Tevatron accelerator may hold the key to cosmic history
SCIENTISTS concerned with the birth of the universe have a new tool with which to explore that inscrutable event. It's the Tevatron particle accelerator now being brought up to full power here at the Fermi National Accelerator Laboratory (Fermilab).
Besides giving new insight into subatomic particles, this machine will also reproduce conditions that have not occurred naturally since the first trillionth of a second in the birth of the cosmos.
''Every collision (between particles) we make here didn't exist, doesn't exist, anywhere in the universe until you go back to shortly after the Big Bang, '' says Fermilab director Leon Lederman. This Bang is the outburst of energy from which the universe emerged 10 to 20 billion years ago, according to popular theories.
This is why particle physicists and astrophysicists have joined together, in recent years, to pursue cosmic history. To use the language of the National Academy of Sciences Report on Astronomy and Astrophysics, they stand on ''the threshold of a revolution in physical thought, in which the properties of elementary particles may hold the key to understanding the early history of the universe.''
For that early epoch, astrophysicists have no adequate theories to guide them. ''We have the history from, say, a hundredth of a second until today well under control,'' says Fermilab astrophysicist Michael Turner.
To explore what happened earlier, cosmologists must wait for discoveries by particle physicists to guide their calculations. As the Tevatron comes up to full power, it will take cosmologists back far earlier than that hundredth-of-a-second benchmark.
The energies involved are measured in terms of electron volts (eV). A proton or electron gains an energy of one eV when it is accelerated by a voltage difference of one volt. Using this unit, it is easy to see that even the energies of protons that fuse together at 15 million degrees to power the sun are puny, compared with those produced by the accelerator.
The solar protons have energies of around a thousand electron volts. Fermilab's new accelerator already has beams of 800 billion eV (800 GeV) protons. Within a year, it should reach its design level of 1,000 GeV or 1 TeV (T for tera, the metric term for a trillion). Hence the accelerator's name - Tevatron. Within two years, it should provide head-on particle collisions of 2 TeV.
Such Tevatron energies correspond to temperatures of many thousands of trillions of degrees. What may be more meaningful, they represent conditions that prevailed in the universe a trillionth of a second after the Big Bang.
Thus Tevatron experiments should provide cosmologists with insight into how matter behaved at that early moment. Certainly, Turner notes, it was a much different situation than prevails anywhere in nature today. Yet, there is debris left over from that epoch, and astronomers can study it. This, in turn, helps elucidate the physicists' theories.
''If we're really clever,'' Turner explains, ''we can use the debris . . . to try to figure out the kind of things that were happening very early on.'' Then these findings can be compared with what accelerator experiments reveal about matter and energy under the primordial conditions. A better understanding both of the early universe and of particle physics may evolve. Debris consists of such relics as a universal bath of microwave (radio) radiation at a temperature of 3 Kelvin (3 degrees above absolute zero), or a form of helium called helium 4 , which accounts for 25 percent of the mass of today's universe.
Facts about such debris can constrain theories spun by physicists. Turner notes that so far physicists have found only three types of neutrinos - particles with no intrinsic mass and no electric charge. If a physicist were to produce a theory embodying a fourth neutrino, this would result in a universe with more than 25 percent helium 4 right after the Bang. The excess helium would have persisted to be detected today. So, Turner says, ''We can tell that theorist, 'Your theory is no good.' ''
Conversely, to understand how the universe began to evolve, cosmologists need to know such basic facts as how many quarks there are and exactly how these interact. Quarks are the units out of which protons and related particles are made. Such knowledge can only come from high-energy-accelerator experiments.
Thus, says Leon Lederman, ''We look down (into the subnuclear world) and we see the birth of the Universe.''