Galaxies Outline Bubble 1 Billion Light-Years Wide in Space

Astronomers mapping the precise distances to tens of thousands of galaxies have found a curious cosmic structure. Galaxies by the thousands lie along a spherical shell that spans 1 billion light-years. The size alone is enough to challenge fundamental ideas about the universe.

Brent Tully (University of Hawaii) and his colleagues happened on the structure while mapping 55,877 galaxies in the local universe as part of a project known as Cosmicflows-4. “We were not looking for it,” Tully explains. “It is so huge that it spills to the edges of the sector of the sky that we were analyzing.” The structure itself, however, had been previously discovered by Maret Einasto and colleagues in 2016, as reported in Astronomy & Astrophysics.

As Tully and colleagues mapped out galaxy positions, they couldn’t help but notice the giant sphere take shape. All around the sphere, the astronomers saw familiar structures, previously discovered in other sky surveys, such as the Sloan Great Wall, the Center for Astrophysics Great Wall, and the Hercules-Corona Borealis Great Wall. Each of these walls is itself made of clusters of hundreds or thousands of galaxies.

“It was an amazing process to construct this map and see how the giant shell structure . . . is composed of elements that were identified in the past as being themselves some of the largest structures of the universe,” says cartographer Daniel Pomarede (Paris-Saclay University).

The scales involved are truly grand: The galactic walls that make up the sphere are themselves millions or even a billion light-years long. Meanwhile, the sphere’s interior is sparse and contains the Boötes Void. Only in the very center does the number of galaxies increase again, due to a collection of galaxy clusters known as the Boötes Supercluster.

The whole sphere lies about 820 million light-years away from us, well outside the Laniākea Supercluster that encompasses the Milky Way Galaxy as well as all of the Local Group that we’re part of. (Tully’s team discovered Laniākea in 2014.)

Explore the sphere with this 3D visualization:

The astronomers worked in partnership with Hawaiian language professor Larry Kimura (University of Hawaii) and Kaʻiu Kimura, director of the ʻImiloa Visitor Center to name the giant galactic sphere Ho‘oleilana. The word stems from a line of the Kumulipo creation chant: “Hoʻoleilei ka lana a ka Pōuliuli,” meaning “From deep darkness come murmurs of awakening.” Ho‘oleilana is drawn from this chant and means “sent murmurs of awakening.”

Origins of the Sphere

The building blocks of Ho‘oleilana are galaxies, which in turn group into clusters and superclusters. Each individual galaxy is couched in dark matter, and dark matter rules the larger groupings via gravity. So, it is possible that the sphere simply grew out of the gravitational mingling of matter. Those interactions have, over time, organized galaxies into a cosmic foam, in which galaxies tend to lie along walls and filaments that outline large regions of mostly empty space known as voids. It could be that the galactic sphere is simply a natural outcome of that process.

However, Tully and colleagues have an alternative suggestion, published in the Astrophysical Journal. They think this spherical structure might be the echo of huge pressure waves that sloshed around the early universe. Known as baryon acoustic oscillations, these waves came about because the early universe was filled with a dense hot plasma not unlike the Sun’s interior. Gravity drew plasma together in denser regions, but photons trapped with the matter tried to push it apart. The result was a sloshing primordial soup.

Eventually, the ions and electrons of the plasma combined to form neutral gas, freeing photons to fly on their merry way. But the effects of the sloshing remained. As the gas collapsed to form stars and galaxies, the waves imprinted a sort of pattern, like ripples in a pond, that then expanded along with the universe over cosmic time. Tully’s team thinks that Ho‘oleilana might be a single one of these ripples, frozen in space.

Not everybody agrees with this conclusion, though. Imagine scattering pebbles into a pond — the resulting ripples would overlap so that the ripple around a single pebble’s drop would be difficult if not impossible to distinguish. Previous studies have found evidence of baryon acoustic oscillations, but only on a statistical basis, by studying huge populations of galaxies.

“The theoretical expectation has been that the density enhancements in individual BAO would be too weak to be registered,” Tully acknowledges.

The only way that a single such ripple might be discerned, Tully and colleagues argue, is if the mass at the center of the ripple were exceptionally dense, like throwing a larger stone into a pond otherwise riddled with pebbles. “We first identified the Ho‘oleilana shell without thinking about what might be at the center,” Tully says. “It was pleasing to find a rich supercluster there.”

A future test of the BAO scenario might be to see if this supercluster really contains enough mass to engender a ripple of its own.

If the sphere comes from a primordial slosh, then with a few mathematical equations astronomers can relate the width of the sphere directly to the current expansion rate of the universe. The sphere is unexpectedly large, about 10% bigger than the BAOs found in statistical studies. As a result, Tully’s team finds a current expansion rate of the universe that’s somewhat high.

(For those interested in the numbers: The Hubble constant they find is 76.9 kilometers per second per megaparsec, but with possible values ranging from 72.1 to 85.1 km/s/Mpc. Other studies have found values between 68 to 73 km/s/Mpc.)

Regardless of Ho‘oleilana’s origin, its sheer size challenges a fundamental idea about how the universe is laid out. While we expect gravitationally organized structure on scales up to some 500 million light-years across, at larger scales astronomers think the universe should start to become relatively smooth. That is, as you slowly zoom out on a map of galaxies, you’d initially make out groups and bubbles and voids, but then everything would start to blend together.

At 1 billion light-years wide, Ho‘oleilana pushes that notion of homogeneity to larger scales, Tully says: “Evidence such as Ho‘oleilana indeed suggests the scale to reach homogeneity is greater.”

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Editorial note: The article has been edited to credit discovery of the giant shell of galaxies, now known as Ho'oleilana, to Maret Einasto and her colleagues in 2016.