A Previously Hidden Supercluster Of Galaxies Reveals Its Presence

The starlit fires of the first galaxies ignited a very long time ago, and began to light up the primordial Universe less than a billion years after its mysterious birth in the exponential inflation of the Big Bang. The favored theory of galactic formation, the bottom-up theory, proposes that large galaxies were rare in the ancient Universe, and that most galaxies reached their immense, mature, and majestic sizes when small primordial galaxies bumped into one another and merged–creating ever larger and larger galactic structures. The first galaxies were probably dark and opaque amorphous clouds that floated down to the centers of halos composed of a ghostly, invisible, non-atomic substance that scientists call dark matter.–and these strange structures hoisted in the first batches of sparkling newborn stars. In July 2017, a team of astronomers from the Inter University Centre for Astronomy & Astrophysics (IUCAA) and the Indian Institute of Science Education and Research (IISER), both in Pune, India, and members of two other universities in India, announced their discovery of a previously unknown, and extremely large, supercluster of galaxies situated in the direction of the constellation Pisces. Named the Saraswati Supercluster, this important discovery represents one of the largest known structures inhabiting the nearby Universe.

“Saraswati” (or “Sarasvati”) is a word with proto-Indo-European roots. The word has been found in ancient Indian texts, and it refers to a river flowing near the place where the people of ancient India lived. It is also the name of a celestial goddess who is the keeper of celestial rivers. In modern India, Saraswati is worshipped as the goddess of music, art, nature, knowledge, and wisdom–in other words, she is the muse of all creativity.

This newly discovered large-scale galactic structure is situated at a distance of about 4,000 million light-years from Earth. A paper describing its discovery is being published in the July 2017 issue of The Astrophysical Journal, the premier research publication of the American Astronomical Society (AAS). Dr. Joydeep Bagchi, from IUCAA, is the lead author of the paper.

According to the bottom-up theory, large-scale structures in the Cosmos are hierarchically constructed. This means that galaxies, along with associated clouds of gas, and dark matter, are bound together in clusters. Clusters of galaxies are organized with other clusters, smaller groups, filaments, sheets and large empty–or almost empty–cavernous spaces called Voids, and this pattern weaves a strange structure called the great Cosmic Web. The Cosmic Web spans the entire observable Universe, and it got its name because it resembles a bizarre web spun by an enormous hidden spider.

Superclusters of galaxies are the largest coherent structures in the Cosmic Web. A supercluster is a chain of galaxies and galaxy clusters that are all bound together by gravity–and they are frequently several hundred times the size of galaxy clusters, consisting of tens of thousands of galaxies. For example, the newly discovered Saraswati Supercluster extends over a scale of 600 million light-years. This enormous collection of galaxies may sport a mass that is equivalent to more than 20 million billion suns.

For astronomers, long ago is the same as far away. This is because the farther away a shining object is in Space, the more ancient it is in Time. It has taken longer for the streaming light of more distant objects to reach us because of the expansion of the Universe. For this reason, astronomers see the Saraswati Supercluster as it was when our almost 14 billion year old Universe was 10 billion years old.

In the ancient Universe, opaque clouds composed mostly of hydrogen gas collected together along the massive and immense filaments of the bizarre and mysterious dark matter of the Cosmic Web. Although scientists have not yet identified the elusive particles that compose the dark matter, they think that it is not composed of the so-called “ordinary” atomic matter that makes up the world human beings find familiar. Extraordinary “ordinary” matter is is the stuff of stars, planets, moons, and people–and literally all of the elements listed in the Periodic Table. Indeed, “ordinary” atomic matter (baryonic matter) accounts for a mere 4% of the mass-energy of the Universe.

There was, long ago, a dark era in our Universe’s past that occurred before the first stars had caught fire, hurling their magnificent light out into Space to chase away the seemingly endless era of featureless blackness. Opaque blobs of gas gathered along the filaments of transparent dark matter that weave their weird way through the vastness of Spacetime. The more massive regions of the dark matter filaments snared these floating clouds of primeval, pristine gas with their powerful gravitational attraction. Dark matter will not dance with atomic matter or electromagnetic radiation except through the force of gravity. However, because it does interact with “ordinary” atomic matter gravitationally, and it warps, bends, and distorts traveling light (gravitational lensing), astronomers realize that it is really there. Gravitational lensing is a phenomenon predicted by Albert Einstein in his Theory of General Relativity (1915), when he came to the realization that gravity could warp light and thus have lens-like attributes.

Imagine how the transparent, mysterious, and ghostly dark matter tugged at the pristine clouds of very ancient gas with the force of its powerful gravity. The pools of collecting gas were destined to become the nurseries of the first generation of fiery stars to light up the primordial Universe–previously a swath of featureless blackness. The gravity of the Cosmic Web tugged on its atomic prey until the captured clouds of pristine gas merged together to create blobs within the transparent halos of the dark matter. The blobs of primordial hydrogen gas floated down into the dark hearts of these invisible halos, stringing themselves out like beads along this majestic, magnificent, mysterious cosmic spider’s web.

Slowly, the churning, writhing sea of primordial gases and the strange phantom-like dark matter wandered throughout the ancient Universe–mixing themselves up together to finally form the familiar structures that we can observe today. The more massive regions within the transparent filaments of dark matter served as the seeds from which the galaxies ultimately formed and grew. The gravitational pull of those ancient seeds slowly attracted the primordial gases into ever tighter and tighter globs. Depending on the size of the dark matter seed, structures of varying sizes began to emerge. If the seed was small, a small protogalactic fragment formed. Conversely, if the seed was large, a large protogalactic fragment emerged. These ancient fragments then began to do a gravitational dance with one another, eventually clustering together. The protogalaxies, of varying sizes, swarmed together like honeybees around a chunk of discarded candy. In this way, they became the galactic building blocks that formed when dark matter halos collapsed under the merciless pull of their own gravity. The newborn protogalaxies interacted with one another gravitationally, merging together, and thus creating ever larger and larger galactic structures that ultimately evolved into the gigantic and majestic galaxies inhabiting the Universe today. Like blobs of clay in the small hands of a playful child, the protogalaxies smacked into one another to form larger and larger amorphous blobs. The ancient Universe was much smaller than it is today, and it was very crowded. The relatively small, shapeless primordial protogalaxies were relatively close to one another. As a result, they frequently bumped into each other, sticking together to form ever larger galactic structures.

Our own large barred-spiral Milky Way Galaxy is a denizen of the Local Group, that is in turn situated close to the outermost region of the Virgo Cluster of galaxies, whose big, bright heart is 50 million light-years from Earth. Our Milky Way’s place in Space is in a galaxy supercluster named Laniakea. The existence of the Laniakea Supercluster was first announced in 2014 by Dr. Brent Tully of the University of Hawaii and his colleagues.

A Previously Hidden Supercluster Of Galaxies Reveals Its Presence

The most popular theory of galactic formation, called the Cold Dark Matter (CDM) model of the evolution of the Universe, predicts that small structures like galaxies are born first, and then merge together to create larger structures. In cosmology and physics, CDM is a hypothetical form of dark matter that travels slowly in comparison to the speed of light (accounting for the “cold” in CDM), and it has been wandering through Spacetime ever since the Universe was about one year old. When the primordial Universe was this age, the cosmic particle horizon harbored the mass of only one typical galaxy. CDM particles interact only very weakly with “ordinary” atomic matter and electromagnetic radiation–which is why it is invisible. Many scientists think that approximately 84.54% of matter in the Universe is dark matter, with only a relatively small fraction of the Universe’s matter being the so-called “ordinary” atomic (baryonic) matter that composes the world that we experience.

According to the CDM theory, structure in the Universe forms hierarchically, with small objects being the first to emerge, collapsing under the relentless squeeze of their own crushing gravity. These smaller objects then bump into one another and merge to create ever larger and more massive objects.

However, most forms of the CDM theory do not predict the existence of enormous structures like the Saraswati Supercluster. This is because, according to the CDM theory, such an enormous structure could not have formed within the current age of our almost 14 billion year old Universe. The discovery of these extremely large galactic structures forces astronomers to re-think the favored theories of how the Universe developed its current form–beginning with a more-or-less uniform distribution of energy after the Big Bang.

Astronomers think that galaxies formed mostly on the sheets and filaments that weave the enormous Cosmic Web throughout Spacetime. According to this viewpoint, galaxies wander along these massive dark matter filaments, ending up in rich clusters, where the crowded environment shuts down the birth of new baby stars, and also results in the metamorphosis of blue spiral disk galaxies into red elliptical galaxies. Because there is a great deal of variation within the environment of a supercluster, their galactic constituents must travel through these differing environments during their “lifetime”. Therefore, in order to understand their formation and evolution, a scientist needs to identify these superclusters and carefully study the effect of their environment on the galaxies that they host. This is a new area of research in astronomy–with the aid of observations of new observational facilities, astronomers are now beginning to gain a new understanding of galactic evolution. The discovery of the Saraswati Supercluster will play an important role in this new field of research.

Dr. Somak Raychaudhury, a co-author of the paper, also discovered the first massive supercluster of galaxies similar in size to the Saraswati Supercluster. Named the Shapley Concentration, Dr. Raychaudhury presented it as part of his doctoral research at the University of Cambridge in the UK. In 1989, Dr. Raychaudhury’s paper on the Shapley Concentration was published in the journal Nature. Currently the Director of IUCAA, Dr. Raychaudhury named the supercluster in honor of the American astronomer Harlow Shapley (1885-1972), in recognition of his pioneering survey of galaxies inhabiting the Southern hemisphere. The Shapley Concentration was first imaged by Harlow Shapley back in 1932.

Study co-author Shishir Sankhyayan, who is a doctoral student at IISER in Pune, said that “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. This supercluster is clearly embedded in a large network of cosmic filaments traced by clusters and large voids. 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 the 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.”

Dark Energy is a mysterious, unidentified substance–thought to be a property of Space itself–that is causing our Universe to accelerate in its expansion.

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