The mysterious Great Attractor thats pulling us in Ars Technica

Our Milky Way galaxy is speeding through the emptiness of space at 600 kilometers per second, headed toward something we cannot clearly see. The focal point of that movement is the Great Attractor, the product of billions of years of cosmic evolution. But we’ll never reach our destination because, in a few billion years, the accelerating force of dark energy will tear the Universe apart.

Whispers in the sky

Beginning as early as the 1970s, astronomers noticed something funny going on with the galaxies in our nearby patch of the Universe. There was the usual and expected Hubble flow, the general recession of galaxies driven by the overall expansion of the Universe. But there seemed to be some vague directionality on top of that, as if all of the galaxies near us were also heading toward the same focal point.

Astronomers debated whether this was a real effect or some artifact of Malmquist bias, the bias we get in our observations because bright galaxies are easier to observe than dim ones (for fans of statistics, its just another expression of a selection effect). It could be that a complete census of the nearby cosmos, including the much more numerous small and dim galaxies, would erase any apparent extra movement and return some sanity to the world.

But then came more detailed observations of the cosmic microwave background (CMB). The CMB is the leftover light from when our Universe cooled from a plasma state and formed neutral atoms when it was a mere 380,000 years olda relative infant compared to its present 13.77 billion years of existence. The CMB absolutely soaks the sky (and, indeed, the entire Universesomething like 99.99 percent of all photons in the cosmos are part of the CMB), coming at us from every direction.

If I were to show you a map of the CMB across the whole sky, it wouldnt look all that impressivejust a uniform blob of photons covering every square degree with a remarkably consistent temperature of around 2.75 Kelvin. But with enough sensitivity, you can detect a subtle, one-part-in-a-thousand difference. The CMB is ever-so-slightly hotter in one direction in the sky, and it’s equally cooler in the opposite direction.

This is the CMB dipole, caused by the movement of the Earth through the Universe. Photons arriving from the forward direction get blueshifted to slightly higher energies, while photons coming up from behind us get redshifted to lower energies. Measuring the strength of that shifting reveals our total current speedroughly 600 kilometers per secondand our direction: somewhere toward the constellation Centaurus.

We can easily account for some of that motion. The Sun is orbiting around the center of the Milky Way galaxy, and our galaxy itself is headed on a collision course with our nearest neighbor, the Andromeda galaxy. Those combined motions account for some of the 600 km/s, but not all of it.

It appears that weand almost all the galaxies around usare barreling toward some random spot in the Universe, compelled to move against our will by a distant and unknown source of immense gravity.

The Great Attractor.

The no-no zone

The Great Attractor wouldnt be that big of a mystery except for an exceptionally unlucky coincidence. By the late 1970s, astronomers had become particularly adept at building surveys of the distant Universe, charting the positions and distances of thousands of galaxies up to hundreds of millions of light-years away. Those surveys revealed a beautiful, intricate interleaving pattern of galaxies known as the cosmic web.

The surveys became ever more complete and comprehensive, except in a particular set of directions on the sky known as the Zone of Avoidance. The problem is that we also live inside a galaxy, and that galaxy is filled with all sorts of dust: big giant clouds of dust in the process of forming stars, small clumps of dust around dead stars, and wandering random dust particles not participating in star formation at all. All this dust creates extinction (the astronomical, not the biological, kind), which is the scattering and reddening of visible light.

Since most of those early surveys (and, to be fair, most contemporary surveys) operate in visible wavelengths, the vast dusty bulk of the Milky Way obscures our view of any galaxies situated beyond the plane of the galactic disk. This is the Zone of Avoidance, where visible light observations fall short, leaving a here-be-dragons blank space in our otherwise thorough surveys.

Of course, the Great Attractor lies within the Zone of Avoidance. For decades, we had little to no information about the structure of the Universe in that direction and hence had almost no clue about the identity or contents of the Great Attractor. Something was over therewe could conclusively determine that based on our movementbut we didnt know exactly what.

Astronomers had two options. First, they could wait for the natural orbit of the Solar System around the center of the Milky Way to wheel us into a better viewing position. But that would take approximately 100 million years (slightly longer than typical grant-funding cycles), so that wasn’t feasible.

The second option was to get creative. Never ones to let a celestial object go unobserved, astronomers turned to other wavelengths of light to peer behind the dust of our galaxy and into the depths of the Universe. X-ray light is great at penetrating dust, but it only reveals the brightest galaxies actively undergoing star formation and the massive-but-rare clusters of galaxies. Thankfully, infrared is much more versatile and is able to peer into great distances, as the James Webb Space Telescope has so aptly demonstrated.

Beginning in the 1990s, astronomers began performing X-ray and infrared surveys within the Zone of Avoidance. Its painstaking work, requiring extensive telescope time to gather the light necessary to estimate distances, and it’s capable of capturing only a handful of galaxies or clusters at a time. Those maps are not yet complete, but they are beginning to give us our first comprehensive picture of the region of the Universe centered on the Great Attractor.

Immeasurable heaven

Lets start with our home. The Milky Way galaxy stretches for roughly 100,000 light-years and contains hundreds of billions of stars. Our nearest neighbor, Andromeda, is an even larger galaxy sitting 2.5 million light-years away from us. Together with Triangulum (a smaller galaxy that doesnt get nearly enough press) and dozens of dwarf galaxies, we comprise the Local Group.

Be sure to make good friends with all the members of the Local Group; were bound together by our mutual gravity forever, and in a few billion years, well all merge together into a single mega-galaxy.

The Local Group is moving as a single unit toward the nearest metropolis in our cosmological neck of the woods: the Virgo cluster. Sitting roughly 50 million light-years away from us, the Virgo cluster is home to over 1,000 individual galaxies compressed into a relatively tight ball about 5 million light-years across. The Virgo cluster is by far the most massive object in our neighborhood; if you combine all its stars, gas, and dark matter, the cluster weighs over a billion trillion solar masses.

The Virgo cluster surrounds itself with a retinue of smaller groups, pulling each one toward it with its immense gravity.

The Virgo cluster and the surrounding groups form whats known as the Virgo supercluster (an unfortunate and confusing duplication of names, but it is what it is). Unlike clusters and groups, superclusters are not gravitationally bound and have not yet completely collapsed. Astronomers once thought that the Virgo supercluster was the largest structure in the nearby Universe, but more extensive surveys within the Zone of Avoidance have revealed the true scope of the picture: The Virgo supercluster is but one branch of an even larger supercluster (thankfully, astronomers resisted the temptation to call it a hypercluster) known as Laniakea, a Hawaiian word roughly translating to immeasurable heaven.

The name is fitting. Laniakea comprises four supercluster branches totaling over 500 groups and clusters with more than 100,000 individual galaxies. The massive, tangled complex stretches for over half a billion light-years.

And the Great Attractor sits at its heart.

Shadow movements

To understand what the Great Attractor is and why its at the center of the Laniakea supercluster, we have to rewind a bit. Say, 13 billion years.

We live in what cosmologists call a hierarchical universe. The cosmos overall is expanding, with the average distance between galaxies at large scales growing ever larger with time. But at smaller scales (and in cosmology, anything less than a hundred million light-years is small), our present-day Universe is a result of a multi-billion-year construction effort.

In the extremely early Universe, everything was pretty much even, with no huge density differences from place to place. But at cosmological scales, the only force at playweak, feeble, but persistent gravitygrabbed hold of the small density differences that did exist. Acting ever so slowly through hundreds of millionsand then billionsof years, gravity worked on those tiny initial density differences. We can see the first evidence of that work in the CMB itself. Beneath the dipole sit one-part-in-a-million temperature differences, a sign of the first density fluctuations that would grow to dominate the entire cosmos.

Through the unceasing efforts of gravity, the rich get richer and the poor get poorer. Regions of higher density have a stronger gravitational pull, allowing them to collect more material. With enriched density, they have an even stronger pull and collect even more mass. Over time, the density differences in our Universe grow higher, with the low-density pockets emptying to become cosmic voids and the high-density regions growing to become stars, galaxies, groups, clusters, and eventually superclusters.

This large-scale construction project is still underway. The galaxies and clusters collapsed and stabilized billions of years ago, but gravity is not done. The patient gravitational force is still constructing the superclusters, including the great Laniakea, pulling on their constituent parts in an attempt to build settled, stable objects bound by their own gravity.

Given this context, its best not to think of the Great Attractor as a thing but as a place. Its a locus, a focal point of gravitational attraction. Its the bottom of the well, so to speak, of our local gravitational environment. Its where everything in this vicinity of the cosmos is headed toward, the happening downtown of happening downtowns. The ultimate place to see and be seen.

The Great Attractor is the end result of billions of years of slow but inexorable gravitational work, the inevitable conclusion to this grand construction project. The Great Attractor isnt just where we happen to be heading; its where weve always meant to be headed. It is our future, our fate.

And yet we will never reach it.

The great loneliness

Cosmologists continue to debate the exact contents and location of the Great Attractor. We can only infer its mass and location based on relatively sparse surveys within the Zone of Avoidance and reconstructions of the movements of those galaxies (which is slightly difficult because we cant watch anything move in real time given the enormous scales involved).

The supposed location of the Great Attractor already contains an immense assemblage of mass known as the Norma Cluster, which is located over 200 million light-years away. Our own Virgo cluster and all its surrounding galaxies are on the move toward Norma, which sits at the center of the flow of all the galaxies within Laniakea.

Recent studies have revealed more complexity. Norma itself appears to be moving toward another supercluster, the Shapley supercluster, which might be even larger than our own Laniakea, indicating that while Norma sits at the bottom of our local gravitational well, there is an even deeper well nearby. Other reconstructions of our local Universe indicate that yet another supercluster unrelated to the Great Attractor, the Vela, contributes to some of our own motion.

Unfortunately, all of this is temporary. Gravitys great engines of creation shut off over 5 billion years ago. The cosmic web, with its vast and interconnected filaments of clusters and superclusters, will never be completed.

The culprit is dark energy, that bastard of the cosmos. Astronomers do not understand what dark energy is, but they do understand what it does. Whatever it is, its causing the expansion of the Universe to accelerate. Over time, any galaxies that are not already gravitationally bound to each other will find themselves flying apart at ever faster rates.

Dark energy has always been here in the background. Early on in the history of the cosmos, the combined gravitational pull of all the mass in the Universe was enough to overwhelm the accelerating effects of dark energy, allowing gravity to build large structures in peace. But 5 billion years ago, the matter contents of the cosmos diluted too much, allowing dark energy to take hold.

Our Local Group will survive the coming expansive annihilation, but Laniakea will not. We will never even reach the Virgo cluster, let alone the site of the Great Attractor. Slowly, over the next few billion years, our motion toward the Great Attractor will slow, then stop, then reverse.

Our far-future descendants will find themselves pushed away from what they will surely call the Great Repeller, with the merged mega-galaxy that was once the Local Group their only refuge in an increasingly dark, cold, and lonely cosmos. The beautiful and intricate cosmic web, with its uncounted superclusters, will unravel, each group and cluster left to fend for itself in the endless night of the distant future.

Nothing lasts forever in this Universe, not even the most powerful gravitational force in the nearby cosmos. Some things were never meant to be, it seems, and some work is always meant to remain unfinished.