The success of the ΛCDM model is tempered by unresolved questions surrounding the nature of dark energy and the cosmological constant. These challenges motivate the exploration of modified theories of gravity as viable alternatives to General Relativity. In this talk, I present the Extended Proca-Nuevo (EPN) framework, a ghost-free vector–tensor theory in which a massive spin-1 field drives...
The nature of dark energy remains one of the central open questions in cosmology. While observational analyses commonly constrain phenomenological equation-of-state parametrizations such as the Chevallier–Polarski–Linder (CPL) model, theoretical interpretations require translating these constraints into physically motivated scalar-field models.
We present a computational framework that...
I will introduce a neat mathematical framework that can be utilized to study, in a unified manner, different aspects of $f(R), f(T), f(G)$ or $f(Q)$ gravity. As an example of the application of this framework, I will show it's applicability for a comparative analysis between $f(R)$ gravity and the trivial FLRW connection branch of $f(Q)$ gravity, with respect to the two following questions:...
We present a unified analysis of the late-time cosmic expansion by bridging model-independent cosmography with modified gravity, using the latest DESI DR2 BAO data. A comparative study of Taylor, Padé, and Chebyshev expansions shows that while Taylor breaks down beyond low redshift, Padé approximants, especially (2,1), deliver stable, high-redshift reconstructions and are strongly preferred by...
We investigate a nonlinear self-interacting spinor field as a dynamical dark-energy candidate capable of driving the late-time accelerated expansion of the Universe. The model is formulated within a spatially flat Friedmann–Lemaître–Robertson–Walker framework, where the spinor field is minimally coupled to gravity and generates an effective dark-energy equation of state through its...
The recent DESI DR2 preference for a time-varying dark-energy equation of state, together with the long-standing difficulty of explaining why an initially anisotropic universe should evolve toward isotropy, motivates the study of interacting dark-sector models in anisotropic backgrounds. We investigate the dynamics of Bianchi type-IX cosmologies containing pressureless dark matter coupled to...
This work investigates wormhole geometries in the framework of symmetric teleparallel (f($\mathcal{Q}$)) gravity by incorporating holographic and mixed dark energy density profiles. Exact wormhole solutions are obtained using rotational velocity profiles to construct the redshift function and dark energy density profiles to determine the shape function. For suitable parameter choices, the...
In this work, we explore the cosmological dynamics of a modified gravity framework based on the function $f(Q,B)=\delta Q^{2}+\beta B$, where $Q$ denotes the nonmetricity scalar and $B$ is the boundary term that relates $Q$ to the Ricci scalar. The matter sector is modeled using the Modified Chaplygin Gas (MCG) with the equation of state $p=A\rho-\frac{D_{1}}{\rho^{\alpha}}$, allowing the...
This work introduces a new gravastar model inspired by the Mazur-Mottola framework within Lorentz-violating gravity, specifically Hořava gravity and Einstein-aether theory with the help of Kuchowicz metric function. The proposed gravastar consists of three distinct regions: a de Sitter-like core, a thin shell filled with ultra-relativistic matter, and an exterior vacuum spacetime. Exact...
Some arguments are considered in favor of the idea that the canonical anticommutation relations for fermions should be modified in curved spacetime near the event horizon of a black hole. Such a modification is expected to lead to a change in the source term of the inhomogeneous Dirac equation describing the two-point Green’s function. By introducing an {\it ad hoc} source into the Dirac...
We explore the cosmological implications of Fractional Holographic Dark Energy within the context of $f(T)=\beta(-T)^n$ gravity, in which the Hubble parameter is determined implicitly. The model incorporates fractional corrections through a generalized entropy-area relation, allowing for the inclusion of nonlocal effects in the dark energy sector. The model parameters are optimized using...
We investigate the possibility of energy exchange in the dark sector for interacting dark energy models, using the latest DESI DR2 baryon acoustic oscillation measurements. Our finding suggest that current observations do not provide significant evidence for dark sector interactions, but they allow the possibility of a weak energy flow that can be further tested with future high-precision...
The redshifted 21-cm line of neutral hydrogen provides a unique probe of the high-redshift Universe, tracing the evolving state of the intergalactic medium (IGM) during the Epoch of Reionization (EoR). The 21-cm signal, however, depends on both the underlying cosmological density field and the astrophysical properties of the sources driving reionization, leading to potentially strong...
In this talk, I present a model-independent spline reconstruction of cosmological distances using latest low-redshift datasets to investigate the late-time expansion history of the Universe. Comparing the reconstructed expansion history with the prediction of the Planck $\Lambda$CDM model, I demonstrate a localized deviation over a certain redshift interval which reaches a maximum significance...
Sub-horizon and quasi-static approximations are widely employed in cosmological simulations to efficiently model the evolution of perturbations in theories beyond $\Lambda$CDM. However, their validity can depend on the underlying dark-sector physics and the scales being considered. In this work, we investigate these approximations in an interacting scalar-field dark-sector model, where dark...
In this paper, we compute the inflationary trispectrum of primordial gauge fields generated through the scalar and tensor exchanges in models with spectator U(1) gauge fields which are kinetically coupled to the inflaton. Focusing on the connected four-point autocorrelation function of gauge fields, we derive exact analytical expressions for the full trispectrum of both electric and magnetic...
The success of the $\Lambda$CDM model and General Relativity has established the standard paradigm of modern cosmology, yet the fundamental nature of spacetime remains an open question. This talk explores three complementary directions for extending the geometric foundations of modern cosmology. I first consider inflation in a fractal spacetime, where non-integer spatial dimensions modify the...
We explore if a late-time cosmic acceleration and a mild variation of fundamental constants can be accommodated within a common scalar-tensor framework. The fundamental constants we focus on are the Newtonian gravitational coupling $G$ and the electron mass $m_e$. The scalar-tensor framework under consideration is a generalized Brans-Dicke theory with an additional matter field. The viability...
Astrophysical black holes are expected to reside in environments enriched by dark matter, raising the possibility that environmental effects may leave observable imprints on gravitational-wave signals. We investigate how the structure of a surrounding dark-matter halo modifies both the ringdown and tidal response of a static, spherically symmetric black hole. The halo is modelled using a...
The Galactic Center (GC) black hole Sgr A* gives us an opportunity to perform horizon scale tests of black hole physics and deviations from General Relativity, thanks to observations carried out by the Event Horizon Telescope (EHT). In this work we study the impact of the scalar degree of freedom (scalaron) of f(R) gravity theory on the size and shape of GC black hole by using a recently...
I would like to give a presentation in which I will discuss the existence and physical implications of homothetic Killing vectors (HKVs) in a broad class of dynamical Vaidya spacetimes, including generalized spherically symmetric Husain-type solutions and rotating Kerr–Vaidya geometries. By solving the conformal Killing equations, it can be demonstrated that these spacetimes admit a unique...
When we consider the the case of chaotic scattering in a region described by a potential or a massive object, we see that the behavior of particle trajectories are heavily dependent on initial conditions, which eventually decide whether particles stay in the scattering region or exit it through different exits. Initially, the H\'enon-Heiles potential is taken as a model to study these...
The accelerated expansion of the Universe poses a serious challenge to our understanding about the dark sector, motivating the investigation of dark matter and dark energy within a unified framework. In this work, we deal with the analysis of two-interacting scalar field model inspired by the hybrid inflation potential. We initially perform the cosmographic reconstruction of the Hubble...
I wish to give a presentation, in which I will talk about a class of horizonless, regular ultra-compact objects which may arise in a theory of gravity where fluid variables may couple to the curvature scalar of the spacetime. This kind of non-minimal interaction between fluid variables and the Ricci scalar generates a vacuum-like equation of state in the interior, while the exterior remains...
We study aspects of black-hole spacetimes in the presence of dark-energy components, focusing on how the equation of state and associated curvature diagnostics constrain physically distinct regimes. Starting from the Schwarzschild geometry, we derive the Kretschmann scalar as a coordinate-invariant measure of curvature and use it to characterize the behavior near the horizon and at the central...
$f(Q, \mathcal{L}_m)$ gravity is a well-known extension of $f(Q)$ gravity, obtained through non-minimal or additive minimal coupling in the action's Lagrangian density. Existing literature within the framework of $f(Q, \mathcal{L}_m)$ has explored the cosmographic scenario through various approaches, including background-level analyses and comparisons with $\Lambda$CDM (Myrzakulov et al.,...
This study offers a comprehensive reconstruction of modified Teleparallel (f(T)) gravity model with three distinct non-linear as well as novel forms employing a hybrid scale factor to depict the expansion history of the universe. We investigate a physically motivated cosmological parametrization through this hybrid scale factor that can explain a smooth transition from an early decelerated...
Despite significant advances in post-Minkowskian gravity, scattering-amplitude methods, and gravitational-wave modeling, existing frameworks largely assume vacuum binary dynamics and do not systematically incorporate coherent environmental effects. We develop a unified Hamiltonian framework for compact binaries embedded in coherent ultralight dark matter, combining post-Minkowskian gravity...
Abstract:
The stellar orbits near the Galactic Center black hole provide a unique probe for understanding the nature of mass distribution as well as carrying out tests of gravity. Inferring the nature of mass distribution in the region is crucial for understanding the formation history of the black hole and how it interacts with its environment. In this work we consider several viable...
We study the late-time cosmological dynamics of a two-field dark-energy model consisting of a canonical quintessence scalar field and a phantom scalar field in a spatially flat Friedmann-Lemaître-Robertson-Walker universe. The fields are minimally coupled to gravity and uncoupled at the level of the potential, with the quintessence sector governed by an exponential potential and the phantom...
