arXiv subjects

Cemsinan Deliduman

Publications and source records attributed to Cemsinan Deliduman.

At least 19 recordsLinked to original sources

arXiv2025

Growth Index in the $γδ$CDM model

To better distinguish the nature of $H_0$ and $S_8$ tensions, it is necessary to separate the effects of expansion and the growth of structure. The growth index $γ$ was identified as the most important parameter that characterizes the growth of density fluctuations independently of the effects of cosmic expansion. In the $Λ$CDM model, analyses performed with various cosmological datasets indicate that the growth index has to be larger than its theoretically predicted value. Cosmological models based on $f(R)$ gravity theories have scale-dependent growth indices, whose values are even more at odds with the growth rate data. In this work, we evaluate the growth index in the $γδ$CDM model both theoretically and numerically. Although based on $f(R)$ gravity theory, we show through several analyses with different combinations of datasets that the growth index in the $γδ$CDM model is very close in value to the $Λ$CDM and the $ω$CDM models. The growth of structure is suppressed in the $γδ$CDM model, which is formulated with the extended gravitational growth framework. Upon analyzing cosmological data, we ascertain that the $γδ$CDM model is equally competitive as the $Λ$CDM and the $ω$CDM models.

astro-ph.CO↗

arXiv2025

Testing $γδ$CDM Model in the Redshift Bins

The Hubble crisis is the discrepancy in the values of the Hubble constant inferred from diverse observations in the late and early Universe, being of the order 5$σ$. Instead of resolution, the conflict is getting larger with further late-time observations. A fundamental constant should be and remain constant throughout the cosmological history and thus at all redshifts. The fact that it turns out to be a function of redshift in the $Λ$CDM model points out that either there is a problem with the current cosmological model, indicating unknown new physics, or there are unknown systematics in some of the observations. In this work, we investigate the redshift dependence of the Hubble constant in the $γδ$CDM cosmological model, which is a new cosmological model based on $f(R)$ gravity in an anisotropic background. Through data analysis with the Pantheon+ type Ia supernovae, the cosmic chronometers Hubble, and both the old and the Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillation data, we establish that the Hubble constant in our model does not evolve with redshift. We also confirm that our model fits the aforementioned data better than the $Λ$CDM model by checking various information criteria. The value of the Hubble constant obtained in the $γδ$CDM model is in the 1$σ$ bound of the late Universe observations.

astro-ph.CO↗

arXiv2024

The Ages of the Oldest Astrophysical Objects in an Ellipsoidal Universe

James Webb Space Telescope's (JWST) observations since its launch have shown us that there could be very massive and very large galaxies, as well as massive quasars very early in the history of the universe, conflicting expectations of the $Λ$CDM model. This so-called ''impossibly early galaxy problem'' requires too rapid star formation in the earliest galaxies than appears to be permitted by the $Λ$CDM model. In fact, this might not be a high masses problem, but a ''time-compression problem'': time too short for the observed large and massive structures to form from the initial seeds. A cosmological model that could allocate more time for the earliest large structures to form would be more conforming to the data than the $Λ$CDM model. In this work we are going to discuss how the recently proposed $γδ$CDM model might ease and perhaps resolve the time-compression problem. In the $γδ$CDM model, different energy densities contribute to the Hubble parameter with different weights. Additionally, in the formula for the Hubble parameter, energy densities depend on the redshift differently than what their physical nature dictates. This new way of relating universe's energy content to the Hubble parameter leads to a modified relation between cosmic time and redshift. We test the observational relevance of the $γδ$CDM model to the age problem by constraining its parameters with the ages of the oldest astronomical objects (OAO) together with the cosmic chronometers (CC) Hubble data and the Pantheon+ Type Ia supernovae data of the late universe at low redshift. We find that, thanks to a modified time-redshift relation, the $γδ$CDM model has a more plausible time period at high redshift for large and massive galaxies and massive quasars to form, whereas the age of the universe today is not modified significantly.

astro-ph.CO↗

arXiv2024

f(R) Gravity in an Ellipsoidal Universe

We propose a new model of cosmology based on an anisotropic background and a specific $f(R)$ theory of gravity. It is shown that field equations of $f(R)$ gravity in a Bianchi type I background give rise to a modified Friedmann equation. This model contains two important parameters: $γ$ and $δ$. We, thus, simply call our model $γδ$CDM. It is distinguished in two important aspects from the $Λ$CDM model: firstly, the contribution of different energy densities to the Hubble parameter are weighted with different weights, and then, dependence of energy densities to redshift is modified as well. This unorthodox relation of energy content to Hubble parameter brings forth a new way of interpreting the cosmological history. This solution does not allow the existence of a cosmological constant component, however, a dark energy contribution with dependence on redshift is possible. We tested observational relevance of the new solution by best fitting to different data sets. We found that our model could accommodate the idea of cosmological coupling of black holes.

gr-qc↗

arXiv2020

Flat Galactic Rotation Curves from Geometry in Weyl Gravity

We searched for a resolution of the flat galactic rotation curve problem from geometry instead of assuming the existence of dark matter. We observed that the scale independence of the rotational velocity in the outer region of galaxies could point out to a possible existence of local scale symmetry and therefore the gravitational phenomena inside such regions should be described by the unique local scale symmetric theory, namely Weyl's theory of gravity. We solved field equations of Weyl gravity and determined the special geometry in the outer region of galaxies. In order to understand the effective description of gravitational phenomena, we compared individual terms of so called Einstein-Weyl theory and concluded that while the outer region of galaxies are described by the Weyl term, the inner region of galaxies are described by the Einstein-Hilbert term.

gr-qc↗

arXiv2020

Spherical and cylindrical solutions in f (T) gravity by Noether Symmetry Approach

We find exact solutions for f (T) teleparallel gravity for the cases of spherically and cylindrically symmetric tetrads. The adopted method is based on the search for Noether symmetries of point-like Lagrangians defined in Jordan and Einstein frames. Constants of motion are used to reduce the dynamical system.We first consider the Lagrangian defined in the Jordan frame for a spherically symmetric tetrad and, by the help of two constants of motion, we eliminate a tetrad potential and integrate the other. The more complicated structure in the Einstein frame is also overcome by the same method. After that we obtain the Jordan frame Lagrangian for a general cylindrically symmetric tetrad. Following the same procedure adopted in the spherically symmetric case, we again obtain the tetrad potentials and then the exact solutions.

gr-qc↗

arXiv2019

Gravitational Lensing in Weyl Gravity

We calculate the deflection angle of light from a distant source by a galaxy cluster in Weyl's conformal gravity. The general method of calculation is first applied to calculate the deflection angle in Schwarzschild-de Sitter (Kottler) spacetime. The deflection angle calculated in Kottler spacetime includes the contribution of the cosmological constant, which quantitatively agrees with one work and disagrees with many works in the literature. We then calculate the deflection angle in Mannheim-Kazanas spacetime in two conformally related coordinate systems and find that the result includes contributions from both the cosmological constant and the Mannheim-Kazanas parameter. There are conflicting results on the deflection angle for light in Weyl gravity in the literature. We point out a possible reason for the discrepancy between our work and the others.

gr-qc↗

arXiv2019

Emergence of correlated proton tunneling in water ice

Several experimental and theoretical studies report instances of concerted or correlated multiple proton tunneling in solid phases of water. Here, we construct a pseudo-spin model for the quantum motion of protons in a hexameric H$_2$O ring and extend it to open system dynamics that takes environmental effects into account in the form of O$-$H stretch vibrations. We approach the problem of correlations in tunneling using quantum information theory in a departure from previous studies. Our formalism enables us to quantify the coherent proton mobility around the hexagonal ring by one of the principal measures of coherence, the $l_1$ norm of coherence. The nature of the pairwise pseudo-spin correlations underlying the overall mobility is further investigated within this formalism. We show that the classical correlations of the individual quantum tunneling events in long-time limit is sufficient to capture the behaviour of coherent proton mobility observed in low-temperature experiments. We conclude that long-range intra-ring interactions do not appear to be a necessary condition for correlated proton tunneling in water ice.

quant-ph↗

arXiv2018

Proton tunneling in hydrogen bonds and its implications in an induced-fit model of enzyme catalysis

The role of proton tunneling in biological catalysis is investigated here within the frameworks of quantum information theory and thermodynamics. We consider the quantum correlations generated through two hydrogen bonds between a substrate and a prototypical enzyme that first catalyzes the tautomerization of the substrate to move on to a subsequent catalysis, and discuss how the enzyme can derive its catalytic potency from these correlations. In particular, we show that classical changes induced in the binding site of the enzyme spreads the quantum correlations among all of the four hydrogen-bonded atoms thanks to the directionality of hydrogen bonds. If the enzyme rapidly returns to its initial state after the binding stage, the substrate ends in a new transition state corresponding to a quantum superposition. Open quantum system dynamics can then naturally drive the reaction in the forward direction from the major tautomeric form to the minor tautomeric form without needing any additional catalytic activity. We find that in this scenario the enzyme lowers the activation energy so much that there is no energy barrier left in the tautomerization, even if the quantum correlations quickly decay.

physics.chem-ph↗

arXiv2018

Quantum entanglement shared in hydrogen bonds and its usage as a resource in molecular recognition

Quantum tunneling events occurring through biochemical bonds are capable to generate quantum correlations between bonded systems, which in turn makes the conventional second law of thermodynamics approach insufficient to investigate these systems. This means that the utilization of these correlations in their biological functions could give an evolutionary advantage to biomolecules to an extent beyond the predictions of molecular biology that are generally based on the second law in its standard form. To explore this possibility, we first compare the tunneling assisted quantum entanglement shared in the ground states of covalent and hydrogen bonds. Only the latter appears to be useful from a quantum information point of view. Also, significant amounts of quantum entanglement can be found in the thermal state of hydrogen bond. Then, we focus on an illustrative example of ligand binding in which a receptor protein or an enzyme is restricted to recognize its ligands using the same set of proton-acceptors and donors residing on its binding site. In particular, we show that such a biomolecule can discriminate between $3^n - 1$ agonist ligands if it uses the entanglement shared in $n$ intermolecular hydrogen bonds as a resource in molecular recognition. Finally, we consider the molecular recognition events encountered in both the contemporary genetic machinery and its hypothetical primordial ancestor in pre-DNA world, and discuss whether there may have been a place for the utilization of quantum entanglement in the evolutionary history of this system.

quant-ph↗

arXiv2015

Absence of Relativistic Stars in f(T) Gravity

In this work we investigate the structure of neutron stars in modified f(T) gravity models. We find that, unlike the f(R) models, the equations of motion put a rather strict constraint on the possible f(T) functions. Specifically, after analyzing the problem in two different choice of coordinates with spherical symmetry, we conclude that the relativistic neutron star solution in f(T) gravity models is possible only if f(T) is a linear function of the torsion scalar T, that is in the case of Teleparallel Equivalent of General Relativity.

gr-qc↗

arXiv2013

Neutron stars in a perturbative $f(R)$ gravity model with strong magnetic fields

We investigate the effect of a strong magnetic field on the structure of neutron stars in a model with perturbative $f(R)$ gravity. The effect of an interior strong magnetic field of about $10^{17 \sim 18}$ G on the equation of state is derived in the context of a quantum hadrodynamics (QHD) model. We solve the modified spherically symmetric hydrostatic equilibrium equations derived for a gravity model with $f(R)=R+αR^2$. Effects of both the finite magnetic field and the modified gravity are detailed for various values of the magnetic field and the perturbation parameter $α$ along with a discussion of their physical implications. We show that there exists a parameter space of the modified gravity and the magnetic field strength, in which even a soft equation of state can accommodate a large ($> 2$ M$_\odot$) maximum neutron star mass through the modified mass-radius relation.

astro-ph.HE↗

arXiv2012

Neutron star solutions in perturbative quadratic gravity

We study the structure of neutron stars in R+β R^{μν} R_{μν} gravity model with perturbative method. We obtain mass--radius relations for six representative equations of state (EoSs). We find that, for |β| ~ 10^11 cm^2, the results differ substantially from the results of general relativity. Some of the soft EoSs that are excluded within the framework of general relativity can be reconciled for certain values of β of this order with the 2 solar mass neutron star recently observed. For values of β greater than a few 10^11 cm^2 we find a new solution branch allowing highly massive neutron stars. By referring some recent observational constraints on the mass--radius relation we try to constrain the value of β for each EoS. The associated length scale \sqrtβ ~ 10^6 cm is of the order of the typical radius of neutron stars implying that this is the smallest value we could find by using neutron stars as a probe. We thus conclude that the true value of β is most likely much smaller than 10^11 cm^2.

gr-qc↗

arXiv2012

A Massive S-duality in 4 dimensions

We reduce the Type IIA supergravity theory with a generalized Scherk-Schwarz ansatz that exploits the scaling symmetry of the dilaton, the metric and the NS 2-form field. The resulting theory is a new massive, gauged supergravity theory in four dimensions with a massive 2-form field and a massive 1-form field. We show that this theory is S-dual to a theory with a massive vector field and a massive 2-form field, which are dual to the massive 2-form and 1-form fields in the original theory, respectively. The S-dual theory is shown to arise from a Scherk-Schwarz reduction of the heterotic theory. Hence we establish a massive, S-duality type relation between the IIA theory and the heterotic theory in four dimensions. We also show that the Lagrangian for the new four dimensional theory can be put in the most general form of a D=4, N=4 gauged Lagrangian found by Schon and Weidner, in which (part of) the SL(2) group has been gauged.

hep-th↗

arXiv2011

Entanglement Swapping Model of DNA Replication

Molecular biology explains function of molecules by their geometrical and electronical structures that are mainly determined by utilization of quantum effects in chemistry. However, further quantum effects are not thought to play any significant role in the essential processes of life. On the contrary, consideration of quantum circuits/protocols and organic molecules as software and hardware of living systems that are co-optimized during evolution, may be useful to overcome the difficulties raised by biochemical complexity and to understand the physics of life. In this sense, we review quantum information-theoretic approaches to the process of DNA replication and propose a new model in which 1) molecular recognition of a nucleobase is assumed to trigger an intrabase entanglement corresponding to a superposition of different tautomer forms and 2) pairing of complementary nucleobases is described by swapping intrabase entanglements with interbase entanglements. We examine possible biochemical realizations of quantum circuits/protocols to be used to obtain intrabase and interbase entanglements. We deal with the problem of cellular decoherence by using the theory of decoherence-free subspaces and subsystems. Lastly, we discuss feasibility of the computational or experimental verification of the model and future research directions.

quant-ph↗

arXiv2011

Constraints on Perturbative f(R) Gravity via Neutron Stars

We study the structure of neutron stars in perturbative f(R) gravity models with realistic equations of state. We obtain mass-radius relations in a gravity model of the form f(R)=R+αR^2. We find that deviations from the results of general relativity, comparable to the variations due to using different equations of state (EoS'), are induced for |alpha| ~ 10^9 cm^2. Some of the soft EoS' that are excluded within the framework of general relativity can be reconciled with the 2 solar mass neutron star recently observed for certain values of alpha within this range. For some of the EoS' we find that a new solution branch, which allows highly massive neutron stars, exists for values of alpha greater than a few 10^9 cm^2. We find constraints on alpha for a variety of EoS' using the recent observational constraints on the mass-radius relation. These are all 5 orders of magnitude smaller than the recent constraint obtained via Gravity Probe B for this gravity model. The associated length scale \sqrt{alpha} ~ 10^5 cm is only an order of magnitude smaller than the typical radius of a neutron star, the probe used in this test. This implies that real deviations from general relativity can be even smaller.

gr-qc↗

arXiv2011

DNA Replication via Entanglement Swapping

Quantum effects are mainly used for the determination of molecular shapes in molecular biology, but quantum information theory may be a more useful tool to understand the physics of life. Organic molecules and quantum circuits/protocols can be considered as hardware and software of living systems that are co-optimized during evolution. We try to model DNA replication in this sense as a multi-body entanglement swapping with a reliable qubit representation of the nucleotides. In our model molecular recognition of a nucleotide triggers an intrabase entanglement corresponding to a superposition state of different tautomer forms. Then, base pairing occurs by swapping intrabase entanglements with interbase entanglements. We examine possible realizations of quantum circuits to be used to obtain intrabase entanglement and swapping protocols to be employed to obtain interbase entanglement. Finally, we discuss possible ways for computational and experimental verification of the model.

quant-ph↗

arXiv2006

Noncommutative Gravity in Six Dimensions

A gauge theory of gravity is defined in 6 dimensional non-commutative space-time. The gauge group is the unitary group U(2,2), which contains the homogeneous Lorentz group, SO(4,2), in 6 dimensions as a subgroup. It is shown that, after the Seiberg-Witten map, in the corresponding theory the lowest order corrections are first order in the non-commutativity parameter θ. This is in contrast with the results found in non-commutative gauge theories of gravity with the gauge group SO(d,1).

hep-th↗