As we know that the Universe has been expanding since its formation. The observations from space research areas reveal that the rate of expansion of the Universe has slow down from the past to the present time.
The Universe might have enough energy density to stop its expansion and recollapse, or it might have so little energy density that it would never stop expanding, but gravity was certainly behind this slow expansion as time went on.
As defined by Wikipedia “dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe”. Dark energy is the most accepted hypothesis to explain observations since the 1990s that indicate that the universe is expanding at an accelerating rate. According to the standard model of cosmology, the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.
The term ‘dark’ has been used to emphasize that its instinct has not been visible so far and its measurement has remained impossible.
There have been many theories and explanations in favor of this hypothetical term ‘dark energy’, some of these are mentioned as below.
- One of the explanation for how space acquires energy comes from the quantum theory of matter. In this theory, “empty space” is actually full of temporary (“virtual”) particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the number came out 10120 times too big. That’s a 1 with 120 zeros after it. It’s hard to get an answer that bad. So the mystery continues.
- Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this “quintessence,” after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don’t know what it is like, what it interacts with, or why it exists. So the mystery continues.
- A last possibility is that Einstein’s theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein’s theory is known to do, and still give us the different prediction for the Universe that we need? There are other supportive theories, but none are compelling. So the mystery continues.
The nature of dark energy is a matter of speculation. The evidence for dark energy is only indirect coming from distance measurements and their relation to redshift. It is thought to be very homogeneous, not very dense and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied—roughly 1/1029 g/cm3—it is unlikely to be detectable in laboratory experiments. Dark energy can only have such a profound effect on the universe, making up 74% of universal density, because it uniformly fills empty space.
Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant.
- The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate. The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics; Energy must be lost from inside a container to do work on the container. A change in volume dV requires work done equal to a change of energy -P dV, where P is the pressure. But the amount of energy in a container full of vacuum actually increases when the volume increases (dV is positive), because the energy is equal to ?V, where ? (rho) is the energy density of the cosmological constant. Therefore, P is negative and, in fact, P = -?.
- Quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force. Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic, changing over time, unlike the cosmological constant which always stays constant. It is suggested that quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Specifically, it is thought that quintessence became repulsive about 10 billion years ago (the universe is approximately 13.8 billion years old).
There are some very speculative ideas about the future of the universe. One suggests that phantom energy
causes divergent expansion, which would imply that the effective force of dark energy continues growing until it dominates all other forces in the universe. Under this scenario, dark energy would ultimately tear apart all gravitationally bound structures, including galaxies and solar systems, and eventually overcome the electrical and nuclear forces to tear apart atoms themselves, ending the universe in a “Big shred”. On the other hand, dark energy might dissipate with time, or even become attractive. Such uncertainties leave open the possibility that gravity might yet rule the day and lead to a universe that contracts in on itself in a “Big Gnaw”.
Some theories, like the cyclic model – which is one of the cosmological models, suggest a state in which universe follows infinite, self sustaining cycles; a universe undergoing a series of oscillations, each beginning with a big bang and ending with a big crunch. The model emphasize that the universe would expand for a period of time before the gravitational attraction of matter causes it to collapse back in and then it rebounds again. It is also possible that the universe may never have an end and continue in its present (i.e expansion) state forever as suggested by the second law of thermodynamics.