is considered as an exponential expansion, lasted from 10^{-36} seconds to 10^{-32} seconds, not only distributed, the then produced, elementary particles across the universe slenderly but also cooled down the temperature of the universe. As the inflation ended, an enormous amount of energy was released which increased the temperature of the universe and transformed it into a hot quark–gluon plasma. This is called reheating which lasted up until 10^{-12} seconds. At the end of this era (10^{-11} seconds), all four forces gained their present identities as separated ruling forces of the nature and with this, the very early universe era ended.

Early universe era started with the quark epoch and lasted up to 10^{-6} seconds. Although all forces had been separated out and despite of very high temperature and energy, in this hot and dense quark–gluon plasma, quarks bounded to form hadrons via gluons. This is the time about which electromagnetic force decoupled from weak nuclear force. Another substantial event happened around this time which is dubbed as baryogenesis . It is responsible for the asymmetry in the baryonic matter content over the anti–matter. It is also recognized as the first instance when matter started dominating anti–matter. Even though, there was annihilation because of matter and anti–matter interactions but still there was some residual matter left behind, which is still a mystery. It is estimated that there was almost one part of extra matter in one billion parts of matter.

Notwithstanding the baryogenesis, because of very temperature, it was also possible that hadrons could have been annihilated again into free quarks and abate the number density of matter. But the very next and very brief epoch, hadron epoch affirmed the matter dominion over the anti–matter. This epoch also superintended the formation of the so called ‘protonic soup’. Since the proton is the least massive hadron, with the threshold temperature of about 10^{13} kelvin, so as soon as the temperature fell below this threshold, this epoch ended. At the end of this epoch, universe was filled with free baryons with a majority of protons, neutrons, photons and some other subatomic particles.

Hadron epoch was followed by lepton epoch whose time spanned from 10^{-4} seconds to 10 seconds. It includes two phases, namely, neutrino decoupling and leptogenesis . Neutrinos started to decouple when universe was only one second old and has a temperature of about ten billion kelvin. During this phase, neutrinos stopped to interact with baryonic content and ceased to affect the dynamics of the universe. Before decoupling, neutrinos and their counterparts were in thermal equilibrium either via the process of annihilation or scattering but they decoupled when their interaction rate via weak force became smaller than the rate of expansion of the universe. Some scientists, on the basis of this decoupling, also suggested that there must be relic neutrinos like relic radiations[1], which should permeate throughout the universe[2] . If it is so, then it can be a very good probe to look that far past in the universe.

Immediately after the neutrino decoupling, leptogenesis supervened. The tau and muon particles were created during this phase but as the temperature fell further, most of these heavy leptons annihilated and decayed into a lighter flavor of lepton, that is, electron. Leptogenesis, likewise baryogenesis, ended on the vanquishment and ascendency of leptons over their counterparts. At this stage of evolution, universe can be fancied as a particle pool (or particle soup) of electrons, positrons, protons, neutrinos, neutrons and photons. Another important consequence of this era is to almost equalize the proton–neutron ratio. During this epoch, there were more neutrinos with a high interaction rate with neutrons, so they produced a large number of protons in contrast to neutrons. But as the temperature went down, a de-escalate in the ratio observed. And once the temperature went below the threshold temperature of electrons (which is approximately 10^{10} kelvin), neutrinos stopped to interact with electrons and other particles and proceeded to an equilibrium state. This era ended when the universe was around fourteen seconds old and it led to the subsequent photon epoch but before going to the details of photon epoch, we will briefly discuss another substantial phase in the evolution history of the universe, the big bang nucleosynthesis, in the next episode.


Hawley, J. F., & Holcomb, K. A. (2005). Foundations of Modern Cosmology (2nd ed.). Oxford: Oxford University Press.
Guth, A. H. (1981). Inflationary universe: A possible solution to the horizon and flatness problems. Physical Review D, 23(2), 347–356. https://doi.org/10.1103/PhysRevD.23.347
Follin, B., Knox, L., Millea, M., & Pan, Z. (2015). First Detection of the Acoustic Oscillation Phase Shift Expected from the Cosmic Neutrino Background. Physical Review Letters, 115(9), 091301–091301. https://doi.org/10.1103/PhysRevLett.115.091301
Elizalde, E., Elizalde, & Emilio. (2018). “All that Matter … in One Big Bang …”, &Other Cosmological Singularities. Galaxies, 6(1), 25–25. https://doi.org/10.3390/galaxies6010025

  1. cosmic microwave background radiations
  2. If the suggestions are ever found to be consistent with the observations, these background ‘sea’ of neutrinos would be a very good probe to study such an early phase of universe and the initial conditions then.