Think big. No, much bigger. At over 650 million light years across, the Saraswati supercluster of galaxies is one of the largest structures in the universe.
Clumps of galaxies are either called groups, which contain up to 50 individual galaxies, or clusters, which can contain thousands. Superclusters, in turn, are collections of these groups and clusters.
Joydeep Bagchi at Savitribai Phule Pune University in India and his colleagues found the supercluster using data from the Sloan Digital Sky Survey, a map of galaxies in the night sky.
“It’s like a geographer discovering a new great mountain range,” says J. Richard Gott III at Princeton University.
The Saraswati supercluster is about 4 billion light years away in the constellation Pisces, much more distant than other superclusters we have seen. It is made up of at least 43 massive groups and clusters, and contains about 400 galaxies in total, giving it a combined mass 20 million billion times that of our sun.
Ancient history
As Saraswati is so far away and its light takes time to reach us, studying it allows us to look back in time to when the universe was only about 10 billion years old.
“Since a structure of this vastness will only grow extremely slowly, taking many billions of years, it carries with it a sort of record of the entire history of its formation,” says Bagchi. As the Saraswati supercluster was formed relatively early, it could give us a unique opportunity to characterise the early universe and probe the tiny fluctuations that expanded after the big bang to form the largest structures.
With more observations, Bagchi hopes that we will also be able to use the supercluster to probe the interactions between dark matter, which helps clump galaxies together with its gravity, and dark energy, which causes space to expand and the galaxies to spread apart. The Saraswati supercluster was formed in an era when it is thought that dark energy was just starting to accelerate the universe’s expansion, making it a product of the delicate balance between dark energy and dark matter.
“This is how we will make a discovery of whether the standard model of cosmology is wrong, which is one of the most important discoveries we can possibly make,” says J. Richard Bond at the University of Toronto in Canada. “The arena where that plays out will be on these large scales.”
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