The supercluster is about 4 billion light years away and spreads over a “great wall” about 600 million light years across.

A group of Indian astronomers have discovered a massive supercluster of galaxies, and have named it Saraswati. The supercluster is about 4 billion light years away and spreads over a “great wall” about 600 million light years across. This makes it one of the largest superclusters to be discovered and also the furthest.

The astronomers belong to Inter-University Centre for Astronomy and Astrophysics (IUCAA) and Indian Institutes of Science Education and Research (IISER) in Pune, besides NIT, Jamshedpur and Newman College, Thodupuzha.

The first supercluster of galaxies, the Shapley Supercluster, was discovered in 1989, and the second, the Sloan Great Wall in 2003. The Milky Way galaxy is part of the Laniakea Supercluster, which was discovered in 2014.

“It is the first time that we have seen a supercluster that is so far away. Even the Shapley is about 8-10 times closer,” says Somak Raychaudhury, Director of IUCAA and one of the authors of the paper being published in the latest issue of The Astrophysical Journal.

Prof. Raychaudhury was part of the team that discovered Shapley, named after American astronomer Harlow Shapley. “We have a habit of naming galaxies after rivers; the Milky Way is referred to as Akash Ganga. So we thought of naming this supercluster after the ancient river Saraswati,” says Prof. Raychaudhury. Along with Dr. Joydeep Bagchi, the lead author of the paper, he began studying this region 15 years ago, using data from the Sloan Digital Sky Survey, and found two clusters close by. He suspected they were part of a larger group.

Clumpy universe

The structure of the universe is not a homogeneous distribution of matter. It is clumpy with galaxies forming clusters and these in turn forming superclusters. There are thin “filaments” that connect galaxies, forming a cosmic web, and there are large voids in between.

The current belief is that infant galaxies form in these filaments and then drift to the intersections of the filaments where they grow. The Saraswati supercluster could challenge this premise, because it had formed so early and building such a big structure far back might have been difficult. When sighting a supercluster of galaxies 4 billion light years away, the observer is looking back about 4 billion years. A light year is the distance travelled by light in one year. Given the belief that the universe is 13.8 billion years old, the discovery implies that such a huge structure existed even when the universe was just about ten billion years old. This raises questions about the formation of large structures and the nature of the universe, astronomers say.

Dark matter and dark energy are invoked to explain the structure of the universe. Dark matter, being massive, binds together the universe while dark energy, exciting the surrounding space, drives it apart; the balance of the two effects helps in maintaining the universe in its present form.

“Theory has always been confounded by nature. It is true that the balance between dark matter and dark energy can produce large structures, but a supercluster of this size does present an enigma,” says Prof. Raychaudhury.