Largest Structures of the Universe
Today I’m going to be talking about the largest structures in our universe that essentially make it up. You can think of these structures like Lego blocks that when pieced together, yield our current universe. These building blocks are namely galaxy superclusters, and voids.
Let’s first look at galaxy superclusters. A supercluster is a large group of smaller galaxy clusters or galaxy groups. Hence, superclusters are extremely large and low density. The number of superclusters in the observable universe is estimated to be 10 million.
The existence of superclusters indicates that the galaxies in the Universe are not uniformly distributed as astronomers originally thought. Instead most of them are drawn together in groups and clusters, with groups containing up to some dozens of galaxies and clusters up to several thousand galaxies. Those groups and clusters and additional isolated galaxies in turn form even larger structures called superclusters. Now, this makes sense when you realize that individual galaxies have very strong gravitational pulls which pull other galaxies together to create groups and clusters, and those groups and clusters have even larger gravitational pulls that hence attract them closer to each other, which creates superclusters.
Superclusters contain massive structures of galaxies, called “filaments”, “supercluster complexes”, “walls” or “sheets”, that may span between several hundred million light-years to 10 billion light-years, covering more than 5% of the observable universe. These are by far the largest structures known to date. Observations of superclusters can give information about the initial condition of the universe, as that is when these superclusters were created. Interspersed among superclusters are large voids of space where few galaxies exist.
Now that we know what a supercluster even is, let’s look at the Virgo supercluster. The Virgo Supercluster, sometimes referred to as the Local Supercluster, is a mass concentration of galaxies containing the Virgo Cluster and Local Group, which in turn contains the Milky Way and Andromeda galaxies. At least 100 galaxy groups and clusters are located within its diameter of 110 million light-years.
The structure of the Virgo Supercluster consists of two components: a substantially flattened disk containing two-thirds of the supercluster’s luminous galaxies, and a roughly spherical halo containing the remaining one-third. The disk itself is a thin ellipsoid with a long axis to short axis ratio of at least 6 to 1, and possibly as high as 9 to 1. The Virgo Supercluster represents a typical poor supercluster of rather small size. This means that it lacks a high density core. It has one rich galaxy cluster in the center, namely the Virgo cluster, and is surrounded by weak filaments of galaxies and poor groups. Our Local Group is located on the outskirts of the Virgo cluster in an especially small filament.
But even though the Virgo supercluster is weak when compared to other clusters, it is still quite large. The Supercluster’s volume is approximately 7000 times that of the Local Group or 100 billion times that of the Milky Way. The number density of galaxies in the Virgo Supercluster falls off with the square of the distance from its center near the Virgo Cluster, suggesting that this cluster is not randomly located. Overall, the vast majority of the luminous galaxies are concentrated in a small number of galaxy cluster groups. Ninety-eight percent can be found in just 11 of these galaxy cluster groups, though the Virgo Supercluster has many, many, cluster groups. This distribution indicates that most of the Virgo supercluster is simply a large void in space.
One would think that since the Virgo Cluster lay at the approximate center of the Virgo supercluster, all of the other galaxies would be attracted to this cluster. However, it turns out that all of the galaxies are actually moving towards a different point, known as the Great Attractor. Even the Virgo supercluster is moving to this mysterious location!
The Great Attractor baffled scientists for decades, until another supercluster, named the Laniakea supercluster, was discovered. It turns out that the great attractor is the central gravitational point of this supercluster and that the Virgo supercluster is just an appendage of this bigger structure. This explains why, when compared to other superclusters, the Virgo supercluster appeared much weaker. Unfortunately, the Great Attractor is obscured by the rest of our Milky Way, so scientists cannot observe it directly. But scientists can observe the effect of the great attractor of other galaxies, with indirectly allows them to learn more about this mysterious point.
Now, let’s learn more about the Laniakea supercluster. First of all, what does Laniakea even mean? Well, the name laniākea means ‘immense heaven’ in Hawaiian, from lani ‘heaven’, and ākea ‘spacious, immeasurable’. The name was suggested by Nawaʻa Napoleon, an associate professor of Hawaiian language at Kapiolani Community College, near the observatory where this supercluster was discovered. The name honors Polynesian navigators, who used knowledge of the heavens to navigate the Pacific Ocean.
The Laniakea Supercluster encompasses approximately 100,000 galaxies stretched out over 520 million light-years. It has the approximate mass of 1017 solar masses, or a hundred thousand times that of our galaxy, which is almost the same as that of the Horologium Supercluster. It consists of four subparts, which were known previously as separate superclusters. These four parts are the Virgo supercluster, the Hydra-Centaurus Supercluster, the Pavo-Indus supercluster, and the Southern Supercluster.
The most massive galaxy clusters of the Laniakea Supercluster are Virgo, Hydra, Centaurus, Fornax, Eridanus and Norma. The entire supercluster consists of approximately 300 to 500 known galaxy clusters and groups, though the real number may be much larger because some of these are traversing the Zone of Avoidance, an area of the sky that is partially obscured by gas and dust from the Milky Way galaxy, making them essentially undetectable. Unlike its constituent clusters, Laniakea is not gravitationally bound and is projected to be torn apart by dark energy. Now, this doesn’t mean that there aren’t gravitational forces at play inside Laniakea, as otherwise the great attractor would not exist. In fact, Laniakea is considered to be a true supercluster, meaning that everything inside the supercluster is pulled towards the great attractor and everything outside the supercluster is repelled away. The reason Laniakea is not gravitationally bound is because of the pull from the universe’s expansion. On a small scale, this expansion isn’t felt and objects such as our sun have no issue holding onto their planets. This is because at this scale the expansion is minute and basically negligible, but when you get to scales as big as the Laniakea supercluster, the expansion gets much more apparent. Due to the expansion, the gravitational pull that is now attracting the galaxy clusters towards the great attractor will slowly diminish until each galaxy cluster or groups becomes their own individually governed object. This means that the galaxy groups and clusters will still exist and the galaxies in these groupings will be closely bonded together, however they will not be bonded with other clusters or groups. Hence, dark matter will take over Laniakea and erase its existence forever.
Let’s now take a closer look at the different clusters that make up Laniakea. We’ve already covered the Virgo supercluster, so let’s look at the closest supercluster, which is the Hydra-Centaurus supercluster. Technically, this supercluster is composed of two other superclusters that were shoved together by scientists so that they could properly organize the universe. There are 4 large galaxies at the center of the Centaurus part of the supercluster, and two in the Hydra portion. Apart from the central clusters, which are 150 to 200 million light years away, several smaller clusters belong to the group, some of which are less than 100 million lightyears away.
What makes this supercluster unique is that it is very close to the Great Attractor. In fact, astronomers believe that the Great Attractor lies inside this supercluster, in a cluster known as the Norma cluster. The Norma cluster is extremely dense and massive and is also very bright. However, it lies in the Milky Way’s “Zone of Avoidance” so it cannot be seen from earth. The density of the Norma cluster is because the great attractor exerts a large gravitational pull on the nearby galaxies, causing them to come extremely close together. Some of the galaxies are thought to only be 1,000 lightyears apart, which is why many of the galaxies in this cluster are actually slowly fusing together. Scientists believe that in 100 billion years or so, the same thing will happen to the Local group and it will simply be one big galaxy.
Next up is the Pavos-Indus supercluster, which is located somewhere in the constellations of Pavos and Indus. It is about 200 to 230 million lightyears away from the Virgo supercluster, making it farther from us when compared to the Hydra-Centaurus supercluster. The supercluster contains three main clusters which form a line in space when observed. This arrangement may seem strange as space is usually so very chaotic. However, the galaxies are actually linked together by a galactic filament, which keeps them in that straightish line. Around this central line are a few smaller clusters.
Finally we have the Southern Supercluster. This supercluster is a lot looser than the other superclusters mentioned, and by that I mean that it is mainly composed of galaxy clouds, which as you can likely recall, are clouds of ionized gas and interstellar dust, interspersed with a few small galaxies here and there. Now, that isn’t to say that the Southern supercluster doesn’t have any proper clusters. At its center lies the Fornax cluster, which is actually only 62 million lightyears away from our position in the solar system, though scientists aren’t quite sure what its length is. It is the second richest galaxy cluster within 100 million lightyears, apart from of course the Virgo cluster. The Fornax Cluster is a particularly valuable source of information about the evolution of such clusters due to its relatively close proximity to the Sun. It also shows the gravitational effects of a merger of a galaxy subgroup with the main galaxy group, which in turn lends clues about the associated galactic superstructure.
At the center lies galaxy NGC 1399. The galaxy itself is 66 million lightyears away from earth and has a diameter of 130,000 lightyears, making it just slightly larger than the Milky Way. It has a bright center and a vast, diffuse envelope surrounding it. It is also an early-type galaxy, the largest one in the Fornax Cluster.
Despite their name, early-type galaxies are much older than spiral galaxies, and mostly comprise old, red-colored stars. However, NGC 1399 is still very bright, which doesn’t make sense as these old red stars tend to be very dull. But when you take a closer look at NGC 1399, things start making a little more sense. The galaxy is extremely rich in globular clusters, which estimates ranging from 5,700, to over 6,500. Scientists believe that NGC 1399 has such a large number of globular clusters because of a past interaction with a neighboring galaxy called NGC 1404, during which NGC 1399 basically stole all of NGC 1404’s globular clusters, which is rather rude if I do say so myself. There are also 37 planetary nebulae in this galaxy, which further contribute to its brightness.
Of course, there are more superclusters on top of Laniakea. Some scientists are actually grouping superclusters together to create what they call supercluster complexes. These scientists say that the Laniakea supercluster is a part of the Pisces-Cetus supercluster complex, home to 4 total superclusters, as well as 60 rogue galaxy clusters that aren’t within any of these 4 superclusters. The Pisces–Cetus Supercluster Complex is estimated to be about 1.0 billion light-years long and 150 million light years wide. It is one of the largest structures known in the observable universe, but is exceeded by a few great walls and huge, large quasar groups. More on those later.
Now, how do these supercluster complexes even work? Scientists say that the structure is linked together with a particularly large galaxy filament. They believe that these filaments formed in the early universe alongside dark matter, and hence were naturally suited to be the birthplaces of galaxies, since galaxies are composed of regular matter and cannot form within dark matter. So the galaxies had to cluster around the galaxy filament, which then created more over densities which prompted more galaxy formation, which lead to the creation of clusters ands superclusters and all of that fun stuff.
Now, as for the structure of the supercluster complex, the first supercluster in this complex is of course the one it was named after: The Perseus–Pisces Supercluster. This supercluster is one of the largest known in the universe. Even at a distance of 250 million light-years, this chain of galaxy clusters extends more than 40° across the northern winter sky. The Perseus-Pisces Supercluster is one of two dominant concentrations of galaxies, the other being the Local supercluster in the nearby universe. This supercluster also borders a prominent void, the Taurus Void.
The other two superclusters are the Hercules Supercluster and the Sculptor supercluster. The Hercules supercluster is about 330 million lightyears across and is unique since scientists have observed it to contain binary galaxies, which are galaxies that seem to actually orbit each other. The sculptor supercluster is sometimes referred to as the sculptor wall, because its galaxy clusters are arranged in a thin, tall line. Scientists estimate that the Sculptor wall is 320 million lightyears long, 23 million lightyears tall, and 3 million lightyears deep.
But wait! There’s one important part of superclusters we haven’t talked about: voids. Voids are basically the second building block of our universe and highly mysterious in nature, and to honor some of that mystery I’m going to talk about them in my next post, forcing my dear readers to wait in anticipation and suspense. Isn’t that just delightful?
