One of the most exciting possibilities offered by PLANCK is that of searching our present horizon volume for cosmic defects, relics of a grand unification phase transition in the earliest moments of the hot big bang. Since (Kibble 1976) realized that such defects were a generic prediction of unified gauge field theories, there has been continued theoretical interest in their cosmological consequences (for recent reviews, see Vilenkin & Shellard 1994, Hindmarsh & Kibble 1995). For some time they have provided the leading alternative scenario for structure formation to inflationary quantum fluctuations. Recent computational breakthroughs (Albrecht et al. 1997; Pen et al. 1997) have made them less attractive in this respect, since when normalized to COBE the theories predict too little power in the galaxy distribution on 100 Mpc scales. Nevertheless it is still extremely important to search for cosmic defects produced in the early universe, since they provide one of the very few available windows into the process of unified gauge symmetry breaking in the very early universe. Defects can be formed at any energy scale, however the most likely possibility is that they were formed at the GUT transition, predicted from the high precision LEP measurements to be at GeV. Such defects would give a substantial contribution to the cosmic microwave background anisotropy, with several key features allowing it to be easily distinguished from an inflationary contribution.
Cosmic defects would produce a nonGaussian pattern of temperature anisotropies on the sky (see figure 1.6). Global textures and monopoles would produce distinctive patterns of hot spots on scales of which PLANCK would be able to map to high accuracy. Cosmic strings would produce a more Gaussian pattern of anisotropies, but with a key difference to inflation in that the Doppler peaks produced by strings are smeared out in l space due to the nonlinear character of the string dynamics.
It is also important to bear in mind that there are several other possibilities for cosmic structure formation, some of which have only begun to be explored. For example, isocurvature perturbations where the total energy density of the universe is smooth but the relative abundance of different species varies spatially have been partially studied (Peebles 1987; Efstathiou & Bond 1986, 1987,Peebles 1999a,b). Calculations for neutrino isocurvature perturbations are only now being performed (Spergel & Turok 1999). More generally, there is the possibility that nonlinear but causal physics occurring before recombination could have seeded the cosmic perturbations. A few such models have been proposed (e.g. Turok 1996), which are able in some respects to mimic inflation but are distinguishable for example by their predictions for the polarization-temperature cross correlation (Spergel & Zaldarriaga 1997).