Sky & Telescope November 1985 [antikvár]

New Physics and the New Big BangPaul Davies, University of Newcastle upon Tyne, EnglandThe idea that the universe began abruptly amid a huge burst of energy has been accepted by almost all cosmolo-gists for a generation. However, describing the origin of this Big Bang was long thought to lie beyond the scope of science. The occurrence of the outburst, and its form and nature, could not be explained on physical grounds; it simply had to happen. The old Big Bang theory was therefore not really an explanation of the universe at all but merely a description of the first historical event.Today, much more ambitious investigations of the very early universe are under way. They could lead to a proper explanation for all the fundamental features of the cosmos in terms of physical processes occurring during the initial second of cosmic existence. As yet only tentative and dimly perceived, this new version of the origin might even answer the ultimate question of all: the cause of the Big Bang itselfThe key to these dramatic developments lies in high-energy particle physics, the branch of science probing the structure of matter on the smallest scales. We now believe that in its earliest moments the universe was like a huge particle-physics laboratory. This view has become so pervasive that some cosmologists are now more at home using giant accelerator machines than telescopes.We can only explain how the universe is structured today by understanding the exotic particle-physics processes that took place in that first brief flash of cosmic existence. Currently existing accelerators, such as those at Fermilab near Chicago or the CERN laboratory in Europe, can simulate the conditions in the universe about 10"" second after the inifial bang. Speculations about still earlier epochs rest on theoretical ideas alone.In the last decade a plethora of ideas about ultra-high-energy matter have been expounded, and they find a natural application in studies of events in the very early universe. Physicists can thus use the great cosmic laboratory as a testing ground for their theories, while cosmologists can use the same ideas to look for clues about the nature of the universe we see today.In its simplest form, the Big Bang model states that the universe exploded into existence from a state of effectively infinite density and temperature. Initially, the universe was a featureless ferment of energy. But as it expanded and cooled, cosmic structures and the forces that shape them406 Sky Sc Telescope, November. 1985"froze" out of the primeval maelstrom one by one. Thus, there was a natural progression from simplicity to complexity.THE OLD BIG BANGThe crucial parameter determining the physics of the early universe at any given instant was temperature or, equivalently, the energy of each particle. At one second, for example, the temperature was 10'° degrees Kelvin, which means that a typical particle had an energy of about 1,000,000 eV (electron vohs: the energy an electron gains when accelerated by a potential difference of 1,000,000 volts). At 10"" second the temperature was 10" degrees Kelvin, corresponding to particle energies of almost a trillion (10") eV. This is the frontier of modem high-energy particle colHders.Cosmologists have long appreciated that the universe has a number of exceedingly special features. On a large scale, matter and energy are distributed very uniformly. Nevertheless there is a hierarchy of structure, ranging from planets and stars, up through galaxies and clusters, to superclus-ters and the huge voids between them.This structure is of the same general form and degree throughout the cosmos. Thus, the universe is in the peculiar state of being almost, but not quite, perfectly ordered. There is regularity on a large scale but irregularity on smaller scales.The expansion of the universe also poses a puzzle. Its rate turns out to be very close to the cosmic escape velocity. A slightly bigger bang and all the material would now be dispersed. A slightly smaller bang and the entire cosmos would have long ago collapsed under its own weight, obliterating itself in a "Big Crunch." Evidently the explosive vigor of the Big Bang was fine-tuned to uncanny precision.Another mystery concerns the origin of matter and energy. On the face of it there seems to be no particular reason why the observable universe should contain the 10" or so protons, neutrons, and electrons it does. Furthermore, the whole cosmos is bathed in the fading glow of the primeval inferno, radiation that now has a temperature of about 3° Kelvin and fills all space. This background accounts for most of the roughly 10'° photons in the observable universe. Any complete cosmological theory must account for the values of these two large numbers.The old Big Bang theory could not explain these features. They simply had tobe put in "by hand"; you had to assume very special initial conditions. The matter and energy, the highly regular large-scale structure, and the small local departures from uniformity necessary for the formation of galaxies all had to be present at the outset, since none could be generated by physical processes.The great triumph of the new Big Bang theory is that these very special features receive natural explanations. It is no longer necessary to assume that the universe began in a highly contrived and peculiar state. On the contrary, there is optimism that the cosmos was inhially rather featureless and unremarkable. Thus, all the special features were generated by physical processes before the briefest fraction of a second had elapsed.