When scientists talk about the space beyond the Milky Way, most of the time, you can only understand that there are hundreds of billions of galaxies out there, similar to our star system. It becomes difficult to follow it all as soon as they start talking about clusters, superclusters, voids, filaments, and walls. However, we will try to figure it out.
The beginning of extragalactic astronomy
One of the most important tasks facing astronomy since ancient times has been understanding how our world works and what place we occupy. Mankind has been looking at the stars and trying to figure out what structures form the celestial bodies based on the results of observations.
The desire to organize the world around us is very important for people – it gives them a sense of control over everything that happens. If the world is predictable, it seems safe. But with astronomy, this “worked” in a rather strange way.
First, we had to abandon the perception of the Earth as the center of the universe. The heliocentric model explained to people that they live on a tiny grain of sand that orbits the Sun in the middle of nowhere. However, almost immediately (by historical standards), it became clear that our Sun itself is only one of billions of stars circling the colossal disk of the Galaxy, which we see in the sky as a band of the Milky Way.
For most astronomers of the 20th century, even this reality was already quite shocking, and they held the view that everything we see in the sky is part of the galactic disk. Although even then, there were people who assumed that some nebulae in the sky were galaxies like ours, and they formed some kind of structure.
In 1924-1926, the American astronomer Edwin Hubble found evidence that objects like the Andromeda Nebula were other galaxies. He later discovered a pattern in the shifts of spectral lines of other star systems, which is now called the law of his name.
Hubble’s law allowed us to estimate intergalactic distances. Once again, it turned out that the Universe is much larger than we had imagined. But this time, the entire Milky Way turned out to be one of hundreds of millions of grains of sand that form colossal structures that fill the void of space.
Local Group
Before we talk about global space structures outside our Galaxy, it is important to explain the scale we are talking about. After all, the dimensions of everything that follows are so large that the human imagination may simply refuse to perceive them.
It is better to take the diameter of the Milky Way as a starting point. It is so large that light travels from one end to the other in 100 thousand years. For comparison, the distance between the Sun and the Earth is covered by electromagnetic waves in an average of 8 minutes and 19 seconds.
However, no matter how incredible this distance may seem, everything that will be discussed below is tens, hundreds, and thousands of times larger than the Milky Way. For example, the smallest structure that our Galaxy is part of is the Local Group. These gravitationally bound star systems together are about 3 million light-years across, which is 30 times the diameter of our disk. They are dominated by two large galaxies: The Milky Way and the already mentioned Andromeda Nebula (M31). The latter is located at a distance of 2.5 million light-years from us, which is about 25 diameters of the galactic disk.
Each of the large galaxies is surrounded by a swarm of much smaller satellites. The closest of these objects is the dwarf galaxy in Sagittarius. Its distance is only 70 thousand light years, meaning that it actually “hangs” over the Milky Way disk.
In addition to the two large galactic subgroups, the Local Group includes several “loners” such as the Triangulum Nebula (M33), located at a distance of 700 thousand light years from the Andromeda Nebula.
There are dozens of groups like the Local around us, and their total number in the Universe is measured in millions. The closest similar group of galaxies to us, the Antlia-Sextans, is located at a distance of 4.3 million light-years, which is approximately equal to 43 Milky Way disks.
Clusters and voids
However, a significant number of galaxies in the Universe are not gathered in such small groups as the Local Group. They form much larger and more massive groupings called clusters. The closest of these is the Virgo Cluster. Its center lies 48 million light-years away, which is 480 times the diameter of the Milky Way.
It should be noted here that all the proper names for extragalactic objects were given mainly by the constellations in which they were found. This can cause some confusion, since a globular cluster, a single galaxy, a group of galaxies, and a cluster can be named after the same constellation. Therefore, when talking about the large-scale structures of the Universe, it is important to add to the name a clarification of what type of object is being discussed.
The Virgo Cluster provides an excellent example of what such structures look like. Its diameter is about 15 million light-years, which means it is 150 times larger than the Milky Way and about five times larger than the Local Group. But this volume contains about 2,000 galaxies, which are interconnected by gravitational forces.
And most of them are not small star systems at all. The diameter of many of them – M90, M86, M49, M98 – significantly exceeds the size of the Milky Way. In addition, between these systems, there are about 11 thousand globular star clusters, each of which may contain tens or hundreds of thousands of stars.
It would be logical to assume that if there is an excess of matter somewhere, there must also be places where there is virtually no matter. Indeed, huge volumes of space outside of galactic clusters remain empty. Where thousands of groups like the Local Group could fit, only single stars and dwarf galaxies can be found.
Such places are called voids, which means cavities. They are usually even larger than galaxy clusters. For example, the largest known voids in the Eridanus constellation are between 500 million and 1 billion light-years in size. The rest are much smaller, but their diameter can be measured in tens or hundreds of millions of light-years, meaning they are hundreds or thousands of times larger than the Milky Way.
Local Sheet
Compared to the Virgo Cluster, the Local Group may seem like a small village lost in the middle of nowhere. However, we have several neighbors with whom we form a separate large-scale structure – the Local Sheet.
The fact is that astronomers have long noticed that voids are predominantly circular in shape, and the clusters of galaxies that separate them tend to form flat or elongated shapes. Here we are, together with the neighboring groups of galaxies Maffei, M81, M94, Centaurus A, and Sculptor, forming the so-called Local Sheet. This rather dispersed structure has a diameter of 23 million and a thickness of 1.5 million light years.
Its biggest attraction is the Council of Giants, a more or less uniform circle of large galaxies with a diameter of about 12.2 million light-years, consisting of ten large spiral and two more elliptical star systems. It includes the Milky Way with the Andromeda Nebula.
The Local Sheet and the Virgo cluster are balanced on the edge of the void closest to us. It is also called “Local”. The diameter of this gigantic empty “bubble” is at least 150 million light-years, which means it is about 1500 times larger than the Milky Way.
The superclusters of Virgo and Laniakea
In turn, clusters and groups of galaxies are part of much larger structures called superclusters. The Milky Way, together with the entire Local Group, the Local Sheet, the Virgo Cluster, as well as the Fornax, Eridanus, Ursa Major, and numerous other groups, forms the Virgo Supercluster.
Astronomers guessed about the existence of this colossal structure in the second half of the 19th century, because it was in the constellation Virgo that a significant number of galaxies were observed at that time. However, since the distance to them remained unknown, the fact that the Milky Way was part of this supercluster was finally recognized only in 1953.
In total, the Virgo supercluster has a diameter of almost 110 million light-years. It is believed to contain about 30 thousand galaxies, gathered in more than 100 clusters and groups. At the same time, it is too dim for its mass, so scientists believe that a significant part of it is represented by dark matter.
For a long time, scientists assumed that the Virgo supercluster was not a gravitationally bound structure. However, research in 2014 showed that it is part of a much larger complex of galaxies called Laniakea, which is also gravitationally bound.
The center of gravity of Laniakea is the so-called Great Attractor, located at a distance of 150-250 million light years in the direction of the constellations of Norm and the Triangulum Australe. It is in this direction that all the nearest galaxies, including the Milky Way, are constantly accelerating.
It is not known for certain what the Great Attractor is. It is located in the so-called “zone of avoidance” – a part of the sky where extragalactic objects are hidden from us by the cluster of stars and interstellar dust around the galactic center. Therefore, scientists can only hypothesize that there is a huge mass of dark and ordinary matter hiding there, of which the Norma cluster is a part.
Be that as it may, Laniakea, whose name translates from Hawaiian as “vast heavens,” is a truly majestic structure. In addition to “our” Virgo supercluster, it contains the Hydra-Centaurus, Pavo-Indus, and South constellations. Each of them forms a separate branch in this gigantic structure, which in general resembles a feather or a fern leaf. This “cosmic fern” stretches over 520 million light-years, meaning it is at least five times larger than the Virgo supercluster. It is believed to contain at least 100 thousand galaxies.
Nevertheless, it is impossible to say that Laniakea is the largest structure in the Universe or occupies a significant part of it. It is just a very large supercluster of galaxies, of which astronomers count at least 10 million more.
Threads, sheets, and walls
The fact that all galactic structures larger than a group or cluster of galaxies resemble threads or thin ribbons has long bothered scientists, as it was difficult to explain this by the gravitational influence of visible objects alone.
However, when the concept of dark matter emerged, this distribution of galaxies in the Universe was explained. Most of the large-scale structure can be described as threads and sheets. According to the prevailing notions of modern astrophysics, they are streams of dark matter within which stellar systems move.
It is believed that this motion affects the orientation of galaxies in space and even star formation in them, because the inflow of ordinary and dark matter from the outside depends on how they are oriented relative to the filament on which they are located.
One of the smallest examples of such a structure of the Universe is the Local Sheet. But there may be much larger ones. In particular, the entire Laniakeus supercluster can be considered an interweaving of “cosmic threads” with the largest galaxy clusters as its nodes. At the same time, it is only one of the “fibers” that form a really large thread known as the Pisces supercluster of galaxies. In addition to it, this colossal structure includes three more similar branches – Perseus-Pegasus, Pegasus-Pisces, and Pisces-Cetus, as well as a separate region of the Sculptor. Each of them, like Laniakea, is a combination of several superclusters.
The Pisces-Cetus Supercluster Complex is a truly gigantic “cosmic thread” that stretches over a billion light-years in space and is on average 50 million light-years thick. It is an extensive chain of superclusters, each link of which contains tens of thousands of galaxies.
However, even the Pisces-Cetus Supercluster Complex is not the largest structure in the Universe. After all, we already know similar but much larger structures. For example, the Sloan Great Wall is 1.3 billion light-years long, and the Clowes-Campusano quasar group is 2 billion. A huge group of quasars stretches for 4 billion light-years, and the Hercules–Corona Borealis Great Wall, for as much as 10 billion.
Is there a limit to the levels of the structure?
Given all of the above, one might expect that the levels of structure in the Universe will grow endlessly, and we will discover larger and larger complexes formed by “lower-level” structures each time. And we will seem smaller and smaller to ourselves. Is there a limit to the levels of the structure?
However, back in the 1990s, it was proved that on a scale of about 300 Mpc, i.e., about a billion light-years, space is generally homogeneous and is a set of different-sized voids surrounded by a web of filaments of galaxy clusters. They form something like a honeycomb or a sponge. And on an even smaller scale, the Universe will take on a uniform appearance. Even if we just make a map of the part of it that we can observe, even there, the irregularities of the structure will be barely noticeable.
However, we know nothing about most of the Universe. The light from there has not yet had time to reach us in the billions of years that it has existed. But we can already say with certainty that we will not see any incredible structures there.
The Universe has no center, so we cannot say that we are somewhere on the outskirts. We simply live on the Pisces-Cetus thread of the Laniakeus supercluster, in the Local Group, in the Orion arm of the Milky Way galaxy, in the Solar System, on planet Earth.
This article was published in Universe Space Tech magazine #1 (189) 2023. You can buy this issue in the electronic version in our store.
