arXiv subjects

Kemal Gültekin

Publications and source records attributed to Kemal Gültekin.

4 recordsLinked to original sources

arXiv2026

Enhanced Gravitational Effects of Radiation and Cosmological Implications

In the momentarily comoving frame of a cosmological fluid, the determinant of the energy-momentum tensor (EMT) is highly sensitive to its pressure. This component is significant during radiation-dominated epochs and becomes naturally negligible as the universe transitions to the matter-dominated era. Here, we investigate the cosmological consequences of gravity sourced by the determinant of the EMT. Unlike Azri and Nasri, Phys. Lett. B 836, 137626 (2023), we consider the most general scenario in which the second order variation of the perfect-fluid Lagrangian does not vanish. We analyze the dynamics of the power-law case and explore the cosmological implications of the scale-free model characterized by dimensionless couplings to photons and neutrinos. We show that, unlike various theories based on the EMT, the present setup, which leads to enhanced gravitational effects of radiation (EGER), does not alter the time evolution of the energy density of particle species. Using current cosmological observations, we constrain the model parameters and show that EGER may offer a viable mechanism for alleviating the Hubble tension. Although it exhibits a phenomenological analogy to tightly-coupled relativistic fluid scenarios, EGER remains purely gravitational in origin and yields distinguishable signatures in the small-scale anisotropies of the cosmic microwave background. The radiation-gravity couplings we propose here are expected to yield testable cosmological and astrophysical signatures, probing whether gravity distinguishes between relativistic and nonrelativistic species in the early universe.

gr-qc↗

arXiv2023

Bose-Einstein Condensation and Black Holes in Dark Matter and Dark Energy

The main aim of this study is to reveal curved space and particle physics effects on the formation of Bose-Einstein condensate (BEC) scalar fields in cosmology and around a black hole. Cosmological scalar fields for dark energy and dark matter may be considered as a result of Bose-Einstein condensation. In this regard, our main attention will be devoted to BECs in curved space. By considering the dynamics of a BEC scalar field at a microscopic level, we first study the initial phase of the formation of condensation in cosmology. To this end, we initially introduce an effective Minkowski space formulation that enables considering only the effect of particle physics processes, excluding the effect of gravitational particle production and enabling us to see cosmological evolution more easily. Then, by using this formulation, we study a model with a trilinear coupling that induces the processes. After considering the phase evolution of the produced particles, we find that they evolve towards the formation of a BEC if some specific conditions are satisfied. In principle, the effective Minkowski space formulation introduced in this study can be applied to particle physics processes in any sufficiently smooth spacetime. In this regard, we also analyse if a BEC scalar field is realized in the spacetime around a Reissner - Nordström black hole. We find that the produced particles of particle physics processes are localized in a region around the black hole and have a tendency toward condensation if the emerged particles are much heavier than ingoing particles. We also find that such a configuration is phenomenologically viable only if the scalars and the black hole have dark electric charges. Finally, we consider gravitational collapse around Schwarzschild black holes and form a first step towards a study in future about the effects of gravitational collapse on Bose-Einstein condensation.

gr-qc↗

arXiv2021

Curved space and particle physics effects on the formation of Bose-Einstein condensation around a Reissner-Nordström black hole

We consider two scalar fields interacting through a $χ^*χϕ^*ϕ$ term in the presence of a Reissner-Nordstrom black hole. Initially, only $χ$ particles are present. We find that the produced $ϕ$ particles are localized in a region around the black hole and have a tendency towards condensation provided that $ϕ$ particles are much heavier than the $χ$ particles. We also find that such a configuration is phenomenologically viable only if the scalars and the black hole have dark electric charges.

gr-qc↗

arXiv2019

A mechanism for formation of Bose-Einstein condensation in cosmology

We introduce a toy model of scalar particles with a trilinear scalar coupling in cosmology. The trilinear coupling $ϕ^2χ$ causes production of non-relativistic $ϕ$ particles through the process $χχ\,\rightarrow\,ϕϕ$ where, initially, only relativistic $χ$ particles are present. We consider the initial times of $χχ\,\rightarrow\,ϕϕ$ and observe that the curved space effects promote formation of Bose-Einstein condensate of $ϕ$ particles.

gr-qc↗