An Essay by Daniel Pomarède
Our Place on the Map of the Universe
I am a cosmographer – literally a cartographer of the Cosmos. With a gang of a few others, we use the information imprinted in the velocities of galaxies to construct a map of the Universe.
In the prevailing theory of the Big Bang Universe and by virtue of the Cosmological Principle, the cosmos should be uniform and isotropic, that is, having the same average properties everywhere and in every direction. However, in its very first instants, quantum fluctuations, acting as primordial seeds, have given rise to what would become the elements of the cosmic cartography we observe today: planets, stars, galaxies, groups of galaxies, galaxy clusters, galaxy superclusters, walls and filaments separating voids, the whole of it organized in a gigantic architecture called the Cosmic Web. This web is composed of the ordinary stuff we, and the familiar astrophysical luminous objects, are made of. Yet it also harbors the mysterious Dark Matter that is known to be dominant in the distribution of matter in the Universe. So, how are we able to map this complex and, to some extent, eluding structure?
Over the course of their lifetime, galaxies are subject to two antagonist phenomena: on one side, as an inheritance of the Big Bang initial singularity and the resulting expansion of the Universe, they are pushed away one from the others with a velocity that is, according to the Hubble’s Law, proportional to their relative distances. It proceeds as if galaxies were caught in a flow, the so-called Hubble Flow, making them recede one from each other. And, on the other side, they are subject to the force of Gravitation, a universal attractive force of infinite range, that make structures collapse on themselves, and galaxies congregate in various ways, following streams of matters we call Cosmic Flows. By separating these two phenomena, we are able to isolate the contribution of the gravitational sources that cause galaxy motions, infer the three-dimensional distribution of these sources, and hence build a Map. The data needed for this research are collected on some of the greatest astronomical instruments on Earth, such as the radio telescopes of Green Bank in the U.S., Arecibo in Puerto Rico, and Parkes in Australia, and in space with the Hubble and Spitzer telescopes.
So, what is it that we have mapped so far? Before going extragalactic, let us review our inner galactic situation. Our Home planet, Earth, the pale blue dot, is a member of the Solar System, orbiting its central star, the Sun. This star, one among the several hundred billion stars making up the Milky Way galaxy, a barred spiral galaxy, is located within one of its minor spiral arms called the Orion Arm, a safe place at a distance of about 30,000 light-years from the crowded galactic center, where a supermassive black hole, Sagittarius A*, is devouring the unfortunate stars that might approach it. As peripheral a location this might be, still we are immersed in a galactic disk-like structure, filled with stars, opaque dust clouds, star-forming regions, molecular clouds. These represent an observational challenge for what cosmography is concerned, making it impossible to completely map the Milky Way galaxy, especially in the region hidden behind the galactic center, and to directly observe the extragalactic Universe in the continuation of this disk-like structure, the so-called “Zone of Galactic Obscuration” or “Zone of Avoidance.” Though this zone is not observed directly, our research program is able to map in an indirect fashion the structures, or the absence of structures (voids), which might be lurking within, thanks to the influence they exert on our mass tracers: the velocities of galaxies.
Now we are ready to leave our Home galaxy and embark for an intergalactic journey. The first starry islands we encounter are the satellite galaxies of the Milky Way: the Large Magellanic Cloud and the Small Magellanic Cloud that southern earthlings can see with the naked eyes, or the Sagittarius Dwarf and the Sextans Dwarf Spheroidal, and a few dozens more. Their formation history, orbits, interactions with our galaxy, are a source of wonder and active research for cosmologists. Pursuing our journey beyond this swarm of little galaxies, we then encounter the Andromeda Galaxy. A splendor of a galaxy, much similar to our own and hosting like her several hundred billions stars. Located currently at a distance of about 2.5 million light-years, it is set on a collision course with the Milky Way, with a gentle contact and merger foreseen in a few billion years. Together, Andromeda and Milky Way, and their swarm of dwarves, form the “Local Group” of galaxies. Around this group, one can draw a roughly spherical “zero-energy surface” where there is an equilibrium between gravitation and expansion: inside this surface all galaxies will merge into a single super-giant elliptical galaxy, while outside this surface, everything will recede away from us into the dark.
Carrying-on our journey, we realize that the Local Group and other nearby groups of galaxies, such as the Centaurus A Group, the Messier 81 Group, or the Maffei Group, belong to a flattened structure called the “Local Sheet.” This thin wall is located at the boundaries of a giant cosmic void: the “Local Void.” Maybe as wide as 200 million light-years, this void expands across the Zone of Avoidance, and hence it is only recently that it has been mapped thanks to our research program. Inside this expanse, a few lonely galaxies have been spotted – Cosmologists study such peculiar galaxies, peculiar in the sense that they have not been nurtured by their environment, as opposed to the vast majority that have grown up in dense surroundings.
We then proceed 50 million light-years ahead toward a local heavyweight champion: the Virgo Cluster, where a thousand spiral and elliptical galaxies are bound together by gravitation. This cluster of galaxies is the dominant object of a greater structure called the “Virgo Supercluster” or “Local Supercluster,” a 100 million light-years wide aggregation comprising many groups of galaxies, including our own Local Group.
Onward to the next level of the Map: the Great Attractor. It is by studying the velocities of 400 elliptical galaxies that, back in the eighties, a bunch of astronomers that posterity would keep under the name of the “Seven Samurai,” so as to honor their boldness, discovered the Great Attractor. This attractor is a region of space that is particularly over dense, with the noted presence of a nest of a handful of galaxy clusters, including the Centaurus Cluster. It is a very special place on the map: a knot of the Cosmic Web, where our research has established that at least five filaments, or strands, are converging. One of them, the Virgo Strand, connects Centaurus to the Virgo Cluster. The Norma-Pavo-Indus Strand, the Southern Supercluster Strand, and the Antlia Strand all run across the Zone of Avoidance to connect with the Southern galactic sky. Back in 2014, we discovered the Laniakea supercluster of galaxies. This structure appeared in our map as a volume inside which cosmic flows converge onto a unique attractor, the Great Attractor, and outside which these flows converge on either three other more distant attractors. Our galaxy is found to be located inside this volume, in a peripheral region, in effect making Laniakea our Home supercluster of galaxies. 500 million light-years in size, this supercluster is the largest fully mapped entity to which we belong. We gave it an Hawaiian name, built on the contraction of the terms lani: sky, heaven and akea: broad, wide, spacious, immeasurable, so as to honor the culture of the Polynesian navigators, who by their knowledge of stars and oceanic flows, where able to travel on the vast expanses of the Pacific Ocean, connecting the three vertices of the giant triangle formed by Hawaii, Easter Island, and New Zealand.
Let’s now turn our eyes on what lies outside Laniakea. The three other attractors we mentioned earlier are associated with three neighboring superstructures: the Perseus-Pisces Supercluster, the Great Wall, and the super-heavy Shapley Concentration of Clusters, made of 28 galaxy clusters. Cosmic Voids do play an important role. Back in 2017 we published the discovery of the “Dipole Repeller,” a void, at a distance of 700 million light-years in a region of the northern sky that has remained uncharted. A void has this remarkable property that it manifests itself in our three-dimensional map as a divergence in the velocity field: a point from which cosmic flows emanate on their way from underdense to overdense regions. So, what’s going on here? It’s just gravitation having its way, resulting in matter flowing from the void under dense region, toward the over dense regions surrounding it, the dense cores and filaments and walls of the Cosmic Web. The void acts effectively as a repeller, and it was found that the Dipole Repeller contributes in a significant manner to the velocity of our Local Group of Galaxies. Its name is a reference to the dipole effect this velocity implies on the cosmic microwave background radiation, the relic light from an early stage of the Universe. Another such major and influential void, dubbed the “Cold Spot Repeller,” was found at a distance of one billion light-years, and in a direction coincident with the coldest point observed in the map of the relic radiation, hence its name.
Finally, our map revealed the existence of a gigantic large scale structure, a 1.4 billion light-years galactic filament, hidden behind the obscured regions on the fringes of the Zone of Avoidance and in the direction of the Chameleon constellation and of the South Celestial Pole. Named the South Pole Wall, this filament is shaped like an arc that embraces the southernmost frontiers of Laniakea. It is one among the largest structures of the Universe.
Many other outstanding structures have been spotted by fellow cosmographers who have turned their attention to some specific and increasingly distant regions of the universe, to name a few: the Sloan Great Wall, the BOSS Great Wall, the Saraswati Supercluster, the Giant GRB (Gamma-Ray Burst) Ring, and the Giant Arc on the Sky, but that’s another story.
Cosmography is a thrilling endeavor, making us in practice the descendants of the antique world cartographers, we experience the thrill of those who have explored uncharted territories and if fortune was kind enough, suddenly discovered a new land, a new continent. It feels like standing on a bridge, connecting the past of humanity’s knowledge building and mapmaking, and the future of the human adventure in the cosmos. Who knows what will become of these maps: how far will they be extended and how will they be used by our own descendants?