Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 1Citation - Scopus: 1Modeling Cosmological Perturbations of Thermal Inflation(IOP Publishing, 2024) Bae, Jeong-Myeong; Hong, Sungwook E.; Zoe, HeeseungWe consider a simple system consisting of matter, radiation and vacuum components to model the impact of thermal inflation on the evolution of primordial perturbations. The vacuum energy magnifies the primordial modes entering the horizon before its domination, making them potentially observable, and the resulting transfer function reflects the phase changes and energy contents. To determine the transfer function, we follow the curvature perturbation from well outside the horizon during radiation domination to well outside the horizon during vacuum domination and evaluate it on a constant radiation density hypersurface, as is appropriate for the case of thermal inflation. The shape of the transfer function is determined by the ratio of vacuum energy to radiation at matter-radiation equality, which we denote by upsilon , and has two characteristic scales, ka and kb , corresponding to the horizon sizes at matter radiation equality and the beginning of the inflation, respectively. If upsilon MUCH LESS-THAN1 , the Universe experiences radiation, matter and vacuum domination eras and the transfer function is flat for kMUCH LESS-THANkb , oscillates with amplitude 1/5 for kbMUCH LESS-THANkMUCH LESS-THANka and oscillates with amplitude 1 for k >> ka . For upsilon >> 1 , the matter domination era disappears, and the transfer function reduces to being flat for kMUCH LESS-THANkb and oscillating with amplitude 1 for k >> kb .Article Citation - WoS: 6Citation - Scopus: 6Variation of the Light and Period of the Magnetic Cataclysmic Variable Am Her(IOP Publishing, 2008) Kalomeni, Belinda; Yakut, KadriGround-based, long-term optic variability of AM Her, covering the period between 2003-2008, has been conducted to study the features seen in both low and high states of the system. Low-state analysis shows the presence of short-term, low-amplitude light variations of about 0.02-0.03 mag with a mean power time between 16 s and 226 s. Brightness variations on the order of 0.7-2 mag, which could be due to the stellar activity of the component in the system, are also detected. A total of 30 years' times of minimum light given in the literature are combined with nine times of minima obtained in this study. We represented the (observed-calculated) diagram by a parabolic curve and also by two broken lines. Under the assumption of a parabolic variation, we estimate an increase in period, dP/dt = 7.5(1.2) x 10(-9) days yr(-1), with a mass transfer rate of. M = 8(2) x 10(-9) M(circle dot) yr(-1), in agreement with the previous findings by a different method.Article Citation - WoS: 6Citation - Scopus: 5A Mechanism for Formation of Bose-Einstein Condensation in Cosmology(IOP Publishing, 2019) Erdem, Recai; Gültekin, KemalWe introduce a toy model of scalar particles with a trilinear scalar coupling in cosmology. The trilinear coupling phi(2)chi causes production of non-relativistic phi particles through the process chi chi -> phi phi where, initially, only relativistic chi particles are present. We consider the initial times of chi chi -> phi phi and observe that the curved space effects promote formation of Bose-Einstein condensate of phi particles.Article Citation - WoS: 607Citation - Scopus: 736Black Holes, Gravitational Waves and Fundamental Physics: a Roadmap(IOP Publishing, 2019) Barack, Leor; Cardoso, Vitor; Nissanke, Samaya; Sotiriou, Thomas P.; Askar, Abbas; Belczynski, Chris; Erdem, RecaiThe grand challenges of contemporary fundamental physics dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'.Article Citation - WoS: 13Citation - Scopus: 11Geometric dark matter(IOP Publishing, 2020) Demir, Durmuş Ali; Puliçe, BeyhanThe dark matter, needed for various phenomena ranging from flat rotation curves to structure formation, seems to be not only neutral and long-living but also highly secluded from the ordinary matter. Here we show that, metric-affine gravity, which involves metric tensor and affine connection as two independent fields, dynamically reduces, in its minimal form, to the usual gravity plus a massive vector field. The vector, which interacts with only the quarks, leptons and gravity, is neutral and long-living (longer than the age of the Universe) when its mass range is 9.4 MeV < M-Y < 28.4 MeV. Its scattering cross section from nucleons, which is some 60 orders of magnitude below the current bounds, is too small to facilitate direct detection of the dark matter. This property provides an explanation for whys and hows of dark matter searches. We show that due to its geometrical origin the Y(mu )does not couple to scalars and gauge bosons. It couples only to fermions. This very feature of the Y-mu it makes it fundamentally different than all the other vector dark matter candidates in the literature. The geometrical dark matter we present is minimal and self-consistent not only theoretically but also astrophysically in that its feebly interacting nature is all that is needed for its longevity.Article Citation - WoS: 37Citation - Scopus: 36Absolute Properties of the Binary System Bb Pegasi(IOP Publishing, 2007) Kalomeni, Belinda; Yakut, Kadri; Keskin, Varol; Değirmenci, Ömer Lütfi; Ulaş, Burak; Köse, OğuzhanWe present ground-based photometry of the low-temperature contact binary BB Peg. We collected all the times of mid-eclipse available in the literature and combined them with those obtained in this study. Analyses of the data indicate a period increase of (3.0 ± 0.1) × 10-8 days yr -1. This period increase of BB Peg can be interpreted in terms of the mass transfer 2.4 × 10-8 M⊙ yr-1 from the less massive to the more massive component. The physical parameters have been determined as Mc = 1.42M⊙, Mh = 0.53 M⊙, Rc = 1.29 R⊙, Rh, = 0.83 R⊙, Lc = 1.86 L⊙, and L h = 0.94 L⊙ through simultaneous solutions of light and of the radial velocity curves. The orbital parameters of the third body, which orbits the contact system in an eccentric orbit, were obtained from the period variation analysis. The system is compared to the similar binaries in the Hertzsprung-Russell and mass-radius diagrams.