arXiv subjects

Seturumane Tema

Publications and source records attributed to Seturumane Tema.

4 recordsLinked to original sources

arXiv2026

Closing the Cosmographic Hierarchy: Dynamical Attractors from Inflation to Reheating

We develop a potential-independent cosmographic framework, in which cosmographic parameters are promoted to dynamical variables within a closed autonomous system. Although the cosmographic hierarchy is formally infinite, we achieve closure by mapping potential slow-roll parameters onto the kinematic phase space within General Relativity with a minimally coupled scalar field. Within this framework, we perform a stability analysis and show that inflationary (quasi-de Sitter) solutions arise as natural attractors, while stiff-fluid configurations act as repellers without invoking the slow-roll approximation. To describe the transition to standard Big Bang evolution, we extend the system to include a radiation component and a phenomenological decay term. This leads to a generalized, potential-independent description of reheating characterized by an effective equation of state $w_{\rm eff}$. We demonstrate that the radiation-dominated phase is the late-time attractor of the extended system. These results provide a unified kinematical description of the expansion history from inflation through reheating, bridging cosmography and scalar field dynamics.

gr-qc↗

arXiv2025

Power-Law Bounces in $f(R)$ Gravity: Analysis of the Ekpyrosis and Accelerating Regimes

We investigate the dynamics of the Friedmann-Lema\^itre-Robertson-Walker spacetime within the framework of $f(R)$ gravity using a compact, model-independent dynamical systems approach. By assuming a power-law scale factor, we explore ekpyrotic and accelerating solutions to address the big bang singularity. Our analysis demonstrates that a cosmological bounce, characterized by a transition from contraction to expansion, possibly avoids the singularity without directly using the Raychaudhuri equation, unlike previous approaches using specific $f(R) \simeq R^n$ forms. We identify a key fixed point in the phase space corresponding to the bounce, supported by perturbation analysis and qualitative description of trajectories in the phase space. The results suggest that $f(R)$ gravity provides a robust framework for non-singular cosmologies.

gr-qc↗

arXiv2024

Relativistic Corrections and Structure Formation in Dark Matter Superfluidity

The theory of dark matter superfluidity has emerged as a compelling framework, in which the dynamics are governed by a non-relativistic $P(X)$ superfluid Lagrangian that naturally leads to Modified Newtonian Dynamics (MOND)-like behavior when coupled to baryons at galactic scales. Notably, at cosmological scales, this effective description reproduces the standard $\Lambda$ Cold Dark Matter ($\Lambda$CDM) model at the background level, suggesting that cold dark matter may undergo Bose--Einstein condensation at galactic scales. In this work, we extend the non-relativistic formulation by systematically incorporating relativistic corrections within the Friedmann--Lema\^itre--Robertson--Walker (FLRW) spacetime. We further perform a linear perturbation analysis in this relativistic setting to investigate the evolution of matter density fluctuations. Our results clarify the viability of the superfluid dark matter scenario in explaining large-scale structure formation and identify the parameter regimes in which it remains consistent with current cosmological observations.

gr-qc↗

arXiv2024

Cosmological Singularity and Power-Law Solutions in Modified Gravity

A bouncing Universe avoids the big-bang singularity. Using the time-like and null Raychaudhhuri equations, we explore whether the bounce near the big-bang, within a broad spectrum of modified theories of gravity, allows for cosmologically relevant power-law solutions under reasonable physical conditions. Our study shows that certain modified theories of gravity, such as Stelle gravity, do not demonstrate singularity resolution under any reasonable conditions, while others including $f(R)$ gravity and Brans-Dicke theory can demonstrate singularity resolution under suitable conditions. For these theories, we show that the accelerating solution is slightly favoured over ekypyrosis.

gr-qc↗