“It’s the gravity that shapes the large scale structure of the universe, even though it is the weakest of four categories of forces.” -Stephen Hawking

On the largest scales, the Universe looks like a vast cosmic web. Stars link up into galaxies, which are clustered together in galactic groups. Many groups bound together lead to galaxy clusters, and occasionally clusters merge together, creating even larger clusters. Many clusters together, spanning hundreds of millions or even billions of light years across, appear to make the largest structures of all: superclusters. Our own supercluster, Laniakea, consists of approximately 100,000 galaxies, more than 10 times richer than the largest known clusters. Yet these superclusters only appear to be structures. As the Universe ages, the individual components of superclusters are being pushed apart, showing that they’re not true structures after all.

There’s a simple recipe for building the Universe as we know it today: take a sea of matter and radiation that starts off hot, dense and expanding, and give it time to cool. Over long enough timescales, atomic nuclei, neutral atoms, and eventually stars, galaxies, and clusters of galaxies will form. The irresistible force of gravity makes this inevitable, thanks to its effects on both the normal (atomic) matter we know and the dark matter filling our Universe, whose nature is still unknown.

Over time, gravitational interactions will turn a mostly uniform, equal-density Universe into one with large concentrations of matter and huge voids separating them. Image credit: Volker Springel.

When we look out into the Universe — beyond our galaxy to the largest known structures beyond — this picture has tremendous supper. At least it appears to, at first glance. While many galaxies exist in isolation, or grouped together in collections of only a few, there are also huge gravitational “wells” in the Universe, that have pulled in hundreds or even thousands of galaxies, creating enormous clusters. Quite often, there are supermassive elliptical galaxies at the center, with the most massive yet discovered shown below: IC 1101, which is more than a thousand times as massive as our own Milky Way.

The giant galaxy cluster Abell 2029, and its central galaxy, IC 1101. Image credit: Digitized Sky Survey / NASA.

So what’s larger than a galaxy cluster? A supercluster, of course. Superclusters are collections of clusters connected by great cosmic filaments of dark-and-normal matter, whose gravitation mutually attracts them towards their common center-of-mass. You wouldn’t be alone if you thought it was only a matter of time — time and gravity, that is — until the clusters comprising a supercluster all merged together. When that happens, the thought goes, you’ll wind up creating a single bound, cosmic structure of unparalleled mass.

A large collection of many thousands of galaxies makes up our nearby neighborhood within 100,000,000 light years. It’s dominated by the Virgo Cluster, but many other mass collections abound. Image credit: Wikimedia Commons user Andrew Z. Colvin.

In our own neighborhood, the local group, made up of Andromeda, the Milky Way, Triangulum and maybe 50 smaller, dwarf galaxies, lies on the outskirts of the Laniakea supercluster. Our location places us some 50,000,000 light years away from the main source of mass in our nearby Universe: the massive Virgo Cluster, which contains over a thousand Milky Way-sized galaxies. Along the way, many other galaxies, groups of galaxies and smaller clusters can be found.

On even larger scales, the Virgo cluster is only one of many in the portion of the Universe we’ve mapped, along with the two next-nearest ones: the Centaurus cluster and the Perseus-Pisces cluster. Where the galaxies are most concentrated represent the largest clusterings of mass; where the lines connect them, along filaments, we find “strings” of galaxies, like pearls strung too thin on a necklace; and in the great bubbles between the filaments, we find huge underdensities of matter, as those regions have given up their mass to the denser ones.

The greatest overdensities (in red) and underdensities (in black) came about as small gravitational differences in the early Universe evolved over billions of years. Image credit: Helene M. Courtois, Daniel Pomarede, R. Brent Tully, Yehuda Hoffman, Denis Courtois, from “Cosmography of the Local Universe” (2013).

If we take a look at our own neighborhood, we find that there’s a large collection of more than 3,000 galaxies that makes up the large-scale structure that includes ourselves, Virgo, Leo and many other surrounding groups. The dense Virgo cluster is the largest part of it, making up a little more than a third of the total mass, but there are many other concentrations of mass within it, including our own local group (shown in blue, below), connected together by the invisible force of gravity and the unseen filaments of dark matter.

The identified galaxies shown here, circled and represented by white dots, compose what was dubbed the Laniakea Supercluster. Image credit: R. Brent Tully (U. Hawaii) et al., SDvision, DP, CEA/Saclay.

We call this supercluster “Laniakea,” the Hawaiian word for immense heaven. It links up our own massive cluster, Centaurus, the Great Attractor and many others, and contains over 100,000 galaxies total. Additionally, it’s a beautiful name, a beautiful idea, and a beautiful collection of galaxies that includes us. But there’s a problem with not only Laniakea, but with the idea of a supercluster in general: it isn’t real.

Outlined in light blue, giant collections of galaxies can be divided up into superclusters. But this classification doesn’t make superclusters real. Image credit: The Laniakea supercluster of galaxies R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède, Nature 513, 71–73 (04 September 2014).

Our Universe isn’t just the combined effects of an initial expansion along with the counteracting, attractive force of gravitation. In addition, there’s also dark energy, or the energy intrinsic to space itself, which causes the recession of distant galaxies to accelerate, or speed up, as time goes on. The struggle between gravitational attraction (which pulls distant masses together) and the expansion of the Universe (dominated by dark energy) actually had its end determined some six billion years ago, when dark energy became the dominant factor in our Universe. At that point, any objects that weren’t already gravitationally bound to one another — where gravitation hadn’t overcome the expansion of the Universe — never would become so.

What we used to identify as superclusters were superseded by even larger structures like Laniakea. But contrary to what we thought, they aren’t structures at all, as they’re gravitationally unbound. Image credit: Richard Powell of http://www.atlasoftheuniverse.com/nearsc.html, under C.C.-by-S.A.-2.5.

It means that all the identified superclusters are unbound from one another, but even worse, it means that the individual groups and clusters that we know within a supercluster like our own are, for the most part, unbound from one another as well. It means we’ll never merge with the Virgo cluster; it means we’ll never merge with the Leo group, the N96 group, or pretty much anything outside of our local group. It means that except for the few groups or clusters which were already gravitationally bound to one another billions of years ago, no new ones ever will become so. What’s bound today is all that will ever be bound together in the future.

Galaxy clusters, like Abell 1689, are the largest bound structures in the Universe. Other, larger collections aren’t actual structures, but merely temporary alignments that will disappear over time. Image credit: NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d’Astrophysique de Marseille, CNRS, France).

Clusters? Yes.

Groups, galaxies and smaller structures? Absolutely.

But superclusters? They’re only visual figments of our imagination.

They’re not real structures. They’re not bound together, and they’ll never become so. The idea of a supercluster and the name for ours, “Laniakea,” will persist for a long time. But just because we named it doesn’t make it real. Billions of years from now, all the different components will simply be strewn farther and farther apart from one another, and in the farthest futures of our imaginings, they’ll disappear from our view and reach entirely. It’s all because of the simple fact that superclusters, despite their names, aren’t structures at all, but merely temporary configurations destined to be torn apart by the expansion of the Universe.