arXiv subjects

Erik Jensko

Publications and source records attributed to Erik Jensko.

12 recordsLinked to original sources

arXiv2026

Non-minimal fluid Lagrangian couplings

Gravitational models with non-minimal couplings involving functions of the matter Lagrangian and curvature have become popular in recent decades. By coupling the matter Lagrangian directly to the gravitational Lagrangian, one hopes to construct theories that can explain dark energy or dark matter without introducing additional sources. When this matter Lagrangian describes a perfect fluid, some technicalities are involved in its variational formulation. We present a careful derivation of the gravitational field equations together with the complete set of fluid equations using two approaches: Schutz's velocity potential and Brown's Lagrange multipliers. We find that with non-minimal couplings, the energy-momentum conservation equations give rise to a non-vanishing contribution which acts in a different direction depending on the approach. This also leads to the emergence of different effective thermodynamic quantities such as the number density, chemical potential, free energy, and temperature. We demonstrate the non-equivalence of the Lagrangian formulations of Schutz and Brown for these types of models and provide a detailed interpretation of our results.

gr-qc↗

arXiv2026

Geometric formulation of $k$-essence and late-time acceleration

We study a class of geometries in which nonmetricity is fully determined by a vectorial degree of freedom and three independent coefficients. Formulating the simplest linear action in this geometry, implemented through Lagrange multipliers, naturally leads to an equivalence with the purely kinetic $k$-essence models with quadratic kinetic terms. A detailed dynamical systems analysis reveals that the $Λ$CDM phenomenology is embedded within the model. Crucially, we find that if stability conditions such as a positive sound speed squared and energy density are not enforced, the model generically exhibits instabilities and divergent behaviour in the phase space. These physical viability criteria allow us to isolate stable regions of the parameter space and derive well-motivated priors for parameter inference. Using Markov Chain Monte Carlo methods and late-time observational data, including cosmic chronometers, Pantheon$^{+}$ Type Ia supernovae, and DESI baryon acoustic oscillations, we constrain the degrees of freedom associated with nonmetricity and demonstrate the viability of the model. We discuss the implications of these results in light of the recent cosmic tensions, and give a possible explanation as to why the equivalent $k$-essence models have been missed as serious competitors to $Λ$CDM in the past. Finally, we review the geometric foundations of the theory and show that the integrable Weyl, Schrödinger and completely symmetric geometries are embedded within our framework as special cases.

gr-qc↗

arXiv2026

Interacting dark sector from intrinsic entropy couplings

We introduce a new class of interacting dark sector models that couple the intrinsic entropy of dark matter to scalar field dark energy. Using the Lagrangian formulation for relativistic perfect fluids, we construct consistent covariant actions that incorporate algebraic and derivative entropy couplings. These interactions leave the expansion history unchanged, rendering the background cosmology indistinguishable from $Λ$CDM or uncoupled quintessence. At the level of cosmological perturbations, the entropy couplings generate scale-dependent modifications to the dark matter Euler equation, while the continuity equation remains unaltered at linear order. The resulting interactions correspond to a pure-momentum exchange within the dark sector. We show that intrinsic entropy perturbations can carry primordial scale dependence, and non-minimal couplings can lead to a scale-dependent suppression or enhancement of structure growth. Finally, we demonstrate that these models are generically compatible with current Cosmic Microwave Background observations, while inducing distinctive signatures in large-scale structure. The framework provides a theoretically well-motivated and observationally viable extension to the standard cosmological model, opening new directions to explore novel interactions in the dark sector.

astro-ph.CO↗

arXiv2025

Cosmological dynamical systems of non-minimally coupled fluids and scalar fields

We study the cosmological dynamics of non-minimally coupled matter models using the Brown's variational approach to relativistic fluids in General Relativity. After decomposing the Ricci scalar into a bulk and a boundary term, we construct new models by coupling the bulk term to the fluid variables and an external scalar field. Using dynamical systems techniques, we study models of this type and find that they can give rise to both early-time inflationary behaviour and late-time accelerated expansion. Moreover, these models also contain very interesting features that are rarely seen in this context. For example, we find dark energy models which exhibit phantom crossing in the recent past. Other possibilities include models that give a viable past evolution but terminate in a matter-dominated universe. The dynamical systems themselves display an array of mathematically interesting phenomena, including spirals, centres, and non-trivial bifurcations depending on the chosen parameter values.

gr-qc↗

arXiv2025

Relativistic interacting fluids in cosmology

Motivated by cosmological applications for interacting matters, an extension of the action functional for relativistic fluids is proposed to incorporate the physics of non-adiabatic processes and chemical reactions. The former are characterised by entropy growth, while the latter violate particle number conservation. The relevance of these physics is demonstrated in the contexts of self-interacting fluids, fluids interacting with scalar fields, and hyperhydrodynamical interactions with geometry. The possible cosmological applications range from early-universe phase transitions to astrophysical phenomena, and from matter creation inflationary alternatives to interacting dark sector alternatives to the $Λ$CDM model that aim to address its tensions. As an example of the latter, a single fluid model of a unified dark sector is presented. The simple action of the model features one field and one parameter, yet it can both reproduce the $Λ$CDM cosmology and predict new phenomenology.

gr-qc↗

arXiv2025

Spatial curvature in coincident gauge $f(Q)$ cosmology

In this work we study the Friedmann-Lemaître-Robertson-Walker cosmologies with arbitrary spatial curvature for the symmetric teleparallel theories of gravity, giving the first presentation of their coincident gauge form. Our approach explicitly starts with the cosmological Killing vectors and constructs the coincident gauge coordinates adapted to these Killing vectors. We then obtain three distinct spatially flat branches and a single spatially curved branch. Contrary to some previous claims, we show that all branches can be studied in this gauge-fixed formalism, which offers certain conceptual advantages. We also identify common flaws that have appeared in the literature regarding the coincident gauge. Using this approach, we find that both the flat and spatially curved solutions in $f(Q)$ gravity can be seen as equivalent to the metric teleparallel $f(T)$ models, demonstrating a deeper connection between these theories. This is accomplished by studying the connection equation of motion, which can be interpreted as a consistency condition in the gauge-fixed approach. Finally, we discuss the role of diffeomorphism invariance and local Lorentz invariance in these geometric modifications of gravity.

gr-qc↗

arXiv2024

A new 2D formulation of modified General Relativity

It is well known that the Einstein-Hilbert action in two dimensions is topological and yields an identically vanishing Einstein tensor. Consequently one is faced with difficulties when formulating a non-trivial gravity model. We present a new, intrinsically two-dimensional, approach to this problem based on the Einstein action. This yields a well defined variational approach which results in new field equations that break diffeomorphism invariance. Our proposed approach does not require the introduction of additional scalar fields, nor the use of conformal transformations. However, we can show how including conformal counter terms leads to equivalent results. In doing so, we can provide an explanation for why previous approaches worked. Solutions to the field equations are briefly discussed.

gr-qc↗

arXiv2024

A Unified Approach to Geometric Modifications of Gravity

This thesis studies modified theories of gravity from a geometric viewpoint. We review the motivations for considering alternatives to General Relativity and cover the mathematical foundations of gravitational theories in Riemannian and non-Riemannian geometries. Then, starting from the decomposition of the Einstein-Hilbert action into bulk and boundary terms, we construct new modifications of General Relativity. These modifications break diffeomorphism invariance or local Lorentz invariance, allowing one to bypass Lovelock's theorem while remaining second-order and without introducing additional fields. In the metric-affine framework, we introduce a new Einstein-Cartan-type theory with propagating torsion. Important comparisons are made with the modified teleparallel theories, and we construct a unified framework encompassing all these theories. The equivalence between theories that break fundamental symmetries in the Riemannian setting and non-Riemannian theories of gravity is explored in detail. This leads to a dual interpretation of teleparallel gravity, one in terms of geometric quantities and the other in terms of non-covariant objects. We then study the cosmological applications of these modified theories, making use of dynamical systems techniques. One key result is that the modified Einstein-Cartan theories can drive inflation in the early universe, replacing the initial cosmological singularity of General Relativity. To conclude, we discuss the viability of these modifications and possible future directions, examining their significance and relevance to the broader field of gravitational physics.

gr-qc↗

arXiv2023

Modified gravity: a unified approach to metric-affine models

The starting point of this work is the original Einstein action, sometimes called the Gamma squared action. Continuing from our previous results, we study various modified theories of gravity following the Palatini approach. The metric and the connection will be treated as independent variables leading to generalised theories which may contain torsion or non-metricity or both. Due to our particular approach involving the Einstein action, our setup allows us to formulate a substantial number of new theories not previously studied. Our results can be linked back to well-known models like Einstein-Cartan theory and metric-affine theories and also links to many recently studied modified gravity models. In particular we propose an Einstein-Cartan type modified theory of gravity which contains propagating torsion provided our function depends non-linearly on a boundary term. We also can state precise conditions for the existence of propagating torsion. Our work concludes with a brief discussion of cosmology and the role of cosmological torsion in our model. We find solutions with early-time inflation and late-time matter dominated behaviour. No matter sources are required to drive inflation and it becomes a purely geometrical effect.

gr-qc↗

arXiv2023

Dynamical systems analysis of $f(Q)$ gravity

Modified gravity theories can be used for the description of homogeneous and isotropic cosmological models through the corresponding field equations. These can be cast into systems of autonomous differential equations because of their sole dependence on a well chosen time variable, be it the cosmological time, or an alternative. For that reason a dynamical systems approach offers a reliable route to study those equations. Through a model independent set of variables we are able to study all $f(Q)$ modified gravity models. The drawback of the procedure is a more complicated constraint equation. However, it allows the dynamical system to be formulated in fewer dimensions than using other approaches. We focus on a recent model of interest, the power-exponential model, and generalise the fluid content of the model.

gr-qc↗

arXiv2022

Cosmological dynamical systems in modified gravity

The field equations of modified gravity theories, when considering a homogeneous and isotropic cosmological model, always become autonomous differential equations. This relies on the fact that in such models all variables only depend on cosmological time, or another suitably chosen time parameter. Consequently, the field equations can always be cast into the form of a dynamical system, a successful approach to study such models. We propose a perspective that is applicable to many different modified gravity models and relies on the standard cosmological density parameters only, making our choice of variables model independent. The drawback of our approach is a more complicated constraint equation. We demonstrate our procedure studying various modified gravity models and show how much generic information can be extracted before a specific model is considered.

gr-qc↗

arXiv2021

Modified gravity: a unified approach

Starting from the original Einstein action, sometimes called the Gamma squared action, we propose a new setup to formulate modified theories of gravity. This can yield a theory with second order field equations similar to those found in other popular modified gravity models. Using a more general setting the theory gives fourth order equations. This model is based on the metric alone and does not require more general geometries. It is possible to show that our new theory and the recently proposed $f(Q)$ gravity models are equivalent at the level of the action and at the level of the field equations, provided that appropriate boundary terms are taken into account. Our theory can also match up with $f(R)$ gravity which is an expected result. Perhaps more surprisingly, we can also show that this equivalence extends to $f(T)$ gravity at the level of the action and its field equations, provided that appropriate boundary terms are taken in account. While these three theories are conceptually different and are based on different geometrical settings, we can establish the necessary conditions under which their field equations are indistinguishable. The final part requires matter to couple minimally to gravity. Through this work we emphasise the importance played by boundary terms which are at the heart of our approach.

gr-qc↗