Two major additional consequences of the Big Bang theory finally wore down the opposition, and now a fair consensus holds. One is the prediction that the light of the original incandescence—assuming it was hot, hot—would still be around as remnant radiation. Recall that light consists of photons, and the energy of photons is related inversely to their wavelength. A consequence of the universe's expansion is that all lengths are expanded. The wavelengths, originally infinitesimal, as befits high-energy photons, were thus predicted to have grown to the microwave region of a few millimeters. In 1965 the embers of the Big Bang, the microwave radiation, were discovered. The entire universe is awash with these photons, moving in all possible directions. Photons that started a journey billions of years ago when the universe was much smaller and hotter ended up on a Bell Laboratories antenna in New Jersey. What a fate!
After this discovery it became crucial to measure the distribution of wavelengths (here, please reread Chapter 5 with the book turned upside down), which was eventually done. Using the Planck equation, this measurement gives you the average temperature of the stuff (space, stars, dust, an occasional beeping satellite that escaped) that has been bathed in these photons. According to the latest (1991) NASA measurements from the COBE satellite, this is 2.73 degrees above absolute zero (2.73 degrees Kelvin). This remnant radiation is also strong evidence of the hot Big Bang theory.
While we are listing successes, we should also point out difficulties, all of which were eventually overcome. Astrophysicists have been carefully examining the microwave radiation in order to measure temperatures in different parts of the sky. The fact that these temperatures matched up with extraordinary precision (better than .01 percent) was a cause for some concern. Why? Because when two objects have exactly the same temperature, it is plausible to assume that they were once in contact. Yet the experts were sure that the different regions having precisely the same temperatures were never in contact. Not "hardly ever," but never.
Astrophysicists are allowed to speak so categorically because they have calculated how far apart two regions of the sky were at the time the microwave radiation observed by COBE was emitted. That time was 300,000 years after the Big Bang, not as early as one would like but as close as we can get. It turns out that these separations were so large that even with the velocity of light there was no time for the two regions to communicate. Yet they have the same temperature, or very close to it. Our Big Bang theory couldn't explain this. A failure? Another miracle? It became known as the causality, or isotropy, crisis. Causality because there seemed to be a causal connection between sky regions that never should have been in contact. Isotropy because everywhere you look on the grand scale you see pretty much the same pattern of stars, galaxies, clusters, and dust. One could live with this in a Big Bang model by saying that the similarity of the billions of pieces of the universe that had never been in contact was a pure accident. But we don't like "accidents." Miracles are okay if you invest in the lottery or are a Chicago Cubs fan, but not in science. When they appear we suspect that something grander is lurking in the shadows. More on this later.
AN ACCELERATOR WITH AN UNLIMITED BUDGET
A second major success of the Big Bang model has to do with the composition of our universe. You think of the world as being made of air, earth, water (I'll leave out fire), and billboards. But if we look up and measure with our spectroscopic telescopes, we find mostly hydrogen, then helium. These account for 98 percent of the universe. The rest is composed of the other ninety or so elements. We know by our spectroscopic telescopes the relative amounts of the lighter elements—and lo!—the BB theorists say that these abundances are precisely what one would expect. Here is how we know.
The prenatal universe had in it all the matter in the presently observed universe—that is, about 100 billion galaxies, each with its 100 billion suns (can you hear Carl Sagan?). Everything we can see today was squeezed into a volume vastly smaller than the head of a pin. Talk about overcrowding! The temperature was high—about 1032 degrees Kelvin, a lot hotter than our current 3 degrees or so. And consequently matter was decomposed into its most primordial components. A plausible picture is of a "hot soup," or plasma, of quarks and leptons (or whatever is inside, if anything) smashing into each other with energies like 1019 GeV, or a trillion times the energy of the biggest collider a post-SSC physicist can imagine building. Gravity roared in as a powerful (but presently little understood) influence at this microscopic scale.
After this fanciful beginning, there was expansion and cooling. As the universe cooled, the collisions became less violent. The quarks, in intimate contact with one another as part of the dense glob that was the baby universe, began to coagulate into protons, neutrons, and the other hadrons. Earlier, any such union would have come apart in the ensuing violent collisions, but the cooling was relentless, the collisions kinder and gentler. By age three minutes, the temperatures had fallen enough to allow protons and neutrons to combine and, where earlier these would quickly have come apart, now stable nuclei formed. This was the nucleosynthesis period, and since we know a lot of nuclear physics, we can calculate the relative abundances of the chemical elements that did form. They are the nuclei of very light elements; the heavier elements require slow "cooking" in stars. Of course, atoms (nuclei plus electrons) didn't form until the temperature fell enough to allow electrons to organize themselves around nuclei. The right temperature arrived at about 300,000 years. Before that time we had no atoms, and we needed no chemists. Once neutral atoms formed, photons could move freely, and that is why we got our microwave photon information late.
Nucleosynthesis was a success: the calculated and the measured abundances agreed. Wow! Since the calculations are an intimate mix of nuclear physics, weak-force reactions, and early universe conditions, this agreement is very strong support for the Big Bang theory.
In the course of telling this story I have also explained the inner space/outer space connection. The early universe was nothing more than an accelerator lab with a totally unconstrained budget. Our astrophysicists need to know all about quarks and leptons and the forces in order to model evolution. And, as we pointed out in Chapter 6, particle physicists are provided data from Her One Great Experiment. Of course, at times earlier than 1013 seconds, we are much less sure of the physics laws.
Nevertheless, we continue to make progress in our understanding of the Big Bang domain and the evolution of the universe. Our observations are made 15 billion years after the fact. Information that has been rattling around the universe for almost that amount of time occasionally stumbles into our observatories. We are also aided by the standard model and by the accelerator data that support it and try to extend it. But theorists are impatient; the hard accelerator data give out at energies equivalent to a universe that has lived 10−13 seconds. Astrophysicists need to know the operative laws at much earlier times, so they goad the particle theorists to roll up their sleeves and contribute to the torrent of papers: Higgs, unification, compositeness (what's inside the quark), and a host of speculative theories that venture beyond the standard model to build a bridge to a more perfect description of nature and a road to the Big Bang.
THERE ARE THEORIES, AND THEN THERE ARE THEORIES
It is 1:15 A.M. in my study. Several hundred yards away, the Fermilab machine is colliding protons on antiprotons. Two massive detectors are receiving data. The battle-hardened CDF group of 342 scientists and students are busy checking out the new pieces of their 5,000-ton detector. Not all of them, of course. On the average, at this time, a dozen people will be in the control room. Partway around the ring the new D-Zero detector with its 321 collaborators, is being tuned up. The run, a month old, had the usual shaky start, but data taking will go on for about sixteen months, with a break for phasing in a new piece of the accelerator designed to increase the collision rate. Although the main thrust is to find the top quark, testing and extending the standard model is an essential part of the drive.
About 5,000 mile
s away, our CERN colleagues are also working hard to test a variety of theoretical ideas about how to extend the standard model. But while this good, clean work is going on, theoretical physicists are working, too, and I propose to give here a very brief, plumber's version of three of the most intriguing theories: GUTs, supersymmetry, and superstrings. This will be a superficial treatment. Some of these speculations are truly profound and can be appreciated only by the creators, their mothers, and a few close friends.
But first a comment on the word "theory," which lends itself to popular misconceptions. "That's your theory" is a popular sneer. Or "That's only a theory." Our fault for sloppy use. The quantum theory and the Newtonian theory are well-established, well-verified components of our world view. They are not in doubt. It's a matter of derivation. Once upon a time it was Newton's (as yet unverified) "theory." Then it was verified, but the name stuck. "Newton's theory" it will always be. On the other hand, superstrings and GUTs are speculative efforts to extend current understanding, building on what we know. The better theories are verifiable. Once upon a time that was the sine qua non of any theory. Nowadays, addressing events at the Big Bang, we face, perhaps for the first time, a situation in which a theory may never be experimentally tested.
GUTs
I have described the unification of the weak and electromagnetic forces into the electroweak force, carried by a quartet of particles: W+, W−, Z0, and the photon. I have also described QCD—quantum chromodynamics—which deals with the behavior of quarks, in three colors, and gluons. These forces are now both described by quantum field theories obeying gauge symmetry.
Attempts to join QCD with the electroweak force are known collectively as grand unification theories (GUTs). The electroweak unification becomes evident in a world whose temperature exceeds 100 GeV (roughly the mass of the W, or 1015 degrees K). As chronicled in Chapter 8, we can achieve this temperature in the lab. GUTs unification, on the other hand, requires a temperature of 1015 GeV, which puts it out of the range of even the most megalomaniacal accelerator builder. The estimate is derived by looking at three parameters that measure the strengths of the weak, electromagnetic, and strong forces. There is evidence that these parameters in fact change with energy, the strong forces getting weaker and the electroweak forces stronger. The merger of all three numbers occurs at an energy of 1015 GeV. This is the grand unification regime, a place where the symmetry of the laws of nature is at a higher level. Again, this is a theory yet to be verified, but the trend of the measured strengths does indicate a convergence near this energy.
There are a number of grand unified theories, a large number and they all have their ups and downs. For example, an early entrant to the GUT contest predicted that the proton was unstable and would decay into a neutral pion and a positron. The lifetime of a proton in this theory is 1030 years. Since the age of the universe is considerably less—somewhat over 1010 years—not too many protons have decayed. The decay of a proton would be a spectacular event. Remember we considered the proton to be a stable hadron—and a good thing, too, because a reasonably stable proton is very important to the future of the universe and the economy. Yet in spite of the very low expected rate of decay, the experiment is doable. For example, if the lifetime is indeed 1030 years, and we watch a single proton for one year, we have only 1 divided by 1030 as our chance of seeing the decay—1030. Instead, we can watch lots of protons. In 10,000 tons of water there are about 1033 protons (trust me). This would mean that 1,000 protons should decay in a year.
So enterprising physicists went underground—into a salt mine under Lake Erie in Ohio, into a lead mine under Mount Toyama in Japan, and into the Mont Blanc tunnel that connects France and Italy—to be shielded from the background of cosmic radiation. In these tunnels and deep mines they placed huge, clear plastic containers of pure water, about 10,000 tons worth. That would be a cube roughly 70 feet on each side. The water was stared at by hundreds of large, sensitive photomultiplier tubes, which would detect the bursts of energy released by the decay of a proton. So far no proton decays have been observed. This doesn't mean that these ambitious experiments have not proved valuable, for they have established a new measure of the proton's lifetime. Allowing for inefficiencies, the proton lifetime, if indeed the particle is unstable, must be longer than 1032 years.
Interestingly, the long and unsuccessful wait for protons to decay was enlivened by unexpected excitement. I have already told about the supernova explosion of February 1987. Simultaneously a burst of neutrino events was seen by the Lake Erie and Mount Toyama underground detectors. The combination of light and neutrinos was in disgustingly good agreement with models of stellar explosion. You should have seen the astronomers preen! But the protons just don't decay.
GUTs have a hard time but, ever resilient, GUT theorists continue to be enthusiastic. One doesn't have to build a GUT accelerator to test the theory. GUT theories have testable consequences in addition to proton decay. For example, SU(5), one of the grand unified theories, makes the postdiction that the electric charge of particles is quantized, and must come in multiples of one third the charge of the electron. (Remember the quark charges?) Very satisfying. Another consequence is the consolidation of the quarks and leptons in one family. In this theory, quarks (inside the proton) can be converted to leptons and vice versa.
GUTs predict the existence of supermassive particles (X bosons) that are one thousand trillion times heavier than protons. The mere possibility that these exist and can appear as virtual particles does have tiny, tiny consequences, such as the rare decay of protons. Incidentally, the prediction of this decay has practical, if very far-out, implications. If the nucleus of hydrogen (a single proton), for example, could be converted to pure radiation, it would provide a source of energy one hundred times more efficient than fusion energy. A few tons of water could provide all the energy needed by the United States in a day. Of course, right now we'd have to heat the water to GUT temperatures, but perhaps some kid now being turned off to science by an insensitive kindergarten teacher might have the idea that would make this more practical. So, help the teacher!
At the temperatures of the GUT scale (1028 degrees Kelvin) symmetry and simplicity have reached the point where there is only one kind of matter (lepto-quark?) and one force with an array of force-carrying particles and, oh yes, gravity, dangling there.
SUSY
Supersymmetry, or Susy, is the favorite of the betting theorists. We were introduced to Susy earlier. This theory unifies the matter particles (quarks and leptons) and the force carriers (gluons, W's...). It makes a huge number of experimental predictions, not one of which has (yet) been observed. But what fun!
We have gravitinos and winos and gluinos and photinos—the matterlike partners of gravitons, W's, and the rest. We have super-symmetric partners of quarks and leptons: squarks and sleptons, respectively. The burden on this theory is to show why these partners, one for every known particle, have not been seen. Oh, say the theorists, remember antimatter. Until the 1930s no one dreamed that every particle would have its twin antiparticle. And remember that symmetries are created to be broken (like mirrors?). The partner particles haven't been seen because they are heavy. Build a big enough machine and they will all appear.
Mathematical theorists assure the rest of us that the theory has a splendid symmetry in spite of its obscene proliferation of particles. Susy also promises to lead us to a true quantum theory of gravity. Attempts to quantize the general theory of relativity—our theory of gravity—have been beset with infinities up the wazoo in a way that could not be renormalized. Susy promises to lead us to a beautiful quantum theory of gravity.
Susy also civilizes the Higgs particle, which, lacking this symmetry, could not do the job it was invented to do. The Higgs particle, being a scalar (zero-spin) boson, is particularly sensitive to the busy vacuum around it. Its mass is influenced by the virtual particles of all masses that fleetingly occupy its space, each one contributing energy and, therefore, mass un
til the poor Higgs would grow far far too obese to save the electroweak theory. What happens with supersymmetry is that the Susy partners influence the Higgs mass with their opposite signs. That is, the W particle makes the Higgs heavier while wino cancels the effect, so the theory allows the Higgs to have a useful mass. Still, all this doesn't prove that Susy is right. It's just beautiful.
The issue is far from settled. Buzz words appear: supergravity, the geometry of superspace—elegant mathematics, dauntingly complex. But one experimentally intriguing consequence is that Susy willin'gly and generously supplies candidates for dark matter stable neutral particles that could be massive enough to account for this pervasive material that haunts the observable universe. Susy particles were presumably made in the Big Bang era, and the lightest of the predicted particles—perhaps the photino, the higgsino, or the gravitino—could survive as stable remnants to constitute the dark matter and satisfy the astronomers. The next generation of machines must either confirm or deny Susy ... but, oh, oh, oh what a gal!
SUPERSTRINGS
I believe it was Time magazine that forever embellished the lexicon of particle physics by trumpeting this as the Theory of Everything, or TOE. A recent book put it even better: Superstrings, Theory of Everything? (This is read with a rising inflection.) String theory promises a unified description of all forces, even gravity, all particles, space and time, free of arbitrary parameters and infinities. In short: everything. The basic premise replaces point particles by short segments of string. String theory is characterized by a structure that pushes the frontiers of mathematics (as physics has very occasionally done in the past) and the conceptual limitations of the human imagination to the extremes. The creation of this theory has its own history and its own heroes: Gabrielle Veneziano, John Schwarz, André Neveu, Pierre Ramond, Jeff Harvey, Joel Sherk, Michael Green, David Gross, and a gifted pied piper by the name of Edward Witten. Four of the prominent theorists worked together at an obscure institution in New Jersey and have become known as the Princeton String Quartet.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 49
