“It is far harder to kill a phantom than a reality.” -Virginia Woolf

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.

When we look out into the Universe — beyond our galaxy to the largest known structures beyond — this picture appears to be supported tremendously, at least, at first glance. While many galaxies exist in isolation, or grouped together (as ours is) in collections of very 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.

Image credit: Digitized Sky Survey 2, NASA, of the giant galaxy cluster Abell 2029, and its central galaxy, IC 1101.

So what’s larger than a galaxy cluster?

A supercluster, of course, or a collection of clusters connected by these 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 these clusters all merged together, creating a single bound, cosmic structure of unparalleled mass.

Image credit: Andrew Z. Colvin, via Wikimedia Commons.

In our own neighborhood, the local group, made up of Andromeda, the Milky Way, Triangulum and maybe 40–50 smaller, dwarf galaxies, lies on the outskirts of the Virgo 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. And on even larger scales, the Virgo supercluster is only one of many in the portion of the Universe we’ve mapped, along with the two next-nearest ones: the Centaurus supercluster and the Perseus-Pisces supercluster.

Image credit: Helene M. Courtois, Daniel Pomarede, R. Brent Tully, Yehuda Hoffman, Denis Courtois, from “Cosmography of the Local Universe” (2013).

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.

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 our own supercluster. 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.

Image credit: R. Brent Tully (U. Hawaii) et al., SDvision, DP, CEA/Saclay, of Laniakea, our local “supercluster” of galaxies.

We call this supercluster “Laniakea,” the Hawaiian word for immense heaven. And it’s a beautiful name, a beautiful idea, and a beautiful group of thousands of galaxies that includes us.

But there’s a problem with not only Laniakea, but with the idea of a supercluster in general: it’s not real.

Image credit: Tully, R. B., Courtois, H., Hoffman, Y & Pomarède, D. Nature 513, 71–73 (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.

Image credit: Richard Powell of http://www.atlasoftheuniverse.com/nearsc.html, under C.C.-by-S.A.-2.5. This image spans roughly 500 million light-years in radius.

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.

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” are only visual figments of our imagination. They’re not real structures. They’re not bound together, and they’ll never become so. You can learn the word “supercluster” or the name for ours, “Laniakea.” But just because we named it doesn’t make it real. Billions of years from now, all the different components will simply be farther and farther apart from one another, and in the farthest futures of our imaginings, they’ll disappear from our view and reach entirely. And it’s all because of the simple fact that superclusters, despite their names, aren’t structures at all, but are temporary configurations destined to be torn apart by the expansion of the Universe.