L. Parker discovered in the early sixties that the expansion of the Universe can lead to creation of particles out of the vacuum. He pulled together quantum mechanics and general relativity and found that the expansion of the Universe, or, in general, a time-variant gravitational field can lead to an impromptus production of (quantum) particles. The Cosmic Microwave Background (CMB) was discovered by Penzias and Wilson around the same time, which brought new insights into Cosmology and which provided the strongest support to the Big-Bang Theory. In 1992, the COBE (Cosmic Background Explorer) satellite detected small fluctuations in the average temperature of the CMB for the first time. This has later been confirmed by many other experiments, including the Planck satellite. Quantum field theory in curved spacetime and, in particular, gravitational particle creation provides the mechanism driving primitive fluctuations which created the tiny perturbations in the CMB temperature. The creation of galaxies and galactic clusters by clumping of matter can also be explained by this mechanism. Parker’s formalism led Hawking to realize that black holes also create particles, in a way consistent with the laws of thermodynamics. Hawking’s beautiful result was very influential. It revealed that the second law of thermodynamics was valid for systems that included black holes. This established a deep connection between thermodynamics and general relativity.
In recent years, gravitational particle creation is being considered as a viable alternative to Dark Energy (DE) models due to difficulties in identifying the true nature as well as the origin of DE which is considered to have exotic properties such as a huge negative pressure. The Cosmological Constant, which is supported by most observations as the driving force behind the late time cosmic acceleration, is also plagued by serious problems such as the Cosmological Constant problem and coincidence problem. At this juncture, the natural process of gravitational particle creation is speculated to explain not only the presently observed accelerated epoch of the Universe but also the inflationary phase in the early Universe as prophesied by Alan Guth in 1981. In fact, there have been several studies which have confirmed that this process is well equipped to explain the evolutionary stages of the Universe and is also thermodynamically stable. However, there are several drawbacks too, the most important one among them is that the exact rate of particle creation has still not been determined. Consequently, researchers mostly resort to phenomenological considerations to tackle this problem. Moreover, there have been a very few observational studies in this direction and more such studies are needed to understand this mechanism at a deeper level.
In this special issue, we have devoted our attention to understanding the significance of the gravitationally induced particle creation mechanism in the context of Cosmology. Several authors have made their contributions to this Special Issue. We hope that the readers find these articles useful for furthering their research and also gain insights into this very important and rapidly emerging research field.
Conflicts of Interest
All the Guest Editors declare that there is no conflict of interests regarding the publication of this paper.
Subhajit Saha
Kazuharu Bamba
Martiros Khurshudyan