It was Einstein who first looked at the problem of the structure and dynamics of the Universe, by using for this study the general theory of relativity he proposed in the years 1913-1915.
Einstein's original cosmological model, introduced by him in 1917, was a static, homogeneous model with spherical geometry. The gravitational effect of matter caused acceleration and an unstable Universe, which Einstein did not want, since at the time the Universe was not known to be expanding. Thus Einstein introduced a cosmological constant into his equations for General Relativity. This term acts to counteract the gravitational pull of matter, and so it has been described as an anti-gravity effect.
Though subsequently the observational data showed that the universe is in fact expanding, and Einstein rejected the modification, calling it "the biggest blunder of my life", on a theoretical basis the question still remained whether the measured cosmological constant is indeed zero. For many years the cosmological constant was assumed to be zero, since no measurement gave positive indication to the contrary.
If gravity were the only force acting to alter the expansion rate, then the universe would be expected to be decelerating. That's why the observation that the Universe is accelerating, initially reported in 1999 by a team of scientists from Berkeley, came as a big surprise for most of the scientists. Many observations and studies in the past few years have confirmed this amazing phenomenon.
The initial evidence for acceleration came from the study of type Ia supernova explosions, which in all cases reach a uniform (or at least easily calibrated) intrinsic luminosity. Observations show that the most distant examples of this type of supernova are intrinsically about 20% fainter than those nearby.
This observation is easily explained, if the universal expansion is accelerating: Our motion away from the distant supernovae has speeded up since the light left them, sweeping us farther away than we would be in a universe with a constant or decelerating rate of expansion and hence a lower luminosity than expectation. Naturally, astronomers sought other explanations for the faintness of these distant supernovae. Intervening dust would also dim the light, for example. But no observational support has been found for any such alternative explanations.
From the evidence collected from type Ia supernova observations and from WMAP (Wilkinson Microwave Anisotropy Probe), it turns out that the content of the Universe consists of ordinary matter (visible or invisible-the invisible matter component of the Universe is called dark matter), representing around 30% of the total content, and of an unknown component, called dark energy, which represents 70% of the content of the Universe! The visible (luminous) amount of matter in the Universe in only 5%. Therefore, data from a wide variety of independent cosmological and astrophysical observations strongly suggest that most of the energy density of the universe today may be contained in empty space!
A first, and perhaps the most convincing candidate for the dark energy, is the cosmological constant, introduced by Einstein in 1917 and later rejected by him. But, once we accept the reality of the cosmological constant as the dark energy, it would be important to find a physical interpretation for this quantity. A very interesting physical interpretation of the dark energy can be obtained as a vacuum energy, from the following line of reasoning.
According to Quantum Theory, vacuum space is not really empty because of virtual particles produced spontaneously in the vacuum. Since conservation of energy is a fundamental principle, creation of such particles can take place only within the limits imposed by the energy-time uncertainty relations. This immediately tells us that the virtual quanta necessarily have to be short lived. Thus the vacuum has a non-zero and very large value of energy density associated with it.
Therefore the vacuum energy cannot be set to zero and it ought to be included as a source for gravitation, where it acts like the cosmological constant. The vacuum energy behaves like a fluid having a negative pressure. The negative pressure causes the acceleration of the Universe, as observed from the type Ia supernova observations.
Although a cosmological constant is an excellent fit to the current observational data, the observations can also be accommodated by any form of '' dark energy ''. This possibility has been extensively explored of late, and a number of candidates have been put forward.
There are many reasons to consider dynamical dark energy as an alternative to a cosmological constant. First and foremost, it is a logical possibility which might be correct, and can be constrained by observation. Secondly, it is consistent with the hope that the ultimate vacuum energy might actually be zero, and that we simply haven't relaxed all the way to the vacuum as yet. But most interestingly, one might wonder whether replacing a constant parameter with a dynamical field could allow us to relieve some of the burden of fine-tuning that inevitably accompanies the cosmological constant.
The simplest physical model for an appropriate dark energy component is a single slowly-rolling scalar field, sometimes referred to as '' quintessence ''. When the field is slowly-varying, the scalar field potential acts like a cosmological constant.
In addition to the quintessence models, many other theories for dark energy have been proposed, including models based on super-symmetric gauge theories, super-gravity, small extra dimensions, large extra dimensions, quantum field theory effects in curved space-time. All these models are essentially based on the existence of a mass less scalar field acting at a cosmic scale.
There are other models of dark energy besides those based on scalar fields. One scenario is "solid" dark matter, typically based on networks of tangled cosmic strings or domain walls. There is also the idea of dark matter particles whose masses increase as the universe expands, their energy thus redshifting away more slowly than that of ordinary matter. However, the cosmological consequences of these scenarios turn out to be difficult to analyze in detail, and work is still ongoing.
And perhaps even the reader will be the future scientist who will unveil the mystery of the dark energy and of the content of the Universe.