In physical cosmology, structural formation is the formation of galaxies, clusters of galaxies and structures larger than small initial density fluctuations. The universe, as it is now known from observations of cosmic microwave background radiation, began in a hot, dense, almost uniform state about 13.8 billion years ago. However, looking at the sky today, we see structures on all scales, from stars and planets to galaxies and, on a larger scale, galaxy clusters and galactic structures similar to sheets separated by large cavities containing few galaxies. The formation of the structure tries to model how this structure is formed by the gravitational instability of the small initial density ripple.
Modern Lambda-CDM models have successfully predicted the observed large-scale distribution of galaxies, clusters and voids; but on the scale of individual galaxies there are many complications due to nonlinear processes involving baryonic physics, heating and cooling of gases, star formation and feedback. Understanding the process of galaxy formation is a major topic of modern cosmology research, both through observations such as Hubble Ultra-Deep Field and through large computer simulations.
Video Structure formation
Overview
Under these models, the structure of the universe appears to be formed in the following stages:
The very early universe
At this stage, some mechanisms, such as cosmic inflation, are responsible for determining the initial conditions of the universe: homogeneity, isotropy, and flatness. Cosmic inflation will also increase minute quantum fluctuations (pre-inflation) into less ripple overdensity and underdensity (post-inflation) density.
Growth structure
The early universe was dominated by radiation; in this case, the greater density fluctuations of the cosmic horizon grow in proportion to the scale factor, since the gravitational potential fluctuations remain constant. The smaller structure of the horizon basically remains frozen because the dominance of the radiation inhibits growth. As the universe expands, radiation density falls faster than matter (due to redshifting of photon energy); this causes a cross called radiation-material equation at ~ 50,000 years after the Big Bang. After this all dark matter ripples can grow freely, forming the seeds where the baryon will later fall. The size of the universe in this day and age forms a turnaround in the spectrum of measurable material forces in large redshift surveys.
Recombination
The universe is dominated by radiation for most of this stage, and because of intense heat and radiation, hydrogen and primordial helium are fully ionized into nuclei and free electrons. In this hot and dense situation, radiation (photons) can not travel long before Thomson scatters electrons. The universe is very hot and dense, but it expands rapidly and therefore cools. Finally, at less than 400,000 years after the 'bang', it becomes quite cold (about 3000 K) for protons to capture negatively charged electrons, forming neutral hydrogen atoms. (Helium atoms are formed somewhat earlier because of their larger binding energy). After almost all charged particles are bonded in neutral atoms, the photons no longer interact with them and are free to spread over the next 13.8 billion years; Currently we detect the photons are revived by a factor of 1090 to 2,725 K as the Cosmic Microwave Background Radiation (CMB) that fills the current universe. Some exceptionally ground-based space missions (COBE, WMAP, Planck), have detected very small variations in CMB density and temperature. This variation is subtle, and CMB looks almost uniformly the same in every direction. However, slight temperature variations from the order of some parts in 100,000 are very important, since they are essentially the early "seeds" from which all subsequent complex structures in the universe eventually develop.
The theory of what happens after the first 400,000 years of the universe is one form of hierarchical structure: the smaller structures that are gravitationally bound like the tops of matter containing the first stars and the first formed star clusters, and these then join the gas and dark matter to form galaxies, followed by groups, groups and superclusters of galaxies.
Maps Structure formation
The very early universe
The early universe was still a time of poor comprehension, from the standpoint of fundamental physics. The prevailing theory, cosmic inflation, does a good job explaining the observed flatness, homogeneity and isotropy of the universe, and the absence of exotic relic particles (such as magnetic monopoles). Another prediction expressed by observations is that small disturbances in the primordial universe stem the structural formation in the future. These fluctuations, when they form the foundations for all structures, appear most clearly as small temperature fluctuations in one portion within 100,000. (To put this in perspective, the same level of fluctuation on the topographic map of the United States will show no features higher than a few centimeters.) These fluctuations are very important, as they provide the seeds from which the largest structures can grow and eventually collapse to form galaxies and stars. COBE (Cosmic Background Explorer) provided the first detection of intrinsic fluctuations in cosmic microwave background radiation in the 1990s.
This disorder is thought to have a very specific character: they form a Gaussian random field whose covariance functions are diagonal and almost scale-invariant. The observed fluctuations appear to have this shape, and in addition to the spectral index as measured by WMAP - the spectral index measuring deviations from the invariant-scale (or Harrison-Zel'dovich) spectrum - is almost predicted by the inflation model the simplest and most powerful. Other important properties of primordial disorders, that they are adiabatic (or isentropic between the various types of matter that make up the universe), are predicted by cosmic inflation and have been confirmed by observation.
Other theories of the early universe have been proposed that are claimed to make similar predictions, such as bran gas cosmology, cyclic models, big bang models and holographic universes, but they are still newborn and not widely accepted. Some theories, such as cosmic strings, have largely been denied by increasingly precise data.
Horizon issue
Sebuah konsep penting dalam pembentukan struktur adalah gagasan dari jari-jari Hubble, sering disebut hanya horizon, karena berkaitan erat dengan cakrawala partikel. Radius Hubble, yang terkait dengan parameter Hubble sebagai , di mana adalah kecepatan cahaya, mendefinisikan, berbicara kasar, volume alam di dekatnya yang baru-baru ini (pada waktu ekspansi terakhir) berada dalam hubungan kausal dengan seorang pengamat. Karena alam semesta terus meluas, kepadatan energinya terus menurun (tanpa adanya materi eksotis seperti energi hantu). Persamaan Friedmann menghubungkan kerapatan energi alam semesta dengan parameter Hubble dan menunjukkan bahwa radius Hubble terus meningkat.
The issue of the big bang cosmology horizon says that, without inflation, perturbations never exist in causal contact before they enter the horizon and thus homogeneity and isotropy, for example, the distribution of large-scale galaxies can not be explained. This is because, in the usual Friedmann-LemaÃÆ'®tre-Robertson-Walker cosmology, Hubble radius rises faster than the expanding space, so the perturbation only enters the Hubble radius, and is not driven by expansion. This paradox is solved by cosmic inflation, which shows that during the phase of rapid expansion in the early universe, Hubble radius is almost constant. Thus, large-scale isotropy is caused by quantum fluctuations generated during cosmic inflation being pushed outside the horizon.
Primordial plasma
The end of inflation is called reheating, when inflation particles rot into heat, hot plasma from other particles. At this time, the energy content of the universe is fully radiated, with standard model particles having relativistic speed. When the plasma cools, baryogenesis and leptogenesis are thought to occur, as the quark-gluon plasma cools, the termination of electroweak symmetry occurs and the universe becomes primarily composed of ordinary protons, neutrons and electrons. As the universe cools further, big bang nucleosynthesis occurs and a small amount of deuterium, helium and lithium nuclei are created. As the universe cools and expands, the energy in the photons begins to turn red, the particles become non-relativistic and ordinary matter begins to dominate the universe. Finally, the atom begins to form because the free electrons bind to the nuclei. It suppresses Thomson scattering photons. Combined with the refinement of the universe (and consequent increase in the free path of photons), this makes the transparent universe and cosmic microwave background emitted in recombination ( the last scattering surface ).
Acoustic Oscillation
Primordial plasmas will have very little material advantages, which are thought to originate from increased quantum fluctuations during inflation. Whatever the source, these advantages gravitate to attract matter. But the intense heat of the near-constant photon-object interaction of this era rather compels the search for thermal equilibrium, which creates a large amount of external pressure. This opposite force of gravity and pressure creates an oscillation, analogous to the sound waves made in the air by the pressure difference.
This disorder is important, as they are responsible for the fine physics that produces anisotropy of cosmic microwave background. In this day and age, the amplitude of the noise entering the horizon oscillates sinusoidally, with the solid region becoming more clear and then becoming solid again, with the frequency associated with the size of the disturbance. If the disorder is volatile once integral or integral half between coming into the horizon and recombination, it appears as an acoustic peak of the cosmic microwave ionisation anisotropy. (A half-oscillation, in which the solid region becomes a cleared region or vice versa, appears as a peak because anisotropy is shown as a power spectrum , so the underdensity contributes to strength as well as overdensities.) Physics determines the detailed peak structure from microwave background is complicated, but this oscillation gives essence.
Linear structure
One of the key consciousness made by cosmologists in the 1970s and 1980s is that much of the material content of the universe is composed not of atoms, but the mysterious form of matter known as dark matter. Dark matter interacts through the force of gravity, but is not composed of baryons, and is known with very high accuracy so as not to radiate or absorb radiation. It may consist of particles that interact through weak interactions, such as neutrinos, but can not consist entirely of three known types of neutrino (although some claim it is a sterile neutrino). Recent evidence suggests that there are about five times as much dark matter as baryonic matter, and thus the dynamics of the universe in this age are dominated by dark matter.
Dark matter plays an important role in the formation of structures because it only senses the force of gravity: instability Jeans gravity that allows compact structures to form is not opposed by any force, such as radiation pressure. As a result, dark matter begins to collapse into a complicated network of dark matter circles long before ordinary matter, which is hampered by force forces. Without dark matter, the period of galaxy formation will occur substantially later in the universe than is observed.
The physics of forming structures in this day and age is very simple, since dark matter disturbances with different wavelengths evolve independently. When the Hubble radius grows in the expanding universe, it encompasses larger and larger disorders. During the dominance of matter, all dark matter disturbances result from growing through the grouping of gravity. However, the shortwave interference that is included during radiation dominance has slowed its growth until material dominance occurs. At this stage, luminous and baryonic matter is expected to reflect the evolution of dark matter simply, and its distribution must track each other.
It is a simple matter to calculate this "spectrum of linear forces" and, as a tool for cosmology, it is of interest that is comparable to the cosmic microwave background. The galaxy survey has measured the power spectrum, such as the Sloan Digital Sky Survey, and based on the Lyman-? Forest. Because these studies observe the radiation emitted from galaxies and quasars, they do not directly measure dark matter, but the large-scale distribution of galaxies (and the uptake lines in Lyman-forest) is thought to reflect the distribution of dark matter closely.. This depends on the fact that galaxies will be bigger and bigger in the denser parts of the universe, whereas they will be relatively rare in the cleared area.
Nonlinear structure
When the disturbance has grown enough, the small area may become much denser than the average density of the universe. At this point, the physics involved becomes much more complicated. When deviations from small homogeneity, dark matter can be treated as a fluid without pressure and develop with a very simple equation. In areas significantly denser than the background, Newton's full theory of gravity should be included. (Newtonian theory is precisely because the involved mass is much less than it takes to form a black hole, and the gravitational speed is negligible because the light crossing time for the structure is still smaller than the typical dynamic time.) One sign that linear approximations and fluids become invalid is that dark matter begins to form caustics in which the paths of adjacent particles cross, or the particles begin to form orbit. This dynamic is best understood using N -the human simulation (although various semi-analytic schemes, such as Press-Schechter formalism, can be used in some cases). While in principle these simulations are quite simple, in practice they are difficult to implement, because they require the simulation of millions or even billions of particles. In addition, although particles in large quantities, each particle usually has a solar mass of 10 9 and discrete effects can be significant. The biggest simulation as in 2005 was the Millennium simulation.
The results of the N simulation show that the universe consists mostly of voids, whose density may be as low as a tenth of the cosmological. This material condenses in large filaments and halos that have complex web-like structures. These form groups of galaxies, groups and superclusters. While the simulations seem to agree broadly with observations, their interpretation is complicated by an understanding of how dark matter's dense accumulation spurs the formation of galaxies. In particular, many forms of halo are smaller than those we see in astronomical observations as dwarf galaxies and spherical clusters. This is known as a galaxy bias problem, and various explanations have been proposed. Most consider it an effect in the complicated physics of galaxy formation, but some say it is a problem with our dark matter model and that some effects, such as warm dark matter, prevent the formation of the smallest halo.
Gas Evolution
The last stage in evolution arises when the baryon condenses in the center of a halo galaxy to form galaxies, stars, and quasars. Dark matter greatly accelerates the formation of solid halo. Because dark matter has no radiation pressure, the formation of smaller structures of dark matter is not possible. This is because dark matter can not eliminate angular momentum, whereas ordinary barononic material can collapse to form solid objects by eliminating angular momentum through radiation radiation. Understanding these processes is a very difficult computational problem, because they can involve the physics of gravity, magnetohydrodynamics, atomic physics, nuclear reactions, turbulence and even general relativity. In many cases, it is not yet possible to perform quantitative comparative simulations with observations, and the best that can be achieved is an approximate simulation that describes the main qualitative features of a process such as star formation.
Modeling of formation structures
Cosmological Disorders
Much of the difficulties, and many disputes, in understanding the large-scale structure of the universe can be solved by better understanding the choice of gauges in general relativity. With scalar-vector-tensor decomposition, the metrics include four scalar disturbances, two vector disturbances, and one tensor disturbance. Only significant scarar disturbances: vectors are exponentially suppressed in the early universe, and tensor modes make only minor (but important) contributions in the form of primordial gravity radiation and B-mode from cosmic microwave background polarization. Two of the four scalar modes can be removed by physically meaningless coordinate transformations. Which mode is eliminated determines the infinite number of gauges possible. The most popular gauge is the Newton gauge (and the closely related Newton converter gauge), where the scalar is maintained is a Newtonian potential? and ?, which correspond exactly to Newton's potential energy of Newton's gravity. Many other measurements are used, including synchronous gauges, which can be an efficient gauge for numerical calculations (used by CMBFAST). Each gauge still includes some degree of unsuitable freedom. There is a so called invariant-measurement formality, where only measuring invariant variable variables are considered.
Inflation and initial conditions
Kondisi awal untuk alam semesta diperkirakan timbul dari escala fluktuasi mechanika kuantum invarian dari inflasi kosmik. Gangguan kepadatan energi latar belakang pada titik tertentu dalam ruang kemudian diberikan oleh isotropik, bidang acak Gaussian homogen dari nol rata-rata. Ini berarti bahwa transformasi Fourier spasial - memiliki fungsi korelasi berikut
- ,
di mana adalah fungsi delta Dirac tiga dimensi dan adalah panjang . Selain itu, spectrum diprediksi oleh inflasi hampir scalar invarian, yang berarti
- ,
di mana adalah angka kecil. Akhirnya, kondisi awal adiabatik atau isentropik, yang berarti bahwa gangguan fraksional dalam entropi setiap spesies partikel adalah sama. Predicsi yang dihasilkan sangat sesuai dengan observasi, namun ada masalah konseptual dengan gambaran fisik yang disajikan di atas. Keadaan would be able to give fluctuation as much as the right hand side, all kenyataannya benar-benar homogen dan isotropik, denotes demographic tidak dapat diperdebatkan bahwa fluktuasi quantum mewakili inhomogeneities primordial dan anisotropi. Interpretasi ketidakpastian kuantum dalam nilai bidang inflasi (yang merupakan apa yang disebut fluktuasi quantum sebenarnya) seolah-olah fluktuasi statistik dalam bidang acak Gaussian tidak mengikuti dari penerapan aturan standar teori kuantum. Masalah ini kadang-kadang disajikan dalam istilah "quantum to classical transition", yang merupakan cara yang membingungkan untuk merujuk pada masalah yang dihadapi, karena sangat sedikit fisikawan, jika ada, yang berpendapat bahwa ada entitas yang benar-benar ada. klasik pada tingkat fundamental. Bahkan, pertimbangan masalah ini membawa kita berhadapan langsung dengan masalah pengukuran yang disebut dalam teori kuantum. Jika ada, masalahnya menjadi semakin buruk dalam konteks cosmologis, karena alam semesta awal tidak mengandung entitas yang dapat dianggap memainkan peran sebagai "pengamat" atau "alat pengukur", keduanya sangat penting untuk penggunaan standar mechanika kuantum.. Sikap yang paling populer from kalangan kosmolog, dalam hal ini, adalah mengandalkan argumen berdasarkan dekoherensi dan beberapa bentuk "Banyak Interpretasi Dunia" dari teori kuantum. Ada perdebatan sengit yang intense tentang kewajaran postur itu.
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