What are we human beings made of—literally? Flesh and blood? Mind and body? To become a cosmic society, we need to appreciate at a much deeper level what “we” are and how this larger we fits into the universe. Our bodies are made of many kinds of complex atoms, most of which were created inside ancient stars or during supernovas and then flung out during the violent deaths of those stars to travel for eons through space. We are 90 percent stardust by weight and 10 percent hydrogen (mostly in our H2O). We and the ground we walk on are literally made of stardust.1
But isn’t everything? Well into the twentieth century, scientists believed that everything is made of atoms—and everything on Earth actually is. But now we know that Earth is extremely atypical of the universe as a whole, and the way things operate here is not a good basis for extrapolation. Most of the universe is not made of atoms. Until astronomers discovered this, we couldn’t begin to comprehend how we humans fit into the whole, because despite having a word for the whole—universe—we had no idea what that really was. But now we know that stardust is the rarest material in the universe and exists only because the conditions for its creation were set up by the universe’s two overwhelmingly dominant ingredients: invisible dark matter and invisible dark energy.
The Pyramid of All Visible Matter represents everything visible in the universe that can be detected with any scientific instrument (fig. 23). We borrowed the symbol from the back of the dollar bill and the Great Seal of the United States. The thirteen brick steps originally represented the thirteen colonies, and the Eye represented the hope that Providence would look kindly on this historic undertaking. The ribbon at the bottom says in Latin, “The New Order of the Ages.” This motto originally referred to the founding of the new country with its new kind of government. We’re radically reinterpreting this entire symbol, but the motto is even truer of the new universe. In the Pyramid of All Visible Matter, the volume of each section of the pyramid is proportional to the amount of the corresponding ingredient in the visible universe. Hydrogen and helium came straight out of the Big Bang and fill the entire bottom section of the pyramid. They’re the lightest atoms that exist, but there are so many of them in space that collectively they vastly outweigh all the other atoms. Hydrogen and helium eventually condensed and ignited, creating the first stars. Inside the stars, nuclear processes created new kinds of atoms that the young universe had never before seen, such as carbon, oxygen, silicon, and iron. These heavy atoms (heavier than hydrogen and helium) are ejected from stars at the ends of their lives and become stardust, floating through interstellar space, perhaps to be pulled into the gravitational field of some newly forming star system, making it possible for such later-generation stars to have rocky planets like Earth.
Fig. 23. The Pyramid of All Visible Matter
The periodic table of the elements is familiar from any high school chemistry classroom, but here it is color-coded to show the origin of each element (fig. 24). Hydrogen and helium, alone at the top, are in some sense the parents of all the others, and thus the great-great-grandparents of us all. The iron atoms in our blood carrying oxygen at this very moment to our cells came largely from exploding white dwarf stars (fig. 25), while the oxygen itself came mainly from massive stars exploding as supernovas (fig. 26).
Fig. 24. The periodic table of the elements, with the origins of each element
Most of the carbon in the carbon dioxide we exhale on every breath came from planetary nebulas, which are the death clouds of middlesize stars like the sun. The Cat’s Eye Nebula is an extraordinarily beautiful planetary nebula. Here it is shown among its neighboring stars (fig. 27).
When we zoom in, we see what looks like an eye at the center (fig. 28). All that’s left of the original star is the tiny white dot at the center of this close-up. The rest of the star has blown off in those colorful clouds of stardust that surround the star in multiple layers and may someday become part of a new planetary system.
Distant worlds may be wildly different from Earth, but there are things that must be true of them all, simply because of the nature of stardust. For example, on any planet in the Galaxy, wherever you find watery seas and land, there will be sandy beaches (fig. 29). This is because oxygen and silicon are two of the most abundant heavy atoms produced before a star explodes in a supernova. Free-floating in space, they combine with each other and the hydrogen that is everywhere, making H2O and SiO2—water and sand—which travel together and become incorporated into new worlds.
It takes a star millions or billions of years to produce a comparatively tiny number of heavy atoms, yet heavy atoms incarnate our world. The capstone of the Pyramid of All Visible Matter—the floating part at the top—represents the total mass of stardust compared to that of hydrogen and helium. The Eye in the capstone is the reason we chose this symbol (rather than, say, a food pyramid or a pie chart). The Eye represents the minuscule amount of stardust in intelligent creatures—on all worlds anywhere. This is the only cosmic ingredient not drawn to scale, since if the Eye were to scale, it would be a microscopic point.
Fig. 25. Kepler’s supernova remnant, from the explosion of a white dwarf
Fig. 26. The Crab Nebula, remnant of the explosion of a massive star
Fig. 27. A large-scale view of the Cat’s Eye Nebula
Fig. 28. The Cat’s Eye Nebula
Fig. 29. Manorbier Beach, Pembrokeshire, Wales
The Pyramid of All Visible Matter represents what was once thought of as the whole universe. Midgard on the Cosmic Uroboros was also once thought of as the whole universe, but in both cases our perspective has since expanded. We now know scientifically that visible matter is only a small part of the new universe.
The Cosmic Density Pyramid represents everything—visible and invisible—that gives the universe density (fig. 30).2 This includes not only matter but energy, which, as Albert Einstein showed with his iconic equation, E = mc2, is convertible to mass. The Pyramid of All Visible Matter is just the aboveground tip of this huge underground pyramid.
In addition to the half a percent of atoms that are visible, the universe includes an extra 4 percent of atoms that are invisible simply because they are floating through space between the galaxies, far from any stars and thus unlit. The two dominant ingredients in the cosmic recipe—the invisible 95 percent of the density of the cosmos—are dark matter and dark energy. Both are unearthly substances whose natures are still not well understood, although thousands of scientists are working on it.
Most of the matter holding the Milky Way and all other galaxies together is cold dark matter. Dark matter is invisible not because it is unlit but because it does not interact with light at all. Dark matter does not emit light as stars do, reflect light as planets, moons, and gas clouds do, or absorb light as dust does. It also doesn’t emit or absorb X-rays, radio waves, or any other form of radiation that astronomers have detected. We know dark matter is there only because of its immense gravitation, which affects the objects around it in ways we can measure. Because dark matter hardly interacts with anything, including itself, it can never evolve into anything complex. It just forms big “halos”—blobs of eternally whizzing dark matter particles that surround and permeate each and every galaxy. Dark matter is not a good name, in fact, since it’s not dark: it’s transparent. But whatever it’s called, it controls the origin and evolution of galaxies, galaxy clusters, and superclusters of galaxies because it concentrates the matter and helps determine how everything else in the galaxy moves.
Fig. 30. The Cosmic Density Pyramid
The largest portion of the Cosmic Density Pyramid, about 70 percent of the density of the universe, is dark energy. This is the most powerful entity in the universe, and yet until 1998 no one knew it existed. Scientists thought of it only as a hypothetical possibility. Dark energy powers the expansion of the universe, and that expansion is a key part of understanding the picture of our universe. Here’s how it works.
All distant galaxies are being carried away from our Milky Way by expanding space itself. The galaxies are not flying away from one another across space; the space between them is expanding, and the farther away from us, the faster it’s expanding. If we look at two galaxies, and one is twice as far away from us as the other, then from our perspective the farther one will be receding twice as fast. Every observer in the universe sees exactly the same pattern in the motion of distant galaxies. That’s how a uniformly expanding universe works.
For much of the twentieth century astronomers assumed that the expansion of the universe must be gradually slowing because of the mutual gravitational attraction of everything in it. But in 1998 the amazing discovery was made that the expansion of the universe is not slowing down at all but is instead accelerating. Dark energy makes space repel itself. The more space there is (and increasing amounts of space are inevitable in an expanding universe), the more repulsion there is. The more repulsion, the faster space expands. The faster it expands, the more space, the more repulsion, and this leads to an exponentially increasing expansion, which may possibly go on forever. Dark energy seems to be a property of space itself.
Imagine that the entire universe is an ocean of dark energy. On that ocean there sail billions of ghostly ships, made of dark matter. At the tips of the tallest masts of the largest ships there are tiny beacons of light, which we call galaxies (fig. 31). With the Hubble Space Telescope, the beacons are all we see. We don’t see the ships, we don’t see the ocean—but we know they’re there through theory, the Double Dark theory. This oceanic imagery has a certain resonance with the ancient Egyptian and biblical cosmologies. Both of them envisioned that the primeval water, an unearthly substance, not normal water, predated the creation and continued to surround it. The world was created in the midst of it. But here’s the twist in our modern version: dark matter is indeed primeval and came out of the Big Bang, and it surrounds and permeates our Local Group of galaxies, but dark energy, which dominates outside our halo of dark matter, only became important later, because it has been creating more of itself all the time. The larger the universe expands, the faster more dark energy gets created.
Until the late 1990s astronomy students were usually taught that there are exactly three possible futures for the universe:
Fig. 31. Dark matter ships on an ocean of dark energy
1. It can expand forever at a constant rate;
2. It can slow down forever asymptotically (that is, the rate of expansion would get closer and closer to zero without ever reaching it); or
3. With enough matter in the universe, the gravitation of matter could eventually slow the expansion to a halt and the universe would begin to contract. This last possibility led people to speculate that everything might come back together again in a Big Crunch, ending the universe symmetrically, and then maybe there would be another Big Bang, ad infinitum.
We bring all this up because these alternatives have been discussed so widely. But all three are now known to be wrong: the expansion of the universe is actually accelerating. How do we “know” this? What’s the evidence?
The real test of a theory is not how logical, beautiful, and satisfying or, on the other hand, how weird, ugly, or unlikely it sounds. The test is, how accurate are its predictions?
The Double Dark theory has made many precise predictions about phenomena that had never been observed or in some cases even looked for. Over the past twenty years huge amounts of data have become available from the Hubble Space Telescope, from powerful new telescopes on the ground, like the Keck Observatory in Hawaii, and from satellites observing not visible light but the heat radiation of the Big Bang. The predictions of the theory have turned out to be right without exception.3
In figure 32, the wavy blue line that looks like a mountain range is the Double Dark theory’s prediction that a specific pattern in the heat radiation from the Big Bang would be discovered.
It is not necessary to master most of the graph in figure 32; its sole purpose here is to illustrate the extraordinarily close match between the predictions of the theory and the observational data. This complex prediction was made before there were any data and before there was even an instrument capable of making the observations. Every white point is a later separate observation. Over the years, as better measurements have been made, every single observation has fallen along the predicted curve. This can’t be a coincidence.
Fig. 32. Big Bang data agree with the Double Dark theory
Fig. 33. The distribution of matter also agrees with the Double Dark theory
In figure 33 the red curve was the Double Dark theory’s prediction of how matter should be distributed in the universe today on all size scales from our local galactic neighborhood out to the cosmic horizon. Once again as the data have come in, every point has fallen on the line.
Hidden in these graphs is the real story of the expansion of our universe. In the early stages of the universe there was the same amount of dark matter as there is now, but there was relatively little dark energy because there was relatively little space—the universe hadn’t had time to expand very much. And so, for the first nine billion years the gravitational attraction of the dark matter did in fact slow down the expansion. But the expansion kept going, producing always more space, so eventually the dark matter thinned out. Dark energy, however, doesn’t thin out, perhaps because it’s a property of space. Its relative importance only increases as the universe expands. Now the repulsive effect of the dark energy has surpassed the gravitational attraction of dark matter as the dominant effect on large scales in the universe. The turning point was about five billion years ago—which was, coincidentally, about when our solar system was forming.
Taking the existence of dark energy seriously means we have to begin thinking in a new way about what “outer space” means. Most people use the phrase to refer to any place outside Earth’s atmosphere, but this communicates a static picture of the universe—it’s missing the whole idea of expansion.
If we think of outer space as starting outside Earth’s atmosphere, then we need to think of it as stopping at the edge of our “Local Group” of galaxies (the Milky Way, Andromeda, and their fifty or so satellite galaxies). Because our Local Group is bound together by gravity, it travels as a unit in the great expansion. It doesn’t expand apart. There’s no expansion happening to people, planets, or even the entire Galaxy, nor between our Galaxy and the rest of the Local Group (fig. 34).
Fig. 34.“Don’t Feel Bad Loretta … The Entire Universe Is Expanding”
Nice excuse, Lockhorns, but sorry. Gravitationally bound clumps like our Local Group are actually contracting and falling together and will merge in a few billion years. But outside the Local Group, space is expanding faster and faster.
So all the space inside the Local Group is a special kind of space, because it’s been tamed by gravity. Outside our Local Group lies the real outer space: wild space. On this huge scale, dark energy is tearing apart all large structures and accelerating the rate of expansion. Wild space is carrying hundreds of billions of galaxies away from our Local Group in all directions.
To show how wild and tame space work, we can visualize the dark matter expanding, and we do this below with a sequence from a computer simulation. Whenever we simulate dark matter, brightness is used to represent density: the brighter a region appears, the more dark matter it contains, although in reality dark matter is completely invisible.
In figures 35–37 we take a small portion of the universe and show it expanding.
Quickly the screen becomes filled with a region that has become bound together by gravity, and that region stops expanding and instead grows by attracting other blobs of dark matter (fig. 35). After about seven billion years (fig. 36), the central region of this dark matter halo has stopped expanding, and some of the dark matter halos around it are starting to fall toward it. But some don’t. In figure 37 the small dark matter halo in the upper left corner of figure 36 is continuing to expand away from the central halo—so the space between that halo and the central halo of figure 36 is wild space.
Yet expansion is not all that’s going on. Regions with slightly more dark matter than average expand slightly more slowly than regions with less dark matter. The “rich” regions (rich in dark matter) get richer and the poor regions get poorer until the differences become substantial. Inequalities that were there in the beginning get magnified. When a region has twice as much dark matter as average, it stops expanding, while the lower density regions around it keep expanding. Once a galaxy-sized region stops expanding, the ordinary matter in it can fall to the center and begin to form stars. The dark matter is forming structures on every large size scale, and the sum of them all is called the cosmic web.4
Fig. 35. A simulation of the expansion of the universe
Suppose that a filmmaker wants to show a scene between two characters inside a dining car on a moving train. If she or he set up the camera outside on the ground and tried to film the scene through a window as the train went by, the camera would catch only a second of the action. Instead the filmmaker films from inside the dining car, thus subtracting out the movement of the train. We’re essentially doing the same in the sequence in figure 38. We want to show how the dark matter is forming structures, not how those structures are simultaneously getting stretched by expanding space, so here the expansion of the universe is subtracted out. In the sequence we have also blown up all the earlier time steps to the final size so that we can focus on what is happening inside. Astronomers call this “working in co-moving coordinates.”
Fig. 36. The end of expansion
In the first box, which represents a cube of the universe shortly after the Big Bang, the dark matter starts out very smooth. But gradually the dark matter is being attracted to wrinkles in space-time (this process is explained in chapter 5) and begins to show some structure. As time passes, the structure becomes sharper: the filaments and intersections of filaments become denser and the voids in between become emptier. Galaxies form inside the filaments; inside the intersections of the filaments, where the dark matter is densest, clusters of galaxies form .
Fig. 37. Wild space, tame space
The highest-resolution large-scale simulation of the evolution of the Double Dark universe yet run is called Bolshoi and is a pathbreaking new supercomputer simulation that lets us see the invisible dark matter in unprecedented detail (fig. 39). In it every single dark matter halo that could host a visible galaxy is shown. The region simulated is about a billion light-years across. The distribution of dark matter halos in these simulations looks statistically just like the actual distribution of galaxies in the universe .5
Fig. 38. The evolution of the cosmic web
In figure 40 the magnificent Aquarius simulation shows the formation of a single Milky Way–sized dark matter halo. How big would the dark matter halo be, compared to the final visible Milky Way? The tiny photo in the middle would be the visible part of the Milky Way. The halo is so huge that it engulfs several smaller galaxies.
There is a connection between events on this immense size scale and our own lives. Dark matter doesn’t cradle the entire Milky Way—and all galaxies—in delicate, invisible hands, protecting it from the cosmic hurricane of dark energy tearing space apart outside because it cares about us. Dark matter didn’t herd a dispersed, fertile mix of hydrogen and helium into a compact region at the center of our Galaxy so that those primal atoms could easily interact and evolve into stars and worlds in preparation for us. Dark matter didn’t commit itself unconditionally for billions of years, with never an instant off, to hold the atoms in its charge safely in a stable home galaxy for us. It does all these things because it has no choice. Its behavior is built into the order of the universe. But we benefit. We minuscule bits of stardust in that tiny Eye at the peak of the Cosmic Density Pyramid are the only ones in this universe who realize and can appreciate the immensity of what dark matter does and has done.
Fig. 39. The Bolshoi simulation—one billion light-years across
Fig. 40. The Aquarius simulation of a Milky Way–size dark matter halo
And yet, beyond shaping and protecting the galaxies, dark matter has evolved very little; it has not become more complex, and the reason is that it doesn’t participate in chemistry. Its particles scarcely interact with one another or anything else, except gravitationally. Chemistry is created by myriad electromagnetic interactions among atoms and is incomparably more complex than physics. Biology is more complex than chemistry, and we civilized, intelligent beings are the most complex things we know of in the entire universe.
Fig. 41. The Eye of the Pyramid of All Visible Matter
We, with the other potential members of the cosmic club of intelligent life, are at the peak of the Cosmic Density Pyramid. An enormous base of material and phenomena in the universe has made and will continue to make our existence possible. Within the floating capstone, the fraction of stardust associated just with living things or the remains of living things is extremely tiny. Within that extremely tiny fraction, the fraction associated specifically with intelligent life anywhere in the universe is vanishingly small—yet it is only that which looks at and grasps this pyramid and the way that time has constructed it. As much as people around the world hope to find alien beings on other planets, the possibility exists that only our eyes see this universe (fig. 41).
Intelligence can burst out only from bits of stardust. Everything we learn about ourselves in the context of the universe as a whole reinforces a fundamental fact: that from a cosmic point of view we intelligent, self-reflective beings are rare and precious beyond calculation—but we are only possible because of the composition of the rest of the universe.
