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A Previously Hidden Supercluster Of Galaxies Reveals Its Presence

Friday, September 15th, 2017

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.

The Mystery Of Our (Mostly) Missing Universe

Friday, September 15th, 2017

Almost 14 billion years ago, our Universe burst into existence in the form of an unimaginably tiny soup of densely packed, searing-hot particles, commonly referred to as “the fireball”. Spacetime has been expanding–and cooling off–from this original brilliant, fiery, glaring state ever since. But what is our Universe made of, and has its composition evolved over time? It is often said that most of our Universe is “missing”, composed as it mostly is of a mysterious substance that we call dark energy. The elusive dark energy is causing our Universe to accelerate in its relentless expansion, and it is generally believed to be a property of Space itself. In August 2017, scientists announced that they now have a new window from which they can study our Universe’s mysterious properties, thanks to an international collaboration of more than 400 scientists called the Dark Energy Survey (DES), that is helping to shed new light on the secretive structure of our mostly missing Cosmos.

On large scales, the entire Universe appears the same wherever we look–displaying a foamy, bubbly appearance, with extremely heavy filaments that braid themselves around each other, weaving a web-like structure that is appropriately called the Cosmic Web. The filaments of the Cosmic Web shine with the fierce fires of a myriad of stars that outline enormous sheets and intertwining braids that host the starlit galaxies of the visible Universe. Immense dark, empty–or almost empty–Voids interrupt this weird, twisting, transparent web-like structure. The Voids contain few galaxies, and this makes them appear to be almost entirely empty. In dramatic contrast, the heavy starry filaments, that compose the Cosmic Web, weave themselves around these dark caverns creating what looks like a convoluted, twisted knot.

We live in a mysterious Universe–most of which we are unable to see. The galaxies, galaxy clusters and superclusters are all imprisoned in halos composed of invisible non-atomic dark matter. This unidentified material knits the heavy filaments of the great Cosmic Web into a remarkable tapestry that extends throughout all of Spacetime. Scientists are almost certain that the dark matter really exists because of its observable gravitational influence on those objects and structures that can be seen–such as stars, galaxies, and clusters and superclusters of galaxies.

The most recent measurements suggest that our Universe is composed of approximately 70% dark energy and 25% dark matter. As of today, the origin and nature of the mysterious dark matter and dark energy remain elusive. A much smaller percentage of our Universe is composed of the badly misnamed “ordinary” atomic matter–the familiar material that composes all of the elements listed in the Periodic Table. “Ordinary matter”–which is really extraordinary stuff–is comparatively scarce in the Cosmos. However, this runt of the Cosmic litter of three is what makes up the stars, planets, moons, people, and all of the rest of the Universe that human beings perceive as familiar. It is also the precious material that allowed life to emerge and evolve in our Universe.

However, the Cosmos may be even more bizarre than we are capable of imagining it to be. Modern scientific cosmology began with Albert Einstein who, in the early decades of the 20th century, applied his theories of RelativelySpecial (1905) and General (1915)–to our “Cosmic habitat”. At the start of the 20th century, our Milky Way was believed to be the entire Universe, and it was also thought that the Universe was both static and eternal. However, we now know otherwise.

Our Universe does evolve in Time, and there is much, much more of the vast Cosmos than our own home Galaxy. It is generally thought that the Universe was born about 13.8 billion years ago, when Space itself ripped apart, in an event scientists call the Inflationary Big Bang. At the moment of its mysterious birth, in the smallest fraction of a second, the Universe expanded exponentially to balloon to macroscopic size–beginning as an incredibly tiny Patch that was smaller than a proton. Spacetime has been expanding from this initial brilliant state, and cooling off, ever since. All of the galaxies are drifting away from one another, and our Universe has no center. Indeed, everything is floating away from everything else, as a result of the expansion of Spacetime. The expansion of the Universe is frequently likened to a loaf of leavening raisin bread. The dough expands, taking the raisins along for the ride. The raisins become progressively more widely separated from one another because the dough is expanding.

Georges Henri Joseph Edouard Lemaitre (1894-1966) was a Belgian astronomer, priest, and professor of physics at the Catholic University of Louvain. Lemaitre was one of the first to suggest that our Universe is not static–that it is expanding. He also formulated the theory that would eventually be termed the Big Bang Universe. Lemaitre once commented that “The evolution of the world may be compared to a display of fireworks that has just ended: some few wisps, ashes, and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanished brilliance of the origins of the worlds.”

When we refer to the observable, or visible, Universe we are referring to the relatively small region of the entire Universe that we can observe. The rest of it–the lion’s share of it–is located far, far beyond what we call the cosmological horizon. The light traveling to us from those unimaginably remote regions of Spacetime, far beyond the horizon of our visibility, has not had sufficient time to reach us since the Big Bang because of the expansion of the Universe. No known signal can travel faster than light in a vacuum, and this sets something of a universal speed limit that has made it impossible for us to directly observe these extremely remote domains of Spacetime.

The temperature throughout that original primordial fireball was almost uniform. This very small deviation from perfect uniformity resulted in the formation of everything that we are, and all that we can ever know. Before the Inflation occurred, that extremely small primordial Patch was completely homogeneous, smooth, and appeared to be the same in every direction. It is generally thought that Inflation explains how that entirely smooth and homogeneous Patch began to ripple.

The extremely tiny fluctuations, the primordial ripples in Spacetime, occurred in the smallest units that we can measure (quantum). The theory of Inflation explains how these quantum fluctuations, in the smooth and isotropic baby Universe, would eventually grow into large-scale structures like galaxies, galaxy clusters, and superclusters. To paraphrase the late Dr. Carl Sagan of Cornell University, we are the eyes of the Universe seeing itself. But, of course, nothing with eyes to see existed as yet in these initial moments of the birth of Spacetime.

The weird quantum world is a foamy, jittery arena, where absolutely nothing can stay perfectly still. The originally smooth and isotropic Universe formed little hills and valleys. The valleys ultimately grew emptier and emptier; the hills higher and heavier. This is because of the force of gravity. Gravity drew the original material of the baby Universe into the heavier hills, that eventually acquired increasingly more and more of the matter making up the primordial soup. The impoverished plains, that were devoid of the same powerful gravitational lure possessed by the hills, became increasingly more barren of this primordial broth. As time passed, larger and larger structures formed within our Universe’s wealthier and more massive hills. This is because the hills exerted an increasingly more powerful pull on the primordial material–and the heavier the hills became, the more powerful their gravitational attraction grew. The large-scale structure of the Universe began as tiny variations in the density of matter in the ancient Cosmos. Some domains of Spacetime received a much higher density of matter than others, simply as a result of mere chance. The rich get richer and the poor get poorer, as a result of jittery quantum fluctuations. The distribution of wealth in the Universe is completely random. Powerful gravitational attraction made more and more matter clump together in the more richly endowed regions of the Cosmos.

Universe Gone “Missing”

Two future space missions depend on data derived from DES: The European Space Agency’s (ESA’s) Euclid mission (which has significant NASA participation) and NASA’s own Wide-Field Infrared Survey Telescope (WFIRST) mission. Both space missions are expected to launch in the 2020s, and they are designed to investigate the myriad mysteries concerning the secretive nature of the Universe.

“With this study, we are showcasing what’s going to be possible with these much more complex observatories,” commented Dr. Andres Plazas Malagon in an August 4, 2017 Jet Propulsion Laboratory (JPL) Press Release. Dr. Malagon is a postdoctoral researcher at JPL, who helped characterize DES’s Dark Energy Camera detectors and who also participated in detector studies for WFIRST. The JPL is in Pasadena, California.

According to Albert Einstein’s Theory of General Relativity, gravity should slow down the rate of the Universe’s expansion. However, in 1998, two teams of astronomers observing distant supermovae made the surprising discovery that the Universe is not slowing down at all–in fact, it is speeding up! In order to explain this puzzling observation, scientific cosmologists were forced to confront two possibilities: either 70% of the Universe is in an exotic form, now termed dark energy, or General Relativity must be replaced by a new theory of how gravity operates on cosmic scales.

DES is designed to search for the origin of the accelerating Universe and help to reveal the true nature of the dark energy by measuring the 14-billion-year-old history of the universal expansion with high precision. More than 400 scientists from over 25 institutions in the United States, the United Kingdom, Brazil, Spain, Germany, Switzerland, and Australia are participating in this project. The collaboration has constructed a very sensitive 570-Megapixel digital camera, dubbed DECam, mounted on the Blanco 4-meter telescope at the Optical Astronomy Observatory’s 4-meter Cerro Tololo Inter-American Observatory, located high in the Chilean Andes. Its derived data are processed at the National Supercomputing Applications at the University of Illinois at Urbana-Champaign

Over five years (2013-2018), the DES collaboration is using 525 nights of observation to carry out a deep, wide-area survey to record new information about 300 million galaxies that are billions of light-years from our planet. The survey is imaging 5000 square degrees of the southern sky in five optical filters in order to obtain detailed information about each galaxy being targeted. A fraction of the survey time is being used to study smaller regions of the sky approximately once a week in order to discover and observe thousands of supernovae and other forms of astrophysical transients.

The most current leading models of the Universe indicate that it is composed primarily of the dark energy and dark matter. The dark matter plays the role of an “invisible glue” that holds galaxies and galaxy clusters together with its powerful gravitational grip, while the dark energy is believed to be responsible for the accelerated expansion of the Universe. Some of the best scientific predictions for the amount of dark matter and dark energy in the Cosmos come from the ESA’s Planck satellite, which observes the light emitted approximately 400,000 years after the Big Bang.

The Mystery Of The (Mostly) Missing Universe

The DES has studied the composition of the more mature Universe. The new results show that there is an agreement with predictions made using Planck measurements of the Universe’s babyhood. This finding helps cosmologists reach a new understanding about how the Universe has evolved since the Big Bang. The DES findings were presented at the American Physical Society’s (APS) Division of Particles and Fields meeting held at the U.S. Department of Energy’s Fermi National Accelerator Laboratory in Batavia, Illinois.

“The Planck results have been the landmark constraints in cosmology. It is truly amazing that you have a model that describes the Universe at 400,000 years old, and now we have a similarly precise measurement of the Universe at 13 billion years [old] that agrees with the model,” commented JPL’s Dr. Tim Eifler in the August 4, 2017 JPL Press Release. Dr. Eifler led the DES analysis team that developed the science software for the interpretation of the results.

The measurements show that approximately 70% of the Universe is contained in the dark energy, about 25% is contained in the dark matter, and that the rest is composed of “ordinary” atomic matter–the “runt” of the cosmic litter. All three measurements agree with other precise measurements made to date. At this point, DES has found no evidence that the quantity of dark energy has changed over time. This finding is consistent with Albert Einstein’s idea of a cosmological constant. Einstein first proposed the concept of a cosmological constant, usually symbolized by the Greek letter lambda (^), as a mathematical fix to General Relativity.

The results are of great importance to scientific cosmologists because they show, for the first time, that observations of the more recent Universe, using gravitational lensing and galaxy clustering, can yield results just as precise as those obtained from the cosmic microwave background (CMB) radiation. The CMB is the primordial light that lingers from the “infant” Universe.

Gravitational lensing is a distribution of matter (such as galaxy clusters) that are situated between a distant source of light and an observer. The foreground object (the lens) bends the light from the background source, as the traveling light wanders in the direction of the observer. Gravitational lensing can reveal the presence of the invisible, ghostly dark matter, because its gravity bends, distorts, and magnifies the path of the light wandering its way through Space from a background object.

“This is the crossover point where gravitational lensing and galaxy clustering measurements and surveys will be the primary driver of what we know about dark energy in the Universe,” noted Dr. Eric Huff in the August 4, 2017 JPL Press Release. Dr. Huff is a JPL researcher who invented a new method of extracting the weak lensing signal that enhances the precision of the DES galaxy shape catalogs. The findings come from the first-year data set collected by the DES, using the Blanco telescope.

In order to measure the dark matter, the researchers first created maps of galaxy positions. Then they measured the shapes of 26 million galaxies to directly map patterns of dark matter over billions of light-years, using gravitational lensing and galaxy clustering.

The DES scientists then went on to develop new methods to detect the very small lensing distortions appearing on the galaxy images. In the process, they created the largest guide ever drawn to help scientists detect the Universe’s mysterious dark matter. The new dark matter map is 10 times the size of the one DES had already released in 2015–and it continues to grow. The DES plans to publish a data set that is even five times larger over the next two years.

Dr. Eifler commented in the August 4, 2017 JPL Press Release: “There is a feeling of true discovery in the collaboration. For the first time, we have the data and tools in hand to see whether Einstein’s cosmological constant prevails. We are all excited to explore the physical nature of dark energy. In particular we want to see if there are hints in the data that suggest modifying the laws of gravity on the largest scales in the Universe.”