Ultimate Fate of the Universe

The first theory about the nature of the universe appeared in second century AD, presented by an Egyptian astronomer and mathematician Ptolemy. The theory named Geocentric. It asserted that the whole universe is centered upon by our planet earth and all the planets and stars move around earth in fixed spheres. The earth was stationary. All the heavenly bodies were assumed to move in fixed circular orbits. The universe was characterized by spheres such that the planets were contained in the sphere nearest to the earth, while in the second sphere sun and in the third sphere all the other stars were contained. In the outermost sphere, there were gods and spirits which were ruling the universe. In such a model, the universe was supposedly created by the gods and the angels and they were looking after the working of the cosmos. This theory was based on several biblical teachings and had no well defined scientific basis. The ultimate end of the universe was envisaged to be in the hands of the gods and the universe was considered a toy in the hands of gods. Such a universe was surely supported by the churches and other religious groups to satisfy and support the exact biblical teachings. Such a theory had no predictive power about the future events in the universe. Later scientists tried to present a more rational explanation about the universe.

The model was later modified according to the motions of planets and stars by several astronomers but the basic assumptions of Ptolemy’s model were not touched. This model was further carried out by Nicholas Copernicus in seventeenth century AD, when he put the sun in the center of the universe rather then the earth while the rest of the model was again carried out here.

The start of twentieth century marks the birth of modern cosmology when Einstein gave the theory of General Relativity in 1915. The theory had the power to describe the structure of space and time. The theory suggested that space and time are not distinct entities but are actually connected in the fabric of space-time thus making universe from the three dimensions to four dimensions. Moreover, the theory gave a new way of understanding the so called gravitational force. It is a force which every massive object experiences in the presence of other massive objects. That is, it is an attractive force which put the objects towards each other. Einstein’s theory suggested that gravity is not a force; rather it is best understood as the curvature of space-time. The massive the object is, the more curvature it will produce around itself. To understand this concept, let us take the example of our solar system. Einstein said that planets don’t move around the sun due to some mysterious gravitational force rather it is the sun that produces strong curvature around its neighboring space-time that constrains the planets to move in their observable way.

Einstein presented the set of the so called Einstein Field Equations, the set of ten coupled differential equations with sever non-linearity. Apparently these equations suggested that matter in the universe and the geometry of space-time is very tightly connected. Given the geometry of space-time one could solve the equations and got the matter distribution in the space-time and vice versa. As the inventor of these equations, Einstein attacked the problem of origin and evolution of the universe by himself. In order to solve the equations, he assumed the matter distribution in the universe to be completely spherical. As a result of his calculations, he obtained a model of the universe which was apparently spherically symmetrical but highly unstable to small perturbations. According to the famous law given by Newton, any spherically symmetrically distribution of matter must collapse towards the center due to gravitational force if the matter is given small perturbation. In order to stabilize the universe, Einstein introduced the so called Cosmological Constant which had the property to act in the opposite direction of the gravitational force. Thus by introducing this constant in his model Einstein stabilized his universe by balancing the attractive gravitational field with the so called repelling force of cosmological constant. Esthetically this universe had a very unappealing property: it was a static universe devoid of any kind of motion. That is all the objects in the universe had to remain still in order to avoid destabilization. If the gravity start to dominate, the universe will collapse otherwise it will eternally expand in an exponentially rate. It was a platonic way of thinking an eternal universe unchanging both in space and time.

As a matter of fact, nature abhors stalemate and only change is permanent. Astronomical observations in 1920s made by well-known astronomer of that time, Edwin Hubble, confirmed that the universe is no longer static rather much more dynamic. The galaxies and nebulae were not only running away from us but there appeared a pattern in their receding velocities. It was deduced empirically that the farther the galaxy from us the greater will be its receding velocity, so called the Hubble Law. So the distant objects are escaping from our view much faster then the nearby ones. After this discovery by Hubble, Einstein had to shirk his view of a static universe and really had to modify his model to comply with observations. The finding of an expanding universe had to provide us another insight about the beginning of the cosmos: we observe the universe to be expanding, if we tune the movie backward, all the objects must be moving closer to each other! Eventually they will combine at some point. Could this be the starting point of the universe? The Big Bang?

One thing should be made clear at this stage: All the matter and energy including space and time were created as a result of Big Bang. There was no matter or space-time pre existed before big bang. There was absolutely nothing. Also, by definition, the universe is self contained. It is not contained into something else. If it is really contained into something else, then that very thing is also part of the universe. So there is nothing outside the universe. Moreover, there was nothing before the Big Bang. If there was something, then the universe had existed at that time.

When other people including Freidman and De Sitter solved the Einstein field equations they found that the solutions represent singular behavior at the starting point of the universe: a point of infinite density. Since the solutions were giving singularities and infinities about the origin of the universe; no law of physics could explain what happened exactly at the starting point. Einstein and other scientists did not like such solutions and their interpretations valid as they did not tell anything about beginning of the universe.

In order to avoid such singularities in the cosmological solutions, Fred Hoyle and Jayant Narlikar presented a modified version of Einstein’s static universe, Steady State Model. Although their model predicted the expansion of the universe, it gave no singularities in the beginning of the universe. The universe had existed forever. Thus according to their model, the universe must go on expanding while creating gigantic spatial distances between galaxies. This empty space was assumed to be filled up by the formation of new galaxies. In the Big Bang picture of the universe, all the matter was formed as a result of that explosion. But if we accept the steady state model of the universe, what is the origin of matter in that universe? Hoyle and Narlikar suggested the idea of continuous creation of matter from the decaying of a C-Field. The theory remained under hot debate for several decades in fifties and sixties but later on was shirked when it did not complied with the observational results of Cosmic Microwave Background (the radiation produced as a result of decoupling of matter from photons 300,000 years after the Big Bang) and deductions from the formation of lighter elements i.e. hydrogen and helium in the universe.

The future evolution of the universe crucially depends on the matter and energy content in the universe: if the density of the matter and energy combined together in the universe is greater then a certain critical density viz 10^(-29) g/cc then the expansion of the universe will terminate at some point and begin to contract until it will collapse onto itself, so called Big Crunch. This model is classified as a ‘Closed Universe’ model as it is bounded by the starting and end point. If the density is exactly equal to the critical, then the expansion will go on indefinitely and uniformly. Moreover, if it is less then the critical value, the otherwise uniform expansion will be replaced by an accelerated expansion. The last two models are classified as ‘Open Universe’ as they possess starting but no future end point. Such universes are practically infinite in extent.

Observations of the supernova (the cataclysmic death of star of mass more then eight times the mass of our sun by exploding itself) carried out by two groups in 1999, one in US by S. Perlmutter and other in Australia by A. Riess showed clearly that the universe was not only expanding but undergoing in a wild accelerated expansion. The brightness of the supernova lasts from few days to several weeks. Observations carried out over a gap of few days suggested that the supernova has moved away/receded to a large distance then normal, which cannot be explained if the universe is expanding uniformly. The expansion was deduced to be not normal or uniform but accelerated. Later calculations showed that this accelerated expansion started just approximately one billion years ago and before that time the universe was undergoing deceleration. Cosmologists all over the world confused what caused the deceleration of the universe into a sudden acceleration. The problem of accelerated expansion also lead to the Coincidence Problem or the Why Now? Problem in cosmology: why the universe is undergoing this acceleration just NOW, when the intelligent beings like us are able to observe it? The question is still under consideration by scientists.

It was deduced that some sort of Dark Energy was responsible to cause and initiate this acceleration. This mysterious sort of energy possesses much more energy content then all the energy contained in the matter of the universe. Empirically, the energy content of matter is only 3% while the dark energy covers 71% of the total energy content of the universe. The rest is Dark Matter which is also yet hypothetical and undiscovered form of matter. It is called ‘dark’ as it does not emit any light. Its nature is still controversial but its existence is deduced and confirmed due to its strong gravitational fields in the universe.

Moreover, the dark energy possesses strong negative pressure which results in stretching the fabric of space-time outwards thus causing the galaxies to rush away from each other. Apart from this, the nature and composition of this mysterious energy is still unresolved. Cosmologists study the strength of dark energy by introducing the parameter namely ‘w’. This parameter is not a universal constant and its value depends on the expansion and the energy content in the universe. Observations of WMAP suggest that currently the value of w is almost -1 which was before one billion years much greater. Extrapolation of the WMAP data points out that the value of w will decrease further thus causing the strength of this dark energy to increase manifolds.

The manifestations of this dark energy are worth consideration: as discussed earlier that this energy possesses negative pressure which works against gravitational force. As the entire giant structures in the universe like stars and galaxies (gravitational systems containing several billion stars), are supported by the gravitational force which keeps them stable). The emergence of dark energy will be bad news rather a death time arrival. It will gradually destabilize all gravitationally bound structures and will start disrupting them. Some 10 billion years from now the clusters of galaxies will be disrupted into individual galaxies. Few million years after, the individual galaxies will be flunked into individual stars. The process will go on until our solar system will be disrupted. As the strength of dark energy will increase exponentially, the stars and earth will start to explode thus reducing to their constituent particles. At last the stage will be reached when the very structure of space-time will be tearing apart thus causing the complete death of the cosmos, so called The Big Rip. Apparently this scenario provides us a very Cold Death of our universe but it is certainly a sure outcome deduced from the current observations and calculations.

Cosmologists still do not exactly know how the value of the parameter w will behave in the future. As the value of w will decrease, the Big Rip will come closer to us. But we need not to worry as apparently all this is very far in future.