Growth Of Density Fluctuations Research Articles

The growth of the density fluctuations in the scale-invariant vacuum theory

The growth of the density fluctuations is considered to be an important cosmological test. In the standard model, for a matter dominated universe, the growth of the density perturbations evolves with redshift z like (11+z)s with s=1. This is not fast enough to form galaxies and to account for the observed present-day inhomogeneities. This problem is usually resolved by assuming that at the recombination epoch the baryons settle down in the potential well of the dark matter previously assembled during the radiation era of the universe. This view is challenged in the present paper by using the recently proposed model of a scale-invariant framework for cosmology, the Scale-Invariant Vacuum Theory (Maeder 2017a,2017b,2017c), that enlarges the invariance group subtending the theory of the gravitation. From the continuity equation, the Euler and Poisson equations are written in the scale-invariant framework, the equation governing the growth of density fluctuations δ is obtained. Starting from δ=10−5 at a redshift around 1000, numerical solutions for various density background are obtained. The growth of density fluctuations is much faster than in the standard EdS model. The s values are in the range from 2.7 to 3.9 for Ωm between 0.30 and 0.02. This enables the density fluctuations to enter the nonlinear regime with δ>1 long before the present time, typically at redshifts of about 10, without requiring the presence of dark matter.

Quintessential scale dependence from separate universe simulations

By absorbing fluctuations into a local background, separate universe simulations provide a powerful technique to characterize the response of small-scale observables to the long-wavelength density fluctuations, for example those of the power spectrum and halo mass function which lead to the squeezed-limit $n$-point function and halo bias, respectively. Using quintessence dark energy as the paradigmatic example, we extend these simulation techniques to cases where non-gravitational forces in other sectors establish a Jeans scale across which the growth of density fluctuations becomes scale dependent. By characterizing the separate universes with matching background expansion histories, we show that the power spectrum and mass function responses depend on whether the long-wavelength mode is above or below the Jeans scale. Correspondingly, the squeezed bispectrum and halo bias also become scale dependent. Models of bias that are effectively local in the density field at a single epoch, initial or observed, cannot describe this effect which highlights the importance of temporal nonlocality in structure formation. Validated by these quintessence tests, our techniques are applicable to a wide range of models where the complex dynamics of additional fields affect the clustering of matter in the linear regime and it would otherwise be difficult to simulate their impact in the nonlinear regime.

Constraints on the neutrino parameters by future cosmological 21 cm line and precise CMB polarization observations

Observations of the 21 cm line radiation coming from the epoch of reionization have a great capacity to study the cosmological growth of the Universe. Besides, CMB polarization produced by gravitational lensing has a large amount of information about the growth of matter fluctuations at late time. In this paper, we investigate their sensitivities to the impact of neutrino property on the growth of density fluctuations, such as the total neutrino mass, the effective number of neutrino species (extra radiation), and the neutrino mass hierarchy. We show that by combining a precise CMB polarization observation such as Simons Array with a 21 cm line observation such as Square kilometer Array (SKA) phase 1 and a baryon acoustic oscillation observation (Dark Energy Spectroscopic Instrument:DESI) we can measure effects of non-zero neutrino mass on the growth of density fluctuation if the total neutrino mass is larger than 0.1 eV. Additionally, the combinations can strongly improve errors of the bounds on the effective number of neutrino species σ (Nν) ∼ 0.06−0.09 at 95 % C.L.. Finally, by using SKA phase 2, we can determine the neutrino mass hierarchy at 95 % C.L. if the total neutrino mass is similar to or smaller than 0.1 eV.

Growth of spinodal instabilities in nuclear matter. II. Asymmetric matter

As an extension of our previous work, the growth of density fluctuations in the spinodal region of charge asymmetric nuclear matter is investigated in the basis of the stochastic mean-field approach in the non-relativistic framework. A complete treatment of density correlation functions are presented by including collective modes and non-collective modes as well.

TOPOLOGY OF A LARGE-SCALE STRUCTURE AS A TEST OF MODIFIED GRAVITY

The genus of the iso-density contours is a robust measure of the topology of large scale structure, and it is relatively insensitive to nonlinear gravitational evolution, galaxy bias and redshift-space distortion. We show that the growth of density fluctuations is scale-dependent even in the linear regime in some modified gravity theories, which opens a new possibility of testing the theories observationally. We propose to use the genus of the iso-density contours, an intrinsic measure of the topology of large scale structure, as a statistic to be used in such tests. In Einstein's general theory of relativity, density fluctuations are growing at the same rate on all scales in the linear regime, and the genus per comoving volume is almost conserved as structures are growing homologously, so we expect that the genus-smoothing scale relation is basically time-independent. However, in some modified gravity models where structures grow with different rates on different scales, the genus-smoothing scale relation should change over time. This can be used to test the gravity models with large scale structure observations. We studied the case of the f(R) theory, DGP braneworld theory as well as the parameterized post-Friedmann (PPF) models. We also forecast how the modified gravity models can be constrained with optical/IR or redshifted 21cm radio surveys in the near future.

Quantal description of instabilities in nuclear matter in a stochastic relativistic model

Spinodal instabilities and early development of density fluctuations are investigated in the stochastic extension of Walecka-type relativistic mean field including non-linear self-interactions of scalar mesons in the quantal framework. Calculations indicate that at low temperatures T = 0–2 MeV, the initial growth of density fluctuations and hence the initial condensation mechanism occur much faster in quantal calculations than those found in the semi-classical framework. However, at higher temperatures T = 4–5 MeV, semi-classical calculations provide a good approximation for quantal description. Calculations show that the typical size of initial condensation regions is not very sensitive to the temperature, but depends on the initial baryon density. These findings are consistent with previous investigations carried out in the non-relativistic approach with an effective Skyrme interaction.

Note on “Continuous matter creation and the acceleration of the universe: the growth of density fluctuations”

Recently, de Roany & Pacheco (2010) performed a Newtonian analysis on the evolution of perturbations for a class of relativistic cosmological models with Creation of Cold Dark Matter (CCDM) proposed by the present authors. In this note we demonstrate that the basic equations adopted in their work do not recover the specific (unperturbed) CCDM model. Unlike to what happens in the original CCDM cosmology, their basic conclusions refer to a decelerating cosmological model in which there is no transition from a decelerating to an accelerating regime as required by SNe type Ia and complementary observations.

Continuous matter creation and the acceleration of the universe: the growth of density fluctuations

Cosmologies including continuous matter creation are able to reproduce the main properties of the standard ΛCDM model, in particular in cases where the particle and entropy production rates are equal. These specific models, characterized by a mass density equal to the critical value, behave like the standard ΛCDM model at early times whereas their late evolution is similar to the steady-state cosmology. The maximum amplitude of density fluctuations in these models depends on the adopted creation rate, related here to the parameter Ω v and this limitation could be a difficulty for the formation of galaxies and large-scale structure in this class of universe. Additional problems are related with predictions either of the random peculiar velocities of galaxies or the present density of massive clusters of galaxies, both being largely overestimated with respect to observational data.

Matter density fluctuations in the running ΛCDM and ΛXCDM models

We have investigated the matter density fluctuations, δM (a ), in the running ΛCDM and ΛXCDM models. The latter was proposed as an interesting solution to the cosmic coincidence problem. It includes an extra dynamical component, the “cosmon” X , which interacts with a running Λ, but not with matter. Adopting a dark energy (DE) “picture”, in which the total DE and matter components are conserved separately, the growth of density fluctuations = δM (a )/a can be written in terms of the effective equation of state. We made use of the measured galaxy fluctuation power spectrum, P GG , and of the linear bias parameter b 2 (z = 0) = P GG /P MM , where P MM ∝ δM 2 is the matter power spectrum of the model under consideration. According to the 2dFGRS survey, b Λ 2 (z 0) = 1 within a 10% accuracy for the ΛCDM model. We adopted this limit to put constraints on the fundamental parameter, ν , of the running ΛCDM model. We found an agreement with previous estimates obtained by a number of different methods and authors. This provided a good test of the procedure, which we used then to determine the physical region of the ΛXCDM parameter space.

CMB-induced cluster polarization as a cosmological probe

Scattering of the temperature anisotropy quadrupole by free electrons in galaxy clusters leads to a secondary polarization signal in the cosmic microwave background (CMB) fluctuations. At low redshifts, the temperature quadrupole contains a significant contribution from the integrated Sachs–Wolfe (ISW) effect associated with the growth of density fluctuations. Using polarization data from a sample of clusters over a wide range in redshift, one can statistically establish the presence of the ISW effect and determine its redshift evolution. Given the strong dependence of the ISW effect on the background cosmology, cluster polarization can eventually be used as a powerful probe of dark energy. As a further application, we also discuss how it might be used to understand the potential lack of power on large scales.

CMB polarization towards clusters as a probe of the integrated Sachs-Wolfe effect

The scattering of the temperature anisotropy quadrupole by free electrons in galaxy clusters leads to a now well-known polarization signal in the cosmic microwave background fluctuations. Using multifrequency polarization data, one can extract the temperature quadrupole and separate it from the contaminant polarization associated with the kinematic quadrupole due to the transverse motion of clusters. At low redshifts, the temperature quadrupole contains a significant contribution from the integrated Sachs-Wolfe effect (ISW) associated with the growth of density fluctuations. Using polarization data from a sample of clusters over a wide range in redshift, one can statistically establish the presence of the ISW effect and determine the redshift dependence of the ISW contribution to the rms quadrupole. Given the strong dependence of the ISW effect on the background cosmology, the cluster polarization can eventually be used as a probe of the dark energy.

Theory of coherent photoassociation of a Bose-Einstein condensate

We study coherent photoassociation, phenomena analogous to coherent optical transients in few-level systems, which may take place in photoassociation of an atomic Bose-Einstein condensate but not in a nondegenerate gas. We develop a second-quantized Hamiltonian to describe photoassociation, and apply the Hamiltonian both in the momentum representation and in the position representation (field theory). Solution of the two-mode problem including only one mode each for the atomic and molecular condensates displays analogs of Rabi oscillations and rapid adiabatic passage. A classical version of the field theory for atoms and molecules is used to demonstrate that, in the presence of photoassociating light, a joint-atom molecule is unstable against growth of density fluctuations. Experimental complications, including spontaneous emission and unwanted "rogue" photodissociation from a photoassociated molecule are analyzed. A two-color Raman scheme is studied as a method to set up an effective two-mode scheme with reduced spontaneous emission losses. We discuss photoassociation rates and photoassociation Rabi frequencies for high-lying vibrational states in alkalis both on the basis of molecular-structure calculations, and by comparing with an experiment [Wynar et al., Science 287, 1016 (2000)].

Simulating the formation of large-scale cosmic structure with particle-grid methods

Numerical simulations are an essential component of the study of the nonlinear growth and evolution of cosmic structures. After a brief description of the standard cosmological model, this work sets up the framework within which we can study the gravitational growth of density fluctuations in the universe and motivates the use of particle methods for the numerical modelling of these processes. The second part contains a description of the numerical techniques used to solve Poisson's equation for a very large number of self-gravitating particles in the cosmological context. The work concentrates on the common grid-based methods that are widely used for the largest cosmological simulations, and discusses the migration of these algorithms to parallel computer architectures.

Dilatometric Studies Across the Isotropic to Smectic-F Transition

Abstract Thermal Microscopy (TM), Differential Scanning Calorimetry (DSC) and dilatometric studies were carried out to characterise the liquid crystalline phases and mesomorphic transitions exhibited by three higher homologues of N-(p-n-dodecyloxy benzylidene)p-n-alkyl anilines, viz., N-(p-n-dodecyloxy benzylidene)p-n-dodecyl aniline, (12O.12), N-(p-n-dodecyloxy benzylidene)p-n-tetradecyl aniline, (12O.14) and (N-(p-n-dodecyloxy benzylidene)p-n-hexadecyl aniline, (12O.16). The three com-pounds exhibit enantiotropic smectic-F and smectic-G liquid crystalline phases between the isotropic liquid and the solid crystal. The change in density across the phase transformations and the thermal expansion coefficient confirm the first order nature of the isotropic to smectic-F (I-SmF) and smectic-F to smectic-G (SmF-SmG) transitions. The dilatometric studies infer strong pre-transitional fluctuations in a relatively large thermal range. The pre-transitional effects are estimated by αeff in the vicinity of the mesomorphic fluctuation-dominated non-linear region (FDNLR) of the I-SmF transition. The strength of non linear growth of density fluctuations is discussed in the wake of reports at the I-N, I-SmA and I-SmC transitions.

Evolution of the two-point correlation function in the Zel'dovich approximation

We study the evolution of the mass autocorrelation function by describing the growth of density fluctuations through the Zel'dovich approximation. The results are directly compared with the predictions of the scaling hypothesis for clustering evolution extracted from numerical simulations (Hamilton et al. 1991), as implemented by Jain, Mo & White (1995). We find very good agreement between the correlations on mildly non-linear scales and on completely linear scales. In between these regimes, we note that the density fields evolved through the Zel'dovich approximation show more non-linear features than predicted by the scaling ansatz which is, however, forced to match the linear evolution on scales larger than the simulation box. In any case, the scaling ansatz by Baugh & Gaztanaga (1996), calibrated against large box simulations agrees better with ZA predictions on large scales, keeping good accuracy also on intermediate scales. We show that mode-coupling is able to move the first zero crossing of xi(r) as time goes on. A detailed fit of the time dependence of this shifting is given for a CDM model. The evolution of the cross correlation of the density fluctuation field evaluated at two different times is also studied. The possible implications of the results for the analysis of the observed correlation function of high redshift galaxies are discussed.

Previrialization: Perturbative and N-Body Results

view Abstract Citations (59) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Previrialization: Perturbative and N-Body Results Lokas, E. L. ; Juszkiewicz, R. ; Bouchet, F. R. ; Hivon, E. Abstract We present a series of N-body experiments which confirm the reality of the previrialization effect. We use also a weakly nonlinear perturbative approach to study the phenomenon. These two approaches agree when the rms density contrast, σ, is small; more surprisingly, they remain in agreement when σ ~ 1. When the slope of the initial power spectrum is n > -1, nonlinear tidal interactions slow down the growth of density fluctuations, and the magnitude of the suppression increases when n (i.e., the relative amount of small-scale power) is increased. For n < -1, we see an opposite effect: the fluctuations grow more rapidly than in linear theory. The transition occurs at n = -1 when the weakly nonlinear correction to σ is close to zero and the growth rate is close to linear. Our results resolve recent controversy between two N- body studies of previrialization. Peebles assumed n = 0 and found strong evidence in support of previrialization, while Evrard & Crone, who assumed n = -1, reached opposite conclusions. As we show here, the initial conditions with n = -1 are rather special because the nonlinear effects nearly cancel out for that particular spectrum. In addition to our calculations for scale-free initial spectra, we show results for a more realistic spectrum of Peacock & Dodds. Its slope near the scale usually adopted for normalization is close to -1, so σ is close to linear. Our results retroactively justify linear normalization at 8 h^-1^ Mpc while also demonstrating the danger and limitations of this practice. Publication: The Astrophysical Journal Pub Date: August 1996 DOI: 10.1086/177580 arXiv: arXiv:astro-ph/9508032 Bibcode: 1996ApJ...467....1L Keywords: COSMOLOGY: THEORY; GALAXIES: FORMATION; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; Astrophysics E-Print: Significantly revised, 25 pages, uuencoded compressed postscript, figures included, to appear in ApJ full text sources arXiv | ADS |

Matter accretion by cosmic string loops and wakes.

We develop quantative methods for the study of structure formation with cosmic strings in an expanding universe. The gravitational effects of arbitrary string configurations are calculated using linearized gravity. The growth of density fluctuations is then studied using these gravitational forces as a source term in the Zeldovich approximation. We use either the adhesion modification or an N-body tree code to project this fluctuation growth into the nonlinear regime. These methods are applied to specific loop and long string solutions beginning at equal matter and radiation ${\mathit{t}}_{\mathrm{eq}}$ on scales corresponding to about 10 Mpc today (h=0.5). We reproduce analytic results for spherical and planar collapse. We show that these methods are applicable to accretion about closed oscillating loops and in the wakes of moving long strings which possess significant small-scale structure, quantitatively confirming the wiggly string approximation with a renormalized string energy density \ensuremath{\mu}\ifmmode \tilde{}\else \~{}\fi{}. We demonstrate the efficiency of the fragmentation of wakes created by wiggly strings by the present day. These methods are sufficiently computationally efficient to employ in the study of an evolving string network. For the cosmic string scenario, we conclude that reliable quantitative predictions must take into account non-Gaussianity, vorticity generation, and nonlinear fragmentation effects.

The growth of density fluctuations in a simplified model of extended inflation

The authors reduce the cosmological model of extended inflation to a spatially flat Robertson-Walker geometry driven by a self-interacting scalar field with a potential of an exponential type. They quantize the gauge invariant scalar perturbations of the metric and the field, and compute the spectrum of the energy density fluctuations.

Non-Linear Growth of Density Fluctuations in the Spherically Symmetric Expanding Universe and Catastrophe Theory

On etudie la croissance non lineaire des fluctuations de densite dans le systeme de symetrie spherique de particules froides sans collision. Le spectre de Fourier de distribution de densite obeit a la loi puissance apres la premiere apparition de singularite et la valeur de l'indice de puissance est la meme independamment des conditions initiales et ne change pas en fonction du temps. Ceci parce que la valeur de l'indice de puissance n'est determinee que par le type de singularite de la densite qui est classifie d'apres la theorie des catastrophes, et le type A2 apparait dans ce systeme. Cette caracteristique est la meme que celle pour un systeme unidimensionnel dans l'univers en expansion

Fluid mixtures in narrow cylindrical pores: Computer simulation and theory

We discuss the simulation results of phase separation of a binary Lennard-Jones mixture in a cylindrical pore induced by a temperature quench. The liquid-vapor phase separation proceeds in two stages involving different time scales. First, following the growth of density fluctuations, mechanical equilibrium is rapidly established when the system splits into a dense and a dilute phase. Material equilibrium, however, is reached via the mutual diffusion of the two components and this proceeds on an appreciably longer time scale. We briefly address the rounding of a first-order phase transition in a cylinder. In particular, we explore the possibility of multiple domains of gas and liquid when the aspect ratio is very large. Finally, we introduce an extension of Tarazona's nonlocal density function to binary mixtures of arbitrary size. The new theory is successfully tested against simulations of an additive hard-sphere mixture against a hard wall.