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İbrahim Semiz

Publications and source records attributed to İbrahim Semiz.

18 recordsLinked to original sources
arXiv2026

The general static spherical perfect fluid solution with EoS parameter w=-1/6

The general analytical solution for the static spherically symmetric metric supported by a perfect fluid with proportional-equation-of-state $p = w ρ$ is not known at the time of this writing, except for the trivial cases $w=0$ and $w=-1$; for $w=-1/3$, and the recently reported $w=-1/5$. We show that the case $w=-1/6$ is also analytically solvable, as predicted in another recent work. The solution is affected by a Buchdahl transformation of a known solution.

gr-qc↗
arXiv2025

Analysis of Pantheon+ supernova data suggests evidence of sign-changing pressure of the cosmological fluid

In this work, we revisit/reinterpret/extend the model-independent analysis method (which we now call spread - luminosity distance fitting, spread-LDF) from our previous work. We apply it to the updated supernova type Ia catalogue, Pantheon+ and recent GRB compilations. The procedure allows us, using only FLRW assumption, to construct good approximations for expansion history of the universe, re-confirming its acceleration to be a robust feature. When we also assume General Relativity ("GR"), we can demonstrate, without any matter/energy model in mind, the need for (possibly nonconstant) dark energy ("GDE"). We find hints for positive pressure of GDE at z>1 with implications on either the complexity of dark energy, or the validity of one of the cosmological principle, interpretation of SN Ia data, or GR.

gr-qc↗
arXiv2022

The general static spherical perfect fluid solution with EoS parameter w=-1/5

The general analytical solution for the static spherically symmetric metric supported by a perfect fluid with isothermal (proportional) equation-of-state $p = w ρ$ is not known at the time of this writing, except for the trivial cases $w=0$ and $w=-1$; and for $w=-1/3$. We show that if Buchdahl coordinates are used, the problem becomes analytically solvable for $w=-1/5$; display and discuss the solution(s), and exhibit the connection of this case to the $w=-1$ case.

gr-qc↗
arXiv2021

Estimation of attenuation of gravitational waves by Bose-Einstein condensate dark matter halos using Bogoliubov-de Gennes theory

We consider a gravitational plane wave passing through a galactic dark matter halo composed of weakly self-interacting, self-gravitating, Bose-Einstein condensate of ultralight particles. Treating the gravitational wave as a time dependent perturbation, we study energy transfer between the gravitational wave and the Bose-Einstein condensate by applying linear response theory to a non-uniform condensate described by the Bogoliubov-de Gennes theory, and compute the fractional loss in gravitational wave energy. We apply our results to investigate the extent to which this loss effects the estimation of the distance between the gravitational wave source and the earth. We show that the effect is negligible.

gr-qc↗
arXiv2020

On the (non)genericity of the Kiselev spacetime

In many works in the literature, the Kiselev spacetime is interpreted as the "blackhole surrounded by quintessence"; an interpretation not really tenable. For example, it has recently been pointed out that the expressions "perfect fluid" and "quintessence" are used improperly in most of the articles that build on that spacetime; hence it is not as relevant as many of those authors might think. Here we point out that the original derivation of the Kiselev spacetime involves the use of too many coordinate conditions as well, again strongly restricting its relevance.

gr-qc↗
arXiv2020

Reply to: "TOV or OV? The Whole Story"

We argued previously that the well-known equation for hydrostatic equilibrium in a static spherically symmetric spacetime supported by an isotropic perfect fluid should be called the Oppenheimer-Volkoff (OV) equation, rather than the Tolman-Oppenheimer-Volkoff (TOV) equation, a later ArXiv manuscript has disagreed. Here we reply to that comment, reaffirming the original argument.

physics.hist-ph↗
arXiv2016

OV or TOV ?

The well-known equation for hydrostatic equilibrium in a static spherically symmetric spacetime supported by an isotropic perfect fluid is referred to as the Oppenheimer-Volkoff (OV) equation or the Tolman-Oppenheimer-Volkoff (TOV) equation in various General Relativity textbooks or research papers. We scrutinize the relevant original publications to argue that the former is the more appropriate terminology.

physics.hist-ph↗
arXiv2016

Hawking radiation as the cosmic censor

Hawking radiation acts as a cosmic censor since it carries away the angular momentum of the black hole, proportionally more than its mass. In this work we first show that an extremal black hole cannot exist since it will be pushed away from extremality by its own Hawking radiation, without being perturbed by any external effect. We evaluate the efficiency of Hawking radiation to prevent overspinning of black holes. We make an order of magnitude estimate to show that evaporation can prevent overspinning of black holes with an upper limit of mass $M\lesssim 10^{17}-10^{18}\rm{g}$, when we take the interaction period to be the age of the universe. Overspinning of black holes of higher masses by test fields remains possible, even if evaporation is taken into account. We also discuss the possibility to attribute a shorter interaction period for the problem which would reduce the effect of evaporation.

gr-qc↗
arXiv2016

The decoupling problem of the Proca equation; and treatment of Dirac, Maxwell and Proca fields on the resulting pp-wave spacetimes

In this work we take a formal approach to the problem of decoupling Proca equations in curved space-times. We use Newman-Penrose (NP) two-spinor formalism to represent the Proca vector by one complex and two real scalars. We show that a decoupled second order differential equation for one of the real scalars can be derived if and only if the background space-time admits a covariantly constant null vector. Thus, the background space-time must be a pp-wave vacuum. We evaluate the separability of Proca, Maxwell and Dirac equations on the resulting pp-wave background.

gr-qc↗
arXiv2016

Random-walk baryogenesis via primordial black holes

Gravitation violates baryon number $B$: A star has a huge amount of it, while a black hole forming from the star has none. Consider primordial black holes before the hadronic annihiliation in the early universe, encountering and absorbing baryons and antibaryons: Each such absorption changes $B$ of the universe by one unit, up or down. But the absorption events are $uncorrelated$ $and$ $random$, hence they amount to a random walk in $B$-space, leading to the expectation of a net $|B|$ at the end. While the scale of this effect is most uncertain, it must exist. We explore some ramifications, including the change of net $|B|$ with expansion, connection with universe topology, and possible observational signatures.

gr-qc↗
arXiv2015

Weak Cosmic Censorship, Superradiance and Quantum Particle Creation

Since 1970's, gedanken experiments have been devised to challenge the weak cosmic censorship conjecture (WCCC), which is the expectation that spacetime singularities will be hidden from faraway observers by event horizons so that classical predictability in a spacetime is preserved. These experiments involve the interaction of an extremal or a slightly sub-extremal black hole with a test particle or field, attempting to destroy the horizon, i.e. to create a so-called naked singularity. They usually conclude that WCCC cannot be violated starting from an extremal black hole, but may be violated starting from a slightly sub-extremal one, if backreaction and self-force effects are neglected. Some other works also analyze these effects. Starting 2007, a string of papers argue if WCCC can be violated by classically forbidden interactions occuring via the quantum nature of the particles associated with the fields; and where backrection and/or superradiance are pointed out as effects working in the direction of preserving the WCCC. We correct/modify a backrection argument, and furthermore point out that superradiance does not prevent {\em single particles} from being captured by the black hole; even if this capture would lead to WCCC violation. Then we consider the spontaneous emission (which we call the Zel'dovich-Unruh effect) of particles by the black hole, and find that at least for scalars, it can be understood without second quantization. It also completely invalidates the mentioned single- or few-particle thought experiments. However, the conclusions of our previous work on (at least) scalar fields interacting with black holes, i.e. that WCCC may be violated starting from slightly subextremal black holes, remains valid in this (semi)classical framework.

gr-qc↗
arXiv2015

What do the cosmological supernova data really tell us?

Not much by themselves, aparently. We try to reconstruct the scale factor $a(t)$ of the universe from the SNe Ia data, i.e. the luminosity distance $d_{L}(z)$, using only the cosmological principle and the assumption that gravitation is governed by a metric theory. In our hence "model-independent," or "cosmographic" study, we fit functions to $d_{L}(z)$ rather than $a(t)$, since $d_{L}(z)$ is what is measured. We find that the acceleration history of the universe cannot be reliably determined in this approach due to the irregularity and parametrization-dependence of the results. However, adding the GRB data to the dataset cures most of the irregularities, at the cost of compromising the model-independent nature of the study slightly. Then we can determine the redshift of transition to cosmic acceleration as $z_{\rm t} \sim 0.50 \pm 0.09$ for a flat universe (larger for positive spatial curvature). If Einstein gravity (GR) is assumed, we find a redshift at which the density of the universe predicted from the $d_{L}(z)$ data is independent of curvature. We use this point to derive an upper limit on matter density, hence a lower limit on the density of dark energy. While these limits do not improve the generally accepted ones, they are derived *only using the $d_{L}(z)$ data*.

gr-qc↗
arXiv2015

Dyson Spheres around White Dwarfs

A Dyson Sphere is a hypothetical structure that an advanced civilization might build around a star to intercept all of the star's light for its energy needs. One usually thinks of it as a spherical shell about one astronomical unit (AU) in radius, and surrounding a more or less Sun-like star; and might be detectable as an infrared point source. We point out that Dyson Spheres could also be built around white dwarfs. This type would avoid the need for artificial gravity technology, in contrast to the AU-scale Dyson Spheres. In fact, we show that parameters can be found to build Dyson Spheres suitable --temperature- and gravity-wise-- for human habitation. This type would be much harder to detect.

physics.pop-ph↗
arXiv2014

Jovian planets as co-detectors of gravitational waves

Acoustic oscillations in stars can be driven by gravitational waves. However, at present it is not feasible to use the helioseismology data for their detection, since it is impossible to disentangle the uncertain driving contributions originating in the Sun itself. We here point out that any such wave will affect also Jupiter and Saturn in a similar $and$ $coordinated$ $way$; after all, they are mostly spheres of gas like the Sun, only one order of magnitude smaller. Hence, akin to the concept of coincidence detection in particle physics experiments, evaluation of the time-correlation function of the measured acoustic velocities of the same mode of oscillation of any two of these three objects will eliminate the (independent) internal effects; and observation of a peak in that correlation function will be tantamount to detection of a gravitational wave. There is a (slight) possibility that such detection has already occured.

gr-qc↗
arXiv2013

Cosmic Censorship, Black Holes and Integer-spin Test Fields

It has been argued that, starting with a slightly sub-extremal Kerr black hole instead of an extremal one, it is possible to overspin a black hole past the extremal limit and turn it into a naked singularity by sending test bodies, if one neglects radiative and self-force effects. In this work we show that (i) an extremal Kerr black hole can not be overspun as a result of the interaction with massless integer spin test fields (scalar, electromagnetic, or gravitational), (ii) overspinning can be achieved if we start with a nearly extremal black hole instead, and (iii) for the scalar field, the argument applies to more general black holes, and also allows use of a more general field configuration. Our analysis also neglects radiative and self-force effects.

gr-qc↗
arXiv2006

Proposal: The Neural Network Telescope

A neural network mechanism that can compensate for poor optical quality was recently discovered in a biological context. We propose that this mechanism can and should be adopted for astronomical purposes. This would shift emphasis away from the quality of the optical equipment to information processing, hence should decrease the cost and make larger instruments feasible.

astro-ph↗
arXiv1992

The Dirac Equation Is Separable On The Dyon Black Hole Metric

Using the tetrad formalism, we carry out the separation of variables for the massive complex Dirac equation in the gravitational and electromagnetic field of a four-parameter (mass, angular momentum, electric and magnetic charges) black hole.

gr-qc↗