Searcharxiv⌕ Search

arXiv subjects

John Miritzis

Publications and source records attributed to John Miritzis.

At least 19 recordsLinked to original sources
arXiv2026

Viable Cosmological Solutions from Hybrid Potentials

We study flat Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) models with a perfect fluid matter source and a scalar field minimally coupled to matter with power-law-exponential \textquotedblleft hybrid\textquotedblright potential. Using expansion-normalised variables, we formulate the field equations as a constrained three-dimensional dynamical system and determine its equilibrium structure. We show that viable cosmological histories, consisting of a transient matter or radiation era followed by late-time accelerated expansion, arise in restricted regions of parameter space. A central result is that the physically relevant trajectories are confined to an invariant plane, which contains both the transient matter point $\mathcal{B}$ and the accelerated point $\mathcal{C}$. We further show, by centre-manifold analysis, that the accelerated point $\mathcal{C}$ is not a global attractor: it attracts trajectories with $ϕ>0$ and repels those with $ϕ<0$. For dust, a standard matter era requires vanishing coupling of the scalar field to matter, while for radiation the interaction term vanishes identically. Finally, we discuss the issue that the qualitative cosmological dynamics may be independent of the precise functional form of the scalar-field potential.

gr-qc↗
arXiv2023

The cosmological frame principle and cosmic acceleration

We discuss implications of the cosmological frame principle which states that cosmological effects of modified gravity must be stable as solutions of each of the corresponding sets of dynamical equations holding in the two conformally-related frames. We show that there are such globally stable, `frame-independent' solutions describing cosmic acceleration, suggesting that they may represent a physically relevant effect. This result highlights the importance of further investigation into the implications of the frame principle for cosmological properties that rely on the use of conformal frames.

gr-qc↗
arXiv2023

Legendre scalarization in gravity and cosmology

We propose a new formulation of $f(R)$ gravity, dubbed scalarized $f(R)$ gravity, in which the Legendre transform is included as a dynamical term. This leads to a theory with second-order field equations that describes general relativity with a self-interacting scalar field, without requiring the introduction of conformal frames. We demonstrate that the quadratic version of scalarized $f(R)$ gravity reduces to general relativity with a massive scalar field, and we explore its implications for Friedmann cosmology. Our findings suggest that scalarized $f(R)$ gravity may lead to simplified descriptions of cosmological applications, while the proposed formulation could offer a new perspective on the relationship between $f(R)$ gravity and scalar-tensor theories.

gr-qc↗
arXiv2023

Trans-Planckian censorship and spacetime singularities

We study the effects of trans-planckian censorship conjecture (TCC) bounds on geodesic completeness of spacetime and the associated existence for an infinite proper time. Using Gronwall's lemma, TCC bounds can be derived directly, leading to a result about the absence of blowup solutions. We show that the TCC provides part of the required criteria for geodesic completeness, and we then provide the remaining ones - the norm of the extrinsic curvature being bounded away from zero. We also discuss the importance of these results for the classical evolution of Friedmann universes under the assumptions of global and regular hyperbolicity.

gr-qc↗
arXiv2022

Localizing branes with bifurcating bulks

We study the problem of evolution of bulk 5-fluids having an embedded braneworld with a flat, de Sitter, or anti-de Sitter geometry. We introduce new variables to express the Einstein equations as a dynamical system that depends on the equation of state parameter $γ$ and exponent $λ$. For linear fluids (i.e., $λ=1$), our formulation leads to a partial decoupling of the equations and thus to an exact solution. We find that such a fluid develops a transcritical bifurcation around the value $γ=-1/2$, and study how this behaviour affects to stability of the solutions. For nonlinear fluids, the situation is more diverse. We find an overall attractor at $λ=1/2$ and draw enough phase portraits to exhibit in detail the overall dynamics. We show that the value $λ=3/2$ is structurally unstable and typical for other forms of $λ$. Consequently, we observe a noticeable dependence of the qualitative behaviour of the solutions on different `polytropic' forms of the fluid bulk. In addition, we prove the existence of a Dulac function for nonlinear fluids, signifying the impossibility of closed orbits in certain subsets of the phase space. We also provide ample numerical evidence of gravity localizing solutions on the brane which satisfy all energy conditions.

hep-th↗
arXiv2022

The conformal cosmological potential

We discuss qualitative features of the conformal relation between certain classes of gravity theories and general relativity, common to different themes such as $f(R)$, Brans-Dicke-type, and string theories. We focus primarily on the frame relations of the fields involved, slice energy, traceless and Palatini extensions, and selected cosmological applications.

gr-qc↗
arXiv2020

Late time evolution of negatively curved FLRW models

We study the late time evolution of negatively curved Friedmann--Le\-ma\^ıtre--Robert\-son--Walker (FLRW) models with a perfect fluid matter source and a scalar field nonminimally coupled to matter. Since, under mild assumptions on the potential $V$, it is already known that equilibria corresponding to non-negative local minima for $V$ are asymptotically stable, we classify all cases where one of the energy components eventually dominates. In particular for nondegenerate minima with zero critical value, we rigorously prove that if $γ$, the parameter of the equation of state is larger than $2/3$, then there is a transfer of energy from the fluid and the scalar field to the energy density of the scalar curvature. Thus, the scalar curvature, if present, has a dominant effect on the late evolution of the universe and eventually dominates over both the perfect fluid and the scalar field. The analysis in complemented with the case where $V$ is exponential and therefore the scalar field diverges to infinity.

gr-qc↗
arXiv2019

Cosmological wave maps

We consider theories of gravity that include many coupled scalar fields with arbitrary couplings, in the geometric framework of wave maps. We examine the possibility of obtaining acceptable cosmological solutions without the inclusion of a potential term to the scalar fields. To illustrate the theory, we study two simple models and compare their solutions to those in General Relativity. We also address the issue of the conditions that must be satisfied by the wave maps for an accelerated phase of the Universe.

gr-qc↗
arXiv2015

Negative potentials and collapsing universes II

Completing a previous analysis started in [1], we study flat Friedmann--Lema\^ıtre--Robertson--Walker (FLRW) models with a perfect fluid matter source and a scalar field nonminimally coupled to matter, self--interacting with a potential that may attain negative values. We prove that the evolution generically forces the Hubble function to diverge to $-\infty$ in a finite time, except in case the potential exhibits a flat plateau at infinity (tending to zero from below); in that case we find conditions which may give rise to ever expanding or recollapsing cosmologies.

gr-qc↗
arXiv2014

Negative potentials and collapsing universes

We study Friedmann--Robertson--Walker models with a perfect fluid matter source and a scalar field nonminimally coupled to matter. We prove that a general class of bounded from above potentials which fall to minus infinity as the field goes to minus infinity, forces the Hubble function to diverge to $-\infty$ in a finite time. This finite-time singularity theorem is true for arbitrary coupling coefficient, provided that it is a bounded function of the scalar field.

gr-qc↗
arXiv2014

Coupled quintessence with double exponential potentials

We study flat Friedmann-Robertson-Walker (FRW) models with a perfect fluid matter source and a scalar field non minimally coupled to matter having a double exponential potential. It is shown that the scalar field almost always diverges to infinity. Under conditions on the parameter space, we show that the model is able to give an acceptable cosmological history of our universe, that is, a transient matter era followed by an accelerating future attractor. It is found that only a very weak coupling can lead to viable cosmology. We study in the Einstein frame, the cosmological viability of the asymptotic form of a class of f(R) theories predicting acceleration. The role of the coupling constant is briefly discussed.

gr-qc↗
arXiv2013

Acceleration in Weyl integrable spacetime

We study homogeneous and isotropic cosmologies in a Weyl spacetime. It is shown that in Weyl integrable spacetime, the corresponding scalar field may act as a phantom field. In this circumstance the Weyl field gives rise to a late accelerated expansion of the Universe for all initial conditions and for a wide range of the parameters.

gr-qc↗
arXiv2013

Energy exchange in Weyl geometry

We study homogeneous and isotropic cosmologies in a Weyl spacetime. We show that the field equations can be reduced to the Einstein equations with a two-fluid source and analyze the qualitative, asymptotic behavior of the models. Assuming an interaction of the two fluids we impose conditions so that the solutions of the corresponding dynamical system remain in the physically acceptable phase space. We show that in Weyl integrable spacetime, the corresponding scalar field acts as a phantom field and therefore, it may give rise to a late accelerated expansion of the Universe.

gr-qc↗
arXiv2010

Energy exchange for homogeneous and isotropic universes with a scalar field coupled to matter

We study the late time evolution of flat and negatively curved Friedmann-Robertson-Walker (FRW) models with a perfect fluid matter source and a scalar field arising in the conformal frame of $f(R)$ theories nonminimally coupled to matter. Under mild assumptions on the potential V we prove that equilibria corresponding to non-negative local minima for V are asymptotically stable, as well as horizontal asymptotes approached from above by V. We classify all cases of the flat model where one of the matter components eventually dominates. In particular for a nondegenerate minimum of the potential with zero critical value we prove in detail that if the parameter of the equation of state is larger than one, then there is a transfer of energy from the fluid to the scalar field and the later eventually dominates in a generic way.

gr-qc↗
arXiv2010

Late time behavior of closed isotropic models in second order gravity theory

Homogeneous and isotropic closed models are studied in both the Einstein and the Jordan frame of the second order gravity theory. The normal form of the dynamical system has periodic solutions for a large set of initial conditions. This implies that an initially expanding closed isotropic universe may exhibit oscillatory behavior.

gr-qc↗
arXiv2010

Expanding universes in the conformal frame of $f(R) $ gravity

The late time evolution of Friedmann-Robertson-Walker (FRW) models with a perfect fluid matter source is studied in the conformal frame of $f(R) $ gravity. We assume that the corresponding scalar field, nonminimally coupled to matter, has an arbitrary non-negative potential function $V(ϕ) $. We prove that equilibria corresponding to non-negative local minima for $V$ are asymptotically stable. We investigate all cases where one of the matter components eventually dominates. The results are valid for a large class of non-negative potentials without any particular assumptions about the behavior of the potential at infinity. In particular for a nondegenerate minimum of the potential with zero critical value we show that if $γ$, the parameter of the equation of state is larger than one, then there is a transfer of energy from the fluid to the scalar field and the later eventually dominates.

gr-qc↗
arXiv2006

Singularities of varying light speed cosmologies

We study the possible singularities of isotropic cosmological models that have a varying speed of light as well as a varying gravitational constant. The field equations typically reduce to two dimensional systems which are then analyzed both by dynamical systems techniques in phase space and by applying the method of asymptotic splittings. In the general case we find initially expanding closed models which recollapse to a future singularity and open universes that are eternally expanding towards the future. The precise nature of the singularities is also discussed.

gr-qc↗