Evolution Of Perturbations Research Articles

Pole-skipping of gravitational waves in the backgrounds of four-dimensional massive black holes

Pole-skipping is a property of gravitational waves dictated by their behaviour at horizons of black holes. It stems from the inability to unambiguously impose ingoing boundary conditions at the horizon at an infinite discrete set of Fourier modes. The phenomenon has been best understood, when such a description exists, in terms of dual holographic (AdS/CFT) correlation functions that take the value of ‘0/0’ at these special points. In this work, we investigate details of pole-skipping purely from the point of view of classical gravity in 4d massive black hole geometries with flat, spherical and hyperbolic horizons, and with an arbitrary cosmological constant. We show that pole-skipping points naturally fall into two categories: the algebraically special points and a set of pole-skipping points that is common to the even and odd channels of perturbations. Our analysis utilises and generalises (to arbitrary maximally symmetric horizon topology and cosmological constant) the ‘integrable’ structure of the Darboux transformations, which relate the master field equations that describe the evolution of gravitational perturbations in the two channels. Finally, we provide new insights into a number of special cases: spherical black holes, asymptotically Anti-de Sitter black branes and pole-skipping at the cosmological horizon in de Sitter space.

Probing early modification of gravity with Planck, ACT and SPT

We consider a model of early modified gravity (EMG) that was recently proposed as a candidate to resolve the Hubble tension. The model consists of a scalar field σ with a nonminimal coupling (NMC) to the Ricci curvature of the form F(σ) = M pl 2+ξσ 2 and an effective mass induced by a quartic potential V(σ) = λσ 4/4. We present the first analyses of the EMG model in light of the latest ACT DR4 and SPT-3G data in combination with full Planck data, and find a ≳ 2σ preference for a non-zero EMG contribution from a combination of primary CMB data alone, mostly driven by ACT-DR4 data. This is different from popular `Early Dark Energy' models, which are detected only when the high-ℓ information from Planck temperature is removed. We find that the NMC plays a key role in controlling the evolution of density perturbations that is favored by the data over the minimally coupled case. Including measurements of supernovae luminosity distance from Pantheon+, baryonic acoustic oscillations and growth factor from BOSS, and CMB lensing of Planck leaves the preference unaffected. In the EMG model, the tension with SH 0ES is alleviated from ∼ 6σ to ∼ 3σ. Further adding SH 0ES data raises the detection of the EMG model above 5σ.

Non-modal behavior in the linear regime of high-speed boundary layer flows: Flow–thermodynamic interactions

The flow–thermodynamic interactions in the transient linear regime of high-speed boundary layers starting from non-modal initial conditions are studied using direct numerical simulation. These simulations are performed at different Mach numbers: M∈[3,6]. The perturbation velocity field is decomposed into solenoidal and dilatational components using the Helmholtz decomposition. It is shown that at high speeds, random pressure perturbations evolve to their asymptotic state in three distinct stages. In stage 1, pressure–dilatation engenders rapid transfer from internal to kinetic energy leading to a balance between the two forms. Pressure–dilatation maintains this balance throughout stage 2 with harmonic exchange of energy between the two forms. During this stage, the stable modes decay and the unstable modes establish ascendancy. Stage 3 behavior is dominated almost exclusively by the most unstable mode. Both internal and kinetic energies grow at the rate predicted by linear stability analysis. At this stage, pressure–dilatation is small and production dominates the flow evolution. This behavior is also observed in narrow-band perturbation evolution. Spatial boundary layer simulations are also performed to examine the non-parallel effects on the observed behavior. It is seen that the role of pressure–dilatation essentially remains the same as observed in the parallel flow case.

Instability of steady states with inhomogeneous field in electron–positron plasma diode

Instability features of steady states of the plasma diode with electron and positron counter flows are studied. There are several types of such states for each value of the inter-electrode distance. The case when charged particles moving in the diode plasma are not reflected from potential extrema is considered. We have solved an equation for the amplitude of the electric field perturbation for steady states with an inhomogeneous field distribution. Studying the dispersion equation has shown that all considered solutions are unstable. We have also confirmed this result when simulating small perturbation evolution of a steady-state solution.

Second-order corrections to Starobinsky inflation

Higher-order theories of gravity are extensions to general relativity (GR) motivated mainly by high-energy physics searching for GR ultraviolet completeness. They are characterized by the inclusion of correction terms in the Einstein–Hilbert action that leads to higher-order field equations. In this paper, we propose investigating inflation due to the GR extension built with all correction terms up to the second-order involving only the scalar curvature R, namely, R2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R^{2}$$\\end{document}, R3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R^{3}$$\\end{document}, R□R\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R\\square R$$\\end{document}. We investigate inflation within the Friedmann cosmological background, where we study the phase space of the model, as well as explore inflation in slow-roll leading-order. Furthermore, we describe the evolution of scalar perturbations and properly establish the curvature perturbation. Finally, we confront the proposed model with recent observations from Planck, BICEP3/Keck, and BAO data.

Impact of freeze-in on dark matter isocurvature

Dark matter freeze-in is a compelling cosmological production mechanism in which all or some of the observed abundance of dark matter is generated through feeble interactions it has with the Standard Model. In this work we present the first analysis of freeze-in dark matter fluctuations and consider two benchmark models: freeze-in through the direct decay of a heavy vector boson and freeze-in through pair annihilation of Standard Model particles in the thermal bath. We provide a theoretical framework for determining the impact of freeze-in on curvature and dark matter isocurvature perturbations. We determine freeze-in dark matter fluid properties from first principles, tracking its evolution from its relativistic production to its final cold state, and calculate the evolution of the dark matter isocurvature perturbation. We find that in the absence of initial isocurvature, the freeze-in production of dark matter does not source isocurvature. However, for an initial isocurvature perturbation seeded by inflation, the nonthermal freeze-in process may allow for a fraction of the isocurvature to persist, in contrast to the exponential suppression it receives in the case of thermal dark matter. In either case, the evolution of the curvature mode is unaffected by the freeze-in process. We show sensitivity projections of future cosmic microwave background experiments to the amplitude of uncorrelated, totally anticorrelated, and totally correlated dark matter isocurvature perturbations. From these projections, we infer the sensitivity to the abundance of freeze-in dark matter that sustains some fraction of the primordial isocurvature.

Linear Stability of Filtration Flow of a Gas and Two Immiscible Liquids with Interfaces

The stability of the vertical flow that occurs when gas displaces oil from a reservoir is investigated. It is assumed that the oil and gas areas are separated by a layer saturated with water. This method of oil displacement, called water-alternating-gas injection, improves the oil recovery process. We consider the linear stability of two boundaries that are flat at the initial moment, separating, respectively, the areas of gas and water, as well as water and oil. The instability of the interfaces can result in gas and water fingers penetrating into the oil-saturated area and causing residual oil. Two cases of perturbation evolution are considered. In the first case, only the gas–water interface is perturbed at the initial moment, and in the second case, small perturbations of the same amplitude are present on both surfaces. It is shown that the interaction of perturbations at interfaces depends on the thickness of the water-saturated layer, perturbation wavelength, oil viscosity, pressure gradient and formation thickness. Calculations show that perturbations at the oil–water boundary grow much slower than perturbations at the gas–water boundary. It was found that, with other parameters fixed, there is a critical (or threshold) value of the thickness of the water-saturated layer, above which the development of perturbations at the gas–water boundary does not affect the development of perturbations at the water–oil boundary.

Gauge fixing in cosmological perturbations of Unimodular Gravity

With focus on the cosmological evolution of linear perturbations of matter and geometry, we calculate the equivalent expressions to that of the Newtonian and Synchronous gauges within the framework of Unimodular Gravity, being these two gauges commonly used and implemented in Boltzmann codes. An important aspect of our analysis is the inclusion of the energy-momentum current violation, as well as its perturbations. Moreover, for the first time we demonstrate that it is possible to fix both gauges consistently, although as it has been already noticed in previous literature, neither of them is recovered in the sense of the dynamics given in General Relativity for matter and metric fluctuations. Specifically, we show that since the unimodular constraint at the level of linear perturbations lead to only one degree of freedom of scalar modes of metric fluctuations, the dynamics in Unimodular Gravity forces to keep the anisotropic stress in the Newtonian gauge, whereas the cold dark matter comoving frame can not be set in the Synchronous gauge. The physical implications on the density contrast of cold dark matter is reviewed, and the Sachs-Wolfe effect is obtained and compared with previous results in the literature of cosmological perturbations in Unimodular Gravity.

A viable f(R) gravity model without oscillations in the effective dark energy

In this study, we propose a reparametrization of a specific viable [Formula: see text] gravity model to represent it as a perturbation of the [Formula: see text]CDM model. The [Formula: see text] gravity model under consideration includes two parameters, [Formula: see text] and [Formula: see text], which control how close the proposed model can be to [Formula: see text]CDM, allowing for arbitrary proximity. Furthermore, it is shown that the Hu–Sawicki (HS) model is a limiting case of this reparametrized model. Following the existing literature, we also derive an analytical approximation for the expansion rate [Formula: see text], which shows an excellent agreement between this analytical approximation and the numerical solution over a wide range of redshifts for realistic values of the deviation parameter [Formula: see text]. By appropriately selecting values for the model parameters, we plot the cosmological parameters [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], as well as the statefinder quantities [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]. We find that their present values (at [Formula: see text]) are consistent with the observations from Planck 2018 and the values predicted by the [Formula: see text]CDM model. It is important to note that the examined cosmological and statefinder parameters do not exhibit significant oscillations of effective dark energy, which could lead to singular and unphysical solutions at high redshifts. This anomalous behavior has been avoided here by utilizing the approximate analytical solution for [Formula: see text]. Additionally, we conduct a detailed analysis of the evolution of matter density perturbations within the introduced [Formula: see text] gravity model. The results demonstrate that this viable [Formula: see text] gravity model is practically indistinguishable from the [Formula: see text]CDM model at the background level.

IDECAMB: an implementation of interacting dark energy cosmology in CAMB

Interacting dark energy (IDE) scenario is a natural and important extension to the standard ΛCDM cosmology. We develop a full numerical routine, called IDECAMB, as a patch to the public Einstein-Boltzmann solver CAMB, to solve the background and perturbation equations of the IDE models. The IDECAMB solver provides a unified interface for the widely studied IDE models by employing a parametrization model with five free functions. By configuring these five functions, one can easily map the coupled quintessence (CQ) and coupled fluid (CF) models into the parametrization. We handle the perturbation evolutions of the CF models with the parametrized post-Friedmann (PPF) approach to avoid the possible large-scale instability. Compared with the previous established PPF approach whose form depends on a specific IDE model, the PPF approach in this work are model-independent, making it easy to use. We constrain a specific CQ model with the IDECAMB package. The fitting results are consistent with those obtained by Planck Collaboration, which confirms the validity of the package.

Evolution of perturbations in a universe with exotic solid-like matter

We study cosmological perturbations in a universe with only one matter component described by a triplet of fields. Configuration of these fields is the same as for body coordinates of a solid, and they enter the matter Lagrangian only through the kinetic term. We restrict ourselves only to cases with constant pressure to energy density ratio w. Superhorizon perturbations have no constant modes with scalar vector and tensor perturbations decaying or growing at different rates, and in cases with pressure to energy density ratio w>(19−87)/3=̇−0.722 perturbations propagate with superluminal sound speed. Regarding our universe, these results illustrate possible challenges with comparing the observational data to models similar to solid inflation, if the inflation is followed by a period during which the studied model is a sufficiently good approximation.

Scale invariant curvature perturbations from a spontaneously decaying scalar field

The evolution of superhorizon curvature perturbations in a two-component interacting Universe is considered. It is found that adiabatic modes conserve the total curvature perturbation , unless there are stages in which the rate of dissipation of one component into another is not constant. Moreover, our result shows that when the rate is varying it is possible for ‘isocurvature’ perturbations generated during reheating to alter the amplitude of an adiabatic curvature mode even when the mode is outside the horizon. Specifically, if an indefinitely large rate Γ for massive particles decaying into photons develops rapidly amid vanishingly small initial values (before decay) of the total curvature and Newtonian potential , such that the product and become a pair of finite and universal constants for all superhorizon scales afterwards, Harrison–Zel’dovich scale-invariant power spectrum could be synthesized from a homogeneous state without inflation at all.

Superradiance instabilities of charged black holes in Einstein-Maxwell-scalar theory

We study time evolutions of charged scalar perturbations on the background of a charged hairy black hole, in which the perturbations can be governed by a double-peak effective potential. By extracting quasinormal modes from the waveform of scalar perturbations, we discover that some quasinormal modes, which are trapped in a potential well between two potential peaks, can be superradiantly amplified. These superradiant modes make the hairy black hole unstable against charged scalar perturbations. Moreover, it is found that the superradiant modes arise from the competition between the superradiant amplification caused by tunneling through the outer potential barrier and the leakage of modes through the inner potential barrier into the black hole.

Origin and Evolution of Synoptic-Scale Vortices Initiated at Low Level Downwind of the Hoggar Mountains

Abstract Numerous low-level vortices are initiated downwind of the Hoggar Mountains and progress toward the Atlantic coast on the northern path of African easterly waves (AEWs). These vortices occur mostly in July and August and more specifically when the northern position of the Saharan heat low (SHL) generates stronger and vertically expanded easterly winds over the Hoggar Mountains. At synoptic time scales, a composite analysis reveals that vortex initiation and westward motion are also statistically triggered by a reinforcement of these easterly winds by a wide and persistent high pressure anomaly developing around the Strait of Gibraltar and by a weak wave trough approaching from the east. The vortices are generated in the lee of the Hoggar, about 1000 km west of this approaching trough, and intensify rapidly. The evolution of the vortex perturbation is afterward comparable with the known evolution of the AEWs of the northern path and suggest a growth due to dry barotropic and baroclinic processes induced in particular by the strong cyclonic shear between the reinforced easterly winds and the monsoon flow. These results show that vortex genesis promoted by changes in orographic forcing due to the strengthening of easterly winds over the Hoggar Mountains is a source of intensification of the northern path of AEWs in July and August. These results also provide a possible mechanism to explain the role of the SHL and of particular midlatitude intraseasonal disturbances on the intensity of these waves.

The Perturbation of Ozone and Nitrogen Oxides Impacted by Blue Jet Considering the Molecular Diffusion

This study investigated the diffusion impact on the chemical perturbation of NOx and O3 caused by the streamer and leader parts of a blue jet in the low stratosphere (18–30 km), using the coupling of a detailed stratospheric chemistry model and a typical diffusion model. The study found that diffusion significantly impacted the evolution of chemical perturbations at both short-term and long-term time scales after the blue jet discharge, with changes in NOx and O3 concentrations observed at different altitudes (18–28 km). At 18 km, the concentrations of NOx and N2O that account for diffusion start to decrease after 1 s, whereas those without diffusion remain at their peak concentrations. Meanwhile, O3 is slowly destroyed with less NOx, rather than dropping to an unrealistic low value immediately after the discharge without diffusion. The perturbation caused by the blue jet discharge disappears within a few tens of seconds at 18 km when molecular diffusion is considered. At 30 km, the chemical perturbation from four point sources was observed through changes in NO2 concentrations. However, the total concentration of NO2 perturbed by the streamer part discharge at the given surface was negligible when considering diffusion. Overall, this study provided a useful model tool for a more accurate assessment of the chemical effects of individual blue jets.

Formula omitted] gravity with broken Weyl gauge symmetry, cosmological backreaction, and its effects on CMB anisotropy

We propose a new class of f(R) theory where its Weyl gauge symmetry is broken in the primordial era of the universe. This symmetry forces one to adopt a new scalar field, namely a Weyl field and a gauge vector boson. Furthermore, an equivalent form of the Einstein–Hilbert Lagrangian with a non-minimally coupled scalar field corresponding to the function f(R) is found. Due to the geometrical feature of the Weyl field, it turns out that the symmetry breaking induces a non-minimal coupling, which cannot be expected in the standard f(R) theories. We explain how this affects the evolution of the universe at cosmological scales. It is shown that there may be a value shift in the Planck constant and the cosmological constant. This can be regarded as a genuine exemplification of the cosmological backreaction. Furthermore, one also finds new features in the evolution of perturbational variables and cosmic microwave background anisotropy. Moreover, we prove that when a specific f(R) model invokes inflation, the amplitude of the primordial gravitational waves affects the evolution of scalar perturbation due to the new non-minimal coupling. As a case study, we explain how this can be embodied in the Starobinsky inflation. Finally, we discuss some impacts that this physics can bear and the possibility of giving a new restriction of the estimation of cosmological variables such as the gravitational wave amplitude with experiments.

Gauge invariance on the light-cone: curvature perturbations and radiative degrees of freedom

We derive the expressions on the observed light-cone for some relevant cosmological gauge invariant variables, such as the Mukhanov-Sasaki variable and E- and B- modes of the tensor perturbations. Since the structure of the light-cone does not reflect in a direct way the FLRW symmetries, we develop a formalism which is coordinate independent and classifies the perturbations according to their helicities. Even though we work with linear perturbations, our formalism can be readily extended to non-linear theory and put the basis to study the evolution of cosmological perturbations, since the early- until the late-time Universe, directly along the observed light-cone.

Evolution of spherical perturbations in the cosmological environment of degenerate scalar-charged fermions with a scalar Higgs coupling

Evolution of spherical perturbations in the cosmological environment of degenerate scalar-charged fermions with a scalar Higgs coupling

Эволюция сферических возмущений в космологической среде вырожденных скалярно заряженных фермионов со скалярным взаимодействием Хиггса

Построена математическая модель эволюции сферических возмущений в космологической однокомпонентной статистической системе полностью вырожденных скалярно заряженных фермионов со скалярным взаимодействием Хиггса. Построена полная система самосогласованных уравнений для малых возмущений, описывающих эволюцию сферических возмущений. Выделены сингулярные части в модах возмущений, соответствующие точечным массе и скалярному заряду. Получены системы обыкновенных дифференциальных уравнений, описывающих эволюцию массы и заряда сингулярного источника, и системы дифференциальных уравнений в частных производных, описывающие эволюцию несингулярных частей возмущений. Коэффициенты уравнений в частных производных описываются решениями эволюционных уравнений для массы и заряда. Задача о пространственно локализованных возмущениях для полиномиальных по радиальной координате решений сведена к рекуррентной системе обыкновенных линейных дифференциальных уравнений на коэффициенты этих полиномов. Изучены свойства решений в случае кубических полиномов, в частности показано, что радиусы локализации гравитационных и скалярных возмущений совпадают и эволюционируют пропорционально масштабному фактору. Проведено численное моделирование эволюции возмущений, подтвердившее экспоненциальный рост центральной массы возмущения, а также выявившее колебательный характер эволюции скалярного заряда.

Cosmology of a Chaplygin Gas Model Under f(T) Gravity and Evolution of Primordial Perturbations

This paper reports a detailed study of generalized Chaplygin gas (GCG) with power law form of scale factor and truncated form of the scale factor using binomial expansion in both interacting and non-interacting scenarios along with its cosmological consequences, studied in terms of equation of state (EoS) parameter. In the non-interacting scenario, the EoS parameter behaves as quintessence in both forms of the scale factor. In the interacting scenario, the EoS parameter behaves as phantom and for the truncated form of the scale factor, it violates the constraints of the positive parameter α. The cosmological implementation of GCG interacting with pressureless dark matter is investigated in the framework of f(T) modified gravity, where T is the torsion scalar in teleparallelism. The interaction term is directly proportional to the GCG density with positive coupling constant. In f(T) gravity, the EoS is behaving like phantom. The stability of the reconstructed model is investigated and it is found to be stable against small gravitational perturbations, i.e., the squared speed of sound is non-negative and an increasing function of cosmic time t. We have observed that our reconstructed f(T) model satisfies one of the sufficient conditions of a realistic reconstructed model and it is consistent with the CMB constraints and primordial nucleosynthesis. Cosmology of primordial perturbations has also been analyzed and the self-interacting potential has been found to be an increasing function of cosmic time t.