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THE MISSING UNIVERSE
We can only account for 4 percent of the cosmos
The Indian tribes around the sleepy Arizona city of Flagstaff have an interesting take on the human struggle for peace and harmony. According to their traditions, the difficulties and confusions of life have their roots in the arrangement of the stars in the heavens — or rather the lack of it. Those jewels in the sky were meant to help us find a tranquil, contented existence, but when First Woman was using the stars to write the moral laws into the blackness, Coyote ran out of patience and flung them out of her bowl, spattering them across the skies. From Coyote's primal impatience came the mess of constellations in the heavens and the chaos of human existence.
The astronomers who spend their nights gazing at the skies over Flagstaff may find some comfort in this tale. On top of the hill above the city sits a telescope whose observations of the heavens, of the mess of stars and the way they move, have led us into a deep confusion. At the beginning of the twentieth century, starlight passing through the Clark telescope at Flagstaff's Lowell Observatory began a chain of observations that led us to one of the strangest discoveries in science: that most of the universe is missing.
If the future of science depends on identifying the things that don't make sense, the cosmos has a lot to offer. We long to know what the universe is made of, how it really works: in other words, its constituent particles and the forces that guide their interactions. This is the essence of the "final theory" that physicists dream of: a pithy summation of the cosmos and its rules of engagement. Sometimes newspaper, magazine, and TV reports give the impression that we're almost there. But we're not. It is going to be hard to find that final theory until we have dealt with the fact that the majority of the particles and forces it is supposed to describe are entirely unknown to science. We are privileged enough to be living in the golden age of cosmology; we know an enormous amount about how the cosmos came to be, how it evolved into its current state, and yet we don't actually know what most of it is. Almost all of the universe is missing: 96 percent, to put a number on it.
The stars we see at the edges of distant galaxies seem to be moving under the guidance of invisible hands that hold the stars in place and stop them from flying off into empty space. According to our best calculations, the substance of those invisible guiding hands — known to scientists as dark matter — is nearly a quarter of the total amount of mass in the cosmos. Dark matter is just a name, though. We don't have a clue what it is.
And then there is the dark energy. When Albert Einstein showed that mass and energy were like two sides of the same coin, that one could be converted into the other using the recipe E=mc2, he unwittingly laid the foundations for what is now widely regarded as the most embarrassing problem in physics. Dark energy is scientists' name for the ghostly essence that is making the fabric of the universe expand ever faster, creating ever more empty space between galaxies. Use Einstein's equation for converting energy to mass, and you'll discover that dark energy is actually 70 percent of the mass (after Einstein, we should really call it mass-energy) in the cosmos. No one knows where this energy comes from, what it is, whether it will keep on accelerating the universe's expansion forever, or whether it will run out of steam eventually. When it comes to the major constituents of the universe, it seems no one knows anything much. The familiar world of atoms — the stuff that makes us up — accounts for only a tiny fraction of the mass and energy in the universe. The rest is a puzzle that has yet to be solved.
How did we get here? Via one man's obsession with life on Mars. In 1894 Percival Lowell, a wealthy Massachusetts industrialist, had become fixated on the idea that there was an alien civilization on the red planet. Despite merciless mocking from many astronomers of the time, Lowell decided to search for irrefutable astronomical evidence in support of his conviction. He sent a scout to various locations around the United States; in the end, it was decided that the clear Arizona skies above Flagstaff were perfect for the task. After a couple of years of observing with small telescopes, Lowell bought a huge (for the time) 24-inch refractor from a Boston manufacturer and had it shipped to Flagstaff along the Santa Fe railroad.
Thus began the era of big astronomy. The Clark telescope cost Lowell twenty thousand dollars and is housed in a magnificent pine-clad dome on top of Mars Hill, a steep, switchbacked track named in honor of Lowell's great obsession. The telescope has an assured place in history: in the 1960s the Apollo astronauts used it to get their first proper look at their lunar landing sites. And decades earlier an earnest and reserved young man called Vesto Melvin Slipher used it to kick-start modern cosmology.
Slipher was born an Indiana farm boy in 1875. He came to Flagstaff as Percival Lowell's assistant in 1901, just after receiving his degree in mechanics and astronomy. Lowell took Slipher on for a short, fixed term; he employed Slipher reluctantly, as a grudging favor to one of his old professors. It didn't work out quite as Lowell planned, however. Slipher left fifty-three years later when he retired from the position of observatory director.
Though sympathetic to his boss's obsession, Slipher was not terribly interested in the hunt for Martian civilization. He was more captivated by the way that inanimate balls of gas and dust — the stars and planets — moved through the universe. One of the biggest puzzles facing astronomers of the time was the enigma of the spiral nebulae. These faint glows in the night sky were thought by some to be vast aggregations of stars — "Island Universes," as the philosopher Immanuel Kant had described them. Others believed them to be simply distant planetary systems. It is almost ironic that, in resolving this question, Slipher's research led us to worry about what we can't see, rather than what we can.
In 1917, when Albert Einstein was putting the finishing touches to his description of how the universe behaves, he needed to know one experimental fact to pull it all together. The question he asked of the world's astronomers was this: Is the universe expanding, contracting, or holding steady?
Einstein's equations described how the shape of space-time (the dimensions of space and time that together make the fabric of the universe) would develop depending on the mass and energy held within it. Originally, the equations made the universe either expand or contract under the influence of gravity. If the universe was holding steady, he would have to put something else in there: an antigravity term that could push where gravity exerted a pull. He wasn't keen to do so; while it made sense for mass and energy to exert a gravitational pull, there was no obvious reason why any antigravity should exist.
Unfortunately for Einstein, there was consensus among astronomers of the time that the universe was holding steady. So, with a heavy heart, he added in the antigravity term to stop his universe expanding or contracting. It was known as the cosmological constant (because it affected things over cosmological distances, but not on the everyday scale of phenomena within our solar system), and it was introduced with profuse apologies. This constant, Einstein said, was "not justified by our actual knowledge of gravitation." It was only there to make the equations fit with the data. What a shame, then, that nobody had been paying attention to Vesto Slipher's results.
Slipher had been using the Clark telescope to measure whether the nebulae were moving relative to Earth. For this he used a spectrograph, an instrument that splits the light from telescopes into its constituent colors. Looking at the light from the spiral nebulae, Slipher realized that the various colors in the light would change depending on whether a nebula was moving toward or away from Earth. Color is our way of interpreting the frequency of — that is, the number of waves per second in — radiation. When we see a rainbow, what we see is radiation of varying frequencies. The violet light is a relatively high-frequency radiation, the red is a lower frequency; everything else is somewhere in between.
Add motion to that, though, and you have what is known as the Doppler effect: the frequency of the radiation seems to change, just as the frequency (or pitch) of an ambulance siren seems to change as it speeds past us on the street. If a rainbow was moving toward you very fast, all the colors would be shifted toward the blue end of the spectrum; the number of waves reaching you every second would get a boost from the motion of the rainbow's approach. This is called a blueshift. If the rainbow was racing away from you, the number of incoming waves per second would be reduced and the frequency of radiation would shift downward toward the red end of the spectrum: a redshift.
It is the same for light coming from distant nebulae. If a nebula were moving toward Slipher's telescope, its light would be blueshifted. Nebulae that were speeding away from Earth would be redshifted. The magnitude of the frequency change gives the speed.
By 1912 Slipher had completed four spectrographs. Three were redshifted, and one — Andromeda — was blueshifted. In the next two years Slipher measured the motions of twelve more galaxies. All but one of these was redshifted. It was a stunning set of results, so stunning, in fact, that when he presented them at the August 1914 meeting of the American Astronomical Society, he received a standing ovation.
Slipher is one of the unsung heroes of astronomy. According to his National Academy of Sciences biography, he "probably made more fundamental discoveries than any other twentieth century observational astronomer." Yet, for all his contributions, he got little more than recognition on two maps: one of the moon, and one of Mars. Out there, beyond the sky, two craters bear his name.
The reason for this scant recognition is that Slipher had a habit of not really communicating his discoveries. Sometimes he would write a terse paper disseminating his findings; at other times he would put them in letters to other astronomers. According to his biography, Slipher was a "reserved, reticent, cautious man who shunned the public eye and rarely even attended astronomical meetings." The appearance in August 1914 was an anomaly, it seems. But it was one that set an English astronomer called Edwin Powell Hubble on the path to fame.
The Cambridge University cosmologist Stephen Hawking makes a wry observation in his book The Universe in a Nutshell. Comparing the chronology of Slipher's and Hubble's careers, and noting how Hubble is credited with the discovery, in 1929, that the universe is expanding, Hawking makes a pointed reference to the first time Slipher publicly discussed his results. When the audience stood to applaud Slipher's discoveries at that American Astronomical Society meeting of August 1914, Hawking notes, "Hubble heard the presentation."
By 1917, when Einstein was petitioning astronomers for their view of the universe, Slipher's spectrographic observations had shown that, of twenty-five nebulae, twenty-one were hurtling away from Earth, with just four getting closer. They were all moving at startling speeds — on average, at more than 2 million kilometers per hour. It was a shock because most of the stars in the sky were doing no such thing; at the time, the Milky Way was thought to be the whole universe, and the stars were almost static relative to Earth. Slipher changed that, blowing our universe apart. The nebulae, he suggested, are "stellar systems seen at great distances." Slipher had quietly discovered that space was dotted with myriad galaxies that were heading off into the distance.
When these velocity measurements were published in the Proceedings of the American Philosophical Society, no one made much of them, and Slipher certainly wouldn't be so vulgar as to seek attention for his work. Hubble, though, had obviously not forgotten about it. He asked Slipher for the data so as to include them in a book on relativity, and, in 1922, Slipher sent him a table of nebular velocities. By 1929 Hubble had pulled Slipher's observations together with those of a few other astronomers (and his own) and come to a remarkable conclusion.
If you take the galaxies moving away from Earth, and plot their speeds against their distance from Earth, you find that the farther away a galaxy is, the faster it is moving. If one receding galaxy is twice as far from Earth as another, it will be moving twice as fast. If it is three times more distant, its speed is three times greater. To Hubble, there was only one possible explanation. The galaxies were like paper dots stuck onto a balloon; blow it up, and the dots don't grow, but they do move apart. The very space in between the galaxies was growing. Hubble had discovered that the universe is expanding.
It was a heady time. With this expansion, the idea of a big bang, first suggested in the 1920s, bubbled to the surface of cosmology. If the universe was expanding, it must once have been smaller and denser; astronomers began to wonder if this was the state in which the cosmos had begun. Vesto Slipher's work had led to the first evidence of our ultimate origins. The same evidence would eventually bring us the revelation that most of our universe is a mystery.
To understand how we know a significant chunk of the cosmos is missing, tie a weight to a long piece of string. Let the string out, and swing the weight around in a circle. At the end of a long string, the weight moves pretty slowly — you can watch it without getting dizzy. Now pull the string in, so the weight is doing tiny orbits of your head. To keep it spinning around in the air, rather than falling down and strangling you, you have to keep it moving much faster — so fast you can hardly see it.
The same principle is at work in the motions of the planets. The Earth, in its position close to the Sun, moves much faster in its orbit than Neptune, which is farther out. The reason is simple: it's about balancing forces. The gravitational pull of the Sun is stronger at Earth's radial distance out from the Sun than at Neptune's. Something with Earth's mass has to be moving relatively fast to maintain its orbit. For Neptune to hold its orbit, with less pull from the distant Sun, it goes slower to keep in equilibrium. If it moved at the same speed as Earth, it would fly off and out of our solar system.