The latest DESI results, suggesting a dynamical dark energy equation of state, have reinvigorated interest in modified teleparallel theories, such as $f(T)$ gravity, as viable alternatives to the standard cosmological model. These models, grounded in torsion rather than curvature, offer elegant alternatives for explaining cosmic acceleration without invoking dark energy. In this talk, I...
Constraining inflationary flow equations with $n_s - 1 = -2/N$
Vineshree Pillay
University of Cape Town | vineshree.pillay013@gmail.com
Supervisor: Prof. Peter Dunsby
Abstract
This work presents an approach to inflationary cosmology that incorporates the spectral index relation $n_s - 1 = -2/N$ directly into the Hubble slow-roll hierarchy. Traditional methods...
We use the PLANCK 2018 and the WMAP data to constraint inflation models driven by a scalar field $\phi$ in the presence of the non-minimal scalar-curvature mixing term $\frac{1}{2}\xi R \phi^2$. We consider four distinct scalar field potentials $\phi^p e^{-\lambda\phi},~(1 - \phi^{p})e^{-\lambda\phi},~(1-\lambda\phi)^p$ and $\frac{\alpha\phi^2}{1+\alpha\phi^2}$ to study inflation in the...
The maximum mass of a neutron star is a fundamental issue in astrophysics, as it critically constrains the equation of state of ultra-dense matter. While the Tolman-Oppenheimer-Volkoff (TOV) limit provides a theoretical upper bound for non-rotating stars, most observed neutron stars are rotating. This talk examines how rapid rotation alters the maximum mass limit, pushing it beyond the...
In this talk, I will begin by providing an overview and substantiation of EGB theory. Then I will explore two critical features of modified gravitational systems: curvature singularities and mass gaps. Investigations into these features are central to our understanding of the structure and stability of compact objects in higher-dimensional gravity settings and for comparisons to classical GR....
Over the last decade, much attention has been given to the study of modified gravity theories to find a more natural explanation for the late-time acceleration of the Universe. Particular attention has focused on the so-called f(R) dark energy models. Instead of focusing on a particular f(R) model, we present a completely model-independent approach to study the background dynamics and the...
In this study, we examine how primordial magnetic fields (PMFs) influence the formation of large-scale structure in the universe. Working within a magnetized extension of the standard ΛCDM framework, we include the contributions of PMF energy density and pressure in the evolution equations governing matter perturbations. Our analysis investigates the resulting modifications to the matter power...
Interacting dark energy (IDE) models are cosmological models in which dark matter and dark energy interact in a non-gravitational manner, and have been introduced to address long-standing theoretical and observational tensions in cosmology. We discuss recent developments in the field, highlighting both the relevance and potential pitfalls of these models. In particular, we discuss how certain...
Gravitational waves (GWs) passing through matter induce shear into the matter flow, and this shear flow acts as a source of GWs. The consequences include GW echoes and damping of the GW signal together with heating of the matter. These effects have astrophysical applications: core collapse supernova, neutron star mergers, black hole mergers, and primordial GWs.
Cosmic strings, which are one-dimensional topological defects formed during early universe symmetry breaking, can generate a stochastic gravitational wave background (SGWB) detectable across LIGO, LISA and PTA bands. The SGWB generated by cosmic string networks described by the Velocity-Dependent One-Scale (VOS) model will be explored. We will consider both existing analytical treatments...
Strong gravitational lensing is the dramatic, observed deflection of light in the presence of massive foreground lenses that perturb the surrounding spacetime. In general, strong gravitational lensing is a versatile probe of the mass distributions and dark matter content of the lenses that also offers a means of constraining the Hubble parameter and testing General Relativity [1]. We exploit...
We present new constraints on the $R_h=ct$ cosmological model derived from redshift space distortion (RSD) data. In contrast to the standard $\Lambda$CDM framework, the $R_h=ct$ universe imposes a linear expansion and enforces the condition $R_h=ct$ at all cosmic times, leading to distinct predictions for the growth rate of cosmic structure. Using a comprehensive sample of RSD measurements...
The Lambda Cold Dark Matter (ΛCDM) model is a well-known cosmological model that has been used to investigate the acceleration of the universe. In our earlier study, we introduced a modified scale factor (Aydiner et al., 2022) to examine the universe’s accelerating expansion without relying on the conventional dark energy framework of the lambda-cold-dark matter model. In order to test the...
We present a robust mechanism, where the geometrical free-gravitational entropy of an isolated astrophysical radiating star undergoing continual gravitational collapse (where gravity dominates over all other fundamental forces), as measured by an external observer, makes a smooth transition to the Bekenstein-Hawking entropy at the onset of the horizon formation and in the late times of black...
The direct imaging of supermassive black holes by the Event Horizon Telescope (EHT) has provided an unprecedented opportunity to test the foundations of gravitational physics. In this work, we present the first Bayesian statistical analysis of the Nexus Paradigm (NP) — a quantum gravity framework that models spacetime as a lattice of entangled spinorial modes — against the EHT’s observations...
In this talk, I will discuss how quantum features of spacetime can leave imprints on classical gravity within a controlled semiclassical framework. Focusing on (2+1)-dimensional gravity with a negative cosmological constant, I use this lower-dimensional setup as a tractable model to explore the role of curved momentum space, a structure often anticipated in quantum gravity. Starting from a...
Tensions between cosmological simulations and astronomical observations may be resolved by extensions to the ΛCDM paradigm. We consider the possibility that dark matter may self-interact and possess viscosity. Including dissipative effects in cosmic fluids will modify their clustering properties which could have observable effects on the formation of large-scale structure. We analyse the...
Following its many successes in developing physically viable stellar models, the Buchdahl ansatz is invoked in the context of 5-dimensional Einstein-Gauss-Bonnet (EGB) gravity accompanied by a strange star equation of state to develop an anisotropic fluid distribution. The analytical model serves as a useful testbed for the phenomenon of tidal deformability in light of data from the GW170817...
In the present work we investigate new class of charged isotropic solutions with a linear barotropic equation of state (EoS) in the framework of Einstein–Gauss–Bonnet (EGB) gravity for 5 and 6 dimensional spherically symmetric static distributions. Imposing such a restriction of the EoS, one more further assumption on the metric potentials must be chosen in order to close the system. For...
We investigate higher-dimensional homogeneous and isotropic perfect fluid spacetimes within the framework of Einstein–Gauss–Bonnet (EGB) gravity. By solving the modified field equations, which include higher-order curvature corrections, we derive evolution equations for the scale factor and show that it admits a strictly positive minimum value. This lower bound depends on the spacetime...
We study the cosmological implications of barotropic fluid satisfying affine equation of state (EoS) in the General relativity and $f(Q)$ gravity framework. We describe the impact of affine EoS on the cosmic evolution in the model and derive the observational constraints on the model parameters. The models of General relativity and $f(Q)$ gravity may unify the scenario in which the universe...
This talk examines scalar field cosmologies driven by a generalized harmonic potential, focusing on Locally Rotationally Symmetric (LRS) Bianchi I, III, and V spacetimes, as well as their isotropic limits.
We begin by reviewing the averaged dynamics of Bianchi I and III models, which offer accessible examples of anisotropic cosmologies. Building on this foundation, we present new results...
We study the observational signature of non-gravitational interaction between the dark components of the cosmic fluids. We explore a phenomenological model of interacting dark energy and dark matter, characterised by a non-linear coupling term of the form $Q = 3H\xi \left( \frac{\rho_{dm} \rho_{de}}{\rho_{dm} + \rho_{de}} \right)$. This form of interaction naturally interpolates between linear...
It is well known that Lovelock polynomials including the special case of second order Gauss-Bonnet (G) invariants are topological and do not contribute to the dynamics of physical entities in four spacetime dimensions. Efforts to include higher curvature effects through dimensional regularization schemes in four dimensions are ongoing though several mathematical hurdles must be overcome....
This study examines mass-gap compact stars formed from neutron star mergers or massive pulsar evolution within the minimal geometric deformation framework. Using a Buchdahl--Vaidya--Tikekar--type metric, we obtain unperturbed static, spherically symmetric solutions with central densities $\sim 10^{15}\,\mathrm{g/cm^3}$ decreasing to zero at radius $R$. Astrophysical effects such as...
In this manuscript, the background and perturbed cosmic dynamics have been investigated using an interacting dark fluid model, which assumes energy exchange between dark matter and dark energy through a diffusion mechanism. After we solve the background expansion history for the late-time Universe, the full set of linear perturbation equations is driven using the $1+3$-covariant approach. We...
In this talk, I’ll introduce Kosmulator — a new, fast, and transparent MCMC framework for cosmological inference currently in development. Kosmulator allows researchers to swap cosmological models on the fly for fitting theoretical predictions to observational data. Modified-gravity models are implemented as simple Python callables, meaning transitions from ΛCDM to linear or fully non-linear...
We investigate the weak gravitational lensing and black hole shadows properties in the vicinity of static regular Frolov black hole.We investigate the bending of light in weak field regimes. The black hole shadow is studied in detail, with constraints on its parameters derived from observational data of the EHT collaboration. Further, we examine the shadow images under a spherically symmetric...
The 21 cm transition from neutral hydrogen is one of the most promising probes of the Epoch of Reionization (EoR). Precise measurements from this era can better constrain cosmological parameters and shed light on the evolution of galaxies across cosmic time. The new generation of low-frequency radio interferometric arrays, including the Hydrogen Epoch of Reionization Array (HERA), have been...
We investigate cosmology-driven modifications to Schwarzschild-like black hole spacetimes and their impact on photon propagation and gravitational lensing. The effective potential for null geodesics exhibits a single unstable maximum defining the photon sphere, with its radius and stability influenced by the deviation parameter (α) and the angular momentum of the photon (L).Photon trajectories...
The formation and evolution of the Universe’s large-scale structure (LSS) are strongly influenced by the statistical properties of primordial density fluctuations. While cosmological hydrodynamical simulations, such as Simba-C, have become indispensable tools for modeling galaxy formation and cosmic web morphology, they generally adopt a fixed concordance $\Lambda$CDM initial power spectrum....
In an effort to contribute to the current search for a modified gravity theory consistent with all observed phenomena, Boehmer and Jensko noted an obscure fact that the Ricci scalar can be decomposed into a bulk and a boundary term. This is then used to construct an action inspired by the idea of nonmetricity where the affine connection does not commute in the lower indices as in GR. The bulk...
The Weak Cosmic Censorship Conjecture (CCC) generally states that all singularities must be hidden behind the event horizon. Consequently, this conjecture forbids the destruction of event horizons in singular black holes, since their removal would expose naked singularities. Regular black holes, by contrast, contain no curvature singularities and are therefore not subject to the consequences...
