Broken Defects

Recent indications of a neutrino mass raise the possibility that the dark matter may be hot, which would be a severe challenge for structure formation theory. We argue that generic defect theories fare very well in hot dark matter cosmologies, and also work in the presence of curvature or a cosmological constant. If this model is correct, then the MAP and PLANCK missions will not measure what people expect them to (oscillations); rather, they will measure a broad hump. The traditional class of scale invariant defect models has severe problems[1, 2]. They do not explain the rise in fluctuation amplitude at scales of a few degrees, and the slope of the perturbations is at conflict with the observed matter power spectrum. We show here that global topological defects generically change behaviour at the matter-radiation transition at a cosmic age of 100000 years. In this case, instead of being at great discrepancy with the cosmic microwave background radiation fluctuations, this model in fact correctly explains several hitherto mysterious phenomena. Recent measurements have indicated that neutrinos have mass, and have opened up that the dark matter may be massive neutri-nos. If we accept that neutri-nos have mass, we have the potential consequence that neutri-nos account for all the dark mass in the universe. This simple scenario is consistent with the simplest massive neutrino extensions of the standard particle physics models[3], and challenges cosmol-ogists to contemplate its implications for structure formation. The scale invariant adiabatic model predicted by inflation is not consistent if massive neutrinos account for the bulk of gravitating matter. We will show that generic defect models would now become the best physically based model of structure formation. The most recent supernovae search[4, 5], concordance arguments[6] and direct angular diameter distance measurements[7] have suggested that the universe is filled with a cosmological constant. This further challenges hot dark matter (HDM) structure formation models , but will turn out to be simple to reconcile in defect models. Let us now address the dynamics of global defects at matter-radiation equality. In modern particle physics, all interactions which are not forbidden actually occur. On this ground, one generically expects couplings between gravity and other quantum fields which are stronger than just minimal coupling to the space time curvature. Global fields experience couplings of the form ǫφ † 1 φ2R, where ǫ is a dimension-less coupling constant of order unity, and R is the scalar …