Making and remaking the universe

There are models of how matter, energy, dark matter, and dark energy could have interacted

One way to think of the universe is in terms of a vast three-dimensional web with vast empty spaces, intergalactic voids, nested in-between filaments consisting of galaxies. There are clusters of galaxies at the nodes of these filaments. 

Cosmologists and astrophysicists are not sure why the universe is organised this way. There are multiple competing models of how matter, energy, dark matter and dark energy could have interacted to set up this structure. Every time new data comes to light, those models are reviewed and modified. 

A cluster of galaxies can contain thousands of separate galaxies forming a sort of wall of stars densely packed together. A supercluster can contain tens of thousands of galaxies. The first of the superclusters was discovered in 1989 and named the “Shapley Concentration” after astronomer Harlow Shapley. A second such supercluster, the Sloan Great Wall, was identified in 2003.

Six Indian astronomers recently announced the discovery of a third new supercluster. The “Saraswati” supercluster has been named after the long-lost river of Indian prehistory in analogy with the Akash Ganga, which is the Indian name for the Milky Way. 

The paper detailing the identification of Saraswati (https://arxiv.org/-abs/1707.03082) was published only last week. But this is a story 15 years in the making. Two of the authors, Joydeep Bagchi and Somak Raychaudhury are now colleagues at the Inter-University Centre for Astronomy and Astrophysics (IUCAA).

Fifteen years ago, the duo collaborated in a paper about magnetic fields in filaments of galaxies (http://adsabs.harvard.edu/abs/2002NewA....7..249B). At that time, they suspected that a supercluster like Saraswati might exist but the data was lacking to confirm their hypothesis. (Raychaudhury was part of the team that discovered the Shapley concentration as well)

Around five years ago, the data they wanted started becoming available. The Sloan Digital Sky Survey (SDSS) collates astronomical data and maps and releases them periodically into the public domain. As Bagchi and Raychaudhury began looking at the SDSS data, the shape of something really big became visible. However, they needed more data in order to make estimates of the size, shape, distance, age etc, of what they suspected was there. 

Four of their colleagues got involved. Shishir Sankhyayan is from the Indian Institute of Science Education and Research, Pune; Pratik Dabate is also at IUCAA; Joe Jacob is at Newman College, Thodupuzha; and Prakash Sarkar is at the National Institute of Technology, Jamshedpur. They also sourced data from the Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, etc.

Spectrographic data had to be crunched to give a 3-D estimate of the size and shape. As stars move away, the light received becomes redder, shifting in wavelength to the red end of the spectrum. By calculating the red shift, estimates can be made of the distance and the speed of dispersion. 

The Saraswati supercluster is about four billion (one billion=1,000 million) light years away, located in the same part of the sky as the Constellation, Pisces. This is much more distant than the other superclusters. Saraswati has at least 43 clusters and it is spread over more than 650 million light years. Given that light (and radio waves) takes four billion years to travel to us, we’re seeing the supercluster as it looked that long ago. The centre appears to be collapsing and merging. The universe is about 13.5 billion years old, which means that Saraswati was in the process of merging about nine billion years ago. It’s the earliest example of a supercluster formation that has been discovered so far.

Dating this structure has important implications for cosmological models. Most models assume that the initial universe was homogeneous (with matter spread evenly across space) after the Big Bang. Einstein’s General Theory of Relativity postulates that the universe should remain “more or less” homogeneous: There may be local clumps but large volumes of space (including voids and clusters) should contain about the same mass on average. 

Any cosmological hypothesis involving energy and matter interactions must be consistent with physical observation. Saraswati helps cosmologists to review their models. It’s believed that dark energy helped to accelerate the universe’s expansion around nine billion years ago, which is when we’re seeing Saraswati. So, it could lead to many new insights about the process by which the universe acquired its structure.