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Sermet Çağan

Publications and source records attributed to Sermet Çağan.

4 recordsLinked to original sources

Universal Nonminimal Coupling-to-Starobinsky Matching and a Single-Field Attractor

We establish an off-shell commutativity theorem in 4D parity-even quadratic gravity that the Hubbard-Stratonovich/Legendre lifts, algebraic elimination of auxiliaries, including the torsionless Palatini connection, and Jordan-Einstein Weyl rescalings commute at the action level up to boundary terms. This yields a frame-independent characterization of the propagating degrees of freedom and isolates a universal scalaron EFT in the metric branch, while clarifying the algebraic nature of the Palatini $f(R)$ scalar. We obtain, as a result, a frame-universal matching from the generic nonminimal couplings to a positive $R^2$ sector and a quantitative single-field attractor bound, enhanced by a $1/ΔN$ selection term, providing sharp and falsifiable CMB targets.

hep-th↗

Supersymmetric Horizons at the Edge of Effective Field Theory

What if supersymmetry is not something to be hidden, but something to be reached? We introduce the Supersymmetric Horizon Stabilization (SHS): a conceptually unified and analytically controlled framework for embedding metastable de Sitter vacua and trans-Planckian inflation within effective $\mathcal{N}=1$ supergravity. Built from a logarithmic Kähler potential modified by nilpotent constraints, SHS yields a framework that connects the early universe's inflationary phase, intermediate metastable acceleration, and a final supersymmetric fixed point. We classify viable models by enforcing four essential criteria: absence of ghosts, de Sitter metastability, smooth Minkowski transitions, and asymptotic supersymmetry. The resulting scalar dynamics exhibit exponential field-space profiles, $τ(φ) \sim e^{-Δφ}$, allowing trans-Planckian excursions with bounded moduli. This structure leads to finite geodesic distance and a stabilized gravitino mass, conditionally avoiding towers of light states and remaining consistent with the Gravitino and Swampland Distance Conjectures. At the same time, the divergent Euclidean cost of reaching the supersymmetric endpoint realizes the AdS and Generalized Swampland Distance Conjectures, situating the infrared boundary at infinite dynamical distance. From this behavior emerges the Supersymmetric Horizon Conjecture: that consistent gravitational EFTs may begin and end with supersymmetry -- both at the origin and the asymptotic edge of cosmic evolution -- not as a fine-tuned remnant, but as a geometric boundary condition. SHS also supports localized features that transiently enhance scalar fluctuations, enabling efficient production of primordial black holes with masses far exceeding those in conventional inflationary scenarios.

hep-th↗

Stability Analysis of the Cosmological Dynamics of $O(D,D)$-complete Stringy Gravity

The massless fields in the universal NS-NS sector of string theory form $O(D, D)$ multiplets of Double Field Theory, which is a theory that provides a T-duality covariant formulation of supergravity, leading to a stringy modification of General Relativity. In this framework, it is possible to write down the extensions of the Einstein field equations and the Friedmann equations in such a way that the coupling of gravitational and matter sectors is dictated by the $O(D, D)$ symmetry universally. In this paper, we obtain the autonomous form of the $O(D, D)$-complete Friedmann equations, find the critical points and perform their stability analysis. We also include the phase portraits of the system. Cosmologically interesting cases of scalar field, radiation, and matter are separately considered and compared with the Chameleon models in a similar setting. Accelerating phases and the conditions for their existence are also given for such cases.

gr-qc↗

Numerical Approach to the Exterior Solution of Spherically Symmetric and Static Configuration in Scalar-Tensor Theories

We numerically examine the exterior solution of spherically symmetric and static configuration in scalar-tensor theories by using the nonminimally coupled scalar field with zero potential as our sample model. Our main purpose in this work is to fit the resulting data of the numerical solutions in the interested region by seeking for approximate analytical expressions which are weakly dependent of the parameters of a model, such as the nonminimal coupling constant in the present case. To this end, we determine the main forms of the mass and the metric functions in terms of the scalar field and their surface values. Then, we provide a function for the scalar field that contains only the mass and the radius of the configuration together with the surface and the asymptotic values of the scalar field. Therefore, we show that the exterior solution can be expressed in a form which does not depend on the parameters of a chosen model up to an order of accuracy around $10^{-5}$.

gr-qc↗