Backreaction Effects Research Articles

Flattened axion monodromy beyond two derivatives

We study string inspired two-field models of large-field inflation based on axion monodromy in the presence of an interacting heavier modulus. This class of models has enough structure to approximate at least part of the backreaction effects known in full string theory, such as kinetic mixing with the axion, and flattening of the scalar potential. Yet, it is simple enough to fully describe the structure of higher-point curvature perturbation interactions driven by the adjusting modulus backreaction dynamics. We find that the presence of the heavy modulus can be described via two equivalent effective field theories, both of which can incorporate reductions of the speed of sound. Hence, the presence of heavier moduli in axion monodromy inflation constructions will necessarily generate some amount of non-Gaussianity accompanied by changes to $n_s$ and $r$ beyond what results from just from the well known adiabatic flattening backreaction.

Space-time waves from a collapse with a time-dependent cosmological parameter

We study the emission of space-time waves produced by back-reaction effects during a collapse of a spherically symmetric universe with a time-dependent cosmological parameter, which is driven by a scalar field. As in a previous work, the final state avoids the final singularity due to the fact the co-moving relativistic observer never reaches the center, because the physical time evolution $$\mathrm{d}\tau =U_{0}\,\mathrm{d}x^0$$, decelerates for a co-moving observer which falls with the collapse. The equation of state of the system depends on the rate of the collapse, but always is positive: $$0< \omega (p) < 0.25$$.

Meissner like effect in holographic superconductors with back reaction

In this article we employ the matching method to analytically investigate the properties of holographic superconductors in the framework of Maxwell electrodynamics taking into account the effects of back reaction on spacetime. The relationship between the critical temperature (Tc) and the charge density (ρ) has been obtained first. The influence of back reaction on Meissner like effect in this holographic superconductor is then studied. The results for the critical temperature indicate that the condensation gets harder to form when we include the effect of back reaction. We also give a detailed discussion about the dependence of our results on the choice of the matching point. The expression for the critical magnetic field (Bc) above which the superconducting phase vanishes is next obtained. It is observed from our investigation that the ratio of Bc and Tc2 increases with the increase in the back reaction parameter. However, the critical magnetic field Bc decreases with increase in the back reaction parameter.

Back reaction effects on the imaginary potential of quarkonia in heavy quark cloud

Applying the AdS/CFT correspondence, we investigate the effect of back reaction on the imaginary part of heavy quarkonia potential in strongly coupled N=4 supersymmetric Yang-Mills (SYM) plasma. The back reaction considered here arises from the inclusion of static heavy quarks uniformly distributed over N=4 SYM plasma. It is shown that the presence of back reaction reduces the absolute value of the imaginary potential thus decreasing the thermal width. Furthermore, the results imply that back reaction enhances the quarkonia dissociation.

Overspinning Kerr-MOG black holes by test fields and the third law of black hole dynamics

We evaluate the validity of the weak form of the cosmic censorship conjecture and the third law of black hole dynamics for Kerr-MOG black holes interacting with test scalar fields. Ignoring backreaction effects, we first show that both extremal and nearly extremal Kerr-MOG black holes can be overspun into naked singularities by test fields with a frequency slightly above the superradiance limit. In addition, nearly extremal Kerr-MOG black holes can be continuously driven to extremality by test fields. Next, we employ backreaction effects based on the argument that the angular velocity of the event horizon increases before the absorption of the test field. Incorporating the backreaction effects, we derive that the weak form of the cosmic censorship and the third law are both valid for Kerr-MOG black holes with a modification parameter alpha lesssim 0.03, which includes the Kerr case with alpha =0.

GRAMSES: a new route to general relativistic N-body simulations in cosmology. Part I. Methodology and code description

We present GRAMSES, a new pipeline for nonlinear cosmological N-body simulations in General Relativity (GR). This code adopts the Arnowitt-Deser-Misner (ADM) formalism of GR, with constant mean curvature and minimum distortion gauge fixings, which provides a fully nonlinear and background independent framework for relativistic cosmology. Employing a fully constrained formulation, the Einstein equations are reduced to a set of ten elliptical equations which are solved using multigrid relaxation with adaptive mesh refinements (AMR), and three hyperbolic equations for the evolution of tensor degrees of freedom. The current version of GRAMSES neglects the latter by using the conformal flatness approximation, which allows it to compute the two scalar and two vector degrees of freedom of the metric. In this paper we describe the methodology, implementation, code tests and first results for cosmological simulations in a ΛCDM universe, while the generation of initial conditions and physical results will be discussed elsewhere. Inheriting the efficient AMR and massive parallelisation infrastructure from the publicly-available N-body and hydrodynamic simulation code RAMSES, GRAMSES is ideal for studying the detailed behaviour of spacetime inside virialised cosmic structures and hence accurately quantifying the impact of backreaction effects on the cosmic expansion, as well as for investigating GR effects on cosmological observables using cosmic-volume simulations.

Multi-time measurements in Hawking radiation: information at higher-order correlations

It is believed that no information can be stored in Hawking radiation, because correlations between quanta of different field modes vanish. However, such correlations have been defined only with reference to a single moment of time. In this article, we develop a method for the evaluation of multi-time correlations. We find that these correlations are highly non-trivial: for a scalar field in the Schwarzschild black hole, multi-time correlations have an explicit dependence on angular variables and on the scattering history of Hawking quanta. This result leads us to the conjecture that some pre-collapse information can be stored in multi-time correlations after backreaction effects have been incorporated in the physical description.

Waves of space–time from a collapsing compact object

Abstract We study the partial time dependent collapse of a spherically symmetric compact object with initial mass M 1 + M 2 and final mass M 2 and the waves of space–time emitted during the collapse via back-reaction effects. We obtain exact analytical solutions for the waves of space–time in an example in which M 1 = M 2 = ( M 1 + M 2 ) ∕ 2 . The wavelengths of the space–time emitted waves during the collapse have the cut (we use natural units c = ħ = 1 ): λ ( 2 ∕ b ) , ( 1 ∕ b ) -being the time scale that describes the decay of the compact object.

Backreaction on an infinite helical cosmic string

To understand the properties of a possible cosmic string network requires knowledge of the structures on long strings, which control the breaking off of smaller loops. These structures are influenced by backreaction due to gravitational wave emission. To gain an understanding of this process, we calculate the effect of gravitational backreaction on the "helical breather": an infinite cosmic string with a helical standing wave. We do the calculation analytically to first order in the amplitude of the oscillation. Our results for the rate of loss of length agree with the total power emitted from this system as calculated by Sakellariadou. We also find a rotation of the generators of the string that leads to an advancement of the phase of the oscillation in addition to that produced by the loss of length.

Superhorizon effects on the gauge invariant effective energy density in f(R) gravity

In this article we study the gauge invariant effective energy momentum tensor for cosmological perturbations in f(R) gravity during the inflationary epoch. Considering the super-horizon regime, we derive the effective energy density up to the one-loop corrections. It describes the back-reaction effect of the fluctuations on the background space-time.

Ghost-free modification of the Polyakov action and Hawking radiation

In this paper we discuss possible effects of non-locality in black hole spacetimes. We consider a two-dimensional theory in which the action describing matter is a ghost-free modification of the Polyakov action. For this purpose we write the Polyakov action in a local form by using an auxiliary scalar field and modify its kinetic term by including into it a non-local ghost-free form factor. We demonstrate that the effective stress-energy tensor is modified and we study its properties in a background of a two-dimensional black hole. We obtain the expression for the contribution of the ghost-free auxiliary field to the entropy of the black hole. We also demonstrate that if the back-reaction effects are not taken into account, such a ghost-free modification of the theory does not change the energy flux of the Hawking radiation measured at infinity. We illustrate the discussed properties for black hole solution of a 2D dilaton gravity model which admits a rather complete analytical study.

Monte Carlo modelling of particle acceleration in collisionless shocks with effective mean electric field

Relativistic particle acceleration in collisionless shocks of supernova remnants is accompanied with magnetic field amplification by cosmic ray (CR) driven plasma instabilities. Bell’s fast CR-current instability is predicted to produce turbulence with a non-zero mean electric field in the shock precursor. We present a Monte Carlo model of Fermi shock acceleration explicitly taking into account an effective mean upstream electric field. Our model is nonlinear and includes the backreaction effects of efficient Fermi acceleration on the shock structure.

Quantum thermodynamics in a static de Sitter space-time and initial state of the universe

Using Relativistic Quantum Geometry we study back-reaction effects of space-time inside the causal horizon of a static de Sitter metric, in order to make a quantum thermodynamical description of space-time. We found a finite number of discrete energy levels for a scalar field from a polynomial condition of the confluent hypergeometric functions expanded around r=0. As in the previous work, we obtain that the uncertainty principle is valid for each energy level on sub-horizon scales of space-time. We found that temperature and entropy are dependent on the number of sub-states on each energy’s level and the Bekenstein–Hawking temperature of each energy level is recovered when the number of sub-states of a given level tends to infinity. We propose that the primordial state of the universe could be described by a de Sitter metric with Planck energy E_p=m_p,c^2, and a B–H temperature: T_{BH}=left( frac{hbar ,c}{2pi ,l_p,K_B}right) .

Conductivity of the one-dimensional holographic p -wave superconductors in the presence of nonlinear electrodynamics

We investigate analytically as well as numerically effects of nonlinear Born-Infeld (BI) electrodynamics on the properties of (1+1)-dimensional holographic $p$-wave superconductor in the context of gauge/gravity duality. We consider the case in which the gauge and vector fields backreact on the background geometry. We apply the Sturm-Liouville eigenvalue problem for the analytical approach as well as the shooting method for the numerical calculations. In both methods, we find out the relation between critical temperature $T_{c}$ and chemical potential $\mu$ and show that both approaches are in good agreement with each other. We find that if one strengthen the effect of backreaction as well as nonlinearity, the critical temperature decreases which means that the condensation is harder to form. We also explore the conductivity of the one-dimensional holographic $p$-wave superconductor for different values of b and $T/T_{c}$. We find out that the real and imaginary parts of the conductivity have different behaviors in higher dimensions. The effects of different values of temperature is more apparent for larger values of nonlinearity parameter. In addition, for the fixed value of $T/T_{c}$ by increasing the effect of nonlinearity we observe larger values for Drude-like peak in real part of conductivity and deeper minimum for imaginary part.

Exponential collapse with variable time scale driven by a scalar field

Abstract We study the dynamic collapse driven by a scalar field, when a relativistic observer falls co-moving with the collapse and cross the horizon of a Schwarzschild black-hole (BH), at t = t 0 . During the collapse the scale of time is considered as variable. Back-reaction effects and gravitational waves produced during the exponential collapse are studied. We demonstrate that back-reaction effects act as the source of gravitational waves emitted during the collapse, and wavelengths of gravitational waves (GW) are in the range: λ ≪ r s ≡ e − 2 h 0 t 0 2 h 0 , that is, smaller than the Schwarzschild radius. We demonstrate that during all the collapse the global topology of the space–time remains hyperbolic when the observer cross the horizon.

Simulating galaxy formation in f(R) modified gravity: matter, halo, and galaxy statistics

ABSTRACT We present an analysis of the matter, halo, and galaxy clustering in f(R)-gravity employing the SHYBONE full-physics hydrodynamical simulation suite. Our analysis focuses on the interplay between baryonic feedback and f(R)-gravity in the matter power spectrum, the matter and halo correlation functions, the halo and galaxy–host–halo mass function, the subhalo and satellite–galaxy count, and the correlation function of the stars in our simulations. Our studies of the matter power spectrum in full-physics simulations in f(R)-gravity show that it will be very difficult to derive accurate fitting formulae for the power spectrum enhancement in f(R)-gravity which include baryonic effects. We find that the enhancement of the halo mass function due to f(R)-gravity and its suppression due to feedback effects do not show significant back-reaction effects and can thus be estimated from independent general relativity-hydro and f(R) dark matter only simulations. Our simulations furthermore show that the number of subhaloes and satellite-galaxies per halo is not significantly affected by f(R)-gravity. Low-mass haloes are nevertheless more likely to be populated by galaxies in f(R)-gravity. This suppresses the clustering of stars and the galaxy correlation function in the theory compared to standard cosmology.

Cosmological backreaction in spherical and plane symmetric dust-filled space-times

We examine the implementation of Buchert’s and Green & Wald’s averaging formalisms in exact spherically symmetric and plane symmetric dust-filled cosmological models. We find that, given a cosmological space-time, Buchert’s averaging scheme gives a faithful way of interpreting the large-scale expansion of space, and explicit terms that precisely quantify deviations from the behaviour expected from the Friedmann equations of homogeneous and isotropic cosmological models. The Green & Wald formalism, on the other hand, does not appear to yield any information about the large-scale properties of a given inhomogeneous space-time. Instead, this formalism is designed to calculate the back-reaction effects of short-wavelength fluctuations around a given ‘background’ geometry. We find that the inferred expansion of space in this approach is entirely dependent on the choice of this background, which is not uniquely specified for any given inhomogeneous space-time, and that the ‘back-reaction’ from small-scale structures vanishes in every case we study. This would appear to limit the applicability of Green & Wald’s formalism to the study of large-scale expansion in the real Universe, which also has no pre-defined background. Further study is required to enhance the evaluation and comparison of these averaging formalisms, and determine whether the same difficulties exist, in less idealized space-time geometries.

Formation of rocky and icy planetesimals inside and outside the snow line: effects of diffusion, sublimation, and back-reaction

Context. Streaming instability is a possible mechanism to form icy planetesimals. It requires special local conditions such as a high solid-to-gas ratio at the midplane and typically more than a centimeter in size (Stokes number &gt;0.01). Silicate grains cannot grow to such a size through pairwise collisions. It is important to clarify where and when rocky and icy planetesimals are formed in a viscously evolving disk. Aims. We wish to understand how local runaway pile-up of solids (silicate and water ice) occurs inside or outside the snow line. Methods. We assumed an icy pebble contains micron-sized silicate grains that are uniformly mixed with ice and are released during the ice sublimation. Using a local one-dimensional code, we solved the radial drift and the turbulent diffusion of solids and the water vapor, taking account of their sublimation and condensation around the snow line. We systematically investigated the effects of back-reactions of solids to gas on the radial drift and diffusion of solids, scale height evolution of the released silicate particles, and possible differences in effective viscous parameters between those for turbulent diffusion (αtur) and those for the gas accretion rate onto the central star (αacc). We also studied the dependence on the ratio of the solid mass flux to the gas (Fp/g). Results. We show that the favorable locations for the pile-up of silicate grains and icy pebbles are the regions in the proximity of, both inside and outside, the water snow line, respectively. We find that runaway pile-ups occur when both the back-reactions for radial drift and diffusion are included. In the case with only the back-reaction for the radial drift, runaway pile-up is not found except in extremely high pebble flux, while the condition of streaming instability can be satisfied for relatively large Fp/g as found in the past literature. If the back-reaction for radial diffusion is considered, the runaway pile-up occurs for a reasonable value of pebble flux. The runaway pile-up of silicate grains that would lead to formation of rocky planetesimals occurs for αtur ≪ αacc, while the runaway pile-up of icy pebbles is favored for αtur ~ αacc. Based on these results, we discuss timings and locations of rocky and icy planetesimals in an evolving disk.

Dark Energy Phenomenon from Backreaction Effect

In this paper, we interpret the dark energy phenomenon as an averaged effect caused by small scale inhomogeneities of the universe with the use of the spatial averaged approach of Buchert. Two models are considered here, one of which assumes that the backreaction term and the averaged spatial Ricci scalar obey the scaling laws of the volume scale factor at adequately late times, and the other one adopts the ansatz that the backreaction term is a constant in the recent universe. Thanks to the effective geometry introduced by Larena et al. in their previous work, we confront these two backreaction models with latest type Ia supernova and Hubble parameter observations, coming out with the results that the constant backreaction model is slightly favoured over the other model and the best fitting backreaction term in the scaling backreaction model behaves almost like a constant. Also, the numerical results show that the constant backreaction model predicts a smaller expansion rate and decelerated expansion rate than the other model does at redshifts higher than about 1, and both backreaction terms begin to accelerate the universe at a redshift around 0.5.

Quantum ergosphere and brick wall entropy

We revisit the “brick wall” model for black hole entropy taking into account backreaction effects on the horizon structure. We do so by adopting an evaporating metric in the quasi-static approximation in which departures from the standard Schwarzschild metric are governed by a small luminosity factor. One of the effects of the backreaction is to create an ergosphere-like region which naturally tames the usual divergence in the calculation of the partition function of the field. The black hole luminosity sets the width of such ‘quantum ergosphere’. We find a finite horizon contribution to the entropy which, for the luminosity associated to the Hawking flux, reproduces remarkably well the Bekenstein-Hawking entropy-area law.