“Cosmology” it is the study of evolution, origin and ultimate fate of universe. According to religious studies, cosmology is about a theistically created world which is governed by supernatural powers. But according to scientists, modern cosmology deals with creating the most complete understanding of the universe that is stable with scientific observations and elucidated with natural forces. The chronology of universe tells us about the history and future of universe in accordance with the big bang cosmology.
Our universe is estimated to be roughly 13.8 billion years old, which is approximately 130 million years whereas in comparison the solar system is only about 4.6 billion years old. Our universe evolved with the Big Bang as an unbelievable hot and dense point. When the universe was just hundredth of a billionth of a trillionth of a trillionth of a second in age, that is, 10-34 second of age it went through an immense burst of expansion which was known as inflation. During this time, universe doubled in size at least about ninety times, transforming from subatomic size to golf ball size. The Big Bang was not an explosion in the usual way because the universe did not expand into space as no space existed before the universe. The universe did not expand only from any single spot after the Big Bang rather the space itself has stretched and carried matter with it.
The first era is termed as the Planck epoch where the temperature was so high that the four fundamental forces that are electromagnetism, weak nuclear interaction, gravitation and strong nuclear interaction were only one fundamental force. As the expansion of the universe continued it cooled down and crossed transition temperatures where forces separated from each other. This was the time of phase transition much similar to freezing and condensation. After this the grand unification epoch began when gravitation was separated from the other forces of nature, which are known as gauge forces. The grand unification epoch came to an end when the grand unified theory forces which described the non- gravitational physics in this epoch further separated into the strong and electroweak forces. After this came the electroweak epoch which was 10-36 seconds after the Big Bang, where the temperature was low enough to separate the strong forces from electroweak forces. After this the inflation came to an end when the inflation field decayed into ordinary particles and the process was called “reheating”. This is usually quoted as the time “after the Big Bang “. This occurred roughly 10-32 seconds after the Big Bang.
After inflation the growth has continued but is at a slower rate and the universe was filled with quark- gluon plasma. Then came the era of quark epoch where the fundamental forces took their present forms and the fundamental particles had mass , but the temperature of the universe was still high to form hadrons . With the expansion of space the universe cooled down and matter was formed. Just a second after the Big Bang universe was filled with protons, neutrons, electrons, photons, neutrinos. This era was termed as the hadron epoch. These hadrons and anti – hadrons then left leptons and anti- leptons which dominated the mass of universe. After 10 seconds of Big Bang, most of the leptons and anti- leptons were wiped out and photons dominated, hence the photon epoch. After the first three minutes lighter elements evolved during the process of Big Bang nucleosysnthesis. Temperature cooled down further from 1032 Kelvin to 109 Kelvin. During this period the protons and neutrons collided to make isotope of hydrogen that is deuterium. Most of the deuterium then combined to form helium and trace amounts of lithium were also produced. Hydrogen and helium atoms were then formed when the density of the universe began to fall. This happened roughly 377,000 years after the Big Bang .This was the period of recombination where free electrons got captured by the ions and formed electrically neutral ions. This was followed by decoupling where the photons travelled freely and the universe became transparent.
About 400 million years after the Big Bang, universe began to move out from the cosmic dark ages, this was during the epoch of reionization. This lasted for more than half- billion years. During this time clumps of gases collapsed to form the first stars and galaxies, whose very energetic ultraviolet light ionised and destroyed most of the neutral hydrogen. The first form of stars was the population III stars. This was then followed by collapsing of large volume of matter which formed a galaxy. This led to formation of population II and then population I star. Our solar system was born about 9 billion years after the Big Bang. A fragment of a molecular cloud comprising of hydrogen and traces of other elements collapsed to form a large sphere in the centre which became the sun. The surrounding accretion disk then came together to form the planets, comets, asteroids.
The ultimate fate of universe depends upon the physical constants: cosmological constant, proton decay and the natural laws. It can happen due to ‘Heat death’ where energy density of universe decreases and reaches thermodynamic equilibrium where no structures are possible, ‘Big rip’ where clusters of galaxies and finally solar system will torn apart, ‘Big crunch’ where the universe would contract to a hot dense state or ‘Vacuum instability’ where the space-time might collapse into a lower energy state leading to a true vacuum which could then expand outward from that point with the speed of light.