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Harlow Shapley was born in 1885 in rural Missouri, along with his twin brother, Horace. Bright and possessing an impressive personal energy that would be a hallmark throughout his life, Harlow still faced the challenges of his times. His early education was rife with obstacles. As a small child he was placed in a one-room schoolhouse where his older sister sometimes acted as teacher. By twelve he had dropped out. Then, after studying at home and working for a newspaper, he returned, newly motivated, and completed his schooling in record time. His aim was to get into college and study to become a journalist. But on arriving at the University of Missouri, he discovered that the journalism department wouldn’t open for another year. Rather than leave, he decided to sign up for astronomy because it was close to the top of the alphabetical list of subjects, and, he later claimed, because it was easier to pronounce than archaeology. On graduating from Missouri, he was awarded a fellowship at Princeton. Remarkably, just a few years later, in 1914, he had obtained his Ph.D. in astronomy.
The freshly minted Dr. Shapley was well versed in many of the latest astronomical techniques. One of these made use of the properties of a peculiar class of old stars called Cepheids, named after the star Delta Cephei. These objects vary in brightness over days to months on a regular basis as their outer layers undergo cycles of heating and cooling. Surprisingly, the timing of the variations reveals the true luminosity, or energy output, of these stars—the slower the variation, the more luminous the Cepheid. Other classes of stars, like the fainter and rather unpoetically named RR Lyrae variables, exhibit very similar properties. Knowing the true brightness of these variable stars allowed astronomers to deduce their actual distance from the Earth, simply by measuring how faint they appeared. It was like having a marvelous cosmic yardstick, and Shapley knew just how to put it to use.
In the early 1900s it was the general consensus that the Sun was somewhere near the center of the universe. This universe consisted of the great wheel-like disk of stars of the Milky Way, along with near and far nebulae and dense spherical groupings of stars known as globular clusters. It was not yet understood that most of the small hazy nebulae seen in the sky were actually other distant galaxies. Something rather odd, however, had attracted Shapley’s attention. The globular clusters, extraordinary orbs of a hundred thousand stars packed close together, were distributed disproportionately across the night sky—most of them were in only one hemisphere. This was extremely puzzling to him. If we stood near the center of everything, then why were the globular clusters off to one side? Shapley, moving on from the halls of Princeton and freshly installed at the Mount Wilson Observatory high in the San Gabriel Mountains above Pasadena, California, set to work to measure the distances to these stellar hives.
His measuring tools were the variable stars they contained. He was determined to map out the three-dimensional locations of the Milky Way’s globular clusters. By 1918, after studying a total of sixty-nine of these stellar baubles, he had his answer. The globular clusters were asymmetrically distributed on the sky because we were not at their common center. Like geographical markers, they plot out a great sphere centered not on us, but on the Milky Way itself. This meant that our solar system was not at the center of the galaxy, it was stuck out in the suburbs. As Shapley himself put it: “The Sun is very eccentrically situated in the general system … the local group is about half way from the center to the edge of the galaxy.”
It was an extraordinary discovery. Surprisingly, it was met with general agreement by other astronomers—it just made sense. But it opened the floodgates for what were far more controversial revelations in the following decades. We would find not only that the Milky Way is merely one of many galaxies, but that there is no center for these objects themselves. All of them are flying apart as the universe expands. Shapley’s great leaps in 1918 were the beginnings of the modern mapping of the cosmos.
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But challenges abound. As if it isn’t enough that beyond the confines of our spherical world the universe seems to stretch on endlessly, even the plainest of places can hide the most extraordinary complexity.
There is a famous issue in mapmaking that illustrates this particular challenge. It’s very much an earthbound problem, but its implications go far beyond. Awareness of this conundrum originated in the 1950s, in the work of an English scientist by the name of Lewis Fry Richardson. Richardson had eclectic tastes in his research, from physics and meteorology to the mathematics of war. As a Quaker born into a well-to-do family in 1881, he believed strongly that morality was paramount, and his experiences as an ambulance driver in France during World War I only reinforced his pacifism. Viewing war as a disease and an affliction, he eventually tried to model armed conflicts with mathematics. In his book Arms and Insecurity, published in 1949, he famously stated: “The equations are merely a description of what people would do if they did not stop and think.”
These studies led Richardson to wonder whether the length of a shared border between two countries was somehow related to the probability that they would go to war. As archaic as this hypothesis may sound in today’s world of transglobal aggression, it nonetheless fit into a much more complex statistical analysis with real bearing on the nature of human conflicts.
To test the predictive power of his sophisticated mathematical models, Richardson needed to research the estimated lengths of real country-to-country borders. To his great surprise, the numbers he found differed wildly depending on his reference source. The lengths of well-established, settled borders, such as those between Spain and Portugal or the Netherlands and Belgium, could differ by hundreds of miles from one map or survey to the next. What could be going on?
Richardson discovered that the answer was related to the smallest unit of measurement used to estimate a border’s length. If a surveyor went along the border and made visual measurements with the telescope of a theodolite every few hundred feet, adding up each distance, he would get a very different answer than if he had trudged every foot with a surveyor’s wheel. The problem, as Richardson eventually surmised, is that natural borders or coastlines have effectively infinite complexity. The smaller your measuring rod, the more of its lengths you can fit in around every kink and turn—and the longer the border appears. Two runners, one with a short stride and one with a long stride, literally have to travel different distances if they race along the very edge of an island, or a country’s border.
Richardson, with his rather morbid but morally driven fascination with war, had stumbled upon one of the key elements of what would later become known as fractal mathematics. His careful analysis of the discovery became a vital touchstone for this field. Many things we see in nature are best described not by straight lines or simple curves, but by endlessly repeating patterns or structures nesting inside one another.
On the face of it, the universe around us may not seem quite this complex. Indeed, there are very distinct objects and structures that appear superficially to be finite and straightforward. Stars are stars, galaxies are collections of stars, and galaxies themselves form greater collections known as clusters and even superclusters. We can readily make a chart showing the positions of bright stars in the sky or the locations of visible galaxies. But this is deceptive. Just as the earliest maps of the world blithely cast entire continents as “desert” or “land of giants,” sweeping cartographical judgments of the sky are invariably incomplete. What may appear to be a single star can very often turn out to be a pair or even a triplet of objects, hiding inside imperfect images made by our eyes or by our telescopes. A galaxy is composed of stars, but it is also composed of a gas of discrete atoms and molecules, forming great interstellar clouds that can be thin and almost invisible, or dense and bulky like the great nebulae in Orion and Carina. Galaxies can also contain vast quantities of microscopic dust composed of silicates and carbon.
There may be planets lurking around at least 50 percent of all stars in a galaxy like ours—cold, dark objects completely lost to vie
w against the glare of their suns. And what about those planets? Don’t they in turn deserve mapping? Don’t they have geographical features that descend into ever smaller levels of detail? Of course, here we are utterly limited. Beyond the worlds of our own solar system we have for now only the most rudimentary information about planets around other stars—the exoplanets. A telescope big enough and sensitive enough to genuinely map the surface of any of these distant worlds has yet to be made, but there’s every reason to believe that eventually it will be.
Clearly, the observable universe shares some of the intractable difficulties we encounter in trying to evaluate the lengths of country borders or coastlines. Complex objects are nested within complex objects. But this is not the only huge challenge for cartography of the cosmos. The great majority of the information we have traditionally sought for maps of the Earth comes from the observation of visible light. Whether by peering through a telescope from the deck of a ship warily sailing offshore or by trudging through the interior of some great continent, we make a map of what we can see and what we can measure by seeing. In many respects, this allows us to capture the information about the Earth that is most useful to us.
Out in the universe, however, an abundance of environments and physical phenomena emit or absorb light at wavelengths our eyes simply cannot respond to. So another critical aspect to mapping the universe is to incorporate not just the colors and spectra of objects as we might perceive them, but to include the vastness of the entire electromagnetic spectrum. This extends from long radio waves through microwaves and far and near infrared to visible light. It carries on to ultraviolet light, to soft and hard X-ray photons, and to gamma rays. And there are even more exotic aspects of matter and energy to probe, from gravitational waves and neutrinos to fast-moving subatomic particles, not to mention the pervasive but still mysterious dark matter, whose presence reveals itself only through the gravitational pull of its mass.
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So what does our current map of the known universe look like? It already contains a vast amount of information, yet it is barely a scrap of parchment compared to what would be the full atlas. By physical necessity, the level of detail in this map drops off as we move farther and farther beyond Earth. The map also contains many, many layers of data from the different photons used in its construction, each revealing a different but related topography. Often these multiple layers exist in tiny patches, the results of someone’s hard work with a telescope on a very particular object or set of objects and no others. Small pinpricks on the sky where the density of information is great are surrounded by areas of the map that are still somewhat blurry and empty, and may contain dragons.
Doing this map justice in words or images is extremely difficult; it already holds so much information, even if it is only a tiny fraction of what there is to know. It is both three-dimensional and four-dimensional, as time becomes linked to what we see. The farther away objects are, the longer their light has taken to reach us, all the way back through the 14-billion-year history of the universe. There are so many categories of objects and phenomena, and so much higgledy-piggledy data from several hundred years of telescopic astronomy. The best we can do to gain some amount of intuition for this atlas is to play out a thought experiment, a parlor game to let us begin to grasp what a map of forever would look like.
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Let us pretend that a very large box has just been delivered to our doorstep and we have hauled it inside. Within the box is an ominous-looking sack filled to bursting with a mysterious shape. The occasional wisp of gas trickles out through the knotted top, and every so often a muffled thump and an obscured glow come from within.
This sack contains what we could regard as a representative portion of the universe. Cosmologists often speak in terms of “fair samples” of the universe. What they mean is a large chunk, or volume, of the stuff Out There. It encompasses enough of the varying lumps and bumps, galaxies, galaxy clusters, empty places, and crowded places to be typical. If you surveyed all the properties inside the volume and took an average, it would be very close to the universal average. For example, if you divided the total mass in the sack by its volume, you would obtain a very good estimate of the average density of the universe as a whole. Equally, if you measured just how lumpy the arrangement of galaxies was within this volume, the answer would be a close match to the universal “lumpiness” of structure.
So let’s take a peek inside the sack and begin to add up what we find. The first thing that happens when you cautiously peel apart the opening is that electromagnetic radiation floods out, together with particles—such as neutrinos and fast-moving protons, atomic nuclei, and electrons—that are born in highly energetic and violent physical environments. There is also a gentler seepage of hydrogen and helium nuclei and atoms, together with something hard to spot: an as yet unknown species of heavy but invisible particle, known as dark matter. All these things, photons and particles, permeate the cosmos and are important to understand.
Photons of light are present in huge quantities. They come in all flavors, from extremely low-frequency radio waves, where a single crest-to-crest distance may span kilometers, to microwaves, infrared, visible, and ultraviolet frequencies, and on to the realm of X-rays and gamma rays. One of the most pervasive types of photons is the kind that originated in the very young universe, when it was extremely dense, hotter than 5,000 degrees Fahrenheit, and temporarily opaque. By 380,000 years after the Big Bang, the expanding cosmos cooled down and thinned out enough for these photons to fly free, skirting the atoms of hydrogen and helium that would otherwise trap and scatter them. They now fill the cosmos, their wavelengths stretched by the expansion of space and time itself following the Big Bang. This has lowered their energy, and today they mostly span radio wavelengths, ranging from the frequencies where cell phones, TVs, and microwave ovens operate to shorter waves, or higher frequencies, entering the beginnings of the infrared spectrum. They are known as cosmic microwave background photons. There are roughly 410 of them per cubic centimeter of the local universe at any single instant. That may not sound like much, but our sack—our fair sample of the cosmos—is hundreds of millions of light-years across and will contain a colossal number. Even the volume of a sphere that encompasses our tiny little solar system, whose very outermost extent may be about one light-year from the Sun, contains at any given instant roughly 1057 of these ancient photons. That is more than a trillion trillion trillion trillion. Adding to this impressive count are all the other photons that have originated from stars and cooling gas out in the universe. Light may not have mass, but the cosmos is thick with it. This is an extremely important component of the universe. We will find that it can play a critical role in a number of processes. Space may appear largely empty to us, but in reality it is a heaving soup of unseen photons racing back and forth across eternity.
The other particles that come pouring out of the sack are more difficult to put definite numbers to. Neutrinos are extremely low-mass subatomic particles, less than about a millionth of the mass of electrons. They play a key role in what are termed weak interactions in physics, and they come in a variety of flavors: electron, muon, and tau. For example, one type of natural radioactivity occurs when a proton in the nucleus of an atom turns into a neutron through a process called beta decay. In this transformation, the atomic nucleus spits out a high-speed anti-electron together with an electron-neutrino.
Neutrinos have been likened to the “ghosts of the cosmos,” since they have very little to do with normal matter, passing through gases, liquids, and solids with minimal likelihood of actually hitting or interacting with anything. Deep inside normal stars, the processes of nuclear fusion make neutrinos in abundance. But to a neutrino the universe is almost completely transparent, and so they immediately escape from stellar cores and stream out into space. Here on Earth, every second roughly 65 billion neutrinos from the Sun’s core pass through every square centimeter of your skin. Eight minutes ago they were produced in the
solar center, and they have raced outward at a rate that comes close to the speed of light. Despite that incredible barrage, the chances of one actually being stopped by your body are so low that it may happen only once or twice during your lifetime. Stars are therefore actively flooding the universe with neutrinos. In addition to these fresh ones, there are ancient neutrinos. These are the remains of a stage in the universe’s earliest evolution, about two seconds after the Big Bang. Because they are lower in energy, these neutrinos have not yet been conclusively detected, but we expect them to be streaking through the cosmos in all directions.
The majority of normal, recognizable matter that emerges from our open sack is in the form of hydrogen and helium, in the proportion of roughly seven hydrogen atoms to every one helium atom. Again, these are remnants of the young, hot universe. Anything else in this mix of normal matter is a tiny trace by comparison, and has been forged inside stars. The next most abundant element we find in our sack of universe is oxygen, and there is only one oxygen atom for roughly every 1,500 hydrogen atoms. All the elements that are so critical in making objects like planets, and the molecules that are part of us and all living things, are rare—quite literally cosmic pollutants.
Some of these elements come zooming out of the sack with considerable speed. In this case they are components of hot gases, often so hot that most of the electrons that usually stick to an atomic nucleus have been stripped away, leaving an electrically positive object known as an ion. A gas in this state is also referred to as plasma, and what escapes from the sack can have a temperature of tens of millions of degrees. Other normal matter seeps out at an extremely slow rate. These are components of much, much colder gases, some barely a few degrees above absolute zero.
Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos Page 5