Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe inner structure of Zeldovich pancakes
51
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The evolution of a planar perturbation in a Einstein-de Sitter Universe is studied using a previously introduced Lagrangian scheme. An approximate discrete dynamical system is derived, which describes the mass agglomeration process qualitatively. Quantitative predictions for the late density profile are obtained therefrom, and validated by numerical simulations. The main result is a scaling regime for the density profile of a collapsing object of mass $M$ around cosmological coordinate $r^*$, $rho(r)sim frac{M}{d}(frac{|r-r^*|}{d})^{-{1/4}}$. The characteristic scale of the agglomeration, $dsim (t/t_0)^{{4/9}}$, is an increasing function of cosmological time $t$. The major part of the mass hence always lies in a region with decreasing mass density. This shows that one-dimensional self-gravitating motion is not sufficient to effectively drive structure formation in an Einstein-de Sitter Universe. These results are compared with analogous investigations for the adhesion model (Burgers equation with positive viscosity), where the agglomeration is faster, and one-dimensional dynamics is effective. We further study the mutual motion of two mass agglomerations, and show that they oscillate around each other for long times, like two ``heavy particles. Individual particles in the two agglomerations do not mix effectively on the time scale of the interagglomeration motion.
rate research
Read More
The present day universe consists of galaxies, galaxy clusters, one-dimensional filaments and two-dimensional sheets or pancakes, all of which combine to form the cosmic web. The so called Zeldovich pancakes, are very difficult to observe, because their overdensity is only slightly greater than the average density of the universe. Falco et al (2014) presented a method to identify Zeldovich pancakes in observational data, and these were used as a tool for estimating the mass of galaxy clusters. Here we expand and refine that observational detection method. We study two pancakes on scales of 10 Mpc, identified from spectroscopically observed galaxies near the Coma cluster, and compare with twenty numerical pancakes. We find that the observed structures have velocity dispersion about 100 km/sec, which is relatively low compared to typical groups and filaments. These velocity dispersions are consistent with those found for the numerical pancakes. We also confirm that the identified structures are in fact two-dimensional structures. Finally, we estimate the stellar to total mass of the observational pancakes to be $2 times 10^{-4}$, within one order of magnitude, which is smaller than that of clusters of galaxies.
If the large scale structure of the Universe was created, even partially, via Zeldovich pancakes, than the fluctuations of the CMB radiation should be formed due to bulk comptonization of black body spectrum on the contracting pancake. Approximate formulas for the CMB energy spectrum after bulk comptonization are obtained. The difference between comptonized energy spectra of the CMB due to thermal and bulk comptonization may be estimated by comparison of the plots for the spectra in these two cases.
One of the components of the cosmic web are sheets, which are commonly referred to as Zeldovich pancakes. These are structures which have only collapsed along one dimension, as opposed to filaments or galaxies and cluster, which have collapsed along two or three dimensions. These pancakes have recently received renewed interest, since they have been shown to be useful tools for an independent method to determine galaxy cluster masses. We consider sheet-like structures resulting from cosmological simulations, which were previously used to establish the cluster mass determination method, and we show through their level of equilibration, that these structures have indeed only collapsed along the one dimension. We also extract the density profiles of these pancake, which agrees acceptably well with theoretical expectations. We derive the observable velocity distribution function (VDF) analytically by generalizing the Eddington method to one dimension, and we compare with the distribution function from the numerical simulation.
Network topologies can be non-trivial, due to the complex underlying behaviors that form them. While past research has shown that some processes on networks may be characterized by low-order statistics describing nodes and their neighbors, such as degree assortativity, these quantities fail to capture important sources of variation in network structure. We introduce a property called transsortativity that describes correlations among a nodes neighbors, generalizing these statistics from immediate one-hop neighbors to two-hop neighbors. We describe how transsortativity can be systematically varied, independently of the networks degree distribution and assortativity. Moreover, we show that it can significantly impact the spread of contagions as well as the perceptions of neighbors, known as the majority illusion. Our work improves our ability to create and analyze more realistic models of complex networks.
We present a new technique which allows the fully {em ab initio} calculation of the chemical potential of a substitutional impurity in a high-temperature crystal, including harmonic and anharmonic lattice vibrations. The technique uses the combination of thermodynamic integration and reference models developed recently for the {em ab initio} calculation of the free energy of liquids and anharmonic solids. We apply the technique to the case of the substitutional oxygen impurity in h.c.p. iron under Earths core conditions, which earlier static {em ab initio} calculations indicated to be thermodynamically very unstable. Our results show that entropic effects arising from the large vibrational amplitude of the oxygen impurity give a major reduction of the oxygen chemical potential, so that oxygen dissolved in h.c.p. iron may be stabilised at concentrations up a few mol % under core conditions.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
