The Strange Case Of The Pushy Void
By Judith E Braffman-Miller
Billions upon billions of galaxies, blazing with the furious, brilliant fires of a host of dazzling stars, trace out the heavy and gigantic filaments of the Great Cosmic Web–a structure that would otherwise be invisible to our eyes, were it not for these sparkling and revealing flecks of starlight. This mysterious Web, which is thought to be composed of the bizarre dark matter, accounts for more than 50% of the volume of the entire Universe, and these enormous filaments surround almost-empty Cosmic Voids–vast and very black spaces that exist between the invisible filaments of the Cosmic Web. Astronomers have known for decades that our barred-spiral Milky Way Galaxy–along with our companion spiral Galaxy, Andromeda–is traveling through intergalactic Space at the breathtaking speed of approximately 1.4 million miles per hour with respect to the expanding Universe. Astronomers have long assumed that dense regions of the Universe, heavily populated by galaxies, are pulling us through Space in the same way that gravity forced Newton’s famous apple to crash down from its tree to the ground below. However, in a groundbreaking study, released in January 2017, and published in the journal Nature Astronomy, a team of astronomers reported their discovery of a previously unknown Void lurking in our galactic neighborhood–and this secretive, long-hidden Void is the real culprit that is pushing our Milky Way, Andromeda, and the rest of the Local Group of galaxies swiftly through intergalactic Space. Even though both our Milky Way and Andromeda are large and majestic spirals, most of the 54 galaxies dwelling in the Local Group are dwarf galaxies.
Largely barren of galaxies, this nearby Void effectively exerts a repelling force that pushes our entire Local Group of galaxies through Space. Originally, astronomers attributed our Milky Way’s speedy journey through the Universe to the Great Attractor. The Great Attractor, situated about 150 million light-years from Earth, is a region of Space that contains about six heavily populated clusters of galaxies. Following closely on the heels of the discovery of the Great Attractor, astronomers were drawn to a considerably larger structure dubbed the Shapley Concentration. The Shapley Concentration is located 600 light-years away, in the same direction as the Great Attractor. However, there is an ongoing controversy about the relative importance of this duo of attractors, and whether or not they can really be the explanation for our Galaxy’s swift travels through Space.
The Universe is heavily populated with enormous collections of galaxies that are arranged within the gigantic Cosmic Web. The Cosmic Web itself is outlined by galaxy clusters and nodes that are bound together by long strings. This large-scale structure is extremely well-organized, and it reveals to the curious eyes of astronomers very busy intersections of galaxies swarming like fireflies around the enormous and almost-empty Voids.
The black, mostly barren, and cavernous Voids have fascinated astronomers for years, and they have frequently been a target for those scientists trying to understand the small population of galaxies that inhabit these regions of near-emptiness. Indeed, the Voids are intriguingly empty, and might harbor only one or two galaxies. This contrasts with the hundreds of galaxies that are commonly observed dwelling within big galaxy clusters.
The Primordial Universe
The mysterious Voids of the Cosmic Web were first discovered back in 1978 in an important study by Dr. Stephen Gregory and Dr. Laird A. Thompson at the Kitt Peak National Observatory in Arizona.
Many scientists think that the Cosmic Voids were formed as a result of baryon acoustic oscillations (BAO) in the early Universe, and this suggests that collapses of mass came on the heels of implosions of the compressed atomic (baryonic) matter. In cosmology, the BAO are periodic and regular fluctuations in the density of the visible atomic matter of the Cosmos. In much the same way that supernovae provide curious astronomers with a standard candle for astronomical observations, BAO matter clustering provides a standard ruler that can be used to measure the length scale in cosmology. The length of this standard ruler–which is about 490 million light years in the Universe that we observe today–can be measured by observing the large scale structure of matter using astronomical surveys.
Starting from what began as very small anisotropies caused by quantum fluctuations in the primordial Universe, the anisotropies grew larger and larger and larger as time passed. In physics, a quantum is the minimum quantity of any physical entity that is involved in an interaction.
The regions of higher density collapsed more rapidly under the extremely heavy pull of their own gravity–eventually resulting in the foam-like, large-scale structure of the Cosmic Web that is composed of Voids and massive dark matter filaments. Voids situated in regions of low-density are larger than the Voids that are located in high-density environments. Voids also seem to correlate with the observed temperature of the Cosmic Microwave Background (CMB) radiation–which is the relic radiation of the beginning of the Universe. Hotter regions of the CMB correlate with filaments and colder regions with the Voids.
The primordial Universe was composed of a dense, searing-hot plasma that was made of electrons and baryons (protons and neutrons). Particles of light called photons, bouncing around brightly in the early Universe, were trapped, and essentially were unable to travel for any great distance before doing a dance with the plasma.
However, as the Universe expanded, the plasma cooled down to a temperature below 3000 Kelvin. This cooler temperature was of a sufficiently low energy to enable the electrons and photons in the primordial plasma to combine and thus create neutral hydrogen atoms. This era of recombination occurred when the baby Universe was a mere 379,000 years old. The photons interacted to a much lesser degree with neutral matter. As a result, during the era of recombination the Universe became transparent to photons, permitting them to decouple from the matter and stream their beautiful and beaming way through the Universe. This newly liberated light was finally free, and it has been dancing through Space and Time ever since. In other words, the mean free path of the dancing photons essentially became the size of the Universe. The CMB is the light that was emitted after recombination–and it is only now finally finding its way to the welcoming telescopes of astronomers on Earth. Therefore, images of this light that lingers, traveling to us from long ago and far away, reveals the Universe the way it was when it was a mere baby of only 379,000 years of age.
The Light That Lingers
On the largest scales, the entire Cosmos looks the same wherever we observe it–showing a foam-like, bubbly appearance, with extremely massive filaments of dark matter weaving themselves around each other to create the mysterious Cosmic Web. The otherwise invisible filaments are traced out by the brilliant light of stellar fires that sparkle within vast sheets of the tangled, twisted, and intertwining structure. The enormous, almost empty, and very black Voids–which interrupt this strange, transparent web-like structure–are traced out by the glittering flames of a multitude of stars. Because the Voids contain very few galaxies, this makes them appear to be almost empty, in dramatic contrast to the brilliantly lit, star-blasted heavy filaments of the Cosmic Web. The filaments braid themselves around these very dark, and almost empty, caverns, creating a twisted, convoluted knot.
Wherever we look in the observable Universe, we see exactly the same thing–the same bizarre pattern, where brilliantly starlit, majestic galaxies are seen swarming like summer fireflies around the borders of the almost, but not quite, empty Voids. This complicated, twisting and transparent Web is generously sprinkled with matter of both the “ordinary” atomic kind, and the exotic and mysterious dark kind. Indeed, observers have found it a challenge to determine whether the regions of luminous matter and dark filaments, lit by the fires of starlit galaxies, encircle the black and almost empty Voids, or if the Voids instead surround these very massive starlit filamentary strands of the twisted, mysterious stuff. Indeed, the two components are so inextricably tangled up together that the entire construction resembles a natural sponge–or, alternatively, a honeycomb. It has been proposed by some cosmologists that the entire large-scale structure of the Universe can be best described as only one immense filament, lit up by the stars, and one huge Void, with both twisted around each other into a mean Cosmic knot.
Our Universe is mysterious. We cannot even see most of it. The myriad of galaxies and enormous galactic clusters and superclusters are all embedded within halos of the exotic, non-atomic, ghostly dark matter. Even though the dark matter is invisible, most cosmologists think that it is really there because it exerts an observable gravitational influence on those objects that are visible–such as stars and clouds of glaring hot gas.
The most current measurements suggest that the Universe is made up of approximately 27% dark matter and 68% dark energy. Dark energy is even more mysterious than dark matter, and it is an unidentified substance that is causing our expanding Cosmos to speed up in its expansion. The origin and nature of the dark energy is not known, but it is frequently thought to be a property of Space itself. Less than 5% of our Universe is composed of the so-called “ordinary” atomic matter. Atomic matter accounts for literally all of the elements listed in the familiar Periodic Table. “Ordinary” atomic matter is truly extraordinary–it composes literally all of the Universe that human beings on Earth find familiar. It is also the stuff of stars, and stars brought life into the Cosmos. We are such stuff as stars are made of.
Modern scientific cosmology began with Albert Einstein who applied his two theories of Relativity–Special Relativity (1905)and General Relativity (1915)–to the Universe. At the start of the 20th century, it was thought that our Milky Way Galaxy was the entire Universe, and that the Universe was eternal and static. But now we know differently. There are billions and billions of galaxies, and our Universe is dynamic–not static. The Universe was born approximately 13.8 billion years ago–and because it had a definite beginning, it might also end.
The large-scale structure of the Universe, as revealed by the mysterious Cosmic Web, may have been born with no true physical differences between areas of higher density and areas of lower density. This is a possibility because if the current large-scale structure of the Universe is really the result of random fluctuations on the quantum level, occurring in the neonatal Universe, this is precisely what the most straightforward models suggest. According to this viewpoint, some domains of the primordial Universe received a much greater density of matter than others simply as the result of chance. The distribution of wealth in the neonatal Universe was random–some regions were lucky, some were not.
The Strange Case Of The Pushy Void
The existence of the newly discovered pushy Void was earlier proposed by astronomers at the Institute for Astronomy at the University of Hawaii in Manoa. However, obtaining important observation confirmation of the absence of galaxies proved to be a difficult endeavor. Until the 2017 study, observations primarily focused on the detailed distribution of galaxies–that is, where the galaxies are situated, and how much pull they inflict on our Milky Way Galaxy. In this recent study, the team of astronomers, led by Dr. Yehuda Hoffman of the Hebrew University’s Racah Institutes of Physics in Israel, working with colleagues in the United States and France, tried an alternative approach. Rather than observing the positions of galaxies, they used the motions of the galaxies instead. The astronomers created a 3-dimensional map of the galaxy flow field, and used this to calculate the underlying mass distribution that is composed of both luminous matter and the ghostly dark matter. This method revealed the overdense regions that pull on our Galaxy–as well as the underdense regions that give it a big push.
The region of the Universe that is traveling coherantly away from the pushy Void and toward the gravitational attractors is enormous–reaching across more than a billion light years. This amounts to a tenth of the radius of the entire observable Universe. The Laniakea supercluster of galaxies that was discovered by the same team, and described in a 2014 Nature paper, is embedded within this flow in a way that has been compared to a “cork in a stream.”
“Through 3-d mapping the flow of galaxies through space, we found that our Milky Way Galaxy is speeding away from a large, previously unidentified, region of low density that we call the Dipole Repeller, as well as towards the known Shapley Concentration. It has become apparent that push and pull are of comparable importance at our location,” Dr. Hoffman explained in a January 30, 2017 University of Hawaii Press Release.
“There was a hint of the Void from studies of the distribution of rich clusters of galaxies that emit X-rays, discussed in articles over a decade ago by Dale Kocevski, Harald Ebeling and myself at the University of Hawaii, but the statistics were not sufficient to be convincing,” commented Dr. Brent Tully in the same Press Release. Dr. Tully is of the University of Hawaii Institute for Astronomy.
The researchers, by identifying the Dipole Repeller, were able to explain both the direction of our Galaxy’s motion and its velocity relative to the rest of the Universe. They expect that ultra-sensitive surveys in the future at optical, near-infrared, and radio wavelengths will directly identify the few galaxies expected to lie in this Void, and directly confirm the Void associated with the Dipole Repeller.
This study appears in the January 30, 2017 issue of Nature Astronomy.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, 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 the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.