Spectacular 3D maps of the universe have revealed one of the biggest cosmic structures ever found — an almost-inconceivable wall stretching 1.4 billion light-years across that contains hundreds of thousands of galaxies.
The South Pole Wall, as it’s been dubbed, has been hiding in plain sight, remaining undetected until now because large parts of it sit half a billion light-years away behind the bright Milky Way galaxy. The South Pole Wall rivals in size the Sloan Great Wall, the sixth largest cosmic structure discovered. (One light-year is roughly 6 trillion miles, or 9 trillion kilometers, so this «biggest cosmic structure» is mind-bendingly humongous.)
Astronomers have long noticed that galaxies are not scattered randomly throughout the universe but rather clump together in what’s known as the cosmic web, enormous strands of hydrogen gas in which galaxies are strung like pearls on a necklace that surround gigantic and largely empty voids.
Mapping these intergalactic threads belongs to the field of cosmography, which is «the cartography of the cosmos,» study researcher Daniel Pomarede, a cosmographer at Paris-Saclay University in France, told Live Science.
Previous cosmographic work has charted the extent of other galactic assemblies, such as the current structural record holder, the Hercules-Corona Borealis Great Wall, which spans 10 billion light-years, or more than a tenth the size of the visible universe.
In 2014, Pomarede and his colleagues unveiled the Laniakea supercluster, a galactic collection in which our own Milky Way resides. Lanaikea is 520 million light-years wide and contains roughly the mass of 100 million billion suns.
For their new map, the team used newly-created sky surveys to peer into a region called the Zone of Galactic Obscuration. This is an area in the southern part of the sky in which the bright light of the Milky Way blocks out much of what’s behind and around it.
Cosmographers typically determine the distance to objects using redshift, the speed at which an object is receding from Earth due to the expansion of the universe, which depends on their distance, Pomarede said. The farther away an object is, the faster it will appear to be receding from Earth, an observation first made by astronomer Edwin Hubble in 1929 and which has held up ever since.
But he and his colleagues used a slightly different technique, looking at the peculiar velocity of galaxies. This measurement includes redshift but also takes into account the motion of galaxies around one another as they tug at each other gravitationally, Pomarede said.
The advantage of the method is that it can detect hidden mass that is gravitationally influencing how galaxies move and therefore uncover dark matter, that invisible stuff that emits no light but exerts a gravitational tug on anything near enough. (Dark matter also makes up the bulk of the matter in the universe.) By running algorithms looking at peculiar motion in galactic catalogs, the team was able to plot the three-dimensional distribution of matter in and around the Zone of Galactic Obscuration. Their findings are detailed today (July 9) in The Astrophysical Journal.
An all-sky map zoomed in on the South Pole, showing dust only. In this view, the South Pole Wall is not visible, though it is near the Chamaeleon complex, a large star-forming region. The bright line ringing the bottom shows the Zone of Galactic Obscuration. (Image credit: D. Pomarede, R. B. Tully, R. Graziani, H. Courtois, Y. Hoffman, J. Lezmy.)
The resulting map shows a mind-boggling bubble of material more or less centered on the southernmost point of the sky, with a great sweeping wing extending north on one side in the direction of the constellation Cetus and another stubbier arm opposite it in the direction of the constellation Apus.
Knowing how the universe looks on such large scales helps confirm our current cosmological models, Neta Bahcall, an astrophysicist at Princeton University in New Jersey who was not involved in the work, told Live Science. But determining where exactly these enormous, crisscrossing structures begin and end is tricky, she added.
«When you look at the network of filaments and voids, it becomes a semantic question of what’s connected,» she said.
In their paper, the team acknowledges that they may not have plotted yet the entirety of the vast South Pole Wall. «We will not be certain of its full extent, nor whether it is unusual, until we map the universe on a significantly grander scale,» they wrote.