Coming of Age in the Milky Way - Timothy Ferris (2003)
ADDENDUM TO THE PERENNIAL EDITION
The science of cosmology has made admirable progress during the past fifteen years. Some of its findings have been surprising, notably the discovery that the expansion of the universe evidently is accelerating, owing to the presence of a mysterious force called “dark energy” (about which more in a moment). Others have confirmed existing theories and built on them: Such results may make for fewer newspaper headlines, but one should keep in mind that observations that robustly confirm theories can be just as remarkable as those that contradict them. The upshot has been to increase the sum of human knowledge about the cosmos, and, no less important, to improve the quality of our cosmological questions.
The Hubble Space Telescope, launched in 1990, turned out to have a misshapen main mirror, but following its repair by a space shuttle team three years later, made observations that substantially clarified our vision of the cosmos. Astronomers using it to chart Cepheid variable stars and other useful distance-measuring landmarks were able to refine their estimates of the cosmic expansion rate, with the result that the universe now appears to be slightly younger than had been thought—just under fourteen billion years old. High-resolution Hubble images of quasars confirmed that they are indeed located at the nuclei of galaxies and are almost certainly powered by black holes. Galaxies imaged by Hubble at vast distances showed evidence of cosmic evolution, with spirals evidently having once been more commonplace and many of them subsequently being stripped of interstellar gas by collisions with one another to turn them into bald-looking elliptical galaxies. The Hubble Deep Field, a patch of sky imaged in a very long exposure over ten full days of telescope time, revealed galaxies more than halfway across the observable universe and became a kind of scientific watering hole to which many other observers repaired to make comparison observations of their own.
Studies of the cosmic background radiation—now more often called the cosmic microwave background, or CMB, to distinguish it from primordial neutrinos, gravity waves, or other sorts of useful big-bang relics that may soon be detected—reaped major insights for cosmologists. The COBE (for Cosmic Background Explorer) satellite, launched on November 18, 1989, mapped the CMB and confirmed two important predictions of the big-bang theory. First, the background radiation does indeed exhibit a black-body spectrum, as theorists had predicted. Indeed the fit of data to prediction was so exact that, when scientists plotted the data over the predicted curve, you couldn’t tell which was which: It was like Robin Hood’s splitting an opponent’s arrow in an archery contest. Second, the CMB proved to be, as expected, homogeneous and anisotropic—that is, evenly distributed and the same in all directions—except for a small hot spot in one direction and a cool spot on the opposite side of the sky caused by Earth’s motion in its local intergalactic field. Within this global smoothness, however, the COBE data also revealed inhomogeneities—lumps in the primordial soup, out of which stars and galaxies were to grow.
These results sparked great interest in learning more about the CMB, which by then was being called a cosmological Rosetta Stone. The microwave background amounts, after all, to a flash photograph of the universe when it was under half a million years old. The COBE satellite resembled a camera that could register its color (the black-body spectrum) and make out a few vague forms (the inhomogeneities) but was somewhat out of focus. Hence scores of experiments were conducted to study the CMB in other ways, at various wavelengths and differing angles of resolution. Warm air scatters CMB microwaves, so many of these observations were conducted from near the South Pole, where the air is cold and dry, and from balloons such as Boomerang, which flew over Antarctica at an altitude of 120,000 feet and detected evidence of sound waves moving through the primordial soup. Then, in 2001, NASA launched MAP, the Microwave Anisotropy Probe, a satellite equipped to scrutinize the CMB in unprecedented detail. Its findings, announced in 2003, confirmed that the geometry of the universe is close to being “flat”—that is, poised on the knife-edge separating closed and open models, as predicted by the inflationary hypothesis—and supported estimates, based on prior investigations, that the majority of cosmic matter/energy takes the form of a field, its nature as yet unknown, that was being called dark energy. The inhomogeneities in the background radiation mapped by MAP (or WMAP as it now was named, in honor of the Princeton cosmologist David Wilkinson, who died in September 2002) fit the theory that they originated as quantum flux events in the early universe. So it really does appear to be the case that subatomic phenomena authored the vast structures of galaxies and galaxy clusters that we see around us in the expanded universe today.
In a sense, the big-bang universe is a high-energy physics experiment, which can be studied at various points in history to understand how the universe evolved. Analysis of this literally universal experiment confirms that most of the stuff of the universe is invisible—the “dark matter” problem—and that much of this invisible stuff cannot be ordinary matter, like planets and stars, but must be of some exotic form, like the particles predicted by string theories. (String theory, along with M-theory, which portrays the strings as membranes, continues to be a promising method for constructing a unified, quantum theory of gravity, but it remains unfinished.)
Important clues to the nature of dark matter were found by astronomers studying supernovae. One class of these exploding stars, the type la supernovae, all seem to reach about the same maximum brightness (once one corrects for idiosyncracies such as differing abundances of nickel and other elements.) This makes them excellent “standard candles” for measuring the distances of remote galaxies and charting the expansion rate of the universe. Professional and amateur astronomers therefore launched ambitious projects that discovered hundreds of supernovae and charted their rise to maximum brightness. When these data were analyzed, the astronomers were astonished to find that the expansion rate of the universe, rather than slowing down as had been expected, appears to be accelerating. Evidently there is something like antigravity after all—a prospect envisioned in Einstein’s general relativity theory, but not previously found in nature.
This newly discovered antigravity field, often called dark energy, could be the same force that caused cosmic inflation in the first place, having now begun to reassert itself by speeding up the expansion rate. Or it could be something entirely unknown so far, perhaps one of the scalar fields long postulated by theoretical physicists although none have yet been clearly identified in the real world. In any event, there is clearly a lot to be learned about the vacuum and the quantum fields it contains.
Given that matter and energy are equivalent (as Einstein’s e = mc2 reveals) an antigravity field pervading space counts as matter in cosmic bookkeeping. By 2003, astronomers could estimate with some confidence that dark energy constitutes two-thirds of the mass of the universe, with dark matter (its nature also unknown) making up almost all of the other third, while planets, stars, and interstellar gas and dust—the “bright matter”—account for less than one percent of the universe by weight. When the first edition of Coming of Age was written, scientists were in something like the situation of accountants who could weigh a locked safe and estimate how much precious metal it contained, but didn’t know whether they were gold bars or silver coins. Now the safe has been cracked. One can see that it contains a few coins (visible matter) plus two other lockboxes, one labeled “dark matter” and the other “dark energy.” The task is to pick the locks on those two boxes.
One remarkable—if slightly unsettling—prospect presented by dark energy is that the expansion rate of the universe may not dictate its destiny. It used to be thought that if the universe today was expanding above a certain rate, it would keep expanding forever. And perhaps it will. But the “dark energy” field that evidently is accelerating cosmic expansion might have other plans: What it giveth, it can taketh away. Theorists find that certain kinds of scalar fields can speed up expansion for a while, so that the universe looks to be eternal, then put the brakes on and induce cosmic collapse. In some of these models, today’s accelerating universe is already well into middle age, and is destined to shrink to a fiery demise in another ten billion years or so. Ten billion years is a long time—it’s twice the age of the Sun and Earth—but the notion does give one pause.
To sum up, cosmologists can with some confidence say today that the universe:
· Went through an initial period of extremely rapid expansion (an “inflationary” period);
· Then got hot, producing the photons seen today as the cosmic microwave background radiation, and proceeded to cool as cosmic space expanded;
· Was originally made of light elements (mostly hydrogen and helium, forged in the hot big bang) from which the heavier elements subsequently were made, inside stars;
· Is geometrically “flat”—that is, that cosmic space has little or no observable curvature;
· Is made mostly of dark energy (two-thirds) and dark matter (one-third) plus a bit of bright matter (all the things that astronomers yet have seen); and
· Is expanding at an accelerating rate, owing to the influence of the dark energy field, which (depending on what sort of field it is) may continue to act in that fashion, or might instead collapse the universe at some future time.
That is certainly not the whole picture, but it’s an impressive amount to have learned, and it provides a sturdy foundation on which future research can build.
The origin of the universe remains a great mystery, and perhaps always will, but the vacuum genesis ideas depicted in Chapter 18 have continued to bear fruit. Several leading cosmologists, notably Andrei Linde of Stanford University, have constructed consistent and physically reasonable models in which our universe is one among many, perhaps an infinite number of universes. In these models, “new” universes bubble up out of the vacuum of preexisting ones. Many never attain a state in which life can exist—some quickly collapse, and others keep expanding at faster-than-light velocities forever, never forming matter—but some, like ours, can harbor life. The anthropic principle begins to make more sense in such models, inasmuch as it simply describes (or attempts to describe) the cosmological conditions required for life to appear in a given universe and for its existence therefore to be registered by intelligent observers. These models may help us understand how our universe got started, but they do so at the price of removing the ultimate question of genesis to a perhaps unattainable distance. Linde, who likes to imagine bubble universes as akin to apples on a tree, has attempted to calculate how many apples there may be, and how far in spacetime the original genesis event—the taproot of the tree—may be. Such calculations are of course based on many admittedly speculative assumptions, but for what it’s worth, Linde usually gets an infinite number of apples and an infinite distance from an average universe to the taproot. If so, the meta-universe looks infinite in both space and historical time, and the question of whether it ever had an origin cannot be answered.
The search for life in our universe continues, with no resolution in sight. SETI projects, operated on modest scales with private funding, have as yet detected no signal from an alien civilization, and nobody yet knows whether there is (or ever has been) life on Mars or elsewhere in the solar system.
The long-debated question of whether there are planets orbiting other stars has, however, been answered: It turns out that there are lots of them. Using a new method that involves carefully studying the motions of stars to detect planets’ gravitational pull on them—rather like a dog walker’s being tugged by the dogs on his leashes—astronomers have found more than one hundred extrasolar planets. The method works best for big dogs, so as of this writing only giant planets have yet been detected, but there is no reason to suppose that Earth-size planets aren’t abundant. Another approach involves monitoring many stars to measure slight reductions in their apparent brightness when a planet passes between them and us. These mini-eclipses should be detectable not only by professionals at mountaintop observatories, but by amateurs using digital cameras attached to backyard telescopes as well. The stage is set for the first amateur astronomer to discover a planet since 1781, when William Herschel discovered Uranus. It’s even possible that an amateur stargazer or a high-school science student will find the first planet beyond Earth that harbors life.
We’re living in fascinating times. Stay tuned …
Three philosophers came together to taste vinegar, the Chinese symbol for the spirit of life. First Confucius drank of it. “It is sour,” he said. Next, Buddha drank. He pronounced the vinegar bitter. Then Lao-tzu tasted it. He exclaimed, “It is fresh!”
—Traditional Chinese tale, repeated by Niels Bohr
For all my pains, I only beg this favor, that whenever you see the sun, the heavens, or the stars, you will think of me.
—Bernard de Fontenelle