Fluctuations in the cosmic microwave background (CMB) temperature are being studied with ever increasing precision. Two competing types of theories might describe the origins of these fluctuations: "inflation" and "defects". Here we show how the differences between these two scenarios can give rise to striking signatures in the microwave fluctuations on small scales, assuming a standard recombination history. These should enable high resolution measurements of CMB anisotropies to distinguish between these two broad classes of theories, independent of the precise details of each.
Domain walls form naturally in the early Universe whenever a discrete symmetry is spontaneously broken at some phase transition. When each vacuum is populated equally, it is well known that the domain wall network comes to dominate the energy density of the Universe and causes excessive anisotropy in the cosmic microwave background. We present results for the initial conditions and dynamical evolution of domain wall networks in which one of the degenerate vacua has a population bias over the other. The initial distribution of domain walls is well described by percolation theory. We find that such networks, although they show evidence of a limited scaling regime for a range of biases, do not persist indefinitely. It follows that biased domain wall networks avoid the energy density and anisotropy problems.
We investigate how the qualitative structure of Doppler peaks in the angular power spectrum of the cosmic microwave anisotropy is affected by basic assumptions going into theories of structure formation.We define the concepts of "coherent" and "incoherent" fluctuations, and also of "active" and "passive" fluctuations. In these terms inflationary fluctuations are passive and coherent while topological defects are active incoherent fluctuations. Causality and scale invariance are shown to have different implementations in theories differing in the above senses. We then extend the formalism of Hu and Sugiyama to treat models with cosmic defects. Using this formalism we show that the existence or absence of secondary Doppler peaks and the rough placing of the primary peak are very sensitive to the fundamental properties defined. We claim therefore that even a rough measurement of the angular power spectrum C l shape at 100 < l < 1500 ought to tell us which are the basic ingredients to be used in the right structure formation theory. We also apply our formalism to cosmic string theories. These are shown to fall into the class of active incoherent theories for which one can robustly predict the absence of secondary Doppler peaks.The placing of the cosmic strings' primary peak is more uncertain, but should fall in l ≈ 400 − 600.
We study the polarization-polarization and polarization-temperature correlations in standard adiabatic scenarios for structure formation. Temperature anisotropies due to gravitational potential wells and oscillations in the photon-baryon-electron fluid on the surface of last scattering are each associated with a correlated polarization pattern. While the `correlated part' of the polarization has an r.m.s. of only a third of the total signal, it may still be measurable by mapping a large area on the sky. We calculate the expected signal to noise ratio for various measures of the polarization in a hypothetical mapping experiment such as those now being planned.Comment: 12 pages of uuencoded compressed postscript (replacing previous uncompressed version), figures included. PUPT-94-147
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