For a long time, people have questioned how our universe was created? For which people start trying to find the answer. Now we have six major theories and concepts that try to explain the origin of our universe. These six theories are like that - Big Bang theory, Steady State theory, Eternal Inflation theory, Oscillating Universe theory - (Cyclic model), Oscillating Universe theory -  (Branes model), and Exotic theories - ( Hologram, Creationism, Simulation). Among them, one of the best theory is the Big Bang theory.

What is Big Bang Theory

In all six theories, the Big Bang theory is the most accepted scientific theory today. Because all experimental data is fitted into this theory or another way this theory is fit into our experimental data.  If you do not know what is big bang theory, this article is for you. Today, I am going to unlock the details of the Big Bang theory. In this article, you will know -

1. What is the Big Bang theory?
2. Big Bang Theory - Timeline
 a) Plank epoch
 b) Inflation Epoch
 c) Cooling Epoch
 d) Structure Epoch
3. History of Big Bang theory

What is the Big Bang theory?

The Big Bang hypothesis states that - Because the universe is expanding today, all matters are assumed to have been condensed and contained in only one point, at the start. Extrapolation of the expansion of the universe yields an infinite density of temperature at a finite time of 13.8 billion years in the past. This is called a singularity and it lasted for a Plank epoch.

In this time (from 0 to 10^-43 seconds called Planck time), gravity and all other forces may have been unified. This was the unstable universe which then started to expand and then cool.

So according to this theory if we go back from the current state of the Universe then we get that it must have originated at a single point of infinite density and finite time that began to expand.

Big Bang Theory - Timeline

Through the theoretical principle, mathematical analysis, experiments with particle accelerators, and astronomical studies, scientists have created a timeline of events that began with the Big Bang and led to the present state of cosmic evolution.

Here's a diagram to show the timeline of how the universe expanded -

What is Big Bang Theory

Plank epoch

At this time, the entire universe was confined in a single point. All matters were condensed at a single point with infinite temperature and infinite density. At this time, gravity and all other fundamental forces were in the unified state.

This Planck epoch of time extends from point 0 to approximately 10^-43 seconds, which is called Planck time. Prior to this time, all the laws of physics could be seen breaking down. For which it is not possible to explain what happened before this time. At this point, the size of the universe is just 10^-35 meters and its temperature was 10^32 K.

Due to the extreme heat and density of matter, the state of the universe was extremely unstable. For this unstable state, it explodes and then begins to expand, cooling down, leading to the manifestation of the fundamental forces of physics.

From approximately 10^-43 seconds to 10^-36 seconds, the fundamental forces began separating from each other. After Plank's time, the force of gravity starts to separate from other fundamental forces (like strong nuclear force, weak nuclear force, and electromagnetic force).

Then, from approximately 10^-36 to 10^-32 seconds after the Big Bang, the temperature of the universe was low enough (10^28 K) that the forces of electromagnetism (strong force) and weak nuclear forces (weak interaction) were able to separate as well, forming two distinct forces.

Inflation Epoch

The Inflation Epoch started, with the creation of the first fundamental forces of the universe and this epoch lasted from 10^-32 seconds in Planck time to an unknown point.

Most cosmological models suggest that in this time the universe was filled homogeneously with a high-energy density and that the incredibly high temperatures and pressure gave rise to rapid expansion and cooling.

At this time, temperatures were so high that the random motions of particles occurred at relativistic speeds. As a result, particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. After inflation stops, the universe starts to cool down and the matter is formed together.

Cooling Epoch

As the density and temperature of the universe continue to decrease, the energy of each particle began to decrease and phase transitions continued until the fundamental forces of physics and elementary particles changed into their present state.

Scientists believe that about 10^-11 seconds after the Big Bang, the energy of the particles is significantly reduced. In about 10^-6 seconds, quarks and gluons combine to form baryons like protons and neutrons, and over a few quarks over the antiquarks cause a small number of baryons than the antibaryon.

Since the temperature was not sufficient to create a new proton-antiproton pair, so a mass annihilation immediately followed. This leads to a mix with about 10000001 protons for every 10000000 antiprotons. 

As a result, particles (matter) dominates over the antiparticles (antimatter). After these annihilations, protons-antiprotons pair produced a large number of photons and the energy density of the universe was dominated by photons.

Nucleosynthesis: Temperatures dropping to 1 billion kelvin and the energy densities dropping to about the equivalent of air. At this time, neutrons and protons began to combine to form the universe’s first deuterium (a stable isotope of Hydrogen) and helium atoms. However, most of the Universe’s protons remained uncombined as hydrogen nuclei.

After about 3,79,000 years, electrons combined with these nuclei to form atoms, while the radiation decoupled from matter and continued to expand through space. This radiation is now known as the Cosmic Microwave Background Radiations (CMBR), which is the oldest light in the Universe today.

Structure Epoch

Then for billions of years, slightly dense regions of matter almost evenly distributed throughout the universe are attracted to each other due to gravity. That is why they become denser, creating gas clouds, stars, galaxies, and other astronomical structures that we now observe. This is known as Structure epoch.

History of Big Bang theory

The Big Bang theory was developed as a result of observational and theoretical considerations of the structure of the universe.

The year 1912:

In 1912, American astronomer Vesto Slipher measured the first Doppler Redshift while observing the spiral galaxies (known as nebula). And the observation was such that the spiral galaxies showed that they were moving away from the earth.

The year 1922:

Vesto Slipher did not grasp the cosmological implications of this fact. 10 years later in 1922, a Russian cosmologist and mathematician Alexander Friedmann developed Friedmann equations from Einstein field equations. His work showed that the universe is likely in a state of expansion.

The year 1924:

In 1924, American astronomer Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. For this Hubble began developing a series of distance indicators using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory.

The year 1929:

In 1929, Edwin Hubble discovered a correlation between distance and recession velocity. This relation is now known as Hubble’s law.

The year 1927:

In 1927, Belgian physicist Georges Lemaitre derived the same results as Friedmann’s equations and proposed that the inferred recession of the nebulae was due to the expansion of the universe.

The year 1931:

In 1931, Lemaitre went further, suggesting that if the apparent expansion of the universe was to return in time, this would mean that there was a smaller universe in the past. And the entire mass of the universe would have been concentrated into a single point from which the fabric of space and time originated.

These discoveries sparked controversy among physicists throughout the 1920s and 1930s. Some physicists advocating that the universe was in a steady state.

According to this model new matters are continuously created as the universe expands, thus preserving the uniformity and density of matter over time. Thus they seemed that the idea of a Big Bang more theological than scientific.

Although, the observational evidence began to support the Big Bang more than Steady State. In 1965, the discovery and confirmation of Cosmic Microwave Background Radiation (CMBR) secured the Big Bang as the best theory of the origin of the universe.