1027, 1026, 1025, 1024, 1023meters
From about 93 billion light-years to 10 million light-years
From the diameter of the cosmic horizon to the size of the Local Group of galaxies
It's a summer morning. You're sitting in a sunlit room holding this book, reading this page, and about to start a journey through cosmic scale.
Looking up, you notice that tiny specks of glowing dust are caught in the beams of light streaming through the windows. These bright pinpoints loft and swirl in the air currents, like a swarm of mysterious creatures.
The specks are microscopic, yet if the whole room were to represent the size of the observable universe, each of these dusty motes would be the size of an entire galaxy of stars.
Now follow a single sunlit speck. This is our galaxy, the Milky Way. It is home to more than 200 billion stars, and at least that number of planets. These stars and planets span a structure that is a hundred thousand light-years across — a distance of more than nine hundred thousand trillion kilometers. At a walking pace, it would take you twenty trillion years to cross this object.
Packed deep inside, hidden among these billions of other worlds, is one particular planet we call Earth. This world is a modest rocky orb with a thin coating of crystallized mineral crust atop a hot interior, lightly painted with water and atmosphere. It orbits a lonesome star that we call the Sun — one star, only 4.5 billion years old in a 10-billion-year-old galaxy.
Now think about all the things that you know and experience in your life. Your family, friends, dogs, cats, small furry rodents, horses, houses, couches, beds, pizza, apples, oranges, trees, flowers, insects, dirt, clouds, water, snow, rain, mud, sunshine, and starry nights.
Then think about all the people who have ever lived (a total of around 110 billion individual, biologically modern humans) and all that they knew in their lives. All these people, billions upon billions of them, experiencing their surroundings for centuries, decades, years, months, days, hours, seconds, and the blink of an eye.
That's a lot of special moments for human beings. But for 3.5 billion years before we came along, living things swarmed the Earth, from bacteria and archaea to multicellular clumps, from trilobites to insects, dinosaurs to cephalopods. Trillions of living entities slithered around every conceivable niche, compelled into existence by varying potentials of chemical energy and chance. During every passing moment of those many years, all these organisms were being sculpted and battered by natural selection, and driven by the restless engines of molecular mechanics.
The sum total of that, every single last bit of it, has existed on this one world — a vanishingly small mineral dot among billions of mineral dots, all held within a single dust mote that you watch floating through a sunny room that is the universe as we know it.
This one-mote galaxy that we call the Milky Way is a microscopic part of a wrinkly, web-like ocean of matter. That ocean sustains more than 200 billion other galaxies. These galaxies range from small to enormous, some isolated, some in the midst of messy collisions. And these are merely the galaxies from which light has had time to reach us in about thirteen billion years — they're within our "horizon," the horizon of light travel time, like the walls of your room.
This cosmic sprawl is also awash in electromagnetic radiation, energy that exhibits both wave- and particle-like behavior, packaged as massless units called photons racing to and fro across space. Some of this radiation is the product of the early and hot history of the place that we generally call the universe. Other photons are from specific, individual sources: stars, supernovas, warmly glowing young planets, cosmic crashes and shock waves, possibly even plaintive missives hurled between technological civilizations — or not.
UNIVERSE TO MULTIVERSE
In truth, we think that our universe must continue on for a while beyond that horizon of the known. The universe from which light has not yet had time to reach us may be far, far larger than the part we can see. Some estimates, based on analyses of the known geometry of space-time, suggest that the "full" universe could extend at least 250 cosmic horizons farther. Other estimates, based on the very rapid expansion — or inflation — of the universe in its very early youth, suggest that the universe may be on the order of 1023 times larger than the part we will ever see, or ever access.
If this is correct, the universe might contain places that are, in effect, repeats of where we find ourselves. Those repeats could even include solar systems, planets, and life-forms that bear an uncanny resemblance to the ones we know.
It's an unsettling thought, that the roll of cosmic dice might have happened enough times to reproduce Earth and its history. But lurking in our hypotheses about the fundamental nature of the universe and its physical origins are ideas that are even stranger.
To produce the cosmos that we observe, including the "shape" of its space-time and its relative uniformity on the largest scales, scientists have invoked the phenomenon of cosmic inflation. Very early in time, a mind-bogglingly brief 10?36 seconds after the Big Bang, there was likely an episode of enormously rapid expansion of space-time — where the universe grew by more than a trillion trillion times until the cosmic clock hit 10?32 seconds. That's like a small pore on your skin inflating to the size of the Milky Way galaxy over a period that is a hundred quadrillion times less than some of the shortest intervals of time human devices have ever registered (about ten-quintillionths of a second).
One outcome that physicists have proposed for this inflation produces an array of "pocket" universes, so many in number that it makes your eyes water; possibly 10-to-the-10-to-the-10-to-the-7 universes. Not only would there be a spectacular number of universes just like ours — there would be a spectacular number of universes not like ours. If true, this "multiverse" would contain other versions of all of us as well as all our evil twins.
There are, to say the least, some disturbing aspects to this possibility. It would mean that any decision you make — good or bad — would be made many times elsewhere, and in many different ways. So does it really matter if I pick up that piece of litter when a trillion other versions of me will pick it up a trillion other times somewhere else in the multiverse? Should we bother discovering the secrets of the universe when it's only one among many, and not as unique as we once thought?
We're left to wonder for now, and to go and make a strong cup of tea.
VIEW FROM A FAR HORIZON
In this book our journey begins at the place that matches the reach of our current knowledge: we'll start our deep dive into powers of ten at a scale of 1027 meters (an octillion meters). We choose this scale because if we could measure the physical diameter, the extent, of the universe between the bounds of the cosmic horizon at this instant in time, we would find it to be about ninety-one billion light-years, or 860,951,000,000,000,000,000,000 kilometers, across — roughly 1027 meters.
If you're quick-witted you might query that number — since the universe is only 13.8 billion years old, how can it be physically larger than the distance light would travel in that time? The answer is universal expansion — that swelling of space-time. As a consequence, at this very instant, you'd measure it to be larger than you might expect.
We can actually map aspects of the cosmic horizon. As the universe expands, it cools, since the photons racing through space get their wavelengths stretched out and lose energy. If we could go back in time, we'd find that the universe gets hotter and hotter. Until the universe was around 379,000 years old, its average temperature was above 3,000 Kelvin (4,940°F) — too hot for electrons to be bonded in atoms. The photons racing through space were continually scattered by these free electrons. This made the young cosmos a "fog" impenetrable by electromagnetic radiation.
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.
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.
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!CHAPTER 2
DARKNESS AND LIGHT
1022, 1021, 1020, 1019, 1018meters
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.
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.
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.