Dark Matter and Dark Energy

The quest for establishing the existence of dark matter and dark energy has been at the forefront of cosmological research since the early twentieth century. Astronomical observations have shown that only five percent of the total mass of the universe is made of visible matter. It is believed that the remaining ninety five percent is made of 'Dark Matter' and 'Dark Energy'.

Dark matter is a form of matter that is believed to account for nearly twenty seven percent of the total mass of the universe. The term "Dark matter" was coined by Fritz Zwicky in 1933. The matter is called 'Dark Matter' because it cannot be observed by any astronomical instruments. Scientists believe that the possible reason for this invisibility is that the substance that makes up this matter interacts very weakly with electromagnetic radiations (like light, X-Rays etc). In fact no experiment has been able to directly detect 'Dark Matter' particles. The gravitational effects that it exerts on cosmic objects are the only indirect evidence that points towards its existence.

'Dark Energy' on the other hand is a much more recent discovery compared to 'Dark Matter'. It is believed to be an unknown form of energy that is responsible for the accelerated expansion of the universe. The term 'Dark Matter' was coined by Michael Turner in 1998 and it is believed to account for nearly sixty eight percent of the total mass of the universe.

The evolution of the concepts of 'Dark Matter' and 'Dark Energy' is not based on one common premise but on separate premises.

The conception of 'Dark Matter' was theorized to explain the difference between the estimated and the observed masses of galaxies. The masses of the galaxies as estimated by observing the luminosities of the galaxies was found to be inadequate to account for the gravitational force needed to hold the components of the galaxies together. Dutch astronomers Jacobus Kapteyn  in 1922 and Jan Oort in 1932 independently suggested that the additional  gravitational pull is probably due to an unknown, invisible form of matter. This matter was finally named 'Dark Matter' by Zwicky during his research on the Coma galactic cluster in 1933.

The conception of 'Dark Energy' on the other hand started developing much later, during 1980's when a proposal of the existence of a repulsive force driving the cosmic inflation, which started almost immediately after the Big Bang, was put forward. This conception bridged the inadequacy of the previous models of the evolution of the universe, which was based only on 'Dark Matter' and visible matter. The early models proposed that the expansion of the universe would eventually slow down because of the immense gravitational force exerted by the 'dark' as well as 'ordinary matter'. This theorization was in stark contrast to what was observed using astronomical instruments which showed that the universe was expanding at an accelerated rate. Modifications were therefore proposed on the early evolutionary models and new models started coming forward. The most popular amongst these models that is currently under active consideration, to explain the evolutionary process of the universe and the accelerated cosmic inflation is the Lambda-CDM Model, which incorporates 'Dark Energy' along with 'Dark matter' and ordinary matter as the three major models of the universe.

Now two questions might arise out of the above mentioned discussion. The first is why do we need these exotic conceptions to understand the evolution of the cosmos?, and the second is whether alternate theories ,that do not require the conceptions of 'Dark Matter' and 'Dark Energy', are available that can satisfactorily explain the  observed cosmological phenomena.  Let us try to answer the questions one by one.

The answer to the first question is that these so called 'exotic' conceptions  provide the most simplistic explanation to a number of cosmological phenomenon. For example the observed indifference in the orbital  speed of stars (speed of movement of stars in their orbit around the centre of the galaxy) located near the centre and near the periphery of the galaxy is believed to be the result of the gravitational influence of the dark matter present in the galaxy.

Again dark matter is believed to be responsible for the distortion of the image of far away galaxies, observed using space telescopes. Einstein's General Theory of Relativity suggests that when light coming from distant cosmic objects (eg. stars, galaxies etc.) passes near a massive object, the path of the light bends due to the gravitational pull exerted by the object. When this light reaches the observer via telescopes, he or she sees a distorted image of the object. This effect known as 'gravitational lensing' is quite distinctly observed in the images of most galactic clusters.

On the other hand the conception of 'Dark Energy' supposedly explains the observed 'flatness' of the universe. Prominent international experimental projects like BOOMERanG, MAXIMA, DASI etc. have independently confirmed that universe is flat. The energy density (energy per unit volume of the universe) needed to create this flatness can only be provided by an unknown form of energy like 'Dark Energy'.

Further studies on supernovae (luminous stellar expansions), indicates that the universe is undergoing expansion and that too at an accelerated rate. This was further confirmed indirectly by experiments such as The Cosmic Microwave Background (CMB) Experiment. This observation is unexpected because the rate of expansion of the universe should have slowed down so many years after the Big Bang on account of the gravitational pull of the cosmic objects. It therefore points to the possibility of the existence of 'Dark Energy' that provides the repulsion effect causing the cosmic inflation.

As to the question of the existence of alternate cosmological theories that exclude 'Dark Matter' and 'Dark Energy', the answer is, yes, there are.

As far as Dark Matter is concerned theories like The Modified Newtonian Dynamics (MOND), Entropic gravity etc are there that uses modifications of General Theory of Relativity to not only refute the conception of 'Dark Matter' but also provides explanation of those cosmic phenomenon that is believed to be the caused by Dark Matter.

Presence of Dark Energy on the other hand is more or less unanimously accepted by the scientific community. Although theories explaining the accelerated rate of expansion of the universe, without involving 'Dark Energy' , exists but most of them have been refuted in favour of Dark Energy model especially by study of the gravitational wave detected for the first time by LIGO and VIRGO detectors. Some theories in this regard include Modified Gravity Theory, Inhomogeneous Cosmology etc.

The wide acceptance of the theories of 'Dark Matter' and 'Dark Energy' despite having a few skepticisms, have led to launch of many international projects to detect dark matter and dark energy. Some prominent names include The Axion Dark Matter  Experiment (ADMX), Cryogenic Dark Matter Search (CDMS), Dark Energy Survey (DES) etc. India has recently built an underground research facility in the caverns of the Jaduguda Mines, in Jharkhand, to search for Dark Matter.

Establishing the existence of 'Dark Matter' and 'Dark Energy' and understanding their nature is extremely important not just to advance the knowledge of cosmology but also to advance  the knowledge of particle physics. Their discovery will cast light not only on the fate of the universe, but also on the nature of matter, space and time. Today the scientists have, at their disposal, advanced astronomical instruments and new approaches that will surely help them to get direct and irrefutable experimental evidence of these two hypothetical concepts.       

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