Bonnet Term Research Articles

Embedding Gauss–Bonnet Scalarization Models in Higher Dimensional Topological Theories

In the presence of appropriate non-minimal couplings between a scalar field and the curvature squared Gauss–Bonnet (GB) term, compact objects such as neutron stars and black holes (BHs) can spontaneously scalarize, becoming a preferred vacuum. Such strong gravity phase transitions have attracted considerable attention recently. The non-minimal coupling functions that allow this mechanism are, however, always postulated ad hoc. Here, we point out that families of such functions naturally emerge in the context of Higgs–Chern–Simons gravity models, which are found as dimensionally descents of higher dimensional, purely topological, Chern–Pontryagin non-Abelian densities. As a proof of concept, we study spherically symmetric scalarized BH solutions in a particular Einstein-GB-scalar field model, whose coupling is obtained from this construction, pointing out novel features and caveats thereof. The possibility of vectorization is also discussed, since this construction also originates vector fields non-minimally coupled to the GB invariant.

Super inflation mechanism and dark energy from a cosmological entropy perspective

Various approaches are developed that provide a unification of the early inflationary epoch of the universe and dark energy (cosmic accelerating expansion case). In one respect, we express this unification in a universal thermodynamic framework within the context of F ( T , T G ) role=presentation> F ( T , T G ) F ( T , T G ) F\left( T,T_{G} \right) gravity, where T role=presentation> T T T represents the torsion invariant and T G role=presentation> T G T G T_{G} is the teleparallel equivalent of the Gauss--Bonnet term. For a Friedmann-Robertson-Walker universe model bounded by the dynamical apparent horizon, we explore the generalized second law of gravitational thermodynamics (GSLT). Finally, we discuss the super inflation mechanism, the deceleration case (radiation and dust regions), and the late time cosmic acceleration of the universe into the validity of the GSLT that we obtain.

Instability analysis of expansion-free sphere in f(𝒢) gravity

The aim of this paper is to study the dynamical instability of expansion-free spherically symmetric anisotropic fluid in the framework of [Formula: see text] gravity. We apply perturbation scheme of the first-order to the metric functions as well as matter variables and construct modified field equations for both static and perturbed configurations using power-law [Formula: see text] model. To discuss the instability dynamics, we use the contracted Bianchi identities to formulate the dynamical equations in both Newtonian and post-Newtonian regimes. It is found that the range of instability is independent of adiabatic index for expansion-free fluid but depends on anisotropic pressures, energy density and Gauss–Bonnet (GB) terms.

Early inflation to late-time acceleration in f(G) model

We present some solutions in Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime in the modified theory of gravity with a general Gauss–Bonnet (GB) term f(G) and R2 including an ideal fluid. We present evolution of the universe introducing an ansatz without a prior choice of f(G) in one approach, while in other class of model, the solutions are obtained assuming few simple forms of f(G). Some of the solutions show early inflationary expansion, further in one solution the fluctuation of the deceleration parameter q is evident at the end of inflation. In all cases, late-time transition to accelerating universe at redshift z [Formula: see text] 0.7 is realizable.

Isotropic cosmological models in F(T,TG) theory

This paper is devoted to study evolution of the isotropic universe models in the framework of [Formula: see text] gravity ([Formula: see text] represents torsion scalar and [Formula: see text] is the teleparallel equivalent of the Gauss–Bonnet (GB) term). We construct [Formula: see text] models by taking different eras of the universe like non-relativistic and relativistic matter eras, dark energy (DE) dominated era and their combinations. It is found that the reconstructed models indicate decreasing behavior for DE dominated era and its combination with other eras. We also discuss stability of each reconstructed model. Finally, we evaluate equation of state (EoS) parameter by considering two models and study its behavior graphically.

Gauss-Bonnet correction to theR-current correlator inN=4theory at strong coupling

We consider higher-curvature corrections to the $R$-current correlator by studying the propagation of a $U(1)$ field in Einstein--Gauss--Bonnet gravity with a negative cosmological constant. We numerically solve the Maxwell equations and plot an $R$-current spectral function with lightlike momenta, which contains pivotal information of thermal photon emission. We also analytically compute the $R$-current correlator in the long-distance limit by using the holographic membrane paradigm. In the high-energy regime, the inelastic scattering between $R$-current and gauge theory plasma is expected to happen, which will reveal the structure of the plasma at strong coupling. It turns out that the Gauss--Bonnet correction effectively rescales all physical quantities considered here by some functions of the Gauss--Bonnet coefficient. In particular, the Gauss--Bonnet terms will enhance or weaken signatures calculated here in accordance with the sign of the Gauss--Bonnet coefficient.

TORSION AND SPONTANEOUS COMPACTIFICATION

Spontaneous compactification of Einstein–Cartan theory with quadratic Gauß–Bonnet term is investigated. For the homogeneous spaces SU (3)/ U (1) × U (1), Sp (2)/ SU (2) × U (1) and G2/ SU (3) we find the most general form of G-invariant metric and torsion and find solutions of spontaneous compactifying type. It turns out that these solutions admit a geometrical interpretation.