Mapping The Invisible Strands Of The Cosmic Web

By Judith E Braffman-Miller

On a clear, dark night, the sky above Earth blazes with the brilliant, distant fires of a million, billion, trillion stars--but starlight can be a liar. In fact, most of the Universe is dark--composed of mysterious, invisible material, the nature of which is unknown. Luminous objects, like stars, account for only a small fraction of the beautiful Cosmos. Indeed, as lovely as the dancing stars are, they are merely the glittering sprinkles on a universal cupcake. This is because the unimaginably enormous galaxies and gigantic clusters and superclusters of galaxies are all embedded within heavy halos of a strange and abundant form of material that astronomers call the dark matter--and this dark stuff weaves a massive web of invisible strands throughout Spacetime. In April 2018, a team of astronomers announced that they have decoded faint distortions in the patterns of the Universe's oldest light, in order to map huge tube-like structures that are invisible to human eyes. These massive structures, known as filaments, serve as "super-highways" for delivering matter to dense hubs, such as galaxy clusters. The myriad stars, that light up these enormous clusters of galaxies, trace out that which otherwise could not be seen--the heavy, otherwise invisible strands, weaving the enormous and mysterious Cosmic Web.

The international science team, which included researchers from the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley, analyzed data from earlier sky surveys using sophisticated image-recognition technology to study the gravity-based effects that identify the shapes of these transparent filaments. The scientists also used models and theories about the nature of these filaments to help guide and interpret their analysis.

Published in the April 9, 2018 edition of the journal Nature Astronomy, the detailed study of these transparent filaments will enable astronomers to better understand how the Cosmic Web formed and evolved through time. This great cosmic construction composes the large-scale structure of matter in the Cosmos, including the unseen dark matter that accounts for approximately 85 percent of the total mass of the Universe.

The astronomers learned that the filaments, composed of the dark stuff, bend and stretch across hundreds of millions of light-years--and the dark halos that host galaxy clusters are fed by this universal network of filaments. Additional studies of these massive filaments could provide valuable new insights about dark energy--another great mystery of the Cosmos that causes the Universe to accelerate in its expansion. The dark energy is thought to be a property of Space itself.

The properties of the filaments have the potential to test theories of gravity--including Albert Einstein's Theory of General Relativity (1915). The filaments could also provide important clues to help solve a nagging mismatch in the amount of visible matter predicted to inhabit the Cosmos--the "missing baryon problem."

"Usually researchers don't study these filaments directly--they look at galaxies in observations. We used the same methods to find the filaments that Yahoo and Google use for image recognition, like recognizing the names of street signs or finding cats in photographs," Dr. Shirley Ho commented in an April 10, 2018 Lawrence Berkeley Lab (LBL) Press Release. Dr. Ho, who led the study, is a senior scientist at Berkeley Lab and Cooper-Siegel associate professor of physics at Carnegie Mellon University. Carnegie Mellon University is in Pittsburgh, Pennsylvania.

A Mysterious Cosmic Web Of Darkness

The dark matter filaments of the Cosmic Web surround almost-empty, vast, and black cavernous Voids, situated between the transparent, massive filaments that host a multitude of galaxies. Clusters of galaxies and nodes that are bound together by long strings trace out the Cosmic Web, and this large-scale structure is very well-organized with bustling intersections where galaxies swarm like sparkling fireflies around the enormous, almost-empty Voids. Although the Voids are almost empty, they could contain one or two galaxies. This stands in dramatic contrast to the hundreds of galaxies that normally dwell within large galactic clusters.

Soon after the Universe's birth there existed only extremely small anisotropies caused by quantum fluctuations in the primeval Universe. However, the anisotropies grew larger and larger through the passage of time--growing in size as a result of the expansion of Space. In physics, a quantum represents the minimum quantity of any physical entity that is involved in an interaction.

The regions of higher density in the very ancient Universe collapsed more rapidly than lower density regions as a result of the merciless pull of their own powerful gravity. Ultimately, this resulted in the foam-like, large-scale structure that astronomers observe today in the Cosmic Web.

The primordial Cosmos was composed of a searing-hot, dense plasma made up of electrons and baryons (protons and neutrons). Packets of light called photons bounced around, unable to escape, within the glaring, opaque ancient Universe. This is because the photons were trapped, and unable to zip freely around for any great distance, before dancing with the plasma--thus becoming imprisoned.

However, as the Universe expanded, the plasma cooled off considerably to reach a temperature below 3000 Kelvin. This cooler temperature was of a sufficiently low energy to allow the trapped electrons and photons in the ancient plasma to merge and thus form neutral hydrogen atoms. This era is termed the recombination, and it occurred when the baby Universe was only 379,000 years old. The photons interacted to a lesser degree with neutral matter. The upshot of this was that the Universe became transparent to photons, allowing them to decouple from the matter and fly freely through the Universe. This newly liberated dancing light has been dazzling its way through Spacetime ever since. The path of the liberated photons grew to become the enormous size of the Universe.

The Cosmic Microwave Background (CMB) radiation is the oldest light in the Universe. It was emitted after the era of recombination, and it is now finding its way to the telescopes of curious astronomers. Images of this light that lingers, traveling to us from long ago and far away, show us the way the Universe was when it was a mere toddler of only 379,000 years old. The CMB is the relic radiation left of the Big Bang birth of the Universe itself, thought to have occurred almost 14 billion years ago.

On the largest scales, the entire Universe appears the same wherever we observe it--displaying a foam-like, bubbly appearace, with extremely massive dark matter filaments braiding themselves around each other to weave the mysterious Cosmic Web. The otherwise invisible filaments are traced out by the brilliant light emitted by fiery stars that shine within enormous sheets of this tangled, twisted, and intertwining structure. The enormous, almost empty, and very black Voids--which interrupt this bizarre, transparent web-like structure--are traced out by the dazzling fires of myriad stars. The filaments of the Cosmic Web weave themselves around the almost-empty Voids, creating a twisted, convoluted knot.

Wherever we look in the visible Universe, we see precisely the same thing--the same bizarre pattern, where brilliantly starlit galaxies are seen swarming like fireflies around the borders of the almost, but not quite, empty Voids. This twisting, transparent, and complicated Web is abundantly splattered with matter of both the so-called "ordinary" atomic kind, and the exotic and mysterious non-atomic "dark" kind. Indeed, observers have found it difficult to determine whether the regions of luminous matter and invisible filaments encircle the black and almost empty Voids, or if the Voids instead surround these extremely massive starlit filamentary strands of the twisted, mysterious stuff. Indeed, the two components are so inextricably tangled up together that the entire edifice resembles a natural sponge--or, perhaps, a honeycomb. Some scientific cosmologists have proposed that the entire large-scale structure of the Universe can best be described as only one enormous filament, speckled with starlight, and one huge cavernous Void, with both twisted around each other into a mean Cosmic knot.

Our Universe presents us with myriad unknowns. We cannot even see most of it with our human eyes. The billions upon billions of starlit galaxies and galactic clusters and superclusters are all embedded within enormous, massive, transparent halos of the non-atomic, exotic dark matter that haunts our Universe with its ghostly, mysterious presence. Even though the dark stuff is invisible to our eyes, most scientific cosmologists think that it really exists in nature because it exerts observable gravitational effects on objects that can be seen--such as stars, galaxies, and clouds of hot glowing gas.

Recent measurements indicate that the Cosmos is composed of approximately 27% dark matter and 68% dark energy. Dark energy is even more mysterious than dark matter, and it is causing our Universe to expand at an increasing rate in the direction of its own doom.

Less than 5% of the Universe is made up of the badly misnamed "ordinary" atomic matter. Yet, this familiar form of visible matter accounts for literally all of the elements listed in the Periodic Table. Even though the quantity of atomic matter is extremely small when compared to the unseen, "dark" components of the Universe, it accounts for literally all of the Universe that human beings on Earth find familiar. It is also the stuff of stars, and stars are responsible for creating, in their nuclear-fusing furnaces, all of the atomic elements that made life possible on our planet--and probably elsewhere in the Cosmos.

Modern scientific cosmology began when Albert Einstein applied his two theories of Relativity--Special (1905) and General (1915)--to explain the way the Universe works. At the beginning of the 20th century, scientists thought that our Milky Way Galaxy was the entire Universe, and that the Universe itself was both static and eternal. But now we know differently--or, at least, we think we know. There are billions and billions of galaxies, and our Universe is dynamic--not static. The Universe began approximately 13.8 billion years ago in the exponential expansion of the Big Bang, where it went from microscopic size to macroscopic size in the tiniest fraction of a second. Something, we don't know precisely what--made the microscopic Patch that grew to become the Universe undergo this runaway inflation. This original Patch that was much too small for a human being to see, so small that it was almost, but not precisely, nothing, was, in fact, so hot and dense that all that we are and all that we will ever know, emerged from it. Because scientists now think that our Cosmos had a definite beginning, it might also come to an end.

However, some other scientific cosmologists speculate that there may have been something undiscovered and, perhaps, undiscoverable, existing before the Big Bang. What this might have been is purely a matter of speculation--at least, at this point. The neonatal Universe was brimming with extremely energetic radiation, a turbulent sea filled with photons. The entire primordial Universe glared brilliantly like the surface of our Star, the Sun. What we now observe, almost 14 billion years later, is the doomed, fading--greatly expanded and still expanding--aftermath of that brilliant primordial beginning. As our Universe expanded to its current immense size, the fires of its brilliant birth cooled. Now we watch from our tiny, rocky, obscure little planet as our Universe grows ever larger and larger, colder and colder, darker and darker, fading like the haunting grin of the Cheshire Cat in the direction of its own heat death.

The Oldest Light

Dr. Shirley Ho and her team's observations of the Universe's oldest light used data obtained from the Baryon Oscillation Spectroscopic Survey (BOSS), an Earth-based sky survey that captured the light emitted from about 1.5 million galaxies in order to study the Universe's expansion and the patterned distribution of matter in the Universe that had been triggered by the propagation of sound waves (baryonic acoustic oscillations), rippling through Spacetime at its birth.

The BOSS survey team created a catalog of probable filament structures that connected clusters of matter together, and the scientists used this for their most recent research study. Dr. Ho's team also used precise, space-based measurements of the CMB--the nearly uniform relic signal from the first light of the newborn Universe. Even though this light signature is very similar across the entire Universe, there are regular fluctuations that have been mapped in earlier surveys.

In this recent study, the scientists focused on patterned fluctuations in the CMB. They also used sophisticated supercomputer algorithms to obtain the imprint of dark matter filaments caused by gravity-based distortions in the CMB. These distortions are known as weak lensing effects, and they are caused by the CMB light traveling through matter.

The phenomenon of light deflection in the presence of massive objects is termed gravitational lensing. This effect was measured for the first time a century ago during a solar eclipse, and this revealed that the apparent positions of stars in the sky change as a result of the deflection of light by our Sun's gravitational field. This was the first successful test of Einstein's Theory of General Relativity and its prediction of gravitational lensing. On extragalactic scales, galaxies, galaxy clusters, and the filamentary structure of the Cosmic Web can serve as gravitational lenses. The gravity of those extragalactic objects causes images of very remote background galaxies to be distorted by a massive object situated in the foreground (the lens). If the distortions are very small, they cannot be observed in the case of individual galaxies. However, they can be determined statistically--by averaging over a large number of galaxies. Weak lensing is a powerful natural tool that astronomers can use to measure the masses of a variety of objects in the Universe. Gravitational lensing is very sensitive to the presence of dark matter, and it can be used to measure the mass profile of galaxies out to large radii, where the remote light traveling from stars and gas is much too faint to be observed. It can trace the mass distribution of galaxy clusters, and on even larger scales, weak lensing can reveal distortions of galaxies. For this reason, weak lensing can be used to measure the properties of the Cosmic Web. By comparing the statistical properties of the distortion pattern to theoretical models, scientific cosmologists can study the expansion history of the Universe--as well as measure the components of the Cosmos, including the mysterious dark matter and dark energy. Therefore, weak lensing can be used to study the evolution of structure in the Universe, and it also has the potential to probe the mysterious origin of the accelerated expansion of Spacetime, as well as distinguishing between dark energy and certain theories of modified gravity.

Since galaxies are located in the regions of the Universe that are the most dense, the weak lensing signal from the deflection of CMB light is strongest from those parts. Because dark matter resides in galactic halos, it is known to travel from those denser areas in the filaments of the Cosmic Web.

"We knew that these filaments should also cause a deflection of the CMB and would also produce a measurable weak gravitational lensing signal," commented Siyu He in the April 10, 2018 LBL Press Release. He and colleagues used statistical techniques to identify and compare the "ridges"-- or points of higher density--that existing theories indicated would point to the presence of the Cosmic Web filaments. He is Dr. Ho's graduate student, and is of Carnegie Mellon University. She is currently at Berkeley Lab, and is also affiliated with UC Berkeley.

"We were not just trying to 'connect the dots'--we were trying to find these ridges in the density, the local maximum points in density," He added. The scientists then checked their findings with other filament and galaxy cluster data, and with what are called mocks (simulated filaments based on observations and theories). The scientists used large cosmological simulations generated at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC), for example, to check for errors in their measurements.

The filaments of the Cosmic Web can change connections and shape over the course of hundreds of millions of years. The two warring forces of the pull of gravity and the expansion of the Universe can serve to shorten or lengthen these massive web-like filaments.

"Filaments are this integral part of the Cosmic Web, though it's unclear what is the relationship between the underlying dark matter and the filaments," and that was a primary motivation for the study, commented Dr. Simone Ferraro in the April 10, 2018 LBL Press Release. Dr. Ferraro is one of the study's co-authors, and a Miller postdoctoral fellow at UC Berkeley's Center for Cosmological Physics.

New data from existing experiments, and next-generation sky surveys such as the Berkeley Lab-led Dark Energy Spectroscopic Instrument (DESI), currently under construction at Kitt Peak National Observatory in Arizona, are expected to provide even more detailed information about these massive, invisible filaments, Dr. Ferraro added.

The scientists also noted that this important step in discovering the locations and shapes of the invisible filaments should prove to be important for future focused studies that try to identify what types of gases are swirling within the filaments, the temperatures of these gases, and the way particles enter and then move around within the filaments.

Siyu He said in the April 10, 2018 LBL Press Release that resolving the filament structure can also provide important clues concerning the properties and contents of the Voids in Space around the filaments, and also "help with other theories that are modifications of General Relativity", she explained.

"We can also maybe use these filaments to constrain dark energy--their length and width may tell us something about dark energy's parameters," Dr. Shirley Ho noted in the April 10, 2018 LBL Press Release.

Dr. Shadab Alam, a researcher at the University of Edinburgh and Royal Observatory in Edinburgh, UK; and Dr. Yen-Chi hen, an assistant professor at the University of Washington (Seattle), also participated in the study.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.