Maybe many people are struggling for many days with the questions in their mind that how the universe was born, what is the present situation and what about the death of our universe. The same questions had been addressed by some of the very old sages of our country and they had come to a self-consistent solution. It was mostly dominated by personal feelings and personal realizations. Unfortunately, it lacked the experimental evidence in the material world as well as mathematical accuracy. Common people can not ignore the daily material world and hence people tried to find out the answer to these questions through science. The branch of science that deals with these problems are called “Cosmology”.

Mainly the motives of this branch are the following: It tries to answer the question: How the universe started its journey. It tries to make a precise observation (sitting at the present time) and analysis of the stages through which our universe had undergone. Based on this mathematical analysis this stream tries to build a model on the future/fate of the universe. It needs hardly be said that not all the parts of this branch are properly enlightened. Still one can not belittle the amount of data we have collected and based on those data the history of the universe that we have unveiled.

Long back, our observatories used to collect the observational data of our universe. Coupled with this information the present day highly powerful telescopes have made us successful in concluding about the size of our universe as well as the rate of expansion of the size of the universe with time. If we consider that, theory of general relativity of Albert Einstein holds good for our universe and if we assume that the universe was filled with only radiation and matter just after its birth, then the mathematical equations lead us to a conclusion that the universe should continuously get increased and the rate of expansion should get decelerated (decelerating universe). Our old data echoes the result. But we have no observational data regarding the events that took place exactly at the time of the beginning of our universe. Hence we can merely guess the birth of our universe just based on different models.

But just at the beginning of this century, we have come to know that after a particular time the rate of expansion of the size of our universe is continually increasing (accelerating universe). That means, it is now clear to us that, just after the birth of our universe the rate of expansion of the size of our universe was decelerating, but due to some reason, after a typical time, this rate of expansion is accelerating. Naturally, the question comes that, what is the cause behind this “late-time acceleration”? There are many theories behind it but still, now we have not been able to come to an answer which is accepted by all. This is known as the Dark Energy Problem.

Now we come to the shape of the universe. Even some years ago there were a lot of confusions among the researchers about the shape of the universe. But some ultra-modern experiments, for example, WMAP in 2005 and PLANCK in 2013 have proved that our universe is “Effectively Flat” i.e. even if the universe had been spherical at its beginning, at present time the radius of the universe has been increased so much that we can safely consider it as flat or our universe is flat from its beginning. But universe it expanding. The continuous expansion of the universe is expressed in terms of “scale factor”(a). Let us consider a measuring stick made up of rubber. The stick contains the centimeter marking as 1,2,3… Now somebody is stretching the scale. Then every marking of the scale is continuously going away from each other. We call that the distance between any two markings is a function of time. Hence scale factor represents the distance between any two points at any particular time. Now the scale factor of the universe was different at different times. Presently we have been able to plot the variation of the scale factor of our universe with time. It is shown in the adjacent figure.

However, some researchers does not support the idea of sudden increment of the rate of expansion of the scale factor ( ). But according to Hubble’s observation the rate of receding of every point from each other is proportional to the distance between the two points i.e. velocity () = Hubble’s Constant () distance between the two points (), where the Hubble’s constant is . – This is the famous Hubble’s Law.

Now we come to the past of the universe. Unfortunately, we have not been able to come to an all-accepted theory that says exactly what had happened at the beginning or at the birth of our universe. The inflation theory of the universe is one of the popular and partially successful theories that simultaneously solve two problems viz. Flatness problem and Horizon / Causality problem, of cosmology (here we have accepted that the general theory of relativity of Einstein is valid at the beginning of the universe and at the beginning of universe the composition was only radiation and matter). In this theory, we assume that the universe at the beginning was encompassed in a very small volume and then without any explosion, it started expanding very fast (Big Bang) and the rate of increment of the expansion was also very high (Inflation). But there are many other theories in parallel.

If we consider the composition of the early universe to be just radiation and matter then we know that suddenly after the rapid expansion the rate of increment of expansion got decelerated and presently it has started accelerating again. Exactly at the point of time when the universe started decelerating, the energy density of the universe was so high that the composition of the universe was the most fundamental elementary particles like Quark and Gluon. This phase is called Radiation Dominated Era. During this period the density of the universe was inversely proportional to the square of a square of the scale factor ( ). Then when the universe expanded more (though the universe was decelerating) the Quarks and Gluons got combined with each other and produced Baryons like Protons, Neutrons etc. After this era when the energy density of universe got more reduced, the Atoms were created and then atoms merged with each other to produce Molecules. This period is called the Matter Dominated Era. In this era, the density of the universe was inversely proportional to the cube of the scale factor of the universe ( ). Our present time is the Dark Energy Dominated Era. Because in the present time the radiation density of the universe is 0.4 of the total density of the universe, the matter density is 27, the Dark Energy density is 72.6. Only the 4 of the matter density is observed/visible and the 23 is still unobserved/invisible and hence is called the Dark Matter. The presence of this Dark Matter is understood from the Rotational Curve of the Galaxies. Even some days ago we used to add another density term with it regarding the Curvature of Universe, but WMAP in 2005 and PLANCK in 2013 have proved that our universe is effectively flat. Presently the density is unchanged with scale factor ( ).

This branch of science is an independent and self-complete stream. Here it should be kept in mind that all the theories and the conclusions of the theories and the background of the conclusions of this branch are mathematically proved. None are the fictitious imagination. If any rational minded person, at any time starts to proceed by proving the equations one by one, can be expected to be sure about the acceptability of every theory and conclusion. Beyond that, the precision and accuracy of all the cosmological experiments as well as the observations that have been done and are being done depending on this theory, with the help of ultra-modern equipment and technology, bears the testimony of this theory.

Here is one fine lecture by Prof. Avi Loeb, Harvard University (© CfA Observatory Nights | YouTube)

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###### References:

- Steven Weinberg, Cosmology, Texas University, Austin, Oxford University Press
- V Mukhanov, LMU Munchen, Physical Foundation of Cosmology, Cambridge University Press
- Daniel Baumann, Harvard University, Institute for Advanced Studies Princeton, Tasi Lectures on Inflation, Arxiv: 0907.5424 (HEP-Th)
- Edmund Copeland, M. Sami, Sinji Tsujikawa – Dynamics of Dark Energy, Int. J. Mod. Phys. D 15 1753-1936,2006, Arxiv: 0603057 (HEP-Th)
- A Bandyopadhyay, D Gangopadhyay, A Moulik – The K-Essence Scalar Field in the Context of Supernova IA Observation, Eur. Phys. J. C 72 1943, 2012, Arxiv: 1102.3554 (Astro Phy)

###### Acknowledgements:

I am wholeheartedly thankful to my Cosmology course instructor Abhijit Bandyopadhyay and my General Relativity course instructor Parthasarathi Majumdar at Ramakrishna Mission Vivekananda University for giving me their valuable time & attention & for providing me a systematic way to complete this article.