Photon Bubbles put X-ray fix into neutron stars. Bubbles of Xrays may form in the matter crashing onto the magnetic poles of a neutron star. \ / \ / Incoming matter from disk. \ / \ / |\ /|Accretion Shock \| |/ Photon Bubbles +----------+ Neutron Star +----------+ || || \/ Xrays /*---------------------------------------------------------------------*/ Keck Telescope in Hawaii Largest optical telescope. Mauna Kea. Keck II 300 tons, 8 stories high, 10meter mirror of 36 hexagons. /*---------------------------------------------------------------------*/ p966 Book Reviews Cosmic Strings and other topological defects. One of the most exciting twists of 20th cen. science has merged the study of the very large and very small. Unification of particle physics and cosmology. Particle physics addresses the structure of matter at the shortest accessible distances . cosmology concerns the structure and the evolution of the whole universe. Particle physics has had great success. A standard model is now firmly in place that can account for all of the available experimental evidence. But the standard model is not a final theory; it leaves unanswered too many fundamental questions. Only studying shorter distances and higher energies can solve this. Cosmology has had comparable success. A standard model is in place that gives a reasonable account of the available observations. But again the standard model leaves unanswered too many questions, especially about initial conditions in the universe. In particles physics, the most energetic current accelerator experiments can resolve the structure of matter at a distance scale of 10^-16cm. But particle theorists boldly speculate about the properties of the fundamental interactions down to the distance scales of 10^-33cm. We crave experimental evidence to constrain these speculations , but no accelerator in the foreseeable future will be sufficiently energetic to provide it. Particle physicists look to the early universe as their accelerator. The universe was once so hot as to evidence the behavior of matter at exceedingly short distances, and we can hope to detect relics of that fiery past in today's universe. Ideas from particle physics can illuminate the central questions of cosmology : Where did the matter of the universe come from? Why is the universe to large, so nearly isotropic and so nearly homogeneous? What explains the origin of galaxies, clusters of galaxies, of superclusters? What is the nature of the "Dark matter " that dominates the halos of spiral galaxies? Particle physics has offered a wealth of appealing answers to these questions. The trouble is that there are too many answers. they cannot all be right. A central theme of modern cosmology is the study of large scale structure - the way the galaxies are distributed in the universe. The very early universe was much more homogeneous than the present universe. But small inhomogeneities in the matter density were present even then and, owing to their gravitational attraction, grew, eventually condensing into the galaxies and other structures we see today. The most fundamental question about the formation of galaxies is: What was the origin of these initial perturbations? Recent measurements of the microwave radiation left over from the big bang allow us to estimate their size. The typical variation of the density about its mean value was only about one part in 10^5, and it is the task of fundamental theory to explain why this variation was so small. Two major competing models for the origin of the density perturbations have been proposed. In the inflationary model, the perturbations arose from quantum fluctuations and can be understood as a consequence of the Heisenberg uncertainty principle. These quantum fluctuations are almost certainly present at some leve, but so far theorists have not offered any unambiguous prediction of their magnitude. The second model of the origins of large-scale structure is the cosmic string model, and it provides the motivation and central focus of Vilenkin and Shellard's Cosmic Strings and other Topological Defects. In this picture, the perturbations arose from a network of linear defects that were created during phase transition that took place during the first 10^-37 after the big bang. Vilenkin is one of the originators of this scenario, and Shellard has been a leading contributor to its development. Their book is a remarkably complete and authoritative review of the broad range of physics issues underlying it. Cosmic strings are closely analogous to the vortex lines that are seen in laboratory experiments with type II superconductors, but with two important differences: the cosmic strings are defects in the vacuum rather than in a bulk state of matter, and their diameter is only or order 10^-30cm. Correspondingly, the strings are enormously heavy- with a mass / unit length of 10^22g/cm (10^10 solar masses / kiloparsec.). These numbers are obtained under the assumption that cosmic strings are indeed responsible for the primordial density perturbations, by fitting to the cosmic microwave observations. The chain of ideas connecting cosmic strings with the large scale structure of the universe is intricate, and Vilenkin and Shellard are compelled to cover a lot of terrain in this treatise. They discuss, for example , the classification of topological defects, the theory of phase transitions and of defect formation in a quanch how strings move and what happens when they collide, the gravitational effects of strings and the gravitational radiation they emit, and of course the effect of strings on the microwave background and on the formation of structure. Indeed, the physics of the cosmic string scenario is so complicated that even after 15years of intensive study , it is difficult to extract precise predictions that can be compared with the observations of the astronomers. Even the qualitative picture of how the scenario works has evolved substantially, especially as a result of improved numerical simulations. vilenkin's original idea was that each galaxy was seeded by a closed loop of string, but the simulations showed that the loops that branch off of the string network are smaller than originally assumed, and this proposal became untenable. Another idea was that gravitational focusing behind a moving open string would cause matter to accrete onto a sheet in the string's wake, enhancing the abundance of galaxies on the sheet. But the simulations showed that the open strings wiggle much more than originally expected, and that picture also had to be modified. Current comparisons of the scenario with observation are still inconclusive, but the situation is bound to be clarified as both the simulations and the observations improve. Actually, what surprised me most is that so little is said about the confrontation of the scenario with the data. I suppose the authors were worried any detailed discussion would be obsolete.