According to our best understanding of the universe, about 13.8 billion years ago, the “Big Bang” took place. Then, a massive expansion occurred, which blew up a possible singularity like a balloon, eventually giving birth to our universe. As every seed needs a specific time to blossom into a full-fledged plant, it took quite a bit more than seven days to create the universe as we know it today. However, the first three minutes were the ones where most of the chief events took place. So, here’s what physicists think happened in the first three minutes after the big bang.

We can divide these first three minutes into seven crucial epochs/eras. Each epoch had a different temperature and density and understanding the physics of each era is an important goal for particle physicists and cosmologists.

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Map of the different eras of the universe | Image: ESA

The Planck epoch

Soon after the big bang, the first period that came into existence was the Planck epoch. At this period, the universe’s temperature was 1032 K, so high that all the four fundamental forces (gravitational force, electromagnetic force, weak force, and the strong force) of nature existed together as one super force. This era lasted for 10-43 seconds. Since, at the Planck scale, the current physical theories can not be applied to calculate what happened, very little is known about the physics of the Planck epoch. 

The GUT era

The GUT or the “Grand Unified Theory” era began when the universe was just 10-43 seconds old and continued till 10-36 seconds after the big bang. After the Plank era, the fundamental force of gravity split apart from the other three fundamental forces of the standard model. So now, the electroweak force, the strong force, and the electromagnetic force were one in the GUT era. Moreover, by the end of this era, the temperature had fallen to 1029 K from 1032K.

The inflationary and electroweak epoch

The electroweak epoch was the third one to fall in place after the Big Bang. In this era, strong force got separated from the other two forces, thus leaving behind the weak and electromagnetic forces as a single force. Moreover, cosmic inflation started when the universe was just 10-33 seconds. During inflation, the universe expanded exponentially and grew up from the size of a proton to a size equivalent to that of a fist. During inflation, the universe expanded at a rate faster than the speed of light. However, the exact physics of this extensively accelerated expansion is still not evident.

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Cosmic inflation ended very soon, and later, the universe started expanding normally. Now, the universe is 10-32 seconds old, the temperature has fallen to 100 trillion trillion (1026) K, and most importantly, W and Z bosons have also been formed.

The quark epoch

The electroweak epoch ended 10-12 seconds after the big bang and then began the quark epoch. By then, the universe had cooled enough for the Higgs field to have a positive value. This caused the electromagnetic force and the weak force to split away from each other. So now, all four fundamental forces have attained their identity. As a result, all the available particles can interact with the Higgs field and can gain mass. However, the temperature is still very high for the quarks to merge to form hadrons such as protons and neutrons. Thus, in the standard model of physics, quarks are one of the most elementary entities.

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A diagrammatic representation of how the forces split apart | Image: abyss.uoregon.edu

The hadron era 

Hadrons are a class of particles that are formed from two or more quarks. Soon after the quark epoch ended, the Hadron era started 1 microsecond after the big bang. By this time, the temperature had fallen to an extent to allow the quarks from the previous era to combine to form hadrons. Although a slight matter-antimatter asymmetry from the earlier phases resulted in an elimination of anti-hadrons, still a majority of hadron/anti-hadron pairs eliminated each other. 

So what mostly remained by the end of this period were only the light stable hadrons: protons and neutrons. The hadron epoch ended 1 second after the big bang.

The lepton epoch:

When the universe became one second old, its temperature became favorable enough to form another class of elementary particles, the leptons. Leptons are a kind of fundamental particles in nature and thus are not composed further of any constituent particles like hadrons. An electron is a classic example of a lepton. So now, by this time, leptons and anti-leptons started forming, and this production continued for 10 seconds. The leptons and anti-leptons remained in thermal equilibrium as the energy of photons was still high enough to produce electron-positron pairs. However, the universe was still opaque as the photons could easily get scattered by these free electrons.

The beginning of nucleosynthesis

By now, the universe contained protons, neutrons, electrons, and photons. The photons outnumbered the massive particles by billions to one. Thus, all four fundamental forces had acquired their present-day form. Now, it was time for the most important process of nucleosynthesis to begin. In simple words, nucleosynthesis is when new atomic nuclei are formed from pre-existing nucleons and smaller nuclei. This is the process via which most of the heavier elements form in our universe. 

So now, at the age of 2 minutes, the universe’s temperature dropped below 1.2 billion degrees kelvin. At this temperature, the average photon energy was 1.8 x 10-14 joules, equivalent to the binding energy of deuterium nuclei. A deuterium nucleus consists of a proton and a neutron held together by the strong nuclear force. So, after two minutes of the big bang, deuterium got formed by the fusion of protons and neutrons. This had happened for the first time after the big bang that the universe contained nuclei more complicated than a single proton. 

Finally, after 3 minutes of the big bang, the temperature of the universe dropped below 1 billion degrees kelvin. At this temperature, the average energy of the photons was 1.5 x 10-14 joules, which was equivalent to the binding energy of helium nuclei. So, at the age of 3 minutes, deuterium, protons, and neutrons combined via different possible processes to form helium nuclei.

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Formation of Deuterium and Helium (Image: medium.com)

In a nutshell, in the first three minutes after the Big Bang, the protons and neutrons began fusing, forming deuterium, and the deuterium atoms then joined up with each other, forming helium-4. The three minutes were followed up by many different epochs and versatile nucleosynthesis processes to create the universe we live in today. But, the first three minutes formed the period which gave us the most fundamental elements of our existence, i.e., hydrogen and helium, and set up the stage for the advanced processes to occur. This undoubtedly makes the first three minutes after the big bang the most crucial minutes in the history of the evolution of our universe.

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