Our Place in the World Wide Universe

Olivier Loose March 2021 Our Place in the World Wide Universe A Matryoshka doll approach to figuring out where we live in the wider cosmos. Taking a step back from our day-to-day activities can sometimes have an invigorating effect, as it allows us to pause for a moment, reflect on where we stand, and see where we want to go from there. This article gives a literal spin to this reflective exercise: Where do we actually stand in the broader Universe? We know that our home planet is whizzing around the Sun and that our Solar System forms part of the Milky Way Galaxy. But what lies beyond that? Let us pause for a while and have a look at what researchers have discovered so far regarding our place in this world wide Universe. Our Sun-Powered Backyard The structure we are probably most familiar with is the Solar System, in which eight planets dwell in a flat disk around the Sun. The four innermost planets closest to the Sun are called the terrestrial or rocky planets – Mercury, Venus, Earth, and Mars. Once having crossed the Main Asteroid Belt, i.e., an area of asteroids circling around the Sun past the orbit of Mars, we find the four outer planets: two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). The reason why the inner planets are rocky relative to the outer ones is that during the formation of the Solar System (which arose roughly 4.6 billion years ago from a mixed cloud of dust particles and hydrogen and helium gases), the heavier chemical elements (referred to as metals, such as iron, nickel and silicon) could better resist the elevated temperatures in closer proximity to the Sun (around 1,500 degrees Kelvin), given their higher boiling points. Moreover, at these stages of planet development, most of the gases present at these distances would not condense and were either burned off due to the heat caused by solar radiation and impact collisions or pushed away from the centre by solar winds and radiation pressure. 1 Olivier Loose March 2021 Therefore, metals were the only available elements that could condense into solids to eventually shape the inner planets. In contrast, lighter materials called ices (e.g., frozen methane, frozen ammonia, and water ice) as well as gases (mostly hydrogen and helium) fared better in farther and cooler regions away from the Sun. Hence, they constituted the building blocks for the giant gaseous and icy planets. Because the lightweight ingredients were much more abundant than metals, it helps explain why the outer planets are considerably larger in size than their rocky counterparts. Fig. 1. Our Solar System with the inner planets (Mercury, Venus, Earth, and Mars) in smaller orbits than the Asteroid Belt and the outer planets (Jupiter, Saturn, Uranus, and Neptune) in orbits between the Asteroid Belt and the Kuiper Belt. (Source: universetoday). Beyond the giant planet Neptune, there is another ring of orbiting objects (the Kuiper Belt), containing over 100,000 icy bodies (at approximately 40 degrees Kelvin), including Pluto, Makemake and Eris. The Kuiper Belt is more extensive than the Main Asteroid Belt: The former is at least eight times wider and almost fifty times more massive than the latter. Still beyond that, we come across the conjectured Oort Cloud, which is a sphere (not a ring) of icy solid objects (called planetesimals) that entirely encapsulates the Solar System and presumably makes up the birthplace of comets. 2 Olivier Loose March 2021 Fig. 2. The spherical Oort Cloud completely surrounds the Solar System. [AU (astronomical unit) is a unit of length and equals the distance between the Sun and planet Earth, i.e., 150 million kilometres]. (Source: socratic). In terms of orbital velocities, the Earth (with a mass of 5.98x1024 kg and at a distance of 150 million km from the Sun) is rushing along its path around the Sun with a breakneck speed of 107,000 km/h (or 66,500 miles/h) to complete one full orbit in 365.25 days. The top speed, however, is reserved for the innermost planet Mercury (172,000 km/h or 107,000 miles/h), while the lowest belongs to Neptune (19,500 km/h or 12,000 miles/h). Our Spiral-Shaped Milky Way Our Solar System sits in a larger structure, i.e., the Milky Way Galaxy, which is brimming with stars (the total number is estimated at 400 billion, one of which is our Sun), dust, gas, globular clusters, planets, dark matter, and other interstellar material. Since the Galaxy’s inception roughly 12 billion years ago, gravity is holding everything together and influences the galactic dynamics. Most of the stars, dust, and gases are gathered in a flattened region of space (the stellar disk), which has an aggregate mass of about 50 billion times the mass of our Sun (expressed as ‘solar masses’ with the mass of the Sun being 2x1030 kg) and a diameter of 100,000 light years (with one light year measuring almost 10 trillion kilometres). 3 Olivier Loose March 2021 Within the stellar disk, stars are usually born in spiral arms, which are characterized by a high density of gas and dust – note that stars can move in and out of these spiral arms. At circa 27,000 light years away from the Milky Way’s centre, our Solar System resides for the moment in the Orion arm and swirls around the Galaxy with an astronomical speed of 720,000 km/h (447,000 miles/h). Notwithstanding such incredible velocities, we still need 240 million years to go round the Milky Way just once. Fig. 3. A schematic view exhibiting the Milky Way’s main components. Left: face-on view. Right: edge-on view. (Source: ESA). At the centre of the stellar disk (the stellar bulge), a very compact collection of primarily older stars, gas, and dust is orbiting a supermassive black hole called Sagittarius A* – a black hole is the densest astrophysical object throughout the Universe from which no light or matter (except perhaps some Hawking radiation) can escape beyond a certain region (the event horizon) due to an ever-increasing curvature of spacetime (gravity) towards its centre (the singularity). What is more, a spherical stellar halo completely surrounds the stellar disk and is home to individual and mostly older stars as well as globular clusters (groups of hundreds of thousands of old stars). Nevertheless, the stellar halo’s mass amounts to barely 1% of the total stellar mass within the entire Galaxy. 4 Olivier Loose March 2021 Extending even more outwards, we encounter a (hypothesized) dark matter halo with a projected diameter of at least 600,000 light years – dark matter is an as yet unidentified type of matter with an attractive gravitational effect that accounts for 28% of all existing matter, with the other types being ordinary matter (4%) and dark energy (68%). When considering all types of matter, then almost 90% of the Milky Way’s mass can be traced back to the dark matter halo – including dark matter, the Galaxy’s mass is roughly 1.5 trillion solar masses. Although it has not been observed directly so far, the existence of this halo is inferred from the dynamics of stars and gas, among other techniques. Fig. 4. Schematic view of the dark matter halo and the innerstellar halo. [Sgr A* = Sagittarius A*, the supermassive black hole at the centre of the Milky Way; kpc = kiloparsec is a unit of length and equals 3,260 light years]. (Source: Matthias Schmitt). Our Local Group Zooming out further, you and I find ourselves being part of an even larger system: The Local Group – in astronomy, a group is generally an assembly of a couple of dozen galaxies, while a 5 Olivier Loose March 2021 cluster typically comprises hundreds or up to thousands of them. This particular group holds about 40 galaxies (some researchers list twice as many) and is to a large extent split into two subgroups, both represented by a main spiral galaxy, i.e., the Milky Way and the Andromeda Galaxy. These two major galaxies are on a collision course and will start merging in the next 3 to 5 billion years – they are currently 2.5 million light years apart and are approaching each other at a speed of 432,000 km/h (268,000 miles/h). Regarding the Local Group as a whole, its total mass is estimated to be at least 2.4 trillion solar masses (some studies mention 3.7 trillion solar masses) and its size would span a region in space measuring approximately 10 million light years across. Fig. 5. The Local Group, including the Milky Way and the Andromeda Galaxy (M31) together with their satellite galaxies. (Source: astronomy). Within the Local Group, the three largest galaxies are Andromeda (with a diameter of 220,000 light years), the Milky Way (100,000 light years), and the Triangulum Galaxy (56,000 light years). When it comes to mass, the top three galaxies are the Milky Way (1.5 trillion solar masses), Andromeda (800 billion solar masses), and the Triangulum Galaxy (80 billion solar 6 Olivier Loose March 2021 masses) – establishing galactic masses is no easy endeavour; for instance, assessments for Andromeda’s mass range from 700 billion to 2.5 trillion solar masses. The remaining galaxies are much smaller in size and are known as dwarf galaxies. Many of them are orbiting either the Milky Way or Andromeda, and aptly referred to as satellite galaxies. For example, the two closest satellite galaxies to the Milky Way are the Canis Major Dwarf Galaxy (42,000 light years from our Galaxy’s centre) and the Sagittarius Dwarf Elliptical Galaxy (at 50,000 light years). Our Virgo Supercluster The Local Group is in turn lodged within a broader galactic arrangement, designated as a supercluster, which is a collective system of many galaxy groups and clusters. About 90% of all the 100 billion individual galaxies distributed throughout the entire Universe are deemed to be inhabiting these large structures, of which there appears to be roughly 10 million. The supercluster we belong to is called the Virgo Supercluster or Local Supercluster, which is classified as a poor supercluster in terms of ‘richness’. This characteristic reflects the number of clusters within a supercluster and is divided into four sub-categories: poor (less than 3 clusters), medium (between 3 and 9), rich (from 10 to 19), and extremely rich (above 20). Richness furthermore positively correlates with density and linearly increases with the size of the supercluster. The Virgo Supercluster is 98 million light years long (this diameter is around 1,000 times longer than that of the Milky Way Galaxy), has a mass of 1.5 quadrillion solar masses (1,000 times the total mass of our Galaxy), and accommodates one major cluster (the Virgo Cluster) as well as dozens of smaller groups (e.g., the Local Group, the Canes Venatici II Group, the M61 Group, the NGC 4697 Group, and the Ursa Major Groups). Our Local Group is situated on the outskirts of the Virgo Supercluster, at 53 million light years away from its centre where the Virgo Cluster is located. 7 Olivier Loose March 2021 Fig. 6. The Virgo Supercluster, here modelled with the Local Group (in yellow) at the centre, although in the real world the Virgo Cluster (the dense galaxy cluster to the right) actually sits at its centre. Every white dot represents a galaxy. (Source: Adapted from Wikimedia). Some of our Local Group’s closest neighbouring galactic groups (which are all part of the Virgo Supercluster) include the Maffei 1 Group (which is the closest to us at a distance of 10 million light years, holding between 5 and 23 galaxies), the M81 Group (the second closest at some 12 million light years, containing a minimum of 34 galaxies), and the Sculptor Group (the third closest at 12.9 million light years, harbouring at least 11 galaxies). At the heart of the Virgo Supercluster, the Virgo Cluster shelters between 1,300 up to 2,000 individual galaxies, of which the supergiant elliptical M87 is the most prominent one – this is 8 Olivier Loose March 2021 the same galaxy whose supermassive black hole’s shadow was captured on camera for the first time in human history in 2019. In addition, not only is the Virgo Cluster the closest cluster to planet Earth (the second and third place go to the Fornax Cluster and the Eridanus Cluster, respectively – see Fig. 6), but we are also moving closer to it: Our Local Group is headed towards the Virgo Cluster with a speed of 972,000 km/h (604,000 miles/h) – a movement which is dubbed the Virgocentric infall. Fig. 7. The Virgocentric flow of the Milky Way (MW, in yellow) and the Andromeda Galaxy (M31, in red) together with many other galaxies towards the Virgo Cluster (in purple). [Mpc = million parsec = 3,262,000 light years; SGX, SGY = supergalactic coordinates X and Y]. (Source: Institute for Astronomy). Our Laniakea Supercluster Yet, the Virgo Supercluster is just one of four major components of a greater astronomical ensemble, i.e., the Laniakea Supercluster. The other three constituents are the Hydra-Centaurus Supercluster (at 204 million light years from us), the Pavo-Indus Supercluster (at an average of 203 million light years), and the Southern Supercluster (the closest supercluster to us at 65 9 Olivier Loose March 2021 million light years which mainly consists of the Fornax Cluster, the Eridanus Cluster, and the Dorado Group). The Laniakea Supercluster stretches out for 522 million light years (which is over 5,000 times longer than the Milky Way), has a mass of 100 quadrillion solar masses (67,000 times the total mass of our Galaxy), and is home to 100,000 galaxies and up to 500 groups and clusters. Some of the more pronounced galaxy clusters include the Norma Cluster (within the HydraCentaurus Supercluster), the Hydra Cluster (the Hydra-Centaurus Supercluster), the Centaurus Cluster (the Hydra-Centaurus Supercluster), the Virgo Cluster (the Virgo Supercluster), the Ophiuchus Cluster, A2870, A3581, and A3656 (Pavo-Indus Supercluster). Other than galaxy clusters, the Laniakea Supercluster equally covers several cosmic voids – areas of low energy-matter density in the Universe that usually show an absence of galaxies and around which galaxies are positioned – such as the Local Void and the Sculptor Void. Fig. 8. A 3D model of the density structure of various superclusters. In the red box: the Laniakea Supercluster with the Local Group underlined in red. (Source: Adapted from Paper Hélène Courtois et al.). 10 Olivier Loose March 2021 Not only are we traveling in the direction of the Virgo Cluster, but we are also moving with an unfathomable speed of 2,160,000 km/h (1,340,000 miles/h) towards the Great Attractor, which is the gravitational centre of the Laniakea Supercluster, sitting right within the Centaurus Cluster. Bear in mind, however, that the Laniakea Supercluster is not a gravitationally bound system since dark energy (which causes the Universe to accelerate its expansion, given its repulsive gravitational effect) will eventually drive some of its galactic members apart from one another. Fig. 9. The Milky Way is headed towards the Virgo Cluster but is at the same time under strong gravitational influence of the Great Attractor in the Centaurus Cluster. (Source: Paper Hélène Courtois et al.). On top of all that, together with the Great Attractor, we are pulled as a whole towards an even greater gravitational well: the Shapley Supercluster. This suggests that part of the speed with 11 Olivier Loose March 2021 which we are drawn to the Great Attractor can be elucidated by the presence of the Shapley Supercluster. This extremely dense region in space is located 652 million light years away from us. In size, it is comparable to the Virgo Supercluster, but in terms of mass, it is 2 to 10 times heavier. As a matter of fact, instead of 1 major cluster (which is the case for the Virgo Supercluster), the Shapley Supercluster harbours 25 of them, A3558 (also called Shapley 8) being the most massive cluster. In other words, more mass in a relatively similar volume of space gives a higher density, which, in turn, according to Albert Einstein’s theory of general relativity, creates a stronger curvature of spacetime, implying a stronger gravitational field and explaining the observed dynamics. Fig. 10. A model of density structure shows that the Local Group (located at the red dot) together with the Great Attractor (underlined in red) tend to flow towards the Shapley Supercluster (in red box). The model depicts as well as the flows of neighbouring superclusters, including Perseus-Pisces, the Lepus region, Hercules, and Coma. (Source: Adapted from Yehuda Hoffman et al.). 12 Olivier Loose March 2021 Our Pisces-Cetus Supercluster Complex The Laniakea Supercluster does not seem to be the end of the story. To the contrary, it is thought to be just one of five segments of the Pisces-Cetus Supercluster Complex, also known as the Pisces-Cetus Filament – a complex is a kind of galaxy filament, which is a thread-like, highdensity cosmic grouping of individual galaxies, groups, clusters, and superclusters. The PiscesCetus Filament is just shy of 1 billion light years long and 163 million light years wide. Besides the Laniakea Supercluster, the other four segments are identified as the Pisces-Cetus Supercluster (the most prominent part within this filament), the Perseus-Pegasus Chain (which comprises the Perseus-Pisces Supercluster), the Pegasus-Pisces Chain, and the Sculptor Region (which contains the Sculptor Supercluster). Fig. 11. The Pisces-Cetus Supercluster Complex (in yellow) with its main components. (Source: Adapted from ESO). 13 Olivier Loose March 2021 What is more, given that superclusters are surrounded by galactic voids, filaments ultimately make up the boundaries between these voids. On larger scales, the Universe as a whole therefore provokes the emergence of a sponge-, foam-, or cellular-like grid of high-density galactic regions (a sort of sea of soap bubbles, if you will). Fig. 12. A density model depicts the filament-like character of several structures, including the Shapley Supercluster, the Sloan Great Wall (a gathering of various superclusters), the Horologium-Reticulum Supercluster, and the Pisces-Cetus Supercluster, which is the major component of the Pisces-Cetus Filament. (Source: Adapted from Chapter 7, PhD Thesis W. Schaap). It becomes clear then that the Pisces-Cetus Supercluster Complex is embedded in a larger interconnected web of galaxies. In that sense, the Pisces-Cetus Filament is to some degree connected to other large structures, e.g., the South Pole Wall – a wall is another type of galactic filament and is usually wider, giving its form a more sheet-like structure. The South Pole Wall has an estimated length of 1.4 billion light years and reaches from the structure Apus+12.5 all the way to the Perseus-Pisces Filament and the Southern Wall via the Lepus region and the Funnel (Eridanus+9.1) – see Fig. 13. 14 Olivier Loose March 2021 The connection with the Pisces-Cetus Supercluster Complex manifests itself when regarding the overlap with two components of the South Pole Wall: the Perseus-Pisces Filament (which holds the Perseus-Pisces Supercluster) and the Southern Wall (of which the Southern Supercluster – which pertains to the Laniakea Supercluster – is the most pronounced feature). Fig. 13. A density model of the South Pole Wall. (Source: Paper by Daniel Pomarède et al.). Our Universal Mother In the same spirit as the Russian nesting Matryoshka dolls, we made a journey starting from Earth within our Solar System on to the Milky Way, the Local Group, the Virgo Supercluster, 15 Olivier Loose March 2021 and the Laniakea Supercluster, to eventually arrive at the mother of all structures to which we belong: the Pisces-Cetus Supercluster Complex. Such vast view on where we live within the cosmos might not only put our life here on Earth in greater perspective, but it also allows us to expand our understanding about a whole gamut of fascinating astronomical structures, despite that some of them are many millions of light years away or not directly accessible to us. With so many more regions to explore across the observable Universe, how long before we find an even larger cosmic Matryoshka doll for our innermost galactic home? 16