The Great Attractor
A dense cluster of galaxies. The colliding galaxies of Stephan’s Quintet.
“Stephan’s Quintet Plus One,” (2014). Photograph. Courtesy Robert Gendler, Judy Schmidt, Subaru Telescope (NAOJ), and Hubble Legacy Archive.
Gaussian Random Field equations derived from Edmund Bertschinger, “Multiscale Gaussian Random Fields for Cosmological Simulations,” Astrophysics Journal Supplement 137.
Calligrapher: Nicolas Ouchenir at nicolasouchenir.com
The Great Attractor
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Consider your Fate.
The fate written in the stars is not just that of an individual, or a family, or a tribe, or even a people—the night sky tells us the fate of all things on Earth, it tells us the fate of the galaxy.
Now consider Avoidance.
There is a place in the sky called the Zone of Avoidance. Here’s how you find it: Let’s say you are out in the middle of the desert on a still, dark night and the Milky Way is arching above you like the glittering spine of heaven. Find the brightest points and look towards the most brilliant center of that grand arch of darkness and light. You are looking right into the disc of the Milky Way. This is the Zone of Avoidance.
In this part of the sky our view is obscured by the swirling dusts and brilliant energies generated by “Sagittarius A,” a black hole four million times the size of the sun that spins at the heart of our galaxy.
Our usage of the word galaxy tends to belie the extraordinary scale of these objects. The size of our Sun—which takes up 99.86% of all of the matter in the solar system—relative to the size of the Milky Way, is equivalent to the size of a single white blood cell relative to the entire United States.
Our galaxy is just the start of the story, for humanity’s collective eyes have evolved into telescopes that allow us to cast our vision across billions of years of time and space. We have learned that rather than the whole of the universe residing within the confines of the Milky Way (as was widely believed until 1924), our home galaxy is just one of at least 100 billion such entities, enmeshed within complex structures of inconceivable scale and grandeur.
Now consider The Attractor.
We are all slaves to our attractions. To the things that bind us, to possessions, sex, family, food, gambling, beauty, love. To our vices and virtues, hopes and dreams, fears and regrets.
To the midnight text from a once-familiar embrace.
But there are more fundamental forces that bind us too—physical forces: the strong, the electroweak, gravity.
Under gravity humans fall towards the earth, the earth falls towards the Sun, the Sun and our entire solar system falls towards Sagittarius A, orbiting it every 230 million years.
The next and obvious question then, is: what is our galaxy falling towards?
Now consider The Great Attractor.
The Zone of Avoidance is a nuisance for deep sky astronomers. In the last few decades—during a time of increasingly sophisticated surveys of the cosmos—a swollen empty hourglass has sheared through these maps.
It is one thing to peer through a telescope and see points of light in the sky, it’s quite another to figure out what these things are and how far away they might be. With the closest stars, simple geometry the ancient Greeks understood is sufficient for deriving distance by measuring angles. To look further than this, astronomers rely on the physics of stars, and use the characteristic behavior of different types of suns as yardsticks. But to measure things really far away—to measure distant galaxies—they need to know that galaxy’s redshift.
Redshift is a measure of the Doppler effect on light. If a galaxy is moving towards us its light is blueshifted (compressed to a higher frequency towards the blue end of the spectrum), while for galaxies moving away from us that light is redshifted (lengthened to a lower frequency towards the red end of the spectrum).
In the 1980s—as the Swedish pop duo Roxette showed us what great hair could really look like—astronomers assembled sophisticated maps of redshift. They discovered something interesting: everything nearby was moving towardsaparticular place. Everything in our “Local Group” of 50-odd nearby galaxies, everything in the local clustersof galaxies, everything in the local superclusters, was moving towards something—often at speeds of more than a million miles per hour.
But, when the maps were plotted and the astronomers looked to that part of the sky, that alluring part of the sky, they discovered that whatever we are all rushing at, whatever this must-visit location is—we can’t see it. It lies in a part of the cosmos that we cannot see through or beyond.
Like so many of the most promising romances, the Great Attractor lies in the Zone of Avoidance.
Now consider Curiosity.
In the same way that we can tune our radio from between the walls of a house, astronomers can tune their telescopes to different bands of the spectrum in order to see through the Zone of Avoidance.
It should be unsurprising that scientists were not satisfied with merely interpreting the Great Attractor as a dubious omen, and ascribing it to a revered place amongst the great mysteries like why The Macarena isn’t popular anymore, or why Michael Jackson and Lisa Marie Presley didn’t work out. And neither was I, so I spoke with Professors Nick Kaiser and Brent Tully—astrophysicists at the University of Hawaii. Professor Kaiser has worked on theoretical issues surrounding large-scale cosmological structure and the Great Attractor, and Professor Tully spearheaded the recent classification of our home supercluster—Laniakea.
Laniakea is a structure that consists of the 100,000 closest galaxies that are gravitationally wedded to the Great Attractor. The word is derived from the Hawaiian language and translates as “ImmeasurableHeaven.” It marks the grandest designator of our location short of the Universe itself. Laniakea subsumes three lesser superclusters and encompasses a roughly spherical volume of space about 522 million light years in diameter—approximately a half of 1% of the visible Universe.
Regarding The Zone of Avoidance.
Nick Kaiser: It was always a problem that the Great Attractor was close to the Zone of Avoidance. So you get around that by infrared, and X-rays that also penetrate the dust.
Brent Tully: Using a variety of telescopes in optical and radio bands we can [circumvent the Zone of Avoidance] to a certain degree.
NK: Fundamentally, we know a couple of things, one of which is that the Local Group is moving at about 600 kilometers a second with respect to the cosmic frame, defined by matter at large distances.
Regarding The Great Attractor.
BT: Everything in Laniakea is going towards the Great Attractor; everything outside of it is going towards something else. It turns out that we’re near the edge actually. So not so far away, let’s say 30 million light years is a boundary, and if you go beyond that you’ll find that the flows are towards another structure called the Perseus-Pisces cluster. We’re out in the suburbs. The Great Attractor is kind of the downtown of Laniakea. [It’s] the basin of gravitational attraction, and the Great Attractor is sort of the central area of that, and Laniakea is the whole ensemble draining into the downtown region. It’s not something that’s like a black hole or a spot that is the center. It’s not really well defined. It’s got a bunch of clusters there that are spread out over a region.
Now consider Some Kind of Conclusion.
So what are the consequences then, of knowing this source of our ultimate attraction?
It seems that we’re so infinitesimally unimportant, that our knowledge of the destination is irrelevant. Tens of billions of years lie between us and our galaxy's final approach. Great attractions are so often a fate beyond manipulation.
Regarding the Shape of Galactic Superclusters in the Cosmic Web
Brent Tully: We talk about what we call the cosmic web [wherein] matter that’s pulled out of the void tends to collapse down first into sheaths and filaments and then drains into clusters [of galaxies]. So that’s the topology of structure in the universe.
Nick Kaiser: The theoretical prejudice is that the initial fluctuations [after the big bang] from which structure grows are like what happens if you threw a rock into a swimming pool—if you left it for a couple of minutes you would just have a random pattern of waves on the surface. There’s a mathematical statement of what those kind of waves look like called Gaussian Random Fields, but that’s the model for structure. So it doesn’t have any preferred shape. There are over-dense regions and there are under-dense regions. So it’s an absence of any geometrical shape, it’s about as random as you can get—but it’s got this large-scale coherence. So that’s why, for instance, the Great Attractor controversy was largely because people were trying to attribute everything we see to one single blob, and it’s not like that, we’re being pushed by local voids and we’re being pulled by local over-densities.
Calligrapher: Nicolas Ouchenir at nicolasouchenir.com
Written by Gus Donohoo