arXiv subjects

Nihan Katirci

Publications and source records attributed to Nihan Katirci.

15 recordsLinked to original sources

arXiv2026

Quadratic energy-momentum squared gravity: constraints from big bang nucleosynthesis

In this work, we extend the standard cosmological model within the quadratic energy-momentum squared gravity (qEMSG) framework, introducing a nonminimal interaction between the usual material field ($T_{μν}$) and its accompanying partner field (qEMSF, $T_{μν}^{\rm qEMSF}$), defined by $f(\mathbf{T}^2)=α\mathbf{T}^2$ with $\mathbf{T^2}=T_{μν}T^{μν}$. Adopting an analytical approach within the qEMSG framework, we present a comprehensive exploration of Big Bang Nucleosynthesis (BBN) dynamics. Our analysis selects the radiation-dominated universe solution compatible with the standard cosmological model limit as $α\rightarrow0$ and reveals that qEMSF interaction model can modify the radiation energy density's evolution, potentially altering neutron-proton interconversion rates and consequently affecting $^4$He abundance in various ways. By explicitly defining modifications to the predicted primordial $^4$He mass fraction, $Y_{\rm p}$, we establish the most stringent cosmological constraints on the parameter $α$ based on recent measurements of $Y_{\rm p}$: $(-8.81\leqα\leq8.14)\times10^{-27}\,\mathrm{eV}^{-4}$ (68% CL) from Aver et al.'s primordial $^4$He abundance measurements, aligning with $α=0$. Additionally, $(3.48\leqα\leq4.43)\,\times 10^{-27}\rm{eV}^{-4}$ (68% CL) from Fields et al.'s estimates, utilizing the Planck-CMB estimated baryon density within the standard cosmological model framework, diverges from $α=0$, thereby lending support to the qEMSF interaction model. The study also highlights the bidirectional nature of energy-momentum/entropy transfer in qEMSF interaction model, depending on the sign of $α$. The implications of qEMSF in the presence of additional relativistic relics are also explored, showcasing the model's potential to accommodate deviations from standard cosmology and the Standard Model of particle physics.

astro-ph.CO↗

arXiv2025

Dynamical analysis of logarithmic energy-momentum squared gravity

We perform the dynamical system analysis of a cosmological model in the energy-momentum squared gravity (EMSG) of the form $f(T_{μν} T^{μν})=α\ln(λT_{μν} T^{μν})$, which is known as energy-momentum log gravity (EMLG). In particular, we show that the analytical cosmological solution of EMLG presented by Akarsu {\it et al.} (Eur. Phys. J. C 79:846, 2019) is a future attractor. It includes new terms in the right-hand side of the Einstein field equations, which yield constant inertial mass density and provide a dynamical dark energy with a density passing below zero at large redshifts, accommodating a mechanism for screening $Λ$ in the past for $α<0$, suggested for alleviating some cosmological tensions. We show that the second law of thermodynamics requires $α\leq0$ that allows the screening mechanism to take place. We also show that the model gives rise to an entire class of new stable late-time solutions with $H\rightarrow\sqrt{(Λ+2α)/3}$ as $a\rightarrow\infty$, where the new term is due to the constant effective inertial mass density that arises from EMLG contribution of dust, whereas $H\rightarrow\sqrt{Λ/3}$ as $a\rightarrow\infty$ in the $Λ$CDM model. We also show the existence of new interesting features and trajectories that are absent in $Λ$CDM with or without spatial curvature.

gr-qc↗

arXiv2023

Weak field and slow motion limits in energy-momentum powered gravity

We explore the weak field and slow motion limits, Newtonian and Post-Newtonian limits, of the energy-momentum powered gravity (EMPG), viz., the energy-momentum squared gravity (EMSG) of the form $f(T_{μν}T^{μν})=α(T_{μν}T^{μν})^η$ with $α$ and $η$ being constants. We have shown that EMPG with $η\geq0$ and general relativity (GR) are not distinguishable by local tests, say, the Solar System tests; as they lead to the same gravitational potential form, PPN parameters, and geodesics for the test particles. However, within the EMPG framework, $M_{\rm ast}$, the mass of an astrophysical object inferred from astronomical observations such as planetary orbits and deflection of light, corresponds to the effective mass $M_{\rm eff}(α,η,M)=M+M_{\rm empg}(α,η,M)$, $M$ being the actual physical mass and $M_{\rm empg}$ being the modification due to EMPG. Accordingly, while in GR we simply have the relation $M_{\rm ast}=M$, in EMPG we have $M_{\rm ast}=M+M_{\rm empg}$. Within the framework of EMPG, if there is information about the values of $\{α,η\}$ pair or $M$ from other independent phenomena (from cosmological observations, structure of the astrophysical object, etc.), then in principle it is possible to infer not only $M_{\rm ast}$ alone from astronomical observations, but $M$ and $M_{\rm empg}$ separately. For a proper analysis within EMPG framework, it is necessary to describe the slow motion condition (also related to the Newtonian limit approximation) by $|p_{\rm eff}/ρ_{\rm eff}|\ll1$ (where $p_{\rm eff}=p+p_{\rm empg}$ and $ρ_{\rm eff}=ρ+ρ_{\rm empg}$), whereas this condition leads to $|p/ρ|\ll1$ in GR.

gr-qc↗

arXiv2021

$Λ$CDM cosmology with a quiescent anisotropy in a higher dimensional steady state universe

In this work, which follows a series of studies on the higher-dimensional steady state universe idea and prepared for Professor Tekin Dereli's Festschrift, we show the influence of the dynamical internal (unobservable) space on the evolution of the possible anisotropy of the external (observable) space. We obtain mathematically exactly the same Friedmann equation of the standard $Λ$CDM model for the external space, but with some remarkable physical differences. In particular, the higher-dimensional negative cosmological constant plays the role of the four-dimensional positive cosmological constant and the expansion anisotropy, viz., the shear scalar, of the external space mimics a negative cosmological constant; it would mimic a stiff fluid when allowed on top of the standard $Λ$CDM model. This latter feature gives us the opportunity to manipulate the CMB quadrupole temperature fluctuation, suggesting a possible answer to the fact that its observed value is lower than that predicted by the standard $Λ$CDM model.

gr-qc↗

arXiv2021

Simple-graduated dark energy and spatial curvature

In this work, we first discuss the possibility that dark energy models with negative energy density values in the past can alleviate the $H_0$ tension, as well as the discrepancy with the baryon acoustic oscillation (BAO) Lyman-$α$ data, both which prevail within the $Λ$CDM model. We then investigate whether two minimal extensions of the $Λ$CDM model, together or separately, can successfully realize such a scenario: (i) the spatial curvature, which, in the case of spatially closed universe, mimics a negative density source and (ii) simple-graduated dark energy (gDE), which promotes the null inertial mass density of the usual vacuum energy to an arbitrary constant--if negative, the corresponding energy density decreases with redshift similar to the phantom models, but unlike them crosses below zero at a certain redshift. We find that, when the Planck data are not included in the observational analysis, the models with simple-gDE predict interesting and some significant deviations from the $Λ$CDM model. In particular, a spatially closed universe along with a simple-gDE of positive inertial mass density, which work in contrast to each other, results in minor improvement to the $H_0$ tension. The joint dataset, including the Planck data, presents no evidence for a deviation from spatial flatness but almost the same evidence for a cosmological constant and the simple-gDE with an inertial mass density of order $\mathcal{O}(10^{-12})\,\rm eV^4$. The latter case predicts almost no deviation from the $Λ$CDM model up until today--so that it results in no improvement regarding the BAO Ly-$α$ data--except that it slightly aggravates the $H_0$ tension. We also study via dynamical analysis the history of the Universe in the models, as the simple-gDE results in futures different than the de Sitter future of the $Λ$CDM model.

astro-ph.CO↗

arXiv2020

Rastall gravity extension of the standard $Λ$CDM model: theoretical features and observational constraints

We present a detailed investigation of the Rastall gravity extension of the standard $Λ$CDM model. We review the model for two simultaneous modifications of different nature in the Friedmann equation due to the Rastall gravity: the new contributions of the material (actual) sources (considered as effective source) and the altered evolution of the material sources. We discuss the role/behavior of these modifications with regard to some low redshift tensions, including the so-called $H_0$ tension, prevailing within the standard $Λ$CDM. We constrain the model at the level of linear perturbations, and obtain the first constraints through a robust and accurate analysis using the latest full Planck CMB data, with and without including BAO data. We find that the Rastall parameter $ε$ (null for general relativity) is consistent with zero at 68\% CL (with a tendency towards positive values, $-0.0001 < ε< 0.0007$ (CMB+BAO) at 68\% CL), which in turn implies no significant statistical evidence for deviation from general relativity, and also a precision of $\mathcal{O}(10^{-4})$ for the coefficient $-1/2$ of the term $g_{μν}R$ in the Einstein field equations of general relativity (guaranteeing the local energy-momentum conservation). We explore the consequences led by the Rastall gravity on the cosmological parameters in the light of the observational analyses. It turns out that the effective source dynamically screens the usual vacuum energy at high redshifts, but this mechanism barely works due to the opposition by the altered evolution of CDM. Consequently, two simultaneous modifications of different nature in the Friedmann equation act against each other, and do not help to considerably relax the so-called low redshift tensions. Our results may offer a guide for the research community that studies the Rastall gravity in various aspects of gravitation and cosmology.

astro-ph.CO↗

arXiv2020

Scalar field emulator via anisotropically deformed vacuum energy: Application to dark energy

We introduce a generalization of the usual vacuum energy, called `deformed vacuum energy', which yields anisotropic pressure whilst preserving zero inertial mass density. It couples to the shear scalar in a unique way, such that they together emulate the canonical scalar field with an arbitrary potential. This opens up a new avenue by reconsidering cosmologies based on canonical scalar fields, along with a bonus that the kinetic term of the scalar field is replaced by an observable, the shear scalar. We further elaborate the aspects of this approach in the context of dark energy.

gr-qc↗

arXiv2020

Anisotropic massive Brans-Dicke gravity extension of the standard $Λ$CDM model

We present an explicit detailed theoretical and observational investigation of an anisotropic massive Brans-Dicke (BD) gravity extension of the standard $Λ$CDM model, wherein the extension is characterized by two additional degrees of freedom; the BD parameter, $ω$, and the present day density parameter corresponding to the shear scalar, $Ω_{σ^2,0}$. The BD parameter, determining the deviation from general relativity (GR), by alone characterizes both the dynamics of the effective dark energy (DE) and the redshift dependence of the shear scalar. These two affect each other depending on $ω$, namely, the shear scalar contributes to the dynamics of the effective DE, and its anisotropic stress --which does not exist in scalar field models of DE within GR-- controls the dynamics of the shear scalar deviating from the usual $\propto(1+z)^6$ form in GR. We mainly confine the current work to non-negative $ω$ values as it is the right sign --theoretically and observationally-- for investigating the model as a correction to the $Λ$CDM. By considering the current cosmological observations, we find that $ω\gtrsim 250$, $Ω_{σ^2,0}\lesssim 10^{-23}$ and the contribution of the anisotropy of the effective DE to this value is insignificant. We conclude that the simplest anisotropic massive BD gravity extension of the standard $Λ$CDM model exhibits no significant deviations from it all the way to the Big Bang Nucleosynthesis. We also point out the interesting features of the model in the case of negative $ω$ values; for instance, the constraints on $Ω_{σ^2,0}$ could be relaxed considerably, the values of $ω\sim-1$ (relevant to string theories) predict dramatically different dynamics for the expansion anisotropy.

gr-qc↗

arXiv2018

Cosmological implications of scale-independent energy-momentum squared gravity: Pseudo nonminimal interactions in dark matter and relativistic relics

In this paper, we introduce a scale-independent energy-momentum squared gravity (EMSG) that allows different gravitational couplings for different types of sources, which may lead to scenarios with many interesting applications/implications in cosmology. In the present study, to begin with, we study a modification of the $Λ$ cold dark matter ($Λ$CDM) model, where photons and baryons couple to the spacetime as in general relativity, while the cold dark matter and relativistic relics (neutrinos and any other relativistic relics) couple to the spacetime in accordance with EMSG. This scenario induces pseudo nonminimal interactions on these components, leading to modification at both the background and perturbative levels. A consequence of this scenario is that the dimensionless free parameter of the theory may induce direct changes on the effective number of the relativistic species, without the need to introduce new extra species. In order to quantify the observational consequences of the cosmological scenario, we use the cosmic microwave background Planck data (temperature, polarization, and lensing power spectrum) and baryonic acoustic oscillations data. We find that the free model parameter is too small to induce statistically significant corrections on the $Λ$CDM model due to EMSG. We deduce that the model presented here is quite rich with promising cosmological applications/implications that deserve further investigations.

gr-qc↗

arXiv2018

Constraint on energy-momentum squared gravity from neutron stars and its cosmological implications

Deviations from the predictions of general relativity due to energy-momentum squared gravity (EMSG) are expected to become pronounced in the high density cores of neutron stars. We derive the hydrostatic equilibrium equations in EMSG and solve them numerically to obtain the neutron star mass-radius relations for four different realistic equations of state. We use the existing observational measurements of the masses and radii of neutron stars to constrain the free parameter, $α,$ that characterizes the coupling between matter and spacetime in EMSG. We show that $-10^{-38}\,\mathrm{cm^{3}/erg}<α<+10^{-37}\,\mathrm{cm^{3}/erg}$. Under this constraint, we discuss what contributions EMSG can provide to the physics of neutron stars, in particular, their relevance to the so called \textit{hyperon puzzle} in neutron stars. We also discuss how EMSG alters the dynamics of the early universe from the predictions of the standard cosmological model. We show that EMSG leaves the standard cosmology safely unaltered back to $t\sim 10^{-4}$ seconds at which the energy density of the universe is $\sim 10^{34}\,\mathrm{erg\,cm^{-3}}$.

gr-qc↗

arXiv2018

Cosmic acceleration in dust only Universe via energy-momentum powered gravity

We propose a modified theory of gravitation constructed by the addition of the term $f(T_{μν}T^{μν})$ to the Einstein-Hilbert action, and elaborate a particular case $f(T_{μν}T^{μν})=α(T_{μν}T^{μν})^η$, where $α$ and $η$ are real constants, dubbed as energy-momentum powered gravity (EMPG). We search for viable cosmologies arising from EMPG especially in the context of the late-time accelerated expansion of the Universe. We investigate the ranges of the EMPG parameters $(α,η)$ on theoretical as well as observational grounds leading to the late-time acceleration of the Universe with pressureless matter only, while keeping the successes of standard general relativity at early times. We find that $η=0$ corresponds to the $Λ$CDM model, whereas $η\neq 0$ leads to a $w$CDM-type model. However, the underlying physics of the EMPG model is entirely different in the sense that the energy in the EMPG Universe is sourced by pressureless matter only. Moreover, the energy of the pressureless matter is not conserved, namely, in general it does not dilute as $ρ\propto a^{-3}$ with the expansion of the Universe. Finally, we constrain the parameters of an EMPG-based cosmology with a recent compilation of 28 Hubble parameter measurements, and find that this model describes an evolution of the Universe similar to that in the $Λ$CDM model. We briefly discuss that EMPG can be unified with Starobinsky gravity to describe the complete history of the Universe including the inflationary era.

gr-qc↗

arXiv2017

Inflation and late time acceleration designed by Stueckelberg massive photon

We present a mini review of the Stueckelberg mechanism, which was proposed to make the abelian gauge theories massive as an alternative to Higgs mechanism, within the framework of Minkowski as well as curved spacetimes. The higher the scale the tighter the bounds on the photon mass, which might be gained via the Stueckelberg mechanism, may be signalling that even an extremely small mass of the photon which cannot be measured directly could have far reaching effects in cosmology. We present a cosmological model where Stueckelberg fields, which consist of both scalar and vector fields, are non-minimally coupled to gravity and the universe could go through a decelerating expansion phase sandwiched by two different accelerated expansion phases. We discuss also the possible anisotropic extensions of the model.

gr-qc↗

arXiv2015

General solution of a cosmological model induced from higher dimensions using a kinematical constraint

In a recent study Akarsu and Dereli (Gen. Relativ. Gravit. 45:1211, 2013) discussed the dynamical reduction of a higher dimensional cosmological model which is augmented by a kinematical constraint characterized by a single real parameter, correlating and controlling the expansion of both the external (physical) and internal spaces. In that paper explicit solutions were found only for the case of three dimensional internal space ($n=3$). Here we derive a general solution of the system using Lie group symmetry properties, in parametric form for arbitrary number $n=1,2,3,\dots$ of internal dimensions. We also investigate the dynamical reduction of the model as a function of cosmic time $t$ for various values of $n$ and generate parametric plots to discuss cosmologically relevant results.

gr-qc↗

arXiv2014

Accelerated expansion of the Universe a la the Stueckelberg mechanism

We investigate a cosmological model in which the Stueckelberg fields are non-minimally coupled to the scalar curvature in a gauge invariant manner. We present not only a solution that can be considered in the context of the late time acceleration of the universe but also a solution compatible with the inflationary cosmology. Distinct behaviors of the scalar and vector fields together with the real valued mass gained by the Stueckelberg mechanism lead the universe to go through the two different accelerated expansion phases with a decelerated expansion phase between them. On the other hand, in the solutions we present, if the mass is null then the universe is either static or exhibits a simple power law expansion due to the vector field potential.

gr-qc↗

arXiv2012

Electromagnetic Origin for Planck Mass and Dark Energy

The origin of dark energy remains to be one of the challenges of modern cosmology. We modify Jordan-Brans-Dicke theory using a vector field instead of a scalar field and theory becomes similar to a simple Einstein-aether theory. The time component of the vector field picks up a cosmological background value. Identifying the vector field to be the photon field, a small photon mass leads to late time inflation. The time dependent background electrical potential of the photon permeates the universe and explains the weakness of the gravitational interaction by coupling to curvature. This theory relates the smallness of the photon mass to the smallness of the Hubble parameter. The model predicted photon mass is far below observational constraints.

gr-qc↗