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The Big Bang Theory Explained

  • Kavya Nambiar
  • Sep 08, 2021
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Scientists, philosophers, and even the common people have for ages pondered over the question- how did the universe come to be? What was the beginning? As more and more scientific mysteries unravelled through the years- with evolution and natural phenomenon being conclusively explained and outer space being explored- the question of the beginning only got more weighted and complicated.


One of the most interesting theories about the universe is none other than the big bang theory. No, not the TV show, the real thing. The big bang is the most widely accepted theory that describes the beginning of the universe.



History of the Big Bang Theory


Before the theory of the big bang, there were innumerable scientific theories, religious myths, folklore, and stories about how it all began. Among them, there were several cosmological theories with scientific backing that were viewed as serious contenders at the time.


In the early 20th century, the idea of an expanding universe was considered absurd, and a steady-state universe model was the one accepted by most scientists.


The history of the big bang theory began in 1912 with Vesto Slipher’s discoveries of the movement of spiral nebulae. Alexander Friedmann derived the Friedmann equations a decade later, which showed an expansion of the universe.


But the contributions of Georges Lemaître and Edwin Hubble formed the basis of the big bang theory, the Hubble–Lemaître law. As stated by the International Astronomical Organisation-


“The Hubble-Lemaître law describes the effect by which objects in an expanding Universe move away from each other with a velocity proportionally related to their distance.”


The discovery of the recession of galaxies, and from that the idea that the universe was undergoing continuous expansion, ultimately led to the idea that there must have been a single point in the beginning.


Lemaître first noted these ideas in 1927 and argued that if the universe is expanding, it was smaller in the past, and extrapolating back in time should lead to an epoch in which all the matter in the universe was packed together in a dense state. Accordingly, the physical universe must have originated from a single particle, which he described as the “primaeval atom.” The explosion of this particle is what gave rise to space and time and a universe that continues to expand and will keep doing so.


Hubble’s experiments in 1929 confirmed the shifting of galaxies and provided evidence for the expanding universe.


But it took several more decades for the acceptance of the big bang theory by the scientific community. Different theories clashed in the coming years. Leading the debate was George Gamow and Fred Hoyle, the former advocating for the big bang and the latter in favour of a steady-state universe. The term big bang theory was coined by Hoyle in 1949. 


The argument was finally settled in 1964 when Arno Penzias and Robert Wilson discovered radio static signals which had no physical sources and were concluded to have a cosmic origin. This radiation called Cosmic Background Radiation is said to be leftover from the early years of the expanding universe. The evidence was considered solid by most and after that, the big bang theory saw widespread acceptance.


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What is the Big Bang Theory?


The big bang theory is the prevailing theory accepted by scientists and scholars that describes a cosmological model explaining the beginning of the universe.


The theory isn’t just about the “bang” though. The explosion that caused the creation of the universe was just the beginning, and everything that comes after that, from mere seconds after to a million years in the future, is of much more intrigue and discussion.


There are several stages in the formation of the universe as dictated by the big bang theory. As each stage passed, the universe cooled down and expanded, by infinitesimal amounts at times, or by leaps and bounds at others. The universe is undergoing expansion and cooling and will do so for the foreseeable future.


Here is a look at the stages of the development of the universe-

The stages of development of the universe are the singularity, the big bang, the radiation era, and the matter era, which includes the present day.

The Stages of Development of the Universe after the Big Bang


  • The Singularity


At the very beginning of everything, there was nothing but an intensely hot and infinitely dense point, a few millimetres wide. The matter that makes up everything that we see and know today was packed into this point. It has been described as being similar to a supercharged black hole.



  • Big Bang


Approximately 13.7 billion years ago, the tiny point that is the singularity violently exploded. It is this explosion that we typically call the big bang. This explosion is the source of all matter, energy, space and time.



  • Radiation Era


The time after the big bang up to the present moment and beyond can be broadly described into two eras, the first of which is the radiation era. This era is so named for the abundance of radiation after the big bang and spanned only the first tens of thousands of years since the explosion.


The era can be said to comprise of smaller epoch stages, most of which lasted less than even a second. It is these earlier epochs that are still subject to much speculation since the universe was not governed by the laws of physics that we understand during that period.


  • Planck Epoch: During this time, there was no matter, only a superforce that would later become the major forces we see today. At the end of the Planck epoch, gravity was formed. The epoch is so named because it took place in the smallest measurable unit of time, Planck time.


  • Grand Unification Epoch: 10-43 seconds after the big bang, strong nuclear force separated from the superforce.


  • Inflationary Epoch: 10-36 seconds after the big bang, during the inflationary epoch, a rapid expansion took place in the universe. The universe was still intensely hot and had only particles like electrons and quarks.


  • Electroweak Epoch: In this epoch, 10-32 seconds after the big bang, electromagnetic force and weak nuclear force broke off the superforce.


  • Quark Epoch: 10-12 seconds after the big bang, the universe was filled with particles needed to form a complex system, but the temperature and density were still too high to support it.


  • Hadron Epoch: At 10-16 seconds after the big bang, the temperature lowered enough to 1010 K for the formation of protons and neutrons from quarks.


  • Lepton Epoch: At about one second after the big bang, the temperature was at 1012 K.


  • Nuclear Epoch: 100 seconds after the big bang, the temperature was at 109 K, and the conditions were sufficient for the formation of nuclei from protons and neutrons. Helium atoms were thus created.


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  • Matter Era


Once the universe was capable of and had sufficient conditions to form elements, the matter era was ushered in, where matter predominates the universe. This era spans the millions of years after the radiation era when the universe has grown and changed and transformed into the form that we see today.


  • Atomic Epoch: The atomic epoch was 50,000 years after the big bang. The universe finally cooled down to 3,000 K, a temperature sufficient to support electrons and nuclei combining to form atoms. The recombination process resulted in the creation of Hydrogen atoms.


  • Galactic Epoch: Fast forward to 200 million years after the big bang, atomic clusters were formed, which would much later become galaxies.


  • Stellar Epoch: 3 billion years after the big bang, stars began forming inside the galaxies. The universe developed into the form that we see now in this epoch. All the other elements were formed, then planets were formed, life took shape. This epoch is also called the Stelliferous Era, and includes the present day, up and until a point in the future when stars will cease being formed.


The stages are also otherwise divided into the inflation, cooling, and structure epochs.


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The Future of the Universe


The big bang theory still leaves behind questions that are not yet answered. Scientists still struggle with explaining phenomena like dark energy and dark matter. There are paradoxes that need to be solved.


One of the pertinent questions is that if the universe is ever-expanding and ever cooling, what does the future hold? Will one day all matter collapse inward into the tiny speck it once was, when the universe runs out of expansive force? Or will the universe expand indefinitely? The former scenario is called the “Big Crunch.”


The latter scenario of infinite expansion is the one considered more likely, and is called the “Big Freeze.” Here the universe expands indefinitely, and also cools indefinitely, reaching a temperature that can no longer sustain life or anything else.


The end of the Stelliferous era, and thus the end of star formation, will lead to a Degenerate Era. This will lead to a Black Hole Era and finally a Dark Era, where the universe will be almost empty of matter.


But these are, of course, speculations, and with advancements in science and more breakthrough technologies we can hope that it is only a matter of time before we have all of- or at the least more of- the answers.

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