What Is Large-Scale Structure Physics?

The large-scale structure (LSS) of the universe refers to patterns of galaxies and matter on a much larger scale than individual galaxies or groups of galaxies. On a large scale, the Universe exhibits a coherent structure, with galaxies in groups and clusters at scales of ~1-3 Mpc/h, which are at the intersection of long filaments of galaxies >10 Mpc/h long. . Through these studies, we see that, on the largest observed scales, galaxies and clusters are found in thin filamentous web-like structures that form walls with large bubbles mostly devoid of galaxies (Fig. 14.5).

The maps show a rich foam-patterned structure containing possible filament walls or filaments of galaxies, star clusters, and large sky regions. On larger scales, massive galaxy clusters contain a large filamentary structure, typically billions of light-years in scale. Superclusters are often referred to as the large-scale structures of the universe, and they can be clearly observed in galaxy redshift studies such as the Australian 2 Degree Field (2dF) project (right). The South Pole Telescope now has a catalog of huge galaxy clusters detected in this way, as well as measurements of background fluctuations produced by smaller clusters, which can be used to track the growth of cosmic structure.

Large-scale structural mapping gives us insight into fluctuations in the early universe, as well as the distribution of dark matter, the formation of galaxies, and an estimate of how fast the universe is expanding. The details of the large-scale structure—the size, density, and distribution of the observed structure—depend on both cosmological parameters, such as the density of matter and dark energy, and the physics of galaxy formation and evolution. The prevailing theoretical paradigm for the existence of large-scale structure is that the initial fluctuations in the energy density of the early universe, observed as temperature deviations in the cosmic microwave background, are built up by gravitational instability in the structure, observed today in the field density of a galaxy. Since the physics describing the nonlinear stage of galaxy formation is extremely complex, numerical simulations have become an absolute necessity for studying the large-scale distribution of galaxies.

Current Lambda-CDM models are able to predict the large-scale distribution of observed galaxies, clusters, and voids; but at the scale of individual galaxies, due to highly nonlinear processes related to baryonic physics, gas heating and cooling, star formation, and feedback, There are many complexities. Because galaxy surveys take into account the radiation emitted by galaxies and quasars, they do not measure dark matter directly, but the large-scale distribution of galaxies (and the absorption lines of Liman Forests) are expected to accurately reflect the distribution of dark matter. case. Statistical analysis of bright galaxies and quasars in the SDSS catalog shows redshifts as high as ~0.6 and 1.8, respectively, making the universe homogeneous on scales of 40-600 million light-years and above. At first glance, the distribution of galaxies in the universe like Figure 14.5 seems to imply that the universe is not homogeneous nor isotropic.

We currently find support for cosmological principles in the distribution of galaxies in the universe. The original CfA review also compared the so-called “complex topology” of large-scale structures observed in galaxy distributions with models observed in N-body dark matter simulations, setting the stage for future research into theoretical structure formation models. These notes describe the motivation and components of the halo model of nonlinear and distorted structures in the universe. The theory is elegant and simple because it links the smallest-scale processes that occurred during the first 10-33 seconds of the universe’s evolution to the formation of the largest structures observed today.

While these patterns cause the observed accelerated expansion, they also affect structure formation on a much smaller scale. Instead of adding yet another dark component to the energy balance of the universe, one might wonder if the observed accelerated expansion might actually be due to the behavior of gravity itself on a larger scale. Most theoretical models of dark energy act to slow down this process of gravity, creating large structures. Radiation dominated this stage; in this case, fluctuations in density above the cosmic horizon increase in proportion to the scale factor, since fluctuations in the gravitational potential remain constant.

The physics of structure formation in the era of galaxy formation is especially simple, since dark matter perturbations with different wavelengths develop independently. The Big Bang cosmology horizon problem states that, without inflation, the perturbations were never in causation before they entered the cosmic horizon, and therefore the uniformity and isotropy of, for example, the distribution of galaxies on large scales cannot be explained. Although the properties of galaxies are primarily determined by the mass of their parent halo, many of the observed correlations with the environment are a simple consequence of these trends. The idea of ​​whether galaxies are evenly distributed in space goes back to Edwin Hubble, who used his catalog of 400 “extra-galactic nebulae” to test the homogeneity of the universe (Hubble 1926), finding that it is broadly uniform on large scales. Hubble Hubble 1926).

Before 1989, it was widely believed that virtual clusters of galaxies were the largest structures in existence, and that they were more or less evenly distributed in all directions throughout the universe. The universe exhibits structure on a wide range of physical scales, from moons orbiting planets to superclusters of galaxies, galaxies, filaments, and voids that cover a large portion of the observable universe. Surveys of the sky and mapping of electromagnetic radiation in various wavelength ranges (especially radiation at 21 cm) provide a wealth of information about the content and nature of the structure of the universe. Large-scale galaxy surveys and intensity mapping experiments directly measure matter density fields, allowing us to image giant filamentary structures with unprecedented precision.

The large-scale rapid motion caused by structure formation remains confined to a few clustered regions, which results in a very steep Mach number distribution with a maximum at M = 1.5 and very few strong shocks, M >= 5-10.

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