With the naked eye, nearly everything visible in the night sky belongs to our own galaxy, with only four extragalactic exceptions — Andromeda, the Large and Small Magellanic Clouds, and the Triangulum galaxy — all within our Local Group. Galaxies cluster into groups, clusters and superclusters, forming enormous cosmic structures. The Milky Way belongs to the Local Group, itself part of the Laniakea Supercluster, spanning hundreds of millions of light-years.
When we look up at the night sky, nearly everything that our naked eyes can perceive is right here in our own galaxy. The planets and our Moon are right here in our own Solar System, as are any asteroids, comets, or meteors that happen to be visible at the time. The stars, nebulae, and plane of the Milky Way are all contained within our own galaxy. Among the naked eye objects, only Andromeda, the Large and Small Magellanic Clouds, and the Triangulum galaxy are extragalactic in nature, and even at that, they’re contained within our Local Group of galaxies. Although there are literally trillions of other galaxies out there in our observable Universe, we cannot see them without the aid of a superior tool, like a long-exposure camera or a telescope. However, there are structures that exist on even larger scales, as galaxies often live together in groups and clusters, and galactic groups and clusters are often linked by cosmic filaments — including in a grand, web-like network — on the largest scales of all. If we were to look at things in detail on those enormous scales, what sorts of structures would we find, and which ones can we consider ourselves to be a part of? That’s what Constantine Deligiannis wants to know, writing in to ask: “I know we are a part of six structures in the universe that [are] bigger than the Milky Way: Local Group, Local Sheet, Local Volume, Virgo Supercluster, Laniakea Supercluster, and Pisces Cetus Supercluster Complex. [Do we] know how many phases are in the history of any of the other structures that are bigger than the Milky Way that we are a part of in the universe?” This is a great question: one that we need to look to the cosmos itself in order to answer. Let’s begin by going back in time to understand how structure forms, and then let’s look at how things are today, zooming out one step at a time, to see where we truly are and how we fit into the grandest picture of all. The density fluctuations in the cosmic microwave background (CMB) provide the seeds for modern cosmic structure to form, including stars, galaxies, clusters of galaxies, filaments, and large-scale cosmic voids. But the CMB itself cannot be seen until the Universe forms neutral atoms out of its ions and electrons, which takes hundreds of thousands of years, and the stars won’t form for even longer: 30-to-100 million years. Credit: E.M. Huff, SDSS-III/South Pole Telescope, Zosia Rostomian Shortly after the hot Big Bang began, there were no structures as we conceive of them today. All we had, at these early times, was a hot, dense, rapidly expanding, and almost perfectly uniform Universe. Those departures from perfect uniformity are incredibly important: they provide the seeds for what will eventually grow into the large-scale structure that pervades the entire Universe. The imperfections that the Universe was born with — seeded by cosmic inflation — consist of overdensities and underdensities in equal amounts, and both play important roles. The overdensities, having more matter-and/or-energy within them, represent regions where the gravitational potentials are greater than average. They preferentially draw matter into them from their surrounding, less dense regions, and even though matter-and-radiation interact, with radiation pushing back to fight against gravitational growth, these initially overdense regions will be the first to accumulate enough matter to gravitationally collapse: forming stars, star clusters, galaxies, galaxy groups, galaxy clusters, and even larger-scale structures. The underdensities, meanwhile, having a below-average cosmic density, preferentially give up their matter to their denser surroundings. While overdense regions gravitationally contract and then collapse, the underdense regions instead expand like bubbles, rarefying and becoming less dense. These become the sparsest regions on cosmic scales: the cosmic voids that separate galaxies, galactic groups, clusters, and filaments. From these initial conditions, plus the ingredients that make up the Universe (normal matter, dark matter, photons, neutrinos, and dark energy), gravity, collisions, cosmic expansion, electromagnetic interactions, and time do the rest. This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Over time, overdense clumps of matter grow richer and more massive, growing into galaxies, groups, and clusters of galaxies, while the less dense regions than average preferentially give up their matter to the denser surrounding areas. The “void” regions between the bound structures continue to expand, but the structures themselves, once they become bound in any fashion, do not. Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn Gravitation and cosmic expansion are arguably the two most important factors in determining the structures that arise on the largest scales of all. The Universe expands globally, with the same number of particles occupying a greater and greater volume as time elapses. But gravitation works locally, as the gravitational influence from any one massive region can only propagate outward at the speed of light. As a result, the smallest cosmic scales are the ones that gravitationally grow the fastest first, and larger-scale structures — although the initial fluctuations are just as large or even a few percent larger on larger scales than smaller ones — only form later, once gravity has had enough time to propagate across those great cosmic distances. Around 30-100 million years after the Big Bang, gravitation is successful enough that the first stars and star clusters begin forming. Within the first 200 million years, the seeds of supermassive black holes have formed, and star clusters begin merging together to form the earliest protogalaxies. By 400 million years of age, a few galaxies have already grown quite large massive, containing billions of stars, and by 700 million years, the earliest proto-galaxy-clusters have begun to assemble. The white boxes outline member galaxies that are part of the most-distant proto-cluster of galaxies ever identified: A2744z7p9OD, found just 650 million years after the Big Bang. These objects are not yet gravitationally bound together, but will become so over time, and will wind up forming a galaxy cluster upward of 1 quadrillion solar masses. Owing to the incredible observations of JWST, we can identify these proto-clusters even when they’re still in the early-stages of the process of formation. Credit: NASA, ESA, CSA, Takahiro Morishita (IPAC); Processing: Alyssa Pagan (STScI) More mature galaxy clusters don’t form until around 2 or 3 billion years after the Big Bang: right around the time that the cosmic star-formation rate reaches its peak. The cosmic web becomes well-defined, with strings of galaxies connecting multiple clusters together across grand cosmic filaments, while the expansion continues and great bubbles largely devoid of matter (with only a small number of stars and galaxies within them) — the cosmic voids — separate the structure-rich regions. Even after the star-formation rate begins dropping, from an age of 4 billion years up until the Universe is more than 7 billion years old, the sizes of the largest cosmic structure that form, which are the grand walls and filaments, continue to grow. And then, about 7.8 billion years after the Big Bang, something else happens: the matter density, including both normal and dark matter, drops to such a low level that a new component of cosmic energy, dark energy, now dominates the cosmic expansion. Instead of gravitation fighting against the initial impetus provided by the expansion that the Big Bang started off with, it now attempts to fight the effects of dark energy as well: effects that become stronger on progressively larger cosmic scales. While smaller-scale structures — gas clouds, galaxies, or galaxy groups and clusters — can still merge together, no gravitationally bound structure that will wind up being larger than 1.4 billion light-years can ever assemble. The Sloan Great Wall is one of the largest apparent, though likely transient, structures in the Universe, at some 1.37 billion light-years across. It may just be a chance alignment of multiple superclusters, but observations definitively indicate that it isn t a single, gravitationally bound structure, as dark energy is in the process of driving it apart. The galaxies of the Sloan Great Wall are depicted at right. In 2025, a structure about 2% longer, Quipu, was discovered, although the uncertainties on their sizes overlap with each other, as well as with the South Pole Wall. Credit: Willem Schaap (L); Pablo Carlos Budassi (R)/Wikimedia Commons And that takes us up to the present day: 13.8 billion years after the Big Bang first occurred. When we look out at the Universe, we see the history of that structure formation story written everywhere we look. Stars and star clusters are common, and almost all of them exist embedded within larger “island universes,” or galaxies, to the tune of several trillion galaxies within our observable Universe. Although a few galaxies are relatively isolated (known as field galaxies), rich collections of galaxies, in groups (like our own Local Group) or clusters (like the nearby Virgo Cluster), are more prominent and more impressive. On larger scales, we often find galaxy clusters located nearby one another, in the process of colliding, or in the aftermath of having recently collided-and-merged: evidence that even on these large cosmic scales, the Universe continues to gravitate. We find direct evidence of baryon acoustic oscillations — or the plasma oscillations from way back before neutral atoms formed — imprinted in our cosmic structure today, and we’ve found many cosmic filaments that approach that theoretical maximum scale: about 1.4 billion light-years in extent. Although others have claimed to find evidence for even larger structures encoded in gamma-ray bursts or quasar signatures, that evidence is weak and heavily disputed; they are taken to be coincident alignments based on incomplete observations, rather than established structures. The structure Ho’oleilana, a candidate for an individual baryon acoustic oscillation, can be identified visually by the human eye as a circular feature around 500 million light-years across. The red circle, shown in animation, makes the presence of this acoustic oscillation even clearer on scales of 155 Mpc or so: about 500 million light-years. This corresponds to the expected acoustic scale with an amplitude that matches the 5-to-1 dark matter-to-normal matter ratio expected from other lines of evidence. Credit. R.B. Tully et al., ApJ, 2023 That’s only what we find overall, of course. How does this relate to what we find when it comes to ourselves and the structures, whether bound or not, that we ourselves are a part of? To answer that, we have to survey our local corner of the cosmos as accurately and comprehensively as possible. That includes not only mapping out the three-dimensional positions of objects to a great degree of accuracy, but also measuring the motions (including the relative motions) of those objects. We need to do that in order to determine just how things are clumped, clustered, and grouped together, and also to find out which structures are gravitationally bound together versus which ones are caught up in the inexorable expansion of the Universe, destined to drift farther and farther apart as time elapses. On small scales, that’s relatively easy. We live on planet Earth: a gravitationally bound structure in hydrostatic equilibrium. Earth is bound to the Sun as part of the Solar System: a gravitationally bound system. The Sun isn’t bound to any other star or any substructure within the Milky Way (like a spiral arm or a spur of one), but is bound to the entire Milky Way galaxy itself, of which we are a part. The Milky Way is just one of hundreds of galaxies within the Local Group, and the Local Group itself is a gravitationally bound entity: stretching for between 3-and-5 million light-years in all directions. This map of many of the galaxies within the Local Group highlights the three biggest members: Andromeda, the Milky Way, and Triangulum. Galaxy WLM, shown at the bottom of the image, lies about 3 million light-years from the Milky Way and is extremely isolated. It contains some of the oldest, most pristine stars within our cosmic backyard, close enough to be resolved by observatories such as JWST. The most distant Local Group members are between 3-and-5 million light-years away; beyond that, no other galaxies or groups of galaxies are gravitationally bound to our own galaxy or group of galaxies. (Credit: Richard Powell; Annotation: E. Siegel) Beyond the extent of the Local Group, however, the gravitationally bound story ends, at least for us. While larger galaxy groups and clusters not only exist, but exist relatively nearby, it turns out that we aren’t ourselves bound to any of them. Every other galaxy group and cluster that we find, both near and far, is receding away from us within the expanding Universe, and is receding at speeds faster too fast for the mutual gravitational force between our galaxy/Local Group and the galaxy/group/cluster in question to overcome. The M81 group, the Leo group, the Centaurus A group, the Virgo cluster, Coma cluster, and Perseus cluster, as well as all other identified galaxy groups and clusters, while gravitationally bound themselves, will only continue to recede away from us, evermore faster and faster, as time marches on. If you start to ask, however, “what is the next-largest structure that the Local Group is a part of,” the answer starts to get murky. Sure, the term “local sheet” was defined way back in 1987, when astronomer Brent Tully (famed for the Tully-Fisher relation) first described it in his book Atlas of Nearby Galaxies, and includes 14 major galaxies that span some 34 million light-years in length, but along a narrow, filament-like path only about 1.5 million light-years wide. The nature of the local sheet, as well as the 14 major member galaxies known as the Council of Giants, is still heavily debated, as some contend that this is all one filamentary structure being pulled in the same overall direction, while others contend that different components have differing motions, and therefore they cannot be considered part of the same structure. This image shows the 14 member galaxies of the so-called “Council of Giants” that represent the massive, bright, major galaxies that line up along the 34 million light-year long structure known as the local sheet. There is a long-standing and still-running debate as to whether the local sheet and the Council of Giants represents a single filamentary structure, or whether these are multiple, independent structures that simply overlap (or nearly overlap) in space. Credit: Piquito veloz/Wikimedia Commons What’s often called the “local volume” is even less of a structure, as it really just focuses on galaxies that happen to be located within around 40 million light-years of the Milky Way. Rather than resulting from some overall shared property, or membership in some sort of bound collection, the local volume is very loosely defined by its proximity to us, and our ability to measure stars and other specific properties within this region of space. When we do conduct those measurements, however, we find — unsurprisingly — that there is indeed a common property that all of these galaxies possess: they appear to be drifting, along with us, in the same general direction. Not only that, but the drift is of approximately the same magnitude, and is even relative to the Hubble flow: the flow of galaxies in the context of the expanding Universe. The reason for this isn’t because the local volume is necessarily a meaningful structure, however. It’s because, when we map out the Universe on still larger scales, we find that there are large clumps of matter — i.e., large collections of mass — that gravitationally tug on all of the galaxies and galactic groups in our vicinity. They include the massive Virgo cluster, the Antlia cluster, the Laniakea supercluster (of which we, and the Virgo cluster are components of), and the enormous Perseus-Pisces supercluster as well. The motions of nearby galaxies and galaxy clusters (as shown by the ‘lines’ along which their velocities flow) are mapped out with the mass field nearby. The greatest overdensities (in red/yellow) and underdensities (in black/blue) came about from very small gravitational differences in the early Universe. Today, a great many nearby galaxies have shifted in position and have had their motions affected by the gravitational effects of matter in their local vicinity, where they depart significantly from the Hubble flow. Credit: H.M. Courtois et al., Astronomical Journal, 2013 On an even deeper level, what’s occurring is that the Universe has not just clumped and clustered to form a vast cosmic web with massive galaxies, galaxy clusters, and cosmic sheets, walls, and filaments lining it, but an even vaster Swiss-cheese-like network of holes: an enormous set of cosmic voids. Just as the overdense clumps of mass and matter pull on (and have pulled on, throughout history) the individual masses and structures like the Milky Way and Local Group, the lack of mass in the underdense regions — in those great cosmic voids — by virtue of being less dense and massive than an average region of space, fails to attract and pull on matter with even an average cosmic force. We don’t often think of gravity in these terms, but because the Universe is so uniform on the large-scale cosmic average, we have to include: the clumped-up masses in the overdense regions and their effective gravitational attraction, and the underdense regions, or voids, and their effective gravitational repulsion, relative to the average cosmic density and the average gravitational force pulling on matter, omnidirectionally. We used to map out our peculiar velocity, or our motion relative to the Hubble flow, and infer the existence of a supremely massive region that we expected was there, but couldn’t see, even giving it an epic name: the Great Attractor. While there are additional sources of mass that do pull on the galaxies and galactic groups in our vicinity, including the Norma cluster, the full extent of Laniakea, and the Shapley supercluster, the truth is that we can only fully understand our actual, observed cosmic motion if we include both overdense and underdense regions. Because matter is distributed roughly uniformly throughout the Universe, it isn’t just the overdense regions that gravitationally influence our motions, but the underdense regions as well. A feature known as the dipole repeller, illustrated here, was discovered only recently and may explain our Local Group’s peculiar motion relative to the other objects in the Universe. Credit: Y. Hoffman et al., Nature Astronomy, 2017 While we can always look to larger scales than galaxies, and talk about “membership” in galactic groups, clusters, filaments, sheets, walls, and superclusters, the reality is much more complex than the naive picture of identifying and grouping collections of matter together. We need to include not only overdense, structure-rich regions, but also the effects of underdense, structure-poor and structure-free regions, to understand how galaxies and other collections of matter move and flow, relative to the expanding Universe, across cosmic time. The most important delineation in the Universe is between structures that are gravitationally bound, both internally and to other structures (like galaxies, groups of galaxies, clusters of galaxies, and separate clusters of galaxies that will someday collide-and-merge), and structures that are unbound and will dissociate as cosmic time marches on: what I’ve called pseudostructures, like filaments, sheets, walls, and yes, even superclusters. And beyond that, when it comes to those pseudostructures, we must take care in delineating what actually is a single, meaningful collection of matter that earnestly represents a collection of matter formed from the influence of the initial gravitational imperfections seeded within the expanding Universe, and what happens to merely be an apparent, chance alignment of objects in space: a very easy way to fool ourselves into believing we’ve found something that’s too big to make sense in the context of modern cosmology. In between the great clusters and filaments of the Universe are great cosmic voids, some of which can span hundreds of millions of light-years in diameter. The long-held idea that the Universe is held together by structures spanning many hundreds of millions of light-years, these ultra-large superclusters, has now been settled, and these enormous web-like features are destined to be torn apart by the Universe s expansion, while the cosmic voids continue to grow. Only individually bound galaxies, groups of galaxies, and clusters of galaxies will persist. Credit: Andrew Z. Colvin and Zeryphex/Astronom5109; Wikimedia Commons Perhaps ironically, it’s somewhat easier to identify the largest cosmic (pseudo)structures at great distances, like the CfA2 Great Wall, the BOSS Great Wall, the Sloan Great Wall or the South Pole Wall, than it is to identify a similar structure that may include us, like the Pisces-Cetus Supercluster Complex. While we have the benefit of being nearby the component members of any structure (or pseudostructure) that we’re a part of, the fact that there are enormous differences in distance, brightness, and dust/obscuration effects for the different components of it make determining the extent, properties, and our membership within these structures an extremely complex task. Nevertheless, the biggest bound structure that we’re a part of is simply the Local Group of galaxies, and the biggest unbound structure that we’re a member of is the supercluster complex known as Laniakea, which now subsumes and includes earlier named superclusters such as the Virgo supercluster and the Hydra-Centaurus supercluster. If we make a larger-scale definition that allows nearby basins of attraction to be considered part of the same structure, regardless of the reason (even if it’s just a random chance alignment or near-overlap), then we may well wind up calling those gravitationally unbound, physically uncorrelated collections of matter things like “supercluster complexes,” where they might truly exceed the cosmic limit for what a formally-defined structure can achieve. What we call things, of course, is always up to us: naming conventions are inherently human, and not necessarily scientific, endeavors. You can continue to talk about the local sheet, the local volume, superclusters, and superclusters complexes all you like; many astronomers do as well. However, if you insist on physically relevant criteria, such as connectedness, gravitationally bound/unbound delineations, or having originated from the same set of initial cosmic imperfections, you’ll wind up stopping — for our own Milky Way and its alleged membership in various categories of “structure — as soon as you exceed scales of the Local Group. No matter how long of a linear feature, how huge of a collection of galaxies, or how many other galaxies flow towards a collection of mass, unless gravity binds you together, you’re ultimately destined to simply dissociate and be torn apart by our relentlessly expanding Universe. Send in your Ask Ethan questions to startswithabang at gmail dot com! This article Ask Ethan: What super-galactic structures are we a part of? is featured on Big Think.
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