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    Reverse-engineered DBI-essence field in the rainbow gravity
    (INESEG Yayıncılık, 2022) Binbay, Fatma Figen
    The current astrophysical data sets have implied that our universe has recently entered in an accelerated expansion phase. In literature, it is commonly thought that the late-time speedy enlargement behaviour is caused by mysterious dark contents (dark matter, dark energy and dark radiation) that cannot be observed directly. Although Einstein's General Theory of Relativity has provided very successful theoretical explanations and predictions, it cannot explain the aforementioned unusual nature of the cosmos. This situation has led scientists to try to create a new theoretical model for the nature of dark contents. The cosmological constant, scalar fields, unified energy densities, additional dimension idea and modified gravity theories are among the prominent point of views. In this research, mainly, the reverse-engineering method is taken into account in the rainbow gravity formalism for the DBI-essence scalar field dark energy model. As it is known, scalar field models can be introduced with the help of some fundamental physics theories, but these theories do not provide a direct way to write explicitly the self-interaction potentials that we encounter in scalar field models. Therefore, the reverse-engineering method used in this research takes significant role while writing the self-interaction potential of a scalar field model.
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    Would an alternative gravity theory developed from an improved gravitational action approach includes negative kinetic energy dynamic degrees of freedom?
    (Elsevier, 2018) Binbay, Veysel; Binbay, Fatma Figen
    The observed unexpected accelerating expansion of the universe, by Riess and his collaborators in 1998, has become one of the most important problems of the contemporary physics. A considerable effort has been spent by theoretical physicists to explain this observation for a while. When one looks at these attempts more closely, two of approaches attract attention: (i) Multi-dimensional alternative gravity models, (ii) Approaches which takes the more general and complex action than it is original Einstein-Hilbert form, which had been given as Ricci scalar R. The second type of these approaches must be examined carefully, because they could be generically involved dynamical degrees of freedom which possess negative kinetic energy (shortly called as ‘ghost states’ or simply ‘ghosts’). In this work, an alternative theory has been studied to understand if it contains ghosts or not. This alternative approach belongs to the second type of the approaches which mentioned above, and it is given as: where . And this model has been examined by this way to see if this specific alternative model could be used to explain the present time acceleration of the universe or not.