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Here’s what you’ll learn when you read this story:
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The Milky Way supposedly lies in the immense Laniākea basin, but even that is now thought to be part of a bigger basin of (gravitational) attraction known as the Shapley Supercluster.
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This supercluster is so expansive that models have not been able to give a remotely accurate idea of how huge it is, and its exact size is still unknown.
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Further observations will determine the size of the Shapley Supercluster, and rule out other superclusters whose gravity could possibly be pulling our galaxy in.
The Milky Way is about 100,000 light-years across, 1,000 light-years thick, and populated by 100 billion stars. That sounds close to infinite—until, that is, you realize that it is just a fragment of an almost unfathomable cosmic megastructure. Space really is wild.
It was already suspected that our galaxy was part of something more gargantuan than itself. Last year, a team of astronomers and international researchers at the University of Hawai’i Institute for Astronomy suggested that the Milky Way—which is part of the Virgo Supercluster—lives on the edge of huge basin known as Laniākea (Hawaiian for “immense heaven”).
But this 500 million-light-year expanse might still not be the limit of what holds our galaxy. Revisiting their findings, the team now thinks that Laniākea exists inside something even larger.
Enormous amounts of both visible and dark matter can be found in what are called basins of attraction. Somewhat like the human nervous system, everything in the universe is connected—except, instead of nerves, the universe has filaments of gas and dust. Galaxies along those filaments are pulled together by gravitational forces, clustering like bunches of neurons into what scientists call basins of attraction.
Within these basins are galaxies in local clusters, which are inside larger clusters, which make up superclusters that lie in even larger superclusters like Laniākea. And that much mass has enough gravity to attract more mass.
Led by astronomer R. Brent Tully of the University of Hawai’i, the research team has become convinced that it is highly likely our galaxy (and everything else in Laniākea) are only specks in the monstrous Shapley Supercluster. It is thought to be ten times the volume of Laniākea and largest known concentration of matter in our region of the universe.
Though something as massive as the Shapley Supercluster can exert huge forces of gravity on matter, gravity itself has a much more difficult time exerting its hold on a whole basin of attraction. Its grip weakens as new stars and star systems continue to form within galaxies. Inflation also keeps objects continually moving as the universe expands—another factor which prevents superclusters from being gravitationally bound.
“The structure in the universe is believed to have evolved from quantum fluctuations seeded by inflation in the early universe,” Tully and his team said in a study published in Nature Astronomy. “These fluctuations lead to density perturbations that grow via gravitational instability into large cosmological structures.”
The Cosmic Microwave Background (CMB)—leftovers of the first light from the dawn of the universe—keeps expanding with inflation. Models can predict how much the CMB (and whatever else is out there) spread out in the past and will continue to expand in the future, but the limits imposed by these models on basins of attraction are far too narrow. Mapping the local universe has revealed that the Shapley Supercluster already seems larger than model predictions.
By figuring out how 56,000 galaxies in our local universe move through space (depending on their velocity), the researchers created a map showing how they are all potentially attracted by Laniākea, and that Laniākea itself is probably attracted by the Shapley Supercluster. Simulations of galaxies and galaxy clusters moving in the supercluster gave the team an idea of how huge the Shapley Supercluster must be, which was beyond the scope of previous models. It is now thought to include the Great Attractor—the supposed central point of gravity within Laniākea that is pulling the Milky Way inward at 600 kilometers (372 miles) per second.
The discovery that Laniākea is a part of the larger Shapley Supercluster, if confirmed, could change how we see the universe. While the cosmological principle states that the universe is uniform and isotropic (meaning its physical proprieties are the same as measured from every direction) when viewed on a large enough scale, megastructures like the Shapley Supercluster seem to defy that. More observations will be needed to reveal exactly how outsized this monster is and to rule out other superclusters that might have power over us.
“The current data are not enough to determine the outer bonds of these dominant [basins of attraction],” Tully said. “It follows that from the point of view of the [basins] that cosmology has not reached its ‘end of greatness.’”
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