arXiv subjects

N. Merve Uzun

Publications and source records attributed to N. Merve Uzun.

9 recordsLinked to original sources

arXiv2026

Gravitating sources with zero energy density in cosmology

A cosmological source can have identically zero comoving energy density and still gravitate through its pressure. This differs from an energy density passing through zero at an isolated time. Maintaining zero density with nonzero pressure requires energy exchange in a nonstatic FLRW universe. We construct proportional-pressure and constant-pressure backgrounds, including steady-state, bouncing, recollapsing, and $Λ$CDM expansion histories, and relate them to creation pressure, bulk viscosity, and time-dependent vacuum descriptions. Scalar fields can realize zero density along a trajectory; imposing it as an identity on a timelike $P(X,ϕ)$ domain selects $P=A(ϕ)\sqrt{X}$, locally a signed potential-free cuscuton for nonzero $A$. We formulate linear perturbations without dividing by the vanishing background density. For a nondegenerate $P(X,ϕ)$ scalar with a timelike field gradient, zero density and negative pressure cannot coexist with positive kinetic and gradient coefficients when the interaction leaves its principal kinetic structure unchanged. Two models couple the scalar to number-conserving dust through a field-dependent mass. The proportional-pressure trajectory is a homogeneous saddle and has no growing power-law dust mode on its accelerating branch in the matter-sourced quasistatic subhorizon approximation. A distinct constant-pressure model reproduces the $Λ$CDM expansion history, but in the same approximation the dust mode that initially grows during matter domination reaches a maximum at low redshift and then decays. These restrictions apply to the stated scalar class and models. Exact Bianchi I solutions demonstrate separately conserved zero-density sources supported by anisotropic stress. An inverse-pressure construction distinguishes the density of an interacting constituent from an inferred dark-energy density that may cross zero.

gr-qc↗

arXiv2026

Defocusing dark energy: Raychaudhuri diagnostics beyond $w<-1/3$ and the phantom divide

In general relativity, cosmic acceleration is timelike defocusing of the comoving congruence and requires a negative total active gravitational mass density, $\mathcal{M}_{\rm tot}=ρ_{\rm tot}+3p_{\rm tot}<0$. The criterion $w\equiv p/ρ<-1/3$ diagnoses sector repulsion only for $ρ>0$: the inequality reverses for $ρ<0$, and ratio variables are ill-defined at $ρ=0$ even when the stress-energy tensor is finite. For sign-changing effective dark energy (DE), as in $Λ_{\rm s}$CDM-type histories, we instead use the signed density $ρ_{\rm de}$ and two branch-independent combinations. The regular null energy condition (NEC) boundary $\mathcal{I}_{\rm de}=ρ_{\rm de}+p_{\rm de}=0$ replaces the phantom divide $w_{\rm de}=-1$, while $\mathcal{M}_{\rm de}=ρ_{\rm de}+3p_{\rm de}<0$ governs sector-level Raychaudhuri repulsion. For a separately conserved DE sector with a smooth negative-to-positive density crossing of finite odd order $n$ at $z_\dagger$, we prove that $\mathcal{I}_{\rm de}$ and $\mathcal{M}_{\rm de}$ are negative in a punctured neighborhood and non-positive at the crossing, while $w_{\rm de}$ develops a kinematic pole with universal residue $n(1+z_\dagger)/3$. If $\mathcal{M}_{\rm de}>0$ at some sufficiently high redshift, continuity requires at least one repulsion boundary $z_{\rm rep}>z_\dagger$: the sector is already repulsive while $ρ_{\rm de}<0$. We derive the exact range of $ρ_{\rm de}'(z_\dagger)$ for acceleration at the crossing with the total NEC satisfied. Under the stated single-impulse and stationary-point assumptions, the deceleration parameter has one or three sign-changing zeros. A smooth $Λ_{\rm s}$CDM profile, an exponential infrared $f(T)$ model, and the minimal phantom brane illustrate the results. These results motivate organizing late-time inference around $(ρ,p,\mathcal{I},\mathcal{M})$ rather than around $w$ alone.

gr-qc↗

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

Unexplored regions in teleparallel $f(T)$ gravity: Sign-changing dark energy density

While $f(T)$ gravity has shown considerable potential in addressing cosmological tensions, we explore previously overlooked solution spaces that hold further promise. We examine the case where the customary assumption of a strictly positive effective DE density may not apply, offering new possibilities. Focusing on $f(T) = T e^{T_*/T}$, we investigate cosmological solutions parametrized by the parameter $β= T_*/T_0$. This parameter uniquely determines $Ω_{\rm m0}$, and its sign plays a crucial role in characterizing deviations from the $Λ$CDM. We elaborate on the structural asymmetry between the positive- and negative-$β$ branches: while the $β_{+}$ leads to dynamics with modest departures from $Λ$CDM, the $β_{-}$ yields more pronounced and nontrivial deviations. Despite these deviations, the negative-$β$ branch can remain consistent with local gravity constraints through an effective chameleon-like mechanism. We also examine the model in the context of dynamical DE. Ensuring consistency with CMB data, the widely studied $β_{+}$ exhibits phantom behavior, while the previously overlooked $β_{-}$ features a sign-changing DE density that transitions smoothly from negative to positive values at $z_{\dagger} \sim 1.5$. Though the sign-changing DE leads to a larger-than-expected enhancement, we extend the analysis by incorporating $Λ$. This extension broadens the solution space consistent with the SH0ES measurement while maintaining consistency with CMB. Additionally, it introduces richer phenomenological possibilities, including the potential moderation or cessation of cosmic acceleration at very low redshifts, aligning with recent observational analyses, such as those from DESI BAO data. Our findings suggest that existing $f(T)$ models, as well as $f(Q)$ models, should be revisited in light of the novel theoretical insights presented here.

gr-qc↗

arXiv2024

Equivalence of matter-type modified gravity theories to general relativity with nonminimal matter interaction

In this study, we first establish that gravity models incorporating matter-related terms, such as $f(\mathcal{L}_{\rm m})$, $f(g_{μν} T^{μν})$, and $f(T_{μν} T^{μν})$, into the usual matter Lagrangian density $\mathcal{L}_{\rm m}$, are equivalent to general relativity with nonminimal matter interactions. Through the redefinition $\mathcal{L}_{\rm m}+f \rightarrow \mathcal{L}_{\rm m}^{\rm tot}$, these models are exactly GR, yet the usual material field $T_{μν}$ and its accompanying partner, the modification field $T_{μν}^{\rm mod}$, engage in nonminimal interactions. Specifically, $\nabla^μT_{μν}=-Q_ν=-\nabla^μT_{μν}^{\rm mod}$, where $Q_ν$ is the interaction kernel that governs the rate of energy transfer. Our focus narrows on the specific model of $f(T_{μν} T^{μν})$, known as Energy-Momentum Squared Gravity, where the usual material field $T_{μν}$ is accompanied by an \textit{energy-momentum squared field} (EMSF), $T_{μν}^{\rm emsf}$, along with a sui generis nonminimal interaction between them. We demonstrate that a particular $T_{μν}^{\rm emsf}$ can be introduced by \textit{removing} $\frac{\partial^2 \mathcal{L}_{\rm m}}{\partial g^{μν} \partial g^{σε}}$ (the new term emerging in models that incorporate scalars formed from $T_{μν}$), thanks to the freedom in determining the interaction kernel, but this approach compromises the Lagrangian formulation of EMSG. Additionally, we address the ambiguities regarding the perfect fluid stemming from this new term. We show the proper way of calculating this term for a perfect fluid, revealing that it is indeed non-zero, contrary to common assumption in the literature. Finally, we re-examine cosmological models within the realm of EMSG, offering new insights into the applicability and interpretation of our findings in EMSG and similar theoretical frameworks.

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↗

arXiv2023

Cosmological models in scale-independent energy-momentum squared gravity

Scale-independent EMSG is a particular model of energy-momentum squared gravity (EMSG) in which the new terms in the Einstein field equations arising from the EMSG theory enter with the same power as the usual terms from Einstein-Hilbert part of the action. However, the model violates the local energy-momentum conservation and matter-current conservation in general and hence, permits a process of matter creation/annihilation in an expanding universe. Consequently, the scale factor dependencies of the energy densities are modified by the dimensionless model parameter $α$. We revisit some nostalgias such as static universes and de Sitter/steady state universes. We reproduce the original ones, moreover, present some novelties, e.g., a spatially flat static universe, de Sitter expansion by negative vacuum energy, steady state universes in the presence of arbitrary fluids with constant equation of state (EoS) parameter other than dust, etc. We also investigate the possible dynamics of dust dominated and radiation dominated universes. Depending on the value of $α$, dust/radiation dominated universe exhibits power-law accelerated/decelerated expansion, corresponds to a steady state model or may end in a big rip. In the framework of anisotropic cosmology, we reproduce Barrow's quiescent universe in the presence of stiff fluid and extend it to fluids with arbitrary constant EoS parameter. We also relax the condition for isotropic initial singularity (big bang) owing to that EMSG effectively allows ultra-stiff EoS parameters.

gr-qc↗

arXiv2020

Screening anisotropy via energy-momentum squared gravity: $Λ$CDM model with hidden anisotropy

We construct a generalization of the standard $Λ$CDM model, wherein we simultaneously replace the spatially flat Robertson-Walker metric with its simplest anisotropic generalization (LRS Bianchi I metric), and couple the cold dark matter to the gravity in accordance with the energy-momentum squared gravity (EMSG) of the form $f(T_{μν}T^{μν})\propto T_{μν}T^{μν}$. These two modifications -- namely, two new stiff fluid-like terms of different nature -- can mutually cancel out, i.e., the shear scalar can be screened completely, and reproduce mathematically exactly the same Friedmann equation of the standard $Λ$CDM model. This evades the BBN limits on the anisotropy, and thereby provides an opportunity to manipulate the cosmic microwave background quadrupole temperature fluctuation at the desired amount. We further discuss the consequences of the model on the very early times and far future of the Universe. This study presents also an example of that the EMSG of the form $f(T_{μν}T^{μν})\propto T_{μν}T^{μν}$, as well as similar type other constructions, is not necessarily relevant only to very early Universe but may even be considered in the context of a major problem of the current cosmology related to the present-day Universe, the so-called $H_0$ problem.

astro-ph.CO↗

arXiv2019

Screening $Λ$ in a new modified gravity model

We study a new model of Energy-Momentum Squared Gravity (EMSG), called Energy-Momentum Log Gravity (EMLG), constructed by the addition of the term $f(T_{μν}T^{μν})=α\ln(λ\,T_{μν}T^{μν})$, envisaged as a correction, to the Einstein-Hilbert action with cosmological constant $Λ$. The choice of this modification is made as a specific way of including new terms in the right-hand side of the Einstein field equations, resulting in constant effective inertial mass density and, importantly, leading to an explicit exact solution of the matter energy density in terms of redshift. We look for viable cosmologies, in particular, an extension of the standard $Λ$CDM model. EMLG provides an effective dynamical dark energy passing below zero at large redshifts, accommodating a mechanism for screening $Λ$ in this region, in line with suggestions for alleviating some of the tensions that arise between observational data sets within the standard $Λ$CDM model. We present a detailed theoretical investigation of the model and then constrain the free parameter $α'$, a normalisation of $α$, using the latest observational data. The data does not rule out the $Λ$CDM limit of our model ($α'= 0$), but prefers slightly negative values of the EMLG model parameter ($α'= -0.032\pm 0.043$), which leads to the screening of $Λ$. We also discuss how EMLG relaxes the persistent tension that appears in the measurements of $H_0$ within the standard $Λ$CDM model.

gr-qc↗