The vast and mysterious great Cosmic Web is composed of dark matter–whose identity we do not know. However, scientists strongly suspect that the dark matter is composed of exotic non-atomic particles that do not interact with light–which is why the Cosmic Web is transparent and invisible. In January 2014, astronomers announced that they have spotted a remote quasar lighting up an enormous nebula of gas, revealing for the first time the web-like network of transparent filaments believed to connect the starlit galaxies embedded in the Cosmic Web. Like sparkling dewdrops suspended on the web of an enormous spider, this arrangement of galaxies traces the large-scale structure of the Universe.
A team of astronomers at the University of California, Santa Cruz, led the study, published in the January 19, 2014 issue of the journal Nature. Using the 10-meter Keck I Telescope Observatory poised atop the Mauna Kea volcano in Hawaii, the team of scientists spotted an enormous, brightly shining nebula composed of gas that extends approximately 2 million light-years across intergalactic space.
“This is a very exceptional object: it’s huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar,” explained Dr. Sebastiano Cantalupo, the lead author of the study. Dr. Cantalupo is a postdoctoral fellow at UC Santa Cruz.
Quasars are exceptionally brilliant objects that are often observed inhabiting the ancient and very remote Universe. These extraordinarily distant bodies are thought to have first caught fire a “mere” few hundred million years after the inflationary Big Bang birth of the Universe almost 14 billion years ago. Quasars dazzle the Cosmos with their fierce, brilliant fires–they are actually the accretion disks encircling young, voracious, and greedy supermassive black holes lurking in the hearts of baby galaxies that were forming in the very early Universe. Supermassive black holes haunt the dark hearts of almost all–if not all–large galaxies, and they weigh-in at millions to billions of times more than our Star, the Sun. Our own large, barred-spiral Galaxy, the Milky Way, holds a supermassive black hole in its secretive heart. It is called Sagittarius A* (Sgr A*, for short), and it is relatively light-weight, by supermassive black hole standards, weighing merely millions–as opposed to billions–of times more than our Star.
The team of astronomers, led by Dr. Cantalupo, used an extremely bright quasar, seen as it appeared when the Universe was “only” about 3 billion years old, to light up the dim gas floating around in this brilliant celestial object’s general neighborhood. The rushing sea of light flowing out from the quasar causes hydrogen atoms in the gas to send forth a tattle-tale wavelength of ultraviolet radiation.
As the Universe continues in its relentless expansion, this radiation is stretched to ever longer wavelengths, ultimately becoming visible light. Dr. Cantalupo, Dr. J. Xavier Prochaska, and their team at UC Santa Cruz, studied that gushing, brilliant, ancient light, with Keck I. The images derived from Keck reveal a cloud of gas that is more than 10 times the diameter of our Galaxy! This represents the very first discovery of radiation flowing from a cloud “on scales far beyond a Galaxy”, Dr. Prochaska said in the January 19, 2014 Nature News.
The Standard Cosmological Model of structure formation in the Universe predicts that galaxies are embedded in the filaments of the great Cosmic Web, most of which (about 84%) is composed of the mysterious, transparent, ghostly dark matter. This Cosmic spider’s web is observed in computer simulations that strive to model the evolution of structure in the Universe. The simulations show the evolution of the dark matter on large scales, including the dark matter halos in which galaxies are born and the Cosmic Web composed of dark matter filaments that connect them.
The force of gravity causes “ordinary” atomic matter to follow the distribution of the dark matter. In this way, filaments of diffuse ionized gas are believed to outline a spidery pattern like the one seen in dark matter computer simulations.
A Cosmic Spider’s Web
At the instant of our Universe’s inflationary Big Bang birth, almost 14 billion years ago, there was a tremendous blast of brilliant light. Particles of light, called photons, of very high-energy electromagnetic radiation flew out from extremely searing-hot matter composing the primordial Cosmos. In the primordial Universe, however, light was unable to blast its way through the Cosmos freely. This is because, at the extraordinarily high temperatures of the very ancient Universe, matter was ionized. Hence, any atoms that managed to congeal in this hot environment were speedily ripped to shreds. The reason for this is that positively charged atomic nuclei could not keep a grip on their encircling clouds of negatively charged electrons. Electrically charged particles constantly emit and absorb photons. For the first 400,000 years of our Universe’s existence, light was being constantly emitted, then absorbed, then emitted, and then absorbed again–and again! This cycle continued for a much longer period of time than human civilization has flourished on Earth! This ancient confusion continued for hundreds of thousands of years–until the extraordinarily high temperatures of the primordial Universe at last fell to less than five thousand degrees Fahrenheit.
All of the billions upon billions of starlit galaxies that blasted the early Universe with light appear to swarm around the perimeters of enormous cavities that can be as much as 150 light-years across. These great Voids are black because they are almost–but not entirely–empty.
These enormous great Cosmic Voids compose about 50% of the volume of the entire Cosmos. On large-scales, the Universe looks frothy, with heavy filaments of dark matter twisting themselves around each other to form the Cosmic Web. These transparent filaments are lit by the brilliant fires of vast sheets of intertwining galaxies. Enormous black Voids, which interrupt this transparent, intertwining, mysterious spidery structure, outlined by the sparkling flames of stars, are almost entirely devoid of galaxies. The Voids contain only a relatively small number of galaxies–which makes them appear to be empty when compared to the star-blasted Cosmic Web, that braids itself around them into a complex and convoluted structure.
Wherever we look in our vast Cosmos, we observe this same strange spidery pattern. Starlit, fiery galaxies are observed to be swarming around the perimeters of immense, black, almost empty Voids. This complex, twisted Web is richly endowed with matter, of both the so-called “ordinary” atomic kind, and the exotic and bizarre non-atomic dark kind.
If we were able to observe the Universe the way it really is, with our Earth-evolved human eyes, we would understand that most of the matter in it is exotic and non-atomic. The starlit galaxies and immense clusters of galaxies are embedded in halos of this mysterious, non-atomic, and invisible dark matter. The transparent dark matter weaves the heavy filaments of the Cosmic Web throughout the vastness of Space and Time.
The dark matter does not reveal its ghostly presence except by way of its gravitational grip on objects that can be observed. The most recent measurements and calculations indicate that the Universe is composed of about 27% dark matter, and approximately 68% dark energy. Dark energy is a mysterious substance that is causing the Universe to accelerate in its expansion. The nature and origin of the dark energy is not currently known.
“For almost a century, the Universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago. However, the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up, the Universe will end in ice,” noted Dr. Saul Perlmutter of the Lawrence Berkeley National Laboratory and a professor of physics at the University of California at Berkeley. Dr. Perlmutter was awarded the 2011 Nobel Prize in Physics with Dr. Brian P. Schnidt and Dr. Adam Riess for providing evidence that the expansion of the Universe is accelerating.
A mere 5% of our Universe is made up of “ordinary” matter–the familiar atomic matter that makes up the Periodic Table of the elements. Although “ordinary” atomic matter is the runt of the Cosmic litter, it is actually quite extraordinary. This is because it is the stuff of stars, planets, moons, and people–and all the rest of our familiar realm of the Universe!
New Light On The Cosmic Web
Until now, the filaments of dark matter that weave the magnificent Cosmic Web have never been directly observed. Although the gas that floats around in intergalactic space has already been spotted, due to its absorption of light gushing from brilliant background sources, those observations do not show how the gas is distributed. In the January 2014 study, however, the scientists spotted the fluorescent glow of hydrogen gas resulting from its illumination by the powerful radiation emanating from the quasar.
“This quasar is illuminating diffuse gas on scales well beyond any we’ve seen before, giving us the first picture of extended gas between galaxies. It provides a terrific insight into the overall structure of our Universe,” noted study co-author Dr. Prochaska in a January 19, 2014 UC Santa Cruz Press Release.
The hydrogen gas, that is lit up by the quasar, sends forth ultraviolet light called the Lyman alpha radiation. The illuminating quasar itself is very remote–about 10 billion light-years away. Indeed, the quasar‘s emitted light is “stretched” by the expansion of the Universe to the point that its original invisible ultraviolet wavelength has become a visible shade of violet to the prying “eyes” of Keck. By knowing the distance to the quasar, the astronomers were able to determine the wavelength of the lyman alpha radiation from that distance, and constructed a special filter for the ‘scope’s LRIS spectrometer to acquire an image at that wavelength.
“We have studied other quasars this way without detecting such extended gas. The light from the quasar is like a flashlight beam, and in this case we were lucky that the flashlight is pointing toward the nebula and making the gas glow. We think this is part of a filament that may be even more extended than this, but we only see the part of the filament that is illuminated by the beamed emission frm the quasar,” Dr. Cantalupo explained in the January 19, 2014 UC Santa Cruz Press Release.
In a previous survey of remote quasars lurking in the ancient Universe, Dr. Cantalupo and others spotted so-called “dark galaxies”, which represent the densest knots of gas floating around in the Cosmic Web. These “dark galaxies” are believed to be either too young or too small to have given birth to fiery baby stars.
“The dark galaxies are much denser and smaller parts of the Cosmic Web. In this new image, we also see dark galaxies, in addition to the much more diffuse and extended nebula. Some of this gas will fall into galaxies, but most of it will remain diffuse and never form stars,” Dr. Cantalupo continued to note.
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