The Saraswati supercluster

On the largest scale, the Universe looks like a vast cosmic web. First, stars, including our Sun, are grouped into galaxies. Then, the galaxies are clustered into galactic groups, bound together to form galaxy clusters. When these galaxy clusters occasionally merge, they create the largest structures called “Superclusters.” They can contain baryonic masses of up to millions or billions of solar masses and stretch tens to hundreds of megaparsecs. And in this manner, they trace the most extensive scale structure in the Universe comprising tens to hundreds of galaxy clusters. And one such supercluster discovered recently is called the Saraswati supercluster.

The Saraswati supercluster was discovered by a group of Indian astronomers from the Inter-University Centre for Astronomy and Astrophysics and the Indian Institute of Science Education and Research in 2017 (see research paper).

Saraswati (or “Sarasvati”), a word that has Proto-Indo-European roots, is a name found in ancient Indian texts to refer to the major river around which the people of the ancient Indian civilization lived. It is also the name of the celestial goddess who is the keeper of the celestial rivers. In modern India, Saraswati is worshipped as the goddess of knowledge, music, art, wisdom, and nature – the muse of all creativity.

The Saraswati Supercluster lies 1.2 gigaparsecs, which is 4 billion light-years away, in the direction of the constellation Pisces. It is one of the largest structures found in the Universe extending over 200 megaparsecs which is roughly equal to 650 million light-years, containing more than 40 confirmed galaxy clusters with a mass of more than 2 x 10¹⁶ solar masses (one solar mass is 2 x 10³⁰ kg). There are only a few known superclusters of this size in the entire Universe. 

The discovery of the Saraswati supercluster

The discovery of Saraswati was made with the help of extensive spectroscopic observations made over a large band of the sky called stripe 82. Later on, Statistical methods were used to spatially link galaxies, eventually revealing the Saraswati supercluster. It contains more than 40 confirmed galaxy clusters and is surrounded by vast galactic voids. The most massive clusters inside Saraswati form a wall-like array with strong filamentary features which evolve with time.

Joydeep Bagchi from IUCAA, the lead author of the paper, and co-author Shishir Sankhyayan (Ph.D. scholar at IISER, Pune) said, “We were very surprised to spot this giant wall-like supercluster of galaxies, visible in a large spectroscopic survey of distant galaxies, known as the Sloan Digital Sky Survey (see figure above). This supercluster is clearly embedded in a large network of cosmic filaments traced by clusters and large voids.’’

He further added, “Previously only a few comparatively large superclusters have been reported, for example, the ‘Shapley Concentration’ or the ‘Sloan Great Wall’ in the nearby universe, while the ‘Saraswati’ supercluster is far more distant one. Our work will help to shed light on the perplexing question; how such extreme large scale, prominent matter-density enhancements had formed billions of years in the past when the mysterious Dark Energy had just started to dominate structure formation.’’

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What makes Saraswati so special?

A supercluster of the size and mass of Saraswati is a prime laboratory for tests of general relativity, gravity, and structure formation and can also be the proof of many cosmological models. It is one of the largest superclusters to be discovered and also the farthest. Looking at a supercluster of galaxies 4 billion light-years away, the observer is looking back about 4 billion years in time. A light-year is a distance traveled by light in one year. Since the Universe is roughly 13.8 billion years old, this discovery implies that such a huge structure existed even when the Universe was about 10 billion years old, about 70% of its present age. Now, this poses a puzzle.

According to present theories, it is difficult for such a huge supercluster to have formed so early in the Universe’s lifetime. Cosmological simulations tell us that finding such an extensive and massive structure is very unlikely. Moreover, the redshift of 0.3 places Saraswati near the region called a cosmic transition meaning a transition from a matter-dominated universe to a dark energy-dominated Universe. This makes the Saraswati supercluster strangely connected to the dark energy as well in some way.

What next?

With great discoveries come great scientific techniques and methods to look into the physics behind them. Discovering more such superstructures can tightly constrain different cosmological models and the power spectrum of primordial density fluctuations which will lead to a better understanding of the Universe in the past. It provides astronomers with huge scope in studying the structure formations in the early Universe.

Scientists are still following up with the spectroscopic and X-ray observations of the Saraswati supercluster that are necessary to confirm the boundaries of the supercluster and refine its total mass content. This is a rare discovery. Since there are only four or five known superclusters of this size in the entire Universe, it can provide solutions to many mysteries in the Universe, along with revealing hidden problems to the scientific community.

This article on the Saraswati supercluster is a guest article by Harshita Gandhi, P.hD. student, Aberystwyth University, Wales.

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