Beyond the Milky Way, a Galactic Wall

The galaxies in the wall cannot be seen, but Dr. 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 the 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 from us, 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,” which is easily measured, as a proxy for relative distance in the universe. By measuring the galaxy distances independently, the “Cosmicflows” team, as Dr. 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!” Dr. 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.”

And yet the South Pole Wall is nearby in cosmological terms. “One might wonder how such a large and not-so-distant structure remained unnoticed,” Dr. 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.

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