Beyond the Milky Way, scientists discover galactic wall hidden in 'plain sight'

Last Updated : 21 July 2020, 03:27 IST
Last Updated : 21 July 2020, 03:27 IST

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Astronomers have discovered that there is a vast wall across the southern border of the local cosmos.

The South Pole Wall, as it is known, consists of thousands of galaxies — beehives of trillions of stars and dark worlds, as well as dust and gas — aligned in a curtain arcing across at least 700 million light-years of space. It winds behind the dust, gas and stars of our own galaxy, the Milky Way, from the constellation Perseus in the Northern Hemisphere to the constellation Apus in the far south. It is so massive that it perturbs the local expansion of the universe.

But don’t bother trying to see it. The entire conglomeration is behind the Milky Way, in what astronomers quaintly call the “zone of avoidance.”

An international team of astronomers led by Daniel Pomarède of Paris-Saclay University and R. Brent Tully of the University of Hawaii announced this new addition to the local universe on July 10 in a paper in Astrophysical Journal. The paper is festooned with maps and diagrams of blobby and stringy features of our local universe as well as a video tour of the South Pole Wall.

It is the latest installment of an ongoing mission to determine where we are in the universe and where we are going.

“The surprise for us is that this structure is as big as the Sloan Great Wall and twice as close, and remained unnoticed, being hidden in an obscured sector of the southern sky,” Pomarède said in an email.

“The discovery is a wonderful poster child for the power of visualizations in research,” Tully said.

The new wall joins a host of other cosmographic features: arrangements of galaxies, or a lack of them, that astronomers have come to know and love over the last few decades, with names like the Great Wall, the Sloan Great Wall, the Hercules-Corona Borealis Great Wall and the Bootes Void.

The new paper was based on measurements, performed by Tully and his colleagues, of the distances of 18,000 galaxies as far away as 600 million light-years. By comparison, the most distant objects we can see are about 13 billion light-years away.

The galaxies in the wall cannot be seen, but Pomarède and his colleagues were able to observe their gravitational effects by assembling data from telescopes around the world.

In the expanding universe, as described in 1929 by astronomer Edwin Hubble and confirmed for almost a century, distant galaxies are flying away from us as if they were dots on an inflating balloon; the farther they are, the faster they recede, according to a relation called the Hubble law.

That motion away from Earth causes their light to be shifted to longer, redder wavelengths and lower frequencies, like retreating ambulance sirens. Astronomers use this “redshift” as a proxy for relative distance in the universe. By measuring the galaxy distances independently, the “Cosmicflows” team, as Pomarède and his colleagues call themselves, was able to distinguish the motion caused by the cosmic expansion from motions caused by gravitational irregularities.

As a result, they found that the galaxies between Earth and the South Pole Wall are sailing away from us slightly faster than they otherwise should be, by about 30 miles per second, drawn outward by the enormous blob of matter in the wall. And galaxies beyond the wall are moving outward more slowly than they otherwise should be, reined in by the gravitational drag of the wall.

One astonishing aspect of the wall is how big it is compared to the volume that the team was surveying: a contiguous filament of light 1.4 billion light-years long, packed into a cloud maybe 600 million in radius. “There is hardly room in the volume for anything bigger!” Tully said in an email. “We’d have to anticipate that our view of the filament is clipped; that it extends beyond our survey horizon.”

Yet the South Pole Wall is nearby in cosmological terms. “One might wonder how such a large and not-so-distant structure remained unnoticed,” Pomarède mused in a statement issued by his university.

But in the expanding universe, there is always something more to see.

On the largest scales, cosmologists attest, the universe should be expanding smoothly, and the galaxies should be evenly distributed. But on smaller, more local scales, the universe appears lumpy and gnarled. Astronomers have found that galaxies are gathered, often by the thousands, in giant clouds called clusters and that these are connected to one another in lacy, luminous chains and filaments to form superclusters extending across billions of light-years. In between are vast deserts of darkness called voids.

From all of this has emerged what some astronomers call our “long address”: We live on Earth, which is in the solar system, which is in the Milky Way galaxy. The Milky Way is part of a small cluster of galaxies called the Local Group, which is on the edge of the Virgo cluster, a conglomeration of several thousand galaxies.

In 2014, Tully suggested that these features were all connected, as part of a giant conglomeration he called Laniakea — Hawaiian for “open skies” or “immense heaven.” It consists of 100,000 galaxies spread across 500 million light-years.

All this lumpiness has distorted the expansion of the universe. In 1986, a group of astronomers who called themselves the Seven Samurai announced that the galaxies in a huge swath of the sky in the direction of the constellation Centaurus were flying away much faster than the Hubble law predicted, as if being pulled toward something — something the astronomers called the Great Attractor. It was the beginning of something big.

“We now see the Great Attractor as the downtown region of the supercluster that we live in — an overall entity that our team has called the Laniakea Supercluster,” Tully said. All the different parts of this supercluster are tugging on us, he added.

As a result, the Great Attractor and its relatives are shedding light on another enduring cosmic mystery — namely, where we are headed.

Astronomers discovered in 1965 that space is suffused with microwave radiation, a bath of heat — with a temperature of 2.7 degrees Kelvin, or minus 455 degrees Fahrenheit — left over from the birth of the universe 14 billion years ago. Subsequent observations revealed that this bath is not uniform: It is slightly warmer in one direction, suggesting that we — Earth, our galaxy and the Local Group — are moving through the microwaves, like a goldfish in a fishbowl, at about 400 miles per second in the approximate direction of Centaurus, but aiming far beyond.

Why? What is over there, on the other side of the fishbowl, compelling us? That is the kind of question that would come up in an Arthur C. Clarke novel, where humanity is always gearing up for some definitive expedition around the curve of the universe.

“A major goal in cosmology is to explain this motion,” Tully said in a series of emails. In theory, the motion arises from the lumpy distribution of matter that grew out of tiny ripples in the density of the early universe.

“The Great Attractor is certainly an important part of the cause of our motion,” Tully said. “The South Pole Wall also contributes but, again, only in part,” he added, listing more local galaxy clusters and voids. “Every hill and valley in the density distribution makes itself felt.”

Most of that is stuff that we cannot see directly. According to the prevailing theory of a confoundingly preposterous universe, the cosmos contains about five times as much invisible dark matter as luminous atomic matter.

Nobody knows exactly what dark matter is made of, but according to cosmologists it provides the gravitational scaffolding for the luminous structures in the universe — galaxies, galaxy clusters, superclusters, voids and chains like the South Pole Wall, all connected by spidery filaments in what is known as the cosmic web. The visible universe of stars and galaxies, cosmologists like to say, is like snow on mountaintops or lights on dark, distant Christmas trees.

But by following the lights and how they are moving, astronomers like Tully and his cosmographers have now been able to probe the shadows on which they sit: galumphing clouds of mass whose gravity shapes the destiny of the visible cosmos, arranging it into shapes and neighborhoods, walls, valleys and voids.

“It’s just dark matter having its way,” Tully said.

“We are like swimmers attempting to swim upstream but being carried downstream faster.”

Published 21 July 2020, 03:13 IST

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