arXiv subjects

Ida M. Rasulian

Publications and source records attributed to Ida M. Rasulian.

7 recordsLinked to original sources

arXiv2026

Static horizons in cosmology

Although previous results have ruled out the possibility of a static event horizon in cosmology, we present black hole and white hole metrics that retain static event horizons while reproducing cosmological behavior at large distances. Using an appropriate coordinate choice, we demonstrate that a static event horizon can exist in a cosmological setting without introducing curvature invariant singularities at the horizon. The resulting metric reduces to the Schwarzschild de Sitter solution when the Hubble parameter is constant. We find that white hole metrics in an expanding universe, or black holes in a contracting universe, are significantly easier to construct, as a black hole in an expanding cosmology requires the velocity function to change sign. Consequently, this work initially examines white holes in expanding cosmologies as a foundation for subsequent analysis of black holes in expanding universes. In later sections, we investigate scenarios involving a white hole coupled with cosmological matter, as well as a white hole with both matter and a cosmological constant. Assuming the pressure component takes its cosmological value, we show that the physical radius of the apparent horizon can asymptotically approach a constant value at late times. This metric avoids pathologies such as a singular horizon in the limit of a vanishing Hubble parameter. Finally, we analyze the realistic case of a black hole embedded in pressureless cosmological matter with and without a cosmological constant and explore its properties. We specifically show that the velocity function can become zero and change sign in the vicinity of a black hole. This means we can smoothly transition from an expanding cosmological phase with a positive velocity function to a contracting black hole phase with a negative velocity function.

gr-qc↗

arXiv2026

Null-strings Gauged, Reloaded and Quantized, I: Canonical Quantization in the Light-Cone Gauge

We study the light-cone quantization of null-strings in $D$ dimensional flat target-space. Incorporating the essential new gauge symmetry and the associated constraint structure of the null-string, allows one to solve for one more degree of freedom (DoF) compared to the standard light-cone gauge, reducing the physical phase space to $(D-3)$ propagating DoF. Quantization is formulated directly in the Schrödinger representation, leading to a Hilbert space of wavefunctions on the reduced configuration space. The space of physical states is built on a reduced phase space associated with the corrected gauge structure. We discuss the ground-state wavefunction and a generic class of excited states. As a direct consequence of the overlooked Carroll-Weyl gauge symmetry of the null-strings, we find the remarkable and perhaps unexpected result that null-strings exhibit a discrete spectrum. Our analysis indicate that there is no critical dimension for null-strings, and $D$ can be arbitrary.

hep-th↗

arXiv2025

Strings, Virasoro Sandwiches and Worldsheet Horizons

We revisit the canonical quantization of free bosonic closed string theory and observe that the physicality of states requires vanishing of the worldsheet Virasoro algebra generators sandwiched between any two physical states. This requirement yields four classes of physical states, depending on discrete worldsheet symmetries: parity and time reversal. The usual string states which are highest weight states of the Virasoro algebra, preserve both, while the other new three classes break one or both. We apply our formulation to an accelerated worldsheet with horizons, initiating the worldsheet formulation of a thermal string theory and strings probing horizon of black holes.

hep-th↗

arXiv2024

Black hole-de Sitter model, a proposal for the de Sitter phases

We expand on the braneworld-black hole de Sitter model introduced in \cite{Rasulian} which is a proposal for constructing an effective de Sitter spacetime with no explicit dependence on brane tension or bulk cosmological constant. In this model the 4D de Sitter space emerges on a brane near the horizon of a 5D black hole. We study the effective gravity on the brane non-perturbatively, with an approximate Z2 symmetry assumption, up to a conformal factor, and find that the evolution of the effective cosmological constant on the brane depends on the flux of energy towards and away from the black hole in the bulk. In this setup the presence of the black hole horizon sets the initial condition for the brane's evolution and the brane approaches its null configuration with de Sitter length $l\lesssim l_5$, where $l_5$ is the 5D Planck's length, as soon as the horizon forms. During the last stages of collapse following this phase (or further flux of matter in the bulk after the horizon is formed), the effective de Sitter length on the brane increases due to the in-falling flux relatively fast. This phase is tentatively the transition between a low scale inflationary phase and the late dark energy phase. Also we observe that the increase in the de Sitter length is accompanied with a flux of energy entering the brane due to the jump in the bulk flux across the brane. Considering the initial state of the brane to be in the $l_5$ neighborhood of the horizon, the configuration which is slightly below the horizon is Euclidean AdS with AdS radius $l\lesssim l_5$. This can be interpreted as a boundary proposal for the resulting cosmology.

hep-th↗

arXiv2023

On Torsion Contribution to Chiral Anomaly via Nieh-Yan Term

In this note we present a solution to the question of whether or not, in the presence of torsion, the topological Nieh-Yan term contributes to chiral anomaly. The integral of Nieh-Yan term is non-zero if topology is non-trivial; the manifold has a boundary or vierbeins have singularities. Noting that singular Nieh-Yan term could be written as a sum of delta functions, we argue that the heat kernel expansion cannot end at finite steps. This leads to a sinusoidal dependence on the Nieh-Yan term and the UV cut-off of the theory (or alternatively the minimum length of spacetime). We show this ill-behaved dependence can be removed if a quantization condition on length scales is applied. It is expected as the Nieh-Yan term can be derived as the difference of two Chern class integrals (i.e. Pontryagin terms). On the other hand, in the presence of a cosmological constant, we find that indeed the Nieh-Yan term contributes to the index with a dimensionful anomaly coefficient that depends on the de Sitter length or equivalently inverse Hubble rate. We find similar result in thermal field theory where the anomaly coefficient depends on temperature. In both examples, the anomaly coefficient depends on IR cut-off of the theory. Without singularities, the Nieh-Yan term can be smoothly rotated away, does not contribute to topological structure and consequently does not contribute to chiral anomaly.

hep-th↗

arXiv2022

Emergent de Sitter Cosmology Near Black Hole Horizon

We propose an effective model for an exponentially expanding universe in the brane-world scenario. The setup consists of a 5D black hole and a brane close to the black hole horizon. In case the brane acquires a specific configuration, which we deduce from stability arguments, the induced metric outside the black hole horizon on the brane becomes de Sitter in static coordinates. Studying the Einstein equations perturbatively we find the effective gravity on the brane at this level and derive the 4D gravitational constant. Considering a homogeneous and isotropic fluid in the corresponding FLRW coordinates we find that the bulk fluid density below the brane, which has the same equation of state as the fluid on the brane, contributes to the energy density in the Friedmann equation and therefore in late time may be attributed to dark matter. Studying the stability of the setup we observe that the brane becomes stabilized, in the presence of matter on the brane, with a de Sitter length that is qualitatively of the order of Schwarzschild radius of the universe due to matter. We briefly discuss effects that can bound the de Sitter lifetime. In particular, this model can provide a lifetime compatible with Trans-Planckian Censorship conjecture for the current de Sitter phase.

hep-th↗

arXiv2021

Swampland de Sitter Conjectures in No-Scale Supergravity Models

It is challenging to construct explicit and controllable models that realize de Sitter solutions in string compactifications. This difficulty is the main motivation for the Refined de Sitter Conjecture and the Trans-Planckian Censorship Conjecture which forbid stable de Sitter solutions but allow metastable, unstable and rolling solutions in a theory consistent with quantum gravity. Inspired by this, we first study a toy de Sitter No-Scale Supergravity model and show that for particular choices of parameters it can be consistent with the Refined de Sitter Conjecture and the Trans-Planckian Censorship Conjecture. Then we modify the model by adding rolling dynamics and show that the theory can become stable along the imaginary direction, where it would otherwise be unstable. We extend the model to multi-field rolling and de Sitter fields, finding the parameter space where they can be compatible with the Refined de Sitter Conjecture . The modified models with rolling fields can be used to construct Quintessence models to accommodate the accelerating expansion of the Universe.

hep-th↗