Big Bang and the Timeline of Universe

Introduction:

Whenever we start thinking about the universe and laws of nature that drive this beyond the imagination world, the first question stuck in our mind is, ‘How was the universe born?’ A few decades ago, this question was considered one of the most challenging questions to answer for the science community. But today, after a series of theories and predictions, we have indeed answered this up to a certain extent.

The Big Bang Theory, a very famous term we all are familiar with. But what exactly is the Big Bang? And what happened during the formation of the universe? You will find the answers very soon.

Singularity and the Big Bang Model:

Big Bang theory is considered one of the most accurate theories for the birth of the universe. The theory is based on the equations of general relativity. According to Big Bang, initially, the universe was a highly dense energy state. The density and temperature of this initial energy state are beyond the imagination of humanity. The origin of this initial energy is still a question. You can imagine this initial state of the universe as a small dense sphere of energy. This initial state is also termed as ‘singularity’.

Singularity means that initially, the distance between objects in the universe was zero, all the physical laws are indistinguishable, or in other words, they were meaningless. There was no existence of space and time during singularity. At the singularity, nothing could escape, not even the light.

The Big Bang model explains how the present state of existence of the universe can be traced back to a singularity. During the Big Bang, this initial state of energy exploded, the universe expanded and gradually cooled, creating space and time. Georges Lemaitre called this singularity a “primaeval atom”. In simple words, a big bang is an event that resulted in the birth of the universe, and singularity is the state just before the big bang. However, many people refer to the singularity itself as ‘Big Bang’. 

The model of Big Bang works on two postulates, which are

(i) The universe is isotopic, which means that the universe is the same in all directions.

(ii) The laws of physics are universal.

The universality of physical laws means that the laws of physics are applicable everywhere in the universe; they remain the same. All planets, stars, galaxies, clusters and superclusters follow the same rules of physics.

Expanding universe and the Big Bang:

The expansion of the universe plays a vital role in the mathematical model of the Big Bang. Mathematically, general relativity and Friedmann-Lemaitre-Robertson-Walker (FLRW) matrix have indirectly proved that the universe is expanding, not necessarily at a uniform rate. FLRW matrix itself is an exact solution of Einstein’s general relativity. For many readers, the big bang is nothing but an explosion of matter in space. That’s not true. Big Bang is the explosion of space itself. The matter within the space (like galaxies and clusters) is not expanding; instead, it is the space itself expanding in all directions.

Thermalization:

Thermalization is the process of the body or matter reaching thermal equilibrium. It is also an essential part of the Big Bang model. During the early stages of the universe, the expansion rate was faster than the rate of thermalization. The Big Bang model still does not entirely explain the formation of the universe. It describes the emergence of matter but does not explain the origin of energy, space and time.

After several observations and mathematical calculations, astronomers concluded that the event of the Big Bang happened 13.8 billion years ago, which is also the age of the observable universe. After the Big Bang, a series of events have taken place.

Timeline of the Universe

Introduction:

As discussed in the last part, the universe itself was a singularity just before the big bang. No laws of physics were applicable at that time. Then what exactly happened after the Big Bang that today we have such a large observable universe existing under different laws of physics. When and how was the gravitational and electromagnetic forces originated? How did atoms and subatomic particles come to existence? All such questions can be answered, up to a certain extent, by understanding the series of events that happened after the Big Bang.

Astronomers divided this timeline into five major stages. They believe that no time exists before these stages. The stages are:

1. Very early universe (just after the big bang)

2. Early universe

3. Dark Age

4. Present day

5. Ultimate fate 

Singularity is not included in these stages. The reason is very simple, no existence of time. Time itself originated after the Big Bang, so singularity cannot be a part of this timeline. However, it is impossible to prove what exactly happened after the big bang. 

(1) The very early Universe:

It is the earliest stage of the universe after the big bang. This period was the first picosecond (10ˉ¹²) of the universe. Time itself began during this period. This stage lasted for such a small fraction of a second, but it still has a significant impact on the entire universe today. The very early universe includes the Planck epoch, unification epoch, inflationary epoch and electroweak epoch. This is a particular time in cosmic history when the universe started expanding. It is expanding still today and will continue to expand till the end of time.

(i) Plank Epoch:

The Planck epoch is the first 10^−43 seconds of the universe. During this epoch, the temperature and density of the universe were so high that ordinary atomic or subatomic particles were impossible to form. Even the fundamental forces like gravitational and electromagnetic forces were unified as one single force. The range of temperature at the time is beyond the capability of human imagination. No laws of physics would work at that temperature. Hence different hypotheses were proposed for the Planck epoch.

The Planck time, 10^−43 seconds, is the shortest possible time in the universe. It is believed that after the Planck time, the spontaneous symmetry breakdown occurred, creating four fundamental forces. During the Planck time, the universe was dominated by quantum effects. The length of the universe during the Planck epoch was 1.6×10^−35 m (Planck Length), and the average temperature was 10^32 K. Planck length is the smallest possible length in the universe.

(ii) Grand Unification Epoch:

This period includes the time between the Planck time and 10^−36 seconds just after the big bang. This period marked the beginning of spontaneous symmetry breakdown. During this time, the universe expanded and cooled enough to cross the transition temperature (The temperature at which fundamental forces can be separated from each other). The four fundamental forces include gravitational force, electromagnetic force, the weak interaction and strong interaction. During the grand unification epoch, three of these forces were still unified as an electronuclear force.

Gravity was now separated from them. There was still no existence of mass, charge and colour charge. The grand unification epoch started with the separation of gravity from the unified force and ended with the separation of strong force from the remaining unified force. Hence, at the end of grand unification, three forces existed: gravity, strong interaction force, and unified force. 

Note: Strong interaction is the same force that binds quarks into a hadron and two protons inside a nucleus, despite having the same charge.

The separation of strong force leads to the formation of elementary particles. So, just 10^−36 seconds after the Big Bang, their formation of particles like quarks began. However, it took much more time to form the first atom, which we will see in later topics. The separation of strong force from the unified force led to cosmic inflation, eventually leading to an inflationary epoch.

(iii) Inflationary Epoch:

This period lasted from 10^-36 seconds to 10^-32 seconds after the Big Bang. During this time, the universe expanded in all directions at a rate greater than the speed of light. The universe expanded by billions of times, possibly 10^27 times the initial size. This sudden expansion of the universe is known as inflation.

The separation of strong force from the unified force is the possible reason for this inflation. Inflation also leads to a sudden drop in the temperature. Despite such large scale inflation, the final width of the universe at the end of the inflationary epoch was just 10 cm! With such rapid expansion, all the elementary particles present were distributed in the vast universe. Inflation released a large amount of potential energy in the form of hot, dense quarks and anti-quarks. Now the universe entered the electroweak epoch. After the inflationary era, the universe continued to expand but at a much slower rate.

(iv) Electroweak Epoch:

The electroweak epoch lasted from 10^-32 to 10^-12 seconds. At this stage, the universe's temperature had fallen enough to separate strong interaction from the unified force but high enough to keep electromagnetism and weak interaction unified. The separation of strong interaction force formed particles like Higgs bosons and W and Z bosons in large numbers. This period also marked the beginning of the modern universe. By the end of the electroweak epoch, electromagnetic force and weak interaction were also separated. 

At the end of this epoch, the universe was cooled enough to support four fundamental forces separately. Also, during this time, the interaction between the fundamental particles gave them common masses. But the temperature was still too high to form other fundamental particles like proton and neutron. 

(2) The early Universe:

Now we have seen what exactly happened during the first picosecond after the big bang. After just one picosecond universe is now holding four fundamental forces. The early universe period lasted for around 370,000 years. During this time, various kinds of elementary, subatomic particles were formed. By the end of this time, the universe would be matter dominated. 

The timeline of the early universe can be divided into four major periods.

1. Quark epoch
2. Hadron epoch
3. Lepton epoch 
4. Photon epoch

The cosmic microwave background (CMB), which we see today, is the photons released during this time. Hence when we look at the CMB, we are looking at the early stage of the universe. CMB is the farthest or says the oldest observation of the universe.

(i) Quark Epoch:

This epoch started just after the electroweak epoch. It lasted from 10^-12 to 10^-5 seconds after the big bang. As discussed earlier, during this time, the universe's temperature was still too high for quarks to bind together as hadrons. Even their collisions were too energetic to bind into a meson. During this period, the universe was filled with hot, dense quarks, leptons and anti-quarks. After about 10^-11 seconds, the energies of particles was equivalent to the energy we attain today at particle accelerators. 

 

At the end of the quark epoch, the energy of these particles was enough to bind into a hadron. That was the beginning of the Hadron Epoch.

Note: Hadrons are particles made up of quarks, anti-quarks and gluons. The two types of hadrons are baryons (made up of three quarks) and mesons (made up of a quark and an anti-quark).  

(ii) Hadron Epoch:

This epoch started with the end of the quark epoch and lasted till 1 second after the big bang. By this time, the temperature of the universe was enough for quarks to form hadrons. The temperature was around 1 trillion degrees! The universe was still expanding and was about the size of the solar system. The decrease in temperature stopped the formation of new hadrons. This lead to an annihilation reaction (reaction in which a particle and antiparticle collide). Most of the hadrons and anti-hadrons were destroyed in this reaction. A small number of hadrons were still left at the end of the epoch.

(iii) Lepton Epoch:

This epoch started at 1 second and lasted till 10 seconds after the big bang. Hadrons and anti-hadrons were annihilated, but still, the temperature was too high to form neutrino and electron-positron pairs. Leptons such as electrons, muons and neutrinos were dominating the universe. Hence this period is known as Lepton Epoch. Electrons were not in a stable state; instead, they were in thermal equilibrium with gamma photons. 

One second after the big bang, the universe was about a thousand times the size of the solar system. One of the major events of the lepton epoch was the neutrino decoupling, where the neutrino stopped interacting with baryonic matter. This resulted in the formation of cosmic neutrino background. At the end of this epoch, when the universe was 10 seconds old, both hadrons and leptons were present. But still, there were no atoms. The universe was still too energetic to form a stable atomic nucleus. 

(iv) Photon Epoch:

This epoch started just after the lepton epoch (10 seconds after the big bang) and lasted for about 370,000 years after the big bang. Ten seconds after the big bang, the temperature of the universe was about a billion kelvin. During this time, photons dominated the universe. The temperature was still too high for photons to interact with charged particles.

At the beginning of this period, nucleosynthesis started, which lasted for another 3 minutes. Nucleosynthesis is the process of the formation of nuclei other than hydrogen. As the universe expanded, the temperature became enough for proton and neutron to bind into a stable nucleus. Hence, the universe was finally undergoing nuclear fusion. 

25% of protons and almost all the neutrons fuse to form a hydrogen isotope called deuterium. The deuterium again fused to form helium. Fusion reaction also created heavier nuclei like beryllium and beryllium-7, and tritium (another isotope of hydrogen). But due to high temperature, those heavier nuclei were unstable. They immediately broke apart again. 2 min after the big bang, the universe was cooled enough for deuterium to exist in a stable form. After 3 min, helium also existed in a steady state. At the end of nucleosynthesis, hydrogen, deuterium, helium-3, helium-4, and lithium-7 were stable.

By mass, the universe was 75% hydrogen, 25% helium. Lithium was formed in a minimal amount. This proportion is almost similar to that of today’s observable universe. Nucleosynthesis also produced a very small (almost negligible) proportion of carbon, nitrogen, oxygen (CNO). One should also note that the big bang never created the heavier elements; they were formed inside the core of stars or during supernova explosions.

The universe continued to expand, and after about 20 minutes, the universe was 600 light-years wide. Matter density was about 4 g/m³. Most of the energy by this time was in electromagnetic radiations. After 47,000 years, the universe was dominated by matter because, by that time, the universe's expansion led the density of matter to exceed that of radiations. The universe was 84% cold dark matter and 16% ordinary matter. We still do not know the exact nature of dark matter; hence the big bang does not explain the formation of dark matter. 

After about 18,000 years, the universe was cooled enough for free electrons to combine with helium nuclei to form He+ atoms. Now the universe was undergoing the process of recombination. It is the process in which the ionized particles combine to create a neutral atom. For the first time after the big bang, electrons and atomic nuclei were combined.

After around 100,000 years, neutral helium atoms began to form. During the same period, the formation of helium hydride also began, making it the first molecule to be formed. The formation of neutral hydrogen atoms began after around 260,000 years. Now the radius of the observable universe was 42 million light-years. The matter density was 500 million hydrogen and helium atoms per m³. 

The newly formed hydrogen and helium atoms and lithium (in negligible amounts) reach their ground states by releasing a photon. The process in which an atom emits a photon to obtain the stable state (ground state) is called decoupling. Decoupled photons created the Cosmic Microwave Background (CMB). Today these photons are detected using radio waves. The CMB is perfect evidence of the early universe. Cosmic Microwave Background is the end of this epoch.

(3) Dark Age: 

The dark age started just after the recombination and decoupling process, about 370,000 years after the big bang and lasted for another billion years. Now the universe was cooled enough for light to travel longer distances, but there were no light-producing sources at that time. The only light source was the photons released during the decoupling reaction, that is, the CMB. Otherwise, the universe was completely dark. 

 

During this time period the temperature of the universe was dropped to 4000 K. Around 150 million years after the big bang the first source of light, that is Population III stars began to form from the gas clouds of hydrogen and helium. The earliest galaxies were formed between 380 to 700 million years after the big bang. The formation of such structures marked the beginning of the end of the dark age. 

The formation of the structure was hierarchical, that is the smaller structures were formed before the larger ones. The earliest structures of the universe were population III stars and dwarf galaxies. The formation of heavier elements started at the core of the Population III stars. However, astronomers are unable to discover any star belonging to the Population III type. 

The concept of structure formation during the dark age was developed by observing the CMB and the quasars. The quasars are also one of the earliest structures after Population III stars and dwarf galaxies. As the population of dwarf galaxies and quasars increased, their intense radiations started the process called Reionization. In Reionization, the hydrogen atoms were broken into free electrons and photons. For the first time, the process of Reionization was happening after the recombination. Expansion of the universe gradually slowed down the Reionization, and now the temperature was dropped to 60 K.

Eight hundred million years after the big bang, galaxies were gravitationally interacting. The gravitational attraction between the galaxies led to the formation of clusters and superclusters. One billion years after the big bang, the dark age finally ended. Now the universe was transparent, with stars and galaxies as significant light sources.