Imagine visiting a far distant galaxy and addressing a postcard to your loved ones back home. You might begin with your house on your street in your hometown, somewhere on Earth, the third planet from our sun. From there the address could list the sun's location in the Orion Spur, a segment of a spiral arm in the Milky Way's suburbs, followed by the Milky Way's residence in the Local Group, a gathering of more than 50 nearby galaxies spanning some seven million light-years of space. The Local Group in turn exists at the outskirts of the Virgo Cluster, a 50-million-light-year-distant cluster of more than 1,000 galaxies that is itself a small part of the Local Supercluster, a collection of hundreds of galaxy groups sprawled across more than 100 million light-years. Such superclusters are believed to be the biggest components of the universe's largest-scale structures, forming great filaments and sheets of galaxies surrounding voids where scarcely any galaxies exist at all.
Until recently, the Local Supercluster would have marked the end of your cosmic address. Beyond this scale, it was thought, further directions would become meaningless as the boundary between the crisp, supercluster-laced structure of galactic sheets and voids gave way to a homogeneous realm of the universe with no larger discernible features. But in 2014 one of us (Tully) led a team that discovered we are part of a structure so immense that it shattered this view. The Local Supercluster, it turns out, is but one lobe of a much larger supercluster, a collection of 100,000 large galaxies stretching across 400 million light-years. The team that discovered this gargantuan supercluster named it Laniakea—Hawaiian for “immeasurable heaven”—in honor of the early Polynesians who navigated the great expanse of the Pacific Ocean by the stars. The Milky Way sits far from Laniakea's center, in its outermost hinterlands.
Laniakea is more than just a new line on our cosmic address. By studying the architecture and dynamics of this immense structure, we can learn more about the universe's past and future. Charting its constituent galaxies and how they behave can help us better understand how galaxies form and grow while telling us more about the nature of dark matter—the invisible substance that astronomers believe accounts for about 80 percent of the stuff in the universe.
Laniakea could also help demystify dark energy, a powerful force only discovered in 1998 that is somehow driving the accelerating expansion of the universe and thus shaping the cosmos's ultimate fate. And the supercluster may not actually be the final line of our cosmic address—it could, in fact, be part of an even bigger structure yet to be discovered.
Probing Mysteries with Galactic Flows
The team that discovered Laniakea did not exactly set out to find it. Instead Laniakea emerged from efforts to answer lingering fundamental questions about the nature of the universe.
Scientists have known for nearly a century that the cosmos is expanding, pulling galaxies away from one another like dots moving apart on the surface of an inflating balloon. Yet in recent decades it has become clear that most galaxies are not separating as fast as would be expected if expansion were the only force acting on them. Another, more local force is at work, too—the gravitational tugs from other nearby accumulations of matter, which can offset a galaxy's flow in the expansion of the cosmos. The difference between a galaxy's motion from cosmic expansion and the motion from its local environment is called peculiar velocity.
If we take all the stars in all the galaxies that we see and add all the gas and other ordinary matter that we know about, we fall short of explaining the gravitational sources of observed peculiar velocities by an order of magnitude. In our ignorance we astronomers call what is missing “dark matter.” We presume this dark matter consists of particles that interact almost exclusively with the rest of the universe through gravity rather than through other forces such as electromagnetism and that the dark matter exerts the “missing” gravitational force needed to account for the observed velocities. Scientists think that galaxies sit within deep pools of dark matter—dark matter being the invisible scaffolding around which galaxies coalesce.
Tully's group and others realized that creating maps of galactic flows and peculiar velocities could reveal dark matter's hidden cosmic distribution, uncovering the mysterious substance's greatest concentrations by their gravitational influence on galaxies' motions. If, say, streams of galaxies are all flowing toward a particular point, one can assume that the galaxies are being gravitationally drawn to that point by a highly dense region of matter.
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They also realized that pinning down the density and distribution of all forms of matter in the universe could help solve another, even deeper mystery: the fact that the cosmos is not only expanding but is doing so at an accelerating rate. This behavior is as counterintuitive as a rock tossed up in the air rushing onward into the heavens rather than falling back to Earth. Whatever powers this bizarre phenomenon is dubbed “dark energy” and has profound implications for the future of the universe. The accelerated expansion suggests the cosmos will ultimately experience a cold death, with most galaxies racing away from one another at accelerating speeds until a final darkness descends on the universe as every star in every galaxy dies and all matter cools to absolute zero. But knowing for sure how it all will end requires not only determining what exactly dark energy is but also how much matter is in the universe: Given a sufficiently high density of matter, in the far future our universe could reverse its expansion to collapse in on itself as a consequence of the self-gravity of its cumulative mass. Or it could instead possess a balanced density of matter that would lead to an infinite but ever slowing expansion.
It was this charting of galactic flows to map the cosmic density of ordinary and dark matter that ultimately led to the unveiling of Laniakea.
Mapping galactic flows requires knowing both a galaxy's motion stemming from cosmic expansion and its motion stemming from nearby matter. As a first step, astronomers measure a galaxy's redshift—the stretching out of the galaxy's emitted light as it recedes away from us through the expanding universe. A whistle or siren moving toward us has a higher pitch than if moving away because its sound waves are compressed to higher frequencies and shorter wavelengths. Similarly, light waves from a galaxy moving away from us are shifted to lower frequencies and longer, redder wavelengths—the faster they are receding, the more redshifted they become. Thus, a galaxy's redshift gives astronomers a measure of its overall velocity and a rough estimate of its distance.
Astronomers can infer how much of a galaxy's velocity is the result of local gravitational tugging by measuring its distance through other techniques besides redshift. For instance, based on rigorous estimates of the universe's expansion rate, a galaxy measured to be 3.25 million light-years away should have a velocity of about 70 kilometers per second. If instead the galaxy's redshift yields a velocity of 60 kilometers per second, astronomers could infer that matter concentrations near that galaxy are giving it a peculiar velocity of 10 kilometers per second. The techniques used to provide redshift-independent distance measurements mostly rely on the fact that light's intensity decreases by the inverse square of the distance from its source. That is, if you see two identical lighthouses but one appears a quarter as bright, then you know the fainter one is two times farther away. In astronomy, such identical lighthouses are called standard candles, astrophysical objects that always shine with the same brightness no matter where they are in the universe. Examples include certain types of exploding or pulsating stars—or even massive galaxies as first proposed by Tully and astronomer J. Richard Fisher in 1977. This Tully-Fisher relation draws on the fact that massive galaxies are both more luminous and rapidly rotating than small galaxies—they have more stars and must spin faster to maintain stability in their stronger gravitational fields. Measure the galaxy's rotation rate, and you learn its intrinsic luminosity; compare that with its apparent luminosity, and you learn its distance.
Each distinct standard candle has a different range where it works best. The pulsating stars called Cepheid variables can only be well observed if the galaxies are close to the Milky Way, and so they are unsuitable for large-scale distance measurements. The Tully-Fisher relation can be used with many spiral galaxies, but the distance estimates they yield have uncertainties of up to 20 percent. Exploding stars called type Ia supernovae yield measurements with half as much uncertainty and shine across vast cosmic distances, but they are rare, only occurring once a century in a good-size galaxy.
If peculiar velocities for a large sample of galaxies in the universe can be obtained, astronomers can then map the largest-scale galactic flows. On these immense scales, the flow of galaxies can be compared with rivers winding through what we call cosmic watersheds, with motions defined by gravitational forces from nearby structures rather than topography. In these “cosmographic” maps, galaxies flow in currents, swirl in eddies and collect in pools to indirectly reveal the structure, dynamics, origins and futures of the largest accumulations of matter in the universe [see box above].
Mapping on the scale needed to address our questions about dark matter and dark energy required cataloguing all the best available data from a large number of observational programs. In 2008 Tully, Hélène M. Courtois, now at the Institute of Nuclear Physics of Lyon in France, and their colleagues published the Cosmicflows catalog, which collated multiple data sets to detail the dynamics of 1,800 galaxies within 130 million light-years of the Milky Way. The team expanded its efforts in 2013 with the Cosmicflows-2 catalog, mapping the motions of about 8,000 galaxies within a volume of about 650 million light-years. One member of the team, Yehuda Hoffman of the Hebrew University of Jerusalem, developed methods to precisely derive the distribution of dark matter from the peculiar velocities of the Cosmicflows data.
As the catalog expanded, we were amazed to find an unexpected pattern hidden in the mass of data: the outlines of a new, previously unseen cosmic structure. Clusters of galaxies across a span of more than 400 million light-years all moved together within a local “basin of attraction,” akin to water pooling at the lowest point of a landscape's topography. Were it not for the universe's incessant expansion, these galaxies would eventually coalesce into one compact, gravitationally bound structure. All together, this vast swarm of galaxies constituted the Laniakea supercluster.
So far studies of the motions of Laniakea's galaxies show them behaving exactly as would be expected from leading models of dark matter's cosmic distribution—although we cannot see it, we can predict with reasonable accuracy where the universe's invisible stuff accumulates. Furthermore, for better or worse, the total density of visible and dark matter within Laniakea suggests that, just as dark energy theorists thought, the universe is destined for a cold death of ever accelerating expansion.
These conclusions remain provisional. The daunting task of mapping galactic flows still has a long way to go. Currently only 20 percent of galaxies within 400 million light-years also have peculiar velocity determinations, and many standard-candle distance measurements still have large uncertainties. Even so, the emerging map of our galactic neighborhood is giving us a new appreciation of our perch in the cosmographic basins and ranges of the universe.
Our Cosmographic Context
Let's take a tour of the flowing, rushing components of our newly discovered home, Laniakea, starting with its most familiar part—you. No matter how slow or fast you are traveling on Earth as you read this, you are spinning around the sun, along with the rest of our planet, at about 30 kilometers per second. The sun in turn orbits the galactic center at roughly 200 kilometers per second, and the entire Local Group, including the Milky Way, is hurtling toward a mysterious concentration of mass in the direction of Centaurus at more than 600 kilometers per second (more on this later). You probably never realized you could move so fast simply by reading a magazine article—or doing nothing at all.
Zooming out from the Milky Way, our journey through Laniakea's expanse begins with two dwarf galaxies, the Small and Large Magellanic Clouds a “mere” 180,000 to 220,000 light-years away. You can glimpse the Magellanics from Earth's Southern Hemisphere, but for the best views you must travel all the way to Antarctica, during the winter. The only other galaxy visible with the naked eye is the giant spiral of Andromeda, although it appears just as a fuzzy patch in a very dark sky.
Andromeda is two and a half million light-years away and is speeding toward us at a peculiar velocity of some 110 kilometers per second. In roughly four billion years it will slam into the Milky Way in a head-on collision and transform both galaxies into a single, featureless ellipsoid of old red stars. It is unlikely that our solar system will be affected during this cosmic car crash—the distance between stars is so large that no two stars are likely to get close enough to collide. The Milky Way, Andromeda and four dozen other galaxies are members of the Local Group, a region where gravity has won the battle against cosmic expansion and is undergoing collapse. Like the Milky Way itself, with its Magellanic Clouds, all these large galaxies have their own entourages of dwarfs.
Just beyond the Local Group, within a volume of about 25 million light-years, three distinctive features appear in our maps. Most of the galaxies here, including our own, live in the unimaginatively named Local Sheet. As “sheet” would imply, it is very thin—most of its galaxies are within three million light-years of this structure, itself the equatorial plane of what is referred to as the supergalactic coordinate system. Below this plane, after a gap, is a filament of galaxies—the Leo Spur—as well as galaxies in the so-called Antlia and Doradus Clouds. Above the plane there is mostly nothing nearby. This emptiness is the domain of the Local Void.
If only the galaxies within the Local Sheet are considered, the situation seems very tranquil. These galaxies are flying apart at the rate of the cosmic expansion, with only small peculiar velocities caused by local interactions. Below the Local Sheet, the galaxies of the Antlia and Doradus Clouds and the Leo Spur have small peculiar velocities, too. They are, however, approaching the Local Sheet at high speed. The Local Void is the probable culprit. Voids expand like inflating balloons, and matter moves from underdense to overdense regions to pile up at their boundaries. We now appreciate that the Local Sheet is a wall of the Local Void and that this void is expanding to push us down toward Antlia, Doradus and Leo.
Zooming farther out, we encounter the Virgo Cluster, which is 300 Local Groups' worth of galaxies squeezed into a volume with a diameter of 13 million light-years. These galaxies whiz around at typical speeds of 700 kilometers per second, and any galaxies within 25 million light-years of the cluster's exterior are falling inward to become part of it within 10 billion years. The full extent of Virgo's domain, the region it will eventually capture, extends to a current radius of 35 million light-years. Interestingly, our Milky Way, at 50 million light-years distant, lies just outside this capture zone.
The Great Galactic Flow
The greater region around the Virgo Cluster that extends to our location is called the Local Supercluster. Almost 30 years ago a group of astronomers who became identified by the convivial moniker “the Seven Samurai” discovered that it is not just the Milky Way moving hundreds of kilometers per second in the direction of Centaurus but rather the entire Local Supercluster. They called the mysterious mass pulling all these galaxies together the Great Attractor. In many ways, the Great Attractor is not so mysterious—the density of matter in that direction of the cosmos is obviously high because it contains seven clusters comparable to the Virgo Cluster lying within a sphere 100 million light-years wide. Three of the largest clusters are called Norma, Centaurus and Hydra.
According to our conception of superclusters as cosmic watersheds, which draws their boundaries based on the divergent movements of galaxies, the so-called Local Supercluster is misnamed. It is only part of something bigger—namely, Laniakea, which encompasses other large structures such as the Pavo-Indus filament and the Ophiuchus Cluster. Imagining Laniakea as a city, our traffic-heavy downtown would be the Great Attractor region. As with most urban cores, it is hard to specify a precise center, but an approximation would place it somewhere between the Norma and Centaurus clusters. This positioning puts our Milky Way far out in the suburbs, near the borders of an adjacent supercluster called Perseus-Pisces. This border is so relatively close in cosmic terms that we can study it in detail to define Laniakea's blobby, roughly round, half-billion-light-year-wide boundary. In total, Laniakea's boundaries encompass a mass from both normal and dark matter equivalent to some 100 million billion suns.
Astronomers have been glimpsing the outlines of what may lie beyond Laniakea for decades. Soon after the discovery of the Great Attractor by the Seven Samurai, something even larger emerged from the intergalactic murk. Directly behind the Great Attractor region, but three times farther away, is a monstrous accumulation of clusters—the densest known within the local universe. Because astronomer Harlow Shapley first spied evidence for its existence in the 1930s, this distant, huge structure became known as the Shapley Supercluster. (Incidentally, just like the Local Sheet, the Virgo Cluster and the main band of the Local Supercluster, as well as the Great Attractor and Shapley Supercluster, all lie on the supergalactic equator. Imagine an immense pancake of galactic superclusters, and you have a good picture of our large-scale local environs.)
So what is causing our Local Supercluster's peculiar velocity of 600 kilometers per second? To some degree, the culprit must be the Great Attractor complex. But we must also consider the gravitational pull of the Shapley Supercluster, which is three times farther away but bears four times the number of rich clusters. Now, according to the Cosmicflows-2 compendium—the same catalog that revealed Laniakea—there is even more to the story. The peculiar velocities of the 8,000 galaxies within this catalog demonstrate a coherent flow toward the Shapley Supercluster. This flow encompasses the entire volume of the Cosmicflows-2 catalog, 1.4 billion light-years from end to end. Does it stop there? We do not yet know. Only even bigger surveys mapping even larger swaths of the universe can reveal the ultimate source—and ultimate structure—behind the epic flow of galaxies in our local universe.