Srinisspace - Big Bang Theory

Have you ever wonder how this universe born? Or how the stars, planets, and galaxies are formed?

Big Bang Theory helps us to understand the Universe better and provides knowledge about the Cosmology. In this post, I tried to explain the Big Bang Theory and Cosmology as much as possible that I can.

Big Bang Theory:

Once scientists believed that the Universe was eternal and not changing which is derived by “Steady State Theory”. Around 1920’s, a proposed theoretical paper suggested expanding Universe and was backed up by observational results by Georges Lemaître.

Big Bang Theory explains that at some point of time, the Birth of the universe happened or atleast the Universe we believe today, started to act as per the laws of physics. Actually it is mentioned as a time at which the universe started to act per laws of physics(specifically General Relativity Theory).

There are five stages derived by scientists to explain the History and Future of the universe according to Big Bank Theory

Very Early Universe:

This is a stage for which we have very limited knowledge or no knowledge. It is described as a stage that our current physical laws were not working. This time also noted as Singularity by Big Bang Theory means. Small ripples in these early stages caused major changes in our current Universe.

Planck Epoch:

It is a time started immediately after Big Bang and in this epoch, The temperature and energies are very high. It is not a favorable situation for the particles to form. The four fundamental forces of our universe also combined in this epoch which is also hard to explain with our current knowledge. So many hypotheses found which leads to different scenarios. Some models trying to describe this Planck Epoch are covered in a category called “New Physics”.

Grand Unification Epoch:

When the Universe started to cool and expand, it reached a point at which the temperature is low enough to gain stability. In this epoch, the fundamental forces separated and started to interact.

Electroweak epoch:

It is the epoch, in which the electronuclear forces begin to separate as strong and weak due to the low temperatures. The chronology of this epoch may come after the Inflationary epoch in some models.

Inflationary epoch:

As the name suggests, in this epoch a rapid expansion of space happened. For example, one nanometer in length expanded to 62 trillion miles approximately in a fraction of second. It is believed to be the result of the separation of strong and electroweak. In this epoch, not the objects but the metrics governing the objects itself changed in scale.

Electroweak symmetry breaking:

This is the final symmetry breaking since the formation universe. After this, the four fundamental forces separated and taken its present form. The fundamental particles had their masses but the temperature is still too high to make stable formations.

Early Universe:

The early universe stage begins after the rapid inflation. In this period, the universe started to act as per the known physics laws and very much understandable.

Quark Epoch:

This epoch started immediately after electroweak symmetric breaking. At this epoch, the fundamental forces started to interact and the universe was filled with quarks, leptons, and their antiparticles. Since the temperature is hot, it prevents the stable formation of particles.

quark – particle contains a fractional electric charge

leptons – elementary particles that don’t undergo strong interactions


When the temperature further cooled, hadrons formed. Hadrons are nothing but the composite structure of quarks by a strong force. Hadrons are classified into buryons and mesons. if an odd number of quarks formed a structure, it is called Buryons(usually three quarks) and even the number of quarks formation is called Mesons(quark and equal anti-quarks). Like Hadron, anti-hadrons were also formed to maintain thermal equilibrium.

Lepton epoch:

At the end of Hadron epoch, all hadrons and anti-hadrons annihilated each other and generated leptons such as electrons, muons & neutrinos, and anti-leptons.

Hadron and anti-hadron collision also resulted in high energy photons.

Photon Epoch:

At the end of Lepton epoch, most of the leptons and anti-leptons annihilated each other. As a result, photons contains the mass-energy in the universe.

Recombination epoch:

The temperature is cooled enough to form stable formations and photons & electrons started to form Hydrogen atoms. And in this epoch, photon decoupling also occurred, in which the high energy electrons combined with Hydrogen atoms and emitted photons to settle in more stable forms.

Dark Ages:

After recombination epoch, the atoms were formed and the universe was transparent. But the stars and galaxies weren’t formed and it required millions of years for the gas structures to form light sources. This period between recombination epoch and Formation of stars known as Dark Ages.

The dark matter and matter both formed clouds of formations due to gravity. Unlike dark matter, the matter can collapse due to many reasons and where the dark matter is dense, the matter collapsed in the clouds of hydrogen gas which formed the stars.

The formation of Universe structure formed from the smaller objects to larger. Multiple stars formed small galaxies. As a result of gravitational forces, new stars born. And small galaxies merged and formed Galaxies. Multiple galaxies formed Groups and Multiple groups formed Clusters. Multiple clusters formed superclusters.

Our solar system resides in Milky-way Galaxy.

Future of the Galaxy:

Our Universe is 13.8 billion old and our current knowledge predicts at least 100 billion years from now.

There are many predictions about the future of the Universe.

Because of its expanding nature, the observable universe will be limited to our local galactic clusters.

Due to its cooling nature, at one point the new stars won't be formed and existing stars will die. The structures will deform into particles like electron and photon.

Some theories suggest that expansion may halt and reverse. Some theories suggest that quantum fields collapse and the forces, particles, and structures take different forms. But all these possibilities are likely to happen after several billion years.

We will discuss some alternative theories in my next post.