Astronomers are working to understand the large-scale structure of the universe—how did they form and how do they affect the expansion of the universe?
On the largest scale, our universe looks like the surface of a windless lake in the afternoon, calm and unruffled. But on a scale of hundreds of millions of light-years or smaller, the universe shows a random and mixed distribution of matter and giant holes . This is a universe where matter clumps together.
Although giant holes occupy most of the volume of the universe, dense areas of fibrous matter are intertwined around these "empty" giant holes. Like strings of water droplets reflecting sunlight on a spider's web, these fibers connect glowing galaxies. Being "glued" to this cosmic web are individual galaxies, the galaxy cluster , and the more mysterious supercluster composed of galaxy clusters.
The large-scale structure of the universe shows that giant holes will be surrounded by fiber-like structures, and these fibers will connect superclusters of galaxies. This numerical simulation shows that the universe is not as uniform as astronomers originally thought. So how did this superstructure form?
Superclusters usually contain at least eight Abell-type galaxy clusters comparable in size to the Virgo cluster (composed of about 2,000 galaxies). In fact, an "ordinary" supercluster typically contains up to about 50,000 galaxies within a range of 200 to 300 million light-years in diameter.
It’s not just the appearance of superclusters that has cosmologists puzzled, but also their formation, evolution and interactions. One of the most fundamental questions related to superclusters is what can the seemingly randomly distributed matter in the universe tell us about the ultimate nature of the universe or our role in it?
Detected Structure
The first evidence for the existence of superclusters came from the Herschel family's nebula surveys in the 18th century. Nearly two centuries later, while searching for galaxy clusters at the Palomar Observatory in the United States, he also discovered clues of superclusters.
Then the first galaxy cluster table was published. Although it is estimated that there are millions of superclusters in the universe, astronomers have discovered only a few dozen of them.
Research on how superclusters affect the large-scale structure of the universe is also gradually making progress.
40 years ago, scientists believed that the larger the scale, the more uniform the universe would be in all directions, that is, "isotropic."
Over the past 30 years, cosmologists' deep-sky surveys of the sector have discovered that the universe is not like this.
From a distance, superclusters look like different cities on a railway line. Their boundaries are also very blurry, as fibrous structures often connect them over a large area.
It is therefore difficult to imagine the extent of a supercluster, and even harder to imagine how large-scale structures grow out of the fabric of space-time. However, superclusters are organized on scales of tens to hundreds of millions of light-years. So cosmologists believe that the initial fluctuations and fluctuations in the structure of the early universe were the seeds for the formation of the galaxy clusters and superclusters we see today.
When the universe was very young, it was very hot, and these embryonic large-scale structures were nothing more than random fluctuations in the very early matter of the universe. Later, when gravity takes over, matter begins to clump together in denser places. But even before that, there were ebbs and flows.
Large-scale structure in the local universe. A total of approximately 1.6 million galaxies were observed in the 2-micron all-sky survey. In this map of galaxies across the sky, the brightest and closest galaxies are shown in purple and blue, and the faintest and farthest galaxies are shown in red. This gives the three-dimensional structure of the universe.
Future arrays of large radio telescopes will be able to detect neutral hydrogen, the basic material from which all galaxies are built, over large areas. By tracking hydrogen, cosmologists can glimpse the structure of the early universe and the formation of the earliest superclusters.
Astronomers also hope to use these telescopes to measure the redshift of early galaxies.The redshift of galaxies is directly related to the expansion of cosmic space-time, so these measurements allow astronomers to track the evolution of superclusters over time and thus search for a causal relationship between neutral hydrogen gas and galaxy formation.
Galaxy clusters, such as the nearby Virgo Cluster, collapse under their own gravity within a billion years of their formation. But now superclusters are still in the expansion and formation stages.
In fact, superclusters are so big that it would take longer than the age of the universe to collapse. While their cores may still collapse, their outer regions don't follow.
Great Wall of Galaxies and Giant Hole
Although superclusters are very large, they are not the largest gravitationally held entities in the universe. It is the huge "Great Wall of Galaxies", and it is superclusters, clusters of galaxies and individual galaxies that make up the wall.
The basic understanding of the structure of superclusters became clear about 40 years ago. Galaxy formation begins in the densest places in the universe, such as the centers of superclusters. It's also where star formation begins. On the outskirts of the supercluster the density is much lower.
In the mid-1970s, there was a big debate among astronomers about whether superclusters were simply "created" by the visual concentration of randomly distributed clusters of galaxies.
But after 1977, astronomers agreed that superclusters were real physical systems. Astronomers have confirmed that all galaxies are members of superclusters. Even the independent galaxies "scattered" in the giant hole are not isolated, but part of a faint galaxy system.
Eternal Motion
Although all matter is part of the never-ending web of the universe, both individual galaxies and superclusters are always in motion. For example, superclusters of galaxies on large scales can interfere with the motion of a galaxy due to the expansion of the universe itself (known as the "Hubble Flow"). It's like a boulder disturbing an icy mountain spring. Therefore, on local small scales, galaxies moving toward superclusters do not "follow the (Hubble) flow." Instead, they may move towards us, manifesting as " blue shift ".
In the late 1980s, astronomers wanted to know how large numbers of galaxies moved as a whole. After subtracting the motion due to the expansion of the universe, they obtained the intrinsic velocity of the galaxy. In other words, what astronomers get at this time is the speed of a certain galaxy relative to other galaxies and even the cosmic background radiation. For example, the velocity of the Milky Way in the Local Group consists of three different components.
On a local scale, we are located on a great wall of galaxies surrounding the giant hole. The giant hole is just opposite the supercluster, and the Milky Way is located on the edge of an expanding giant hole. Astronomers have discovered that the Milky Way is leaving this giant hole at high speed. This motion is caused by the gravitational pull of nearby superclusters.
At the intermediate scales, we are being pulled toward the nearest large collection of matter, the Virgo Cluster, 55 million light-years away. The Virgo galaxy cluster is part of the Virgo supercluster .
The Virgo Cluster is one of the closest galaxy clusters to us. Along with the Local Group of galaxies, it is also part of the supercluster of galaxies in which we are located.
On larger scales, a larger Centaur-Hydra-Centaur supercluster is pulling us toward it. Our Local Group of galaxies and the Virgo Supercluster belong to it.
And the Centaur-Hydra-Centaur supercluster is being attracted by the gravity of the Shapley Aggregation located 600 million light-years away in the southern constellation Centauri . The Shapley Agglomerate is approximately four times more massive than the Virgo supercluster and is the densest known supercluster.
Large-Scale Sky Surveys
One way to better determine the dynamics of superclusters is to conduct large-scale sky surveys.Australia's 6 Degree Field Galaxy Survey (6dFGS) covers 17,000 square degrees of sky, or 80% of the southern sky, and provides the most detailed observations of the large-scale structure of the southern sky. While the observing team is still analyzing the data, they report that 6dFGS has obtained redshifts for about 125,000 galaxies, including more than 400 Abell galaxy cluster in the southern sky.
The method used to find galaxy clusters is similar to "find a friend". Select a galaxy, see if it has neighboring galaxies, and then see if there are any galaxies around those galaxies. When there are no more galaxies around a galaxy, it is a galaxy cluster.
But repeating this process in the observational data of deep sky surveys is very difficult. Fifty years ago, astronomers knew nothing about the overall distribution of galaxies. Astronomers now know that galaxies and galaxy clusters are dense concentrations of matter. Superclusters are not (just) collections of galaxy clusters, but high-density areas in the cosmic web.
In the near future, data from the 6dFGS and Webb Space Telescope will help us better understand superclusters. But no numerical simulations have been able to reproduce the details of superclusters. To improve these computer simulations, astronomers need better observational data. 6dFGS in the north will help with this.
The distribution map of galaxies observed by the 6-degree field of view galaxy survey.
In addition, a new sky survey project will also be added. The American Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) is the optical survey with the largest field of view so far and was fully put into use in 2013. It can cover 75% of the sky. Pan-STARRS will greatly enhance our understanding of the three-dimensional structure of galaxies within 5 billion light-years and the large-scale structure around them.
self-similar universe?
90% of the universe is occupied by giant holes, and the remaining 10% is made up of fiber structures and supergalaxy clusters. What does this mean? The answer may lie in "fractals." Although deep sky surveys like Pan-STARRS should give a global picture of how superclusters are distributed in the sky, at relatively large local scales (100 to 300 million light-years) the universe exhibits a fractal structure.
Simply put, a fractal is a mathematical method of describing uneven shapes that repeat themselves at different scales. It is infinitely complex, yet has a self-similar structure. Examples of fractals include the branching structures of trees, coral reefs, the human body's blood vessel system, coastlines, stone and metal fragments, mountains, snowflakes, and superclusters of galaxies. Even though fractals cannot reveal how these structures are formed, they do provide us with a new perspective on how the universe is organized.
A few years ago, cosmologists said that fractals did not apply to the universe. But now they have changed their minds, but still believe that it does not apply to infinitely small and infinitely large fractals.
However, although the vast majority of cosmologists disagree, as some cosmologists now believe, if the universe is infinite, then fractals may eventually prove to extend to infinite scales. Even if some "slices" of the universe have a fractal structure, it still tells us that there is order in the universe.
The laws of nature operate under very different conditions, and it is mathematics and physics that write these laws. Things that seem to have no rules at all are actually very regular. Likewise, our universe is governed by these most fundamental laws. However, it is still unknown what physical laws create these fractal structures.
big picture
For those willing to think about the universe from a philosophical level, connecting the largest scale fractal structures in the universe to rock fragments is a mind-numbing task. Even so, theoretical and observational astronomers will continue to conduct deeper surveys and more comprehensive computer simulations to help them understand at least two other puzzles.
Can the large-scale structure of the universe really be understood? What role did large-scale structures play in the emergence of life?
Astronomers believe that details about the formation and evolution of large-scale structures in the universe will be broken within decades. But it will take longer to figure out the connection between the large-scale structure of the universe and life on Earth.
In the meantime, the subtleties of the organization of the universe will remain intriguing and mysterious.
