Closed, singularity-free, inflationary cosmological models have recently been studied in the context of general relativity. Despite their appeal, these so called emergent suffer from a number limitations. These include fact that they rely on an initial Einstein static state to describe past-eternal phase universe. Given instability such within relativity, this amounts very severe fine tuning. Also order be able study dynamics universe set conditions for classical phase. Here we existence and...

We make a systematic study of the cosmological dynamics for number $f(R)$ gravity theories in Palatini formalism, using phase space analysis as well numerical simulations. Considering homogeneous and isotropic models, we find interesting results: (i) models based on type (a) $f(R)=R\ensuremath{-}\ensuremath{\beta}/{R}^{n}$ (b) $f(R)=R+\ensuremath{\alpha}\mathrm{ln}R\ensuremath{-}\ensuremath{\beta}$, unlike metric are capable producing sequence radiation-dominated, matter-dominated, de Sitter...

We construct a family of viable scalar-tensor models dark energy (DE) which possess phase late-time acceleration preceded by standard matter era, while at the same time satisfying local gravity constraints (LGC). The coupling $Q$ between scalar field and nonrelativistic in Einstein frame is assumed to be constant our scenario, generalization $f(R)$ theories corresponding $Q=\ensuremath{-}1/\sqrt{6}$. find that these can made compatible with even when $|Q|$ order unity through chameleon...

We describe annual and seasonal changes in air temperatures over high latitudes of the Northern Hemisphere during period 1881–1980. Trends (that is, fluctuations on time scales greater than 20 years) average temperature Arctic are compared with those Hemisphere. Seasonal regional departures from long-term trends identified. Spatial patterns variation field determined by principal component analysis major characteristics series dominant summarized. have been broadly similar to for study...

We make a detailed study of matter density perturbations in both metric and Palatini formalisms. Considering general theories whose Lagrangian is function, $f(R)$, the Ricci scalar $R$, we derive equation each case, number gauges, including comoving, longitudinal uniform gauges. show that for viable $f(R)$ models satisfy cosmological local gravity constraints (LGC), perturbation equations derived under subhorizon approximation are valid even super-Hubble scales provided oscillating mode...

We study chaotic inflation in the context of modified gravitational theories. Our analysis covers models based on (i) a field coupling ω(ϕ) with kinetic energyX = −(1/2)gμν∂μϕ∂νϕ and nonmimimal ζϕ2R/2 Ricci scalar R, (ii) Brans-Dicke (BD) theories, (iii) Gauss-Bonnet (GB) gravity, (iv) gravity Galileon correction. Dilatonic energy and/or negative nonminimal are shown to lead compatibility observations Cosmic Microwave Background (CMB) temperature anisotropies for self-coupling inflaton...

We compute the covariant three-point function near horizon-crossing for a system of slowly-rolling scalar fields during an inflationary epoch, allowing arbitrary field-space metric. show explicitly how to its subsequent evolution using covariantized version separate universe or "delta-N" expansion, which must be augmented by terms measuring curvature manifold, and give nonlinear gauge transformation comoving perturbation. Nonlinearities induced are new potentially significant source...

Positively curved oscillatory universes are studied within the context of loop quantum cosmology subject to a consistent semiclassical treatment. The effects reformulated in terms an effective phantom fluid with variable equation state. In cosmologies sourced by massless scalar field, these lead universe that undergoes ever-repeating cycles expansion and contraction. presence self-interaction potential for field breaks symmetry can enable oscillations establish initial conditions successful...

We study inflationary perturbations in multiple-field models, for which zeta typically evolves until all isocurvature modes decay--the "adiabatic limit". use numerical methods to explore the sensitivity of nonlinear parameter fNL process by this limit is achieved, finding an appreciable dependence on model-specific data such as time at slow-roll breaks down or timescale reheating. In models with a sum-separable potential where decay before end phase we give analytic criterion asymptotic...

An important open question in cosmology is the degree to which Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) solutions of Einstein's equations are able model large-scale behavior locally inhomogeneous observable universe. We investigate this problem by considering a range exact n-body constraint equations. These contain discrete masses, and so allow arbitrarily large density contrasts be modeled. restrict our study regularly arranged distributions masses topological 3-spheres. This has...

We propose a thermodynamically motivated measure of gravitational entropy based on the Bel–Robinson tensor, which has natural interpretation as effective super-energy–momentum tensor free fields. The specific form this differs depending whether field is Coulomb-like or wave-like, and reduces to Bekenstein–Hawking value when integrated over interior Schwarzschild black hole. For scalar perturbations Robertson–Walker geometry we find that goes like Hubble weighted anisotropy field, therefore...

We generalise Wesson's procedure, whereby vacuum $(4+1)-$dimensional field equations give rise to $(3+1)-$dimensional with sources, arbitrary dimensions. then employ this generalisation relate the usual $(2+1)-$dimensional sources and derive analogues of classes solutions obtained by Ponce de Leon. This way viewing lower dimensional gravity theories can be importance in establishing a relationship between such 4-dimensional general relativity, as well giving producing exact $(2+1)$...

Analyses of recent cosmic microwave background (CMB) observations have provided increasing indications for the existence large scale anisotropy in universe. Given far reaching consequences such an our understanding universe, it is important to employ alternative indicators order determine whether reported cosmological origin, and if so extract further information that may be helpful identifying its causes. Here we propose a new directional indicator, based on angular-separation histograms...

Self-consistent convective dynamo simulations in wedge-shaped spherical shells are presented. Differential rotation is generated by the interaction of convection with rotation. Equatorward acceleration and action obtained only for sufficiently rapid The angular velocity tends to be constant along cylinders. Oscillatory large-scale fields found migrate poleward direction. Comparison earlier full Cartesian domains made.

A little known theorem due to Campbell is employed establish the local embedding of a wide class four-dimensional spacetimes in five-dimensional Ricci-flat spaces. An for n-dimensional Einstein spaces also found. The nature Campbell's highlighted by studying some lower-dimensional

In loop quantum gravity, modifications to the geometrical density cause a self-interacting scalar field accelerate away from minimum of its potential. principle, this mechanism can generate conditions that subsequently lead slow-roll inflation. The consequences for various quantization ambiguities arising within cosmology are considered. For case quadratic potential, it is found some procedures more likely phase general, however, robust in and extends range initial

Two approaches to the study of cosmological density perturbations in modified theories Palatini gravity have recently been discussed. These utilize, respectively, a generalization Birkhoff's theorem and direct linearization gravitational field equations. In this paper these are compared contrasted. The general form Lagrangian for which two frameworks yield identical results long-wavelength limit is derived. This class models includes case where power-law Ricci curvature scalar. evolution...

Abstract We use direct numerical simulations of forced MHD turbulence with a forcing function that produces two different signs kinetic helicity in the upper and lower parts domain. show mean flux magnetic from small‐scale field between domain can be described by Fickian diffusion law coefficient is approximately independent Reynolds number about one third estimated turbulent diffusivity. The data suggest diffusive only expected to alleviate catastrophic quenching at numbers more than...

We present direct numerical simulations of the equations compressible magnetohydrodynamics in a wedge-shaped spherical shell, without shear, but with random helical forcing which has negative (positive) helicity northern (southern) hemisphere. find large-scale magnetic field that is nearly uniform azimuthal direction and approximately antisymmetric about equator. Furthermore, each hemisphere oscillates on dynamical time scales reversals polarity equatorward migration. Corresponding...

We study the effects of inhomogeneities on evolution Universe, by considering a range cosmological models with discretized matter content. This is done using exact and fully relativistic methods that exploit symmetries in about submanifolds spacetimes themselves possess no continuous global symmetries. These allow us to follow our throughout their entire history, far beyond what has previously been possible. find while some space-like curves collapse anisotropic singularities finite time,...

We consider the behaviour of attractors near invariant subspaces on varying a parameter that changes dynamics in subspace dynamical system. refer to such as `non-normal'. In presence chaos is fragile, we find blowout bifurcations are blurred over range values. demonstrate this can occur set positive measure space.