by Caleb Scharf
The next step, a descent into the cosmic web
But shortly after this time, as the universe cooled, electrons and protons could bond to form neutral atoms of hydrogen that seldom scattered visible light. The fog lifted and the photons could travel without interacting with any particles. We can detect these same photons today—now greatly stretched out in wavelength and causing an unavoidable microwave hum across space. That background noise is, in effect, a glimpse of the cosmic horizon; it’s as far back as we can see.
With our most sensitive telescopes we can peer into the universe to find the first stars and galaxies (also close to thirteen billion years ago). These infant condensations of matter were seeded by the tiny irregularities of the early inflating cosmos, irregularities that gravity grew by pulling more and more matter together. And all the way from these galactic infants to the galaxies sharing our patch of the universe, we’ve succeeded in constructing great maps of the cosmic landscape.
These maps tell us that the cosmos is both foamy and granular. A bit like the soapy scum left from an emptying sink or bath, the foam is seen in outline, traced by a three-dimensional web of dark as well as luminous matter.
Peppering that web are dense condensations where gravity has overwhelmed the expansion of space-time. Here and there are superclusters of galaxies a trillion trillion meters, or hundreds of millions of light-years, across. Within these superclusters are distinct clusters of galaxies, broad and deep wells of gravity that hold swarms of hundreds, sometimes thousands, of galaxies, all orbiting in and out of the center, together with vast pools of hot gas and cold dark matter spanning tens of millions of light-years.
Specks of galaxies
Galaxies along a filament of large-scale structure
Matter flows in large gravitating structures.
There are tiny galaxies, big galaxies, and really big galaxies. Within these galaxies are the minuscule dense forms of stars and other stellar remnants, as well as the most compact objects in nature: black holes that range from a mass ten times that of our Sun to masses tens of billions of times larger.
Remarkably, on the journey from 1027 meters to 1023 meters, in just five factors of ten we transition down from the size of the observable universe (the cosmic horizon) to our own cosmic neighborhood of Local Group galaxies.
To put that another way, we’ve gone from a volume containing almost Everything (more than 200 billion galaxies) to a volume containing between 50 and 60 galaxies in our vicinity. Or think about a distance of one kilometer (half a mile, perhaps your morning walk) compared to the size of a coin in your pocket. That’s the contrast between the scale of the entirety of the known universe and our intergalactic patch within it.
Galaxies are small dots.
Yet you’ve also only just begun this cosmic voyage. In the rest of this book are another fifty-seven orders of magnitude in scale we need to travel through. Ready? Then turn the page and follow on down!
In only five orders of magnitude, we reach the pocket-change scale of our local universe.
Our Local Group of galaxies
Black holes tear up matter and make light.
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DARKNESS AND LIGHT
1022, 1021, 1020, 1019, 1018 meters
From about 1.06 million light-years to 106 light-years
From about 8 to 10 Milky Way diameters to the size of a giant molecular cloud
Imagine that you are an all-powerful alien being who decides to scrunch up all the stars in the Milky Way so that they are packed next to one another. By getting rid of all the space in between, you can fit these stars into a cube only about 8 billion kilometers (or fifty-four times the radius of Earth’s orbit) on each side. That cube, containing some 200 billion stars, fits neatly within the orbital diameter of Neptune in our solar system. In other words, the galaxy has plenty of surplus room between its stars.
Of course, physics wouldn’t actually let you do this, at least not without making a lot of mess. The problem with putting this much mass in one place is that you’d wind up making a black hole. Why? Because the gravitational pull of all those stars on one another would be irresistible. Weirdly, though, the size of the black hole containing the mass of the 200 billion or more stars of the Milky Way would be much bigger than our imaginary cube of stars by a factor of about 146.
That’s because very massive black holes are actually rather low density if you treat their outermost extent as the measure of their size. This may feel counterintuitive, but the size of a black hole—the event horizon (the point of no return, the radius surrounding the hole’s mass from within which nothing can escape)—increases in lockstep with the hole’s mass. In other words, if you double the black hole mass, you double the radius of the event horizon.
That’s very different from what happens with regular objects. For example, add two identical balls of dough together, and the new radius of the combined ball is not twice what it was; it’s only about 26 percent larger. Why? Because for ordinary materials in a sphere the radius grows as the cube root of the mass—double the mass and you only increase the size by 26 percent. So if we treat the event horizon of a black hole as a measure of its physical size, the average density of matter within its bounds can end up being very low. A black hole with a mass three billion times that of the Sun would appear to be only as dense as the air we breathe! But this is a bit of cosmic misdirection, because our formal understanding of these objects tells us that all of a black hole’s mass is actually concentrated in a tiny, hidden, infinitely dense region at its center.
Compact matter in black holes strongly warps space and time.
Where such giant singularities exist, in the cores of most large galaxies, they also often look like the precise opposite of what we might expect. Yes, black holes are black, but you might not think so, because they can generate enormous amounts of light. Gas, dust, stars, planets, and who knows what else gets accelerated, shredded to bits, and heated if close enough to a black hole. In the process, energy spews outward, from above the event horizon and before the point of no return. With enough infalling matter, a spinning black hole can convert mass to energy with higher efficiency than even nuclear fusion. The most luminous cases across the universe shine with the power of hundreds of trillions of suns.
Matter at its most compact, like in a black hole, can surprise even the most scientific among us. At the other extreme, all that empty space in a galaxy like the Milky Way is also surprising.
NECESSARY EMPTINESS
Most of us experience physical loneliness at some point in our lives: lost in an unfamiliar city, alone in a house, or abandoned in a deep, dark wood by scheming relatives. But intergalactic space and interstellar space—between the galaxies or between the stars in the typical parts of a galaxy—are actually the two loneliest places you might ever wind up in. In these “inter-zone” environments it can be a very, very long distance between safe havens, devoid of much of anything at all.
If you were a hapless cosmic hitchhiker stranded between the stars of the Milky Way, your body would represent a concentration of matter a hundred million trillion times greater than the sparse interstellar space around you. To put that another way, take a look at the tip of your little finger. That pinky end contains about 1023 atoms. That number is the same as the total number of atoms in a sphere of about 100 million cubic kilometers of typical interstellar void.
Consequently, as a stranded traveler you can take a small comfort from the fact that cosmically you are also quite special. If you randomly stuck a pin in a map of the observable universe, there is an exceedingly low probability that it would land anywhere with as much matter richness as your body, or even any planet or star.
The strange emptiness of galaxies makes for some other interesting properties. Imagine that two galaxies are colliding—something that will happen in four billion years to the Milky Way and our neighbor Andromeda. Will the stars themselves come crashing together in such a titanic event? No, they won’t. Stars are so sma
ll compared with the gulfs of space between them that it’s very improbable that any will actually collide, even though their vast galactic parents are lumbering through each other.
The gravitational pull of matter in the approaching galaxies will distort and disrupt their shapes and stellar orbits, but otherwise it’s as if two swarms of bugs or birds are crossing paths. The tiny stars simply slip through all the gaps between.
The scarcity of normal matter in intergalactic space is even more extreme than in interstellar space. Get stranded out there—perhaps on a reckless trip from the Milky Way to our neighbor the Andromeda galaxy—and you’d need to scoop up at least a million times more volume to concentrate matter to the same level as in your body.
At worst you’d need to scoop up ten million times more volume if you were in an intergalactic “void.” Within the web-like distribution of matter on intergalactic scales, there are places where very little luminous material exists. These bubble-like cosmic voids can span scales of more than thirty million light-years (3 × 1023 meters). In these regions the density of matter is less than one-tenth of the average density of the universe, which makes these places quite depressing, unless you enjoy emptiness.
That’s not to say that voids are useless. Far from it. The expansion of space can actually take place a little faster in these places, without the gravitational pull of so much matter inside them. As a result, cosmic voids can “self-clean,” piling material up around their perimeters where it pushes into the surrounding denser intergalactic space. In this way they contribute directly to the gathering of matter into the bright webs of galaxies and stars.
Our galaxy in view
Inside the Local Group of galaxies
The Milky Way (far left) and Andromeda to scale
OUR (SLIGHTLY DYSFUNCTIONAL) GALACTIC FAMILY
The typical separation experienced by the brighter clusters and groupings of galaxies themselves is a little less isolating—only about 3 million light-years, or 3 × 1022 meters. For example, from the center of our Milky Way galaxy to the center of the nearest large galaxy, Andromeda (also known as Messier 31, or M31), there is a relatively modest gulf of about 2.5 million light-years, or 2.5 × 1022 meters.
There is also evidence of a tenuous cloud of plasma (a gas of positively charged ions and electrons) surrounding Andromeda out to a distance of about a million light-years (1022 meters). It’s awfully thin stuff, to our human senses indistinguishable from the harshest of vacuums. But some of the components of this gaseous mixture are at a temperature of nearly a million degrees Kelvin and include carbon and silicon, as well as hydrogen and helium. We don’t yet know if the Milky Way has its own similar halo.
What we have discovered is that the Milky Way plays ringmaster to a swarm of smaller satellite galaxies. The ones with the most stars in them are the familiar Large and Small Magellanic Clouds, a pair of dwarf irregular galaxies containing about thirty billion stars and three billion stars, respectively. But there are at least another thirty dwarf galaxies within about a million and a half light-years, most of which are likely in orbit around the Milky Way.
We’ve also discovered that this is not an entirely happy family. The orbit of these dwarf galaxies can result in their stars being gravitationally pulled away and stripped out into colossal “tidal streams” that wrap around the Milky Way. These stellar debris fields are clues to understanding galaxy growth. A big galaxy can sometimes put on weight by cannibalizing its smaller associates over billions of years.
These tidal streams around the Milky Way are extremely faint because they represent only a small number of stars spread across the gulf of intergalactic space. But sensitive telescopic data can reveal them. A very good example is the Sagittarius Stream—a vast and messy loop of stars that literally encircles our galaxy, running from pole to pole.
Streams like these can also reveal clues to the fundamental shape of the gravitational field of our Milky Way. In that sense the stellar trails are natural gravity probes—measuring rods strewn across tens to hundreds of thousands of light-years.
These measurements also provide a striking reminder of what is still a central mystery of our universe: the fact that in a structure like our galaxy, the visible, luminous, normal matter of stars, gas, and dust is just a minor component. There is between ten and thirty times more dark matter than normal matter in our galaxy.
What is that dark matter? Studying the behavior of individual galaxies is one good way to tackle the puzzle. The currently favored answer is that dark matter is a species of subatomic particle that only interacts via gravity and the weak force. And this stuff doesn’t reflect or absorb electromagnetic radiation.
Individually, these particles should be massive relative to other subatomic particles. That combination of properties has led to the acronym WIMPs—Weakly Interacting Massive Particles. The idea of the existence of WIMPs fits with many cosmic measurements, including the inferred gravitational fields of galaxies and clusters of galaxies, the observation of gravitational lensing, and the patterns of the cosmic microwave background radiation. The snag is that no WIMP has ever been detected directly, and a number of Earth-bound experiments are actively searching for them. It’s possible that instead of there being dark matter, there is something incomplete in our understanding of the nature of gravity itself.
OLD MILKY
Whether there are WIMPs or not, our galaxy is a magnificent example of how the universe assembles matter. Spanning 100,000 light-years (1021 meters) and containing a total mass of visible and dark matter a trillion times the mass of the Sun, it’s quite a beast.
The flattened, disk-like shape of the galaxy is a consequence of its history of formation—a process of gravitational infall, energy dissipation, and conservation of angular momentum.
The Milky Way rises above the horizon of a rogue planet lost in space.
A clash of galaxies and tidal streams of stars
There are other visible clues to our galaxy’s past, including the central bulge of older stars. These are spaced closer together than stars out in the galactic suburbs of the Sun’s location. Down here, close to the geometric center of the Milky Way, the spatial density of stars rises dramatically. Within 300 light-years of the center, there are about 100 times more stars in any given volume than around the Sun’s neighborhood. If you go closer in, the stellar density keeps on rising—to an eye-watering peak where stars are packed a million times closer together than we’re used to.
That means that stars are separated not by light-years but by light-weeks (about 1014 meters), and for any planets in this environment the sky will be filled with brilliant points of light. If we lived this deep in the galactic core, our night sky would be filled with a million stars as bright as Sirius, flooding our world with 200 times the light of our familiar full Moon.
In the depths of this brilliant core lies an enigmatic zone. There is a ring-like structure of gas, dust, and stars surrounding the inner few light-years of the galactic center. This helps shroud a black hole we call Sagittarius A Star (or *), a beast containing approximately four million times the mass of our Sun and surrounded by its own tightly held, swirling disk of accreting matter.
But this is just one special part of the galaxy. Old Milky is a sprawling and complicated place, where more than 200 billion stars make their way around and through a great well of gravity and interstellar filth. The most iconic features are the bright spiral arms. These are the shifting zones where new hot and luminous stars are formed. The arms are linked to slowly propagating changes in the density of stars and gas around the galactic disk, sets of ripples. In this respect the grand structures of the Milky Way, and of other similar galaxies, are somewhat illusory, the consequence of large but comparatively gentle waves of matter.
Out in our neck of the woods—26,000 light-years (2.5 × 1020 meters) from the galactic center—stars follow orbital paths that take them around the galaxy roughly every 230 million years. Our Sun is thought to be in the middle of its twen
tieth trip, or “galactic year.” But our neighboring stars are not all perfectly synchronized in this motion. We’re more like a slightly discombobulated flock of birds.
Approaching the galactic center
Above the Orion Spur of the Milky Way
A planet close to the core of the Milky Way
Sparse stars, a nebula, and a lost planet in the outer Milky Way
The stars around us drift in different directions at velocities of a few tens of kilometers a second. Over time these motions will mix things up, so what’s close to us today may not be in a million years. In fact, we don’t know where the suns are that were born with our Sun, back in a cluster of stars 4.5 billion years ago. Perhaps they are still grouped together somewhere else. Perhaps they are long dispersed.
Today we live in a relatively unexceptional part of the Milky Way. Within fifty light-years of us are some 130 other stars that are visible at night to the naked eye, and at least ten times more stars that are faint but still detectable with our telescopes. It’s not much. It even sounds quite manageable, quite parochial. Yet our species presently has no way of crossing to even the closest stellar neighbor in less than about 40,000 years. In that sense we are an island in the interstellar barrens.
As your journey continues, we’ll find out whether this is an island worth visiting.
Sister stars, 4.5 billion years ago
At the center, barely noticeable, is our Sun.
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THE SLOW, THE FAST, AND THE FANTASTIC
