String theory is a theory about a very distant place, almost as far away as Atlantis or Oz. We are talking about the Planck domain, and if it ever existed (like Oz), it would have been in the very earliest flicker of Big Bang cosmology. There is no way we can imagine experimental data from that epoch. That doesn't mean we shouldn't persevere. Suppose one finds a mathematically consistent (no infinities) theory that somehow describes Oz and has as its low, low energy consequence our standard model? If it is also unique—that is, has no competitors that do the same thing—then we will all rejoice and lay down our pencils and trowels. Uniqueness is what superstrings doesn't enjoy. Within the major assumptions of superstrings are an enormous number of possible paths to the world of data. Let's see what else characterizes this stuff without pretending to explain it. Oh yes, as mentioned in Chapter 8, it requires ten dimensions: nine space dimensions and one time.
Now we all know there are only three space dimensions, although we have warmed up to the issue by imagining living in a two-dimensional world. So why not nine? "Where are they?" you rightly ask. Curled up. Curled up? Well, the theory started with gravity, which is based upon geometry, so one can visualize that six of the dimensions got curled up into a tiny ball. The size of the ball is typical of the Planck regime, 10−33 centimeters, about the size of the string that replaces the point particle. The particles we know emerge as vibrations of these strings. A stretched string (or wire) has an infinite number of vibration modes. That is the basis of the violin—or the lute, if you remember way back when we met Galileo's old man. The vibrations of real strings are classified in terms of a fundamental note and its harmonies or frequency modes. The mathematics of micro-strings is similar. Our particles come from the lowest-frequency modes.
There is no way I can describe what has excited the leaders of this theory. Ed Witten gave a fantastic, gripping lecture about all this at Fermilab some years ago. For the first time in my experience, when he concluded there was almost ten seconds of silence (that's a lot!) before the applause. I rushed over to my lab to explain what I had learned to my colleagues on shift, but by the time I got there I had lost most of it. The artful lecturer makes you think you understand it.
As the theory met increasingly more difficult mathematics and a proliferation of possible directions, the progress and the intensity surrounding superstrings dropped to a more sensible level, and now we can only wait. There continues to be interest on the part of very capable theorists, but I suspect it will be a long time before TOE reaches the standard model.
FLATNESS AND DARK MATTER
Waiting for a theory rescue, Big Bang still has puzzles. Let me select one more problem that has confounded physicists even as it has led us—experimenters and theorists alike—to some tantalizing notions about the Very Beginning. It is known as the flatness problem, and it has a very human content—the morbid interest in whether the universe will continue to expand forever or whether it will slow down and reverse to a period of universal contraction. The issue is how much gravitational mass there is in the universe. If there is enough, the expansion will be reversed and we will have the Big Crunch. This is known as the closed universe. If there isn't enough, the universe will continue to expand forever, growing colder—an open universe. Between these two regimes is a "critical mass" universe, one that has just enough mass to slow the expansion but not enough to reverse it—a so-called flat universe.
Time for a metaphor. Think about sending a rocket up off the surface of the earth. If we give the rocket too small a velocity, it falls back to earth (closed universe). The earth's gravitation is too strong. If we give it a huge velocity, it can escape the earth's gravitation and soar into the solar system (open universe). Obviously there is a critical velocity such that ever so slightly less speed results in fallback, and ever so slightly more results in escape. Flatness occurs when the velocity is right on. The rocket escapes, but with ever-decreasing velocity. For rockets on our earth the critical velocity is 11.3 kilometers per second. Now, following the example, think of a fixed-velocity rocket (the Big Bang) and ask how heavy a planet (total mass density of the universe) results in escape or fallback.
One can estimate the gravitational mass of the universe by counting the stars. People have done this, and, taken alone, the number is too small to halt the expansion; it predicts an open universe, by a very wide margin. However, there is very strong evidence for the existence of a distribution of nonradiating matter "dark matter" pervading the universe. When observed matter and estimated dark matter are combined, measurements indicate that the mass in the universe is close to — not less than 10 percent of nor more than two times—the critical mass. Thus it is still an open question whether the universe will continue to expand or will contract eventually.
There are many speculative candidates for dark matter. Most of them are particles, of course, with fancy names—axions, photinos —given by loving theorist-inventors. One of the most fascinating possibilities for dark matter is one or more of the standard-model neutrinos. There should be an enormous density of these elusive objects left over from the Big Bang era. They would be ideal candidates if ... if they had a finite rest mass. We already know that the electron neutrino is too light, leaving two candidates, of which the tau neutrino is the favorite. Two reasons: (1) it exists, and (2) we know almost nothing about its mass.
Not long ago at Fermilab we carried out an ingenious and subtle experiment designed to detect whether the tau neutrino has a finite mass that would serve to close the universe. (Here cosmological needs drove an accelerator experiment, an indication of the particle-cosmology union.)
Imagine a graduate student on shift on a bleak winter's night, imprisoned in a small electronics hut on the wind-swept prairie of Illinois. Data have been accumulating for eight months. He checks the progress of the experiment, and as part of his routine he examines the data on the neutrino mass effect. (You don't measure the mass directly, but an influence the mass would have on some reactions.) He runs the entire sample of data through the calculation.
"What's this?" He becomes instantly alert. He can't believe the screen. "Oh, my God!" He runs computer checks. All are positive. There it is—mass! Enough to close the universe. This twenty-two-year-old graduate student experiences the incredible, breath-stopping conviction that he alone on the planet, among 5.32 billion of his fellow sapiens, knows the future of the universe. Talk about a Eureka moment!
Well, it's a nice story to think about. The part about the graduate student was true, but the experiment failed to detect any mass. That particular experiment just wasn't good enough, but it could have been, and ... perhaps someday it will be. Colleague reader, please read this to your uncertain teenager con brio! Tell him or her that (1) experiments often fail, and (2) they don't always fail.
CHARLTON, GOLDA, AND GUTH
But even if we don't yet understand how the universe contains the critical mass needed for a flat universe, we're pretty sure that it does. We'll see why. Of all the masses nature could have chosen for Her universe (say 106 times critical mass or 10−16 times critical mass), She chose something nearly flat. In fact, it's worse than that. It appears to be a miracle that the universe has survived the two opposing fates—immediate runaway expansion or immediate crunch—for 15 billion years. It turns out that the flatness at age one second had to be close to perfect. If it deviated by ever so little, on one side we would have had the Big Crunch even before we made a single nucleus; if the deviation were on the other side, the expansion of the universe would have progressed by this time to a stone-cold dead thing. Again a miracle! Much as scientists may envision the Wise One, der Alte, a Charlton Heston type with fake long flowing beard and a strange laser-induced glow, or (as in my own view) a Margaret Mead or Golda Meir or Margaret Thatcher type of deity, the contract clearly says that the laws of nature are not to be amended, that they are what they are. Thus the flatness problem is too much of a miracle and one seeks causes to make the flatness "natural." That's why my graduat
e student was freezing his ass off trying to determine whether neutrinos are dark matter or not. Infinite expansion or Big Crunch. He wanted to know. So do we.
The problem of flatness, the problem of the uniform 3-degree radiation, and several other problems of the Big Bang model were solved, at least theoretically, in 1980 by Alan Guth, an MIT particle theorist. His improvement is known as the Inflationary Big Bang model.
INFLATION AND THE SCALAR PARTICLE
In this brief history of the past 15 billion years I forgot to mention that the evolution of the universe is pretty much all contained in Einstein's equations of general relativity. Once the universe cools to a temperature of 1032 degrees Kelvin, classical (nonquantum) relativity prevails, and the subsequent events are indeed consequences of Einstein's theory. Unfortunately, the great power of the theory of relativity was discovered, not by the master but by his followers. In 1916, before Hubble and Knubble, the universe was thought to be a much more sedate, static object, and Einstein in his self-proclaimed "greatest blunder" added a term to his equation to prevent the expansion that the equation predicted. Since this is not a book on cosmology (and there are some excellent ones around), we will hardly do justice to the concepts, many of which are above my salary level.
What Guth discovered was a process, allowed by the Einstein equations, that generated an explosive force so huge as to produce a runaway expansion; the universe inflated from a size much smaller than that of a proton (10−15 meters) to the size of a golf ball in a time interval of 10−33 seconds or so. This inflationary phase arose through the influence of a new field, a nondirectional (scalar) field—a field that looks and acts and smells like ... Higgs!
It is Higgs! The astrophysicists have discovered a Higgs thing in a wholly new context. What is the role of the Higgs field in promoting this bizarre pre-expanding-universe event that we call inflation?
We have noted that the Higgs field is closely tied to the concept of mass. What induces the wild inflation is the assumption that the pre-inflationary universe is suffused with a Higgs field whose energy content is so large that it drives a very rapid expansion. So "In the beginning there was a Higgs field" may not be too far from the truth. The Higgs field, which is constant throughout space, changes over time—in accordance with the laws of physics. These laws (added to the Einstein equations) generate the inflationary phase, which occupies the enormous time interval of 10−35 seconds to 10−33 seconds after Creation. Theoretical cosmologists describe the initial state as a "false vacuum" because of the energy content of the Higgs field. The ultimate transition to a true vacuum releases this energy to create the particles and the radiation, all at the enormous temperature of the Beginning. Following this, the more familiar Big Bang phase of relatively serene expansion and cooling begins. The universe is confirmed at the age of 10−33 seconds. "Today I am a universe," one intones at this phase.
Having donated all of its energy to the creation of particles, the Higgs field retires temporarily, reappearing several times in various disguises in order to keep the mathematics consistent, suppress infinities, and supervise the increasing complexity as the forces and particles continue to differentiate. Here is the God Particle in all its splendor.
Now wait. I didn't make any of this up. The originator of the theory, Alan Guth, was a young particle physicist trying to solve what appeared to be a totally different problem: the standard Big Bang model predicted the existence of magnetic monopoles—isolated single poles. North and south would then be related as matter and antimatter are. Looking for monopoles was a favorite game of particle hunters, and every new machine had its monopole search. But all proved unsuccessful. So at least monopoles are very rare, in spite of the absurd cosmological prediction that there should be enormous numbers of them. Guth, an amateur cosmologist, hit on the idea of inflation as a way of modifying the Big Bang cosmology to eliminate monopoles; then he discovered that by improving his inflation idea, he could solve all the other defects of that cosmology. Guth later commented on how lucky he was to make this discovery because all the components were known—a comment on the virtue of innocence in the creative act. Wolfgang Pauli once complained about his loss of creativity, "Ach, I know too much."
To complete this final tribute to Higgs, I should briefly explain how this rapid expansion solves the isotropy, or causality, and flatness crises. The inflation, which takes place at speeds vastly greater than the speed of light (the theory of relativity sets no limit on how fast space can expand), is just what we need. In the beginning, small regions of the universe were in intimate contact. Inflation vastly expanded these regions, separating their parts into causally disconnected regions. After inflation the expansion was slow compared to light velocity, so we continually discover new regions of the universe as their light finally reaches us. "Ah," the cosmic voice says, "we meet again." Now it is not a shock to realize that they are just like us: isotropy!
Flatness? The inflationary universe makes a clear statement: the universe is at critical mass; the expansion will continue to slow forever, but it will never reverse. Flatness: in Einstein's general theory of relativity, all is geometry. The presence of mass causes space to be curved; the more the mass, the greater the curvature. A flat universe is a critical condition between two opposing types of curvature. Large mass generates inward curvature of space, like the surface of a sphere. This is attractive and tends to a closed universe. Small mass produces an outward curvature, like the surface of a saddle. This tends to an open universe. Flatness represents a universe with a critical mass, "in between" inward and outward curvature. Inflation has the effect of stretching a tiny amount of curved space to so huge a domain as to make it effectively flat—very flat. The prediction of exact flatness, a universe that is critically poised between expansion and contraction, can be tested by identifying the dark matter and continuing the process of measuring the mass density. This, we are assured by the astros, will be done.
Other successes of the inflationary model have given it wide acceptance. For example, one of the "minor" annoyances of Big Bang cosmology is that it doesn't explain the lumpiness of the universe—the existence of galaxies, stars, and the rest. Qualitatively that lumpiness seems okay. By chance fluctuation, some matter clumps together out of a smooth plasma. The slight extra gravitational attraction draws other stuff to it, making the gravity even stronger. The process continues, and sooner or later we have a galaxy. But the details show that the process is far too slow if it is dependent on "chance fluctuations," so the seeds of galaxy formation must have been implanted during the inflationary phase.
Theorists who have thought about these seeds imagine them as small (less than 0.1 percent) density variations in the initial distribution of matter. Where did these seeds come from? Guth's inflation provides a very attractive explanation. One has to go to the quantum phase of the universe's history, in which spooky quantum mechanical fluctuations during inflation can lead to the irregularities. Inflation enlarges these microscopic fluctuations to a scale commensurate with galaxies. Recent observations (announced in April 1992) by the COBE satellite of ever so small differences in the temperature of the microwave background radiation in different directions are delightfully consistent with the inflationary scenario.
What COBE saw reflects conditions when the universe was young—only 300,000 years old—and stamped with the imprint of the inflation-induced distributions that made the background radiation hotter where it was less dense, cooler where it was more dense. The observed temperature differences thus provide experimental evidence for the existence of the necessary seeds for galaxy formation. No wonder the news made headlines all over the world. The temperature differences were only a few millionths of a degree and required extraordinary experimental care, but what a payout! One could detect, in the homogenized goop, evidence of the dumpiness that presaged the galaxies, suns, planets, and us. "It was like seeing the face of God," said exuberant astronomer George Smoot.
Heinz Pagels stressed the philosoph
ical point that the inflationary phase is the ultimate Tower of Babel device, effectively cutting us off from whatever went before. It stretched and diluted all the structures that preexisted. So although we have an exciting story about the beginning, from time 10−33 seconds to time 1017 seconds (now), there are still those pesky kids out there who say, Yes, but the universe exists and how did it start?
In 1987 we had a "face of God" sort of conference at Fermilab when a group of astro/cosmo/theorists gathered to discuss how the universe began. The official title of the conference was Quantum Cosmology, and it was called so that the experts could commiserate about the domain of ignorance. No satisfactory theory of quantum gravity exists, and until one does, there will be no way of coping with the physical situation of the universe at the earliest moments.
The conference roster was a Who's Who of this exotic discipline: Stephen Hawking, Murray Geli-Mann, Yakov Zeldovich, André Linde, Jim Hartle, Mike Turner, Rocky Kolb, and David Schramm, among others. The arguments were abstract, mathematical, and very lively. Most of it was over my head. What I enjoyed most was Hawking's summary talk on the origin of the universe, given Sunday morning at about the time when 16,427 other sermons on roughly the same subject were being delivered from 16,427 pulpits around the nation. Except. Except that Hawking's talk was delivered through a voice synthesizer, giving it just that extra authenticity. As usual, he had a lot of interesting and complicated things to say, but he expressed the most profound thought quite simply. "The universe is what it is because it was what it was," he intoned.
Hawking was saying that the application of quantum theory to cosmology has as a task the specifications of initial conditions that must have existed at the very moment of creation. His premise assumes that the proper laws of nature—which, we hope, will be formulated by some genius now in third grade—will then take over and describe the subsequent evolution. The new great theory must integrate a description of the universe's initial conditions with a perfect understanding of the laws of nature and so explain all cosmological observations. It must also have as a consequence the standard model of the 1990s. If, before this breakthrough, we have achieved, via data from the Super Collider, a new standard model with a much more concise accounting for all of the data since Pisa, so much the better. Our sarcastic Pauli once drew an empty rectangle and claimed he had replicated the finest work of Titian—only the details were missing. Indeed, our painting "The Birth and Evolution of the Universe" requires a few more brushstrokes. But the frame is beautiful.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 50
