CMBFAST

Abstract: I consider the growth of inhomogeneities in a low-density, baryonic, vacuum energy-dominated universe in the context of modified Newtonian dynamics (MOND). I first write down a two-field Langrangian-based theory of MOND (nonrelativistic) that embodies several assumptions, such as constancy of the MOND acceleration parameter, association of a MOND force with peculiar accelerations only, and the deceleration of the Hubble flow as a background field that influences the dynamics of a finite-size region. In the context of this theory, the equation for the evolution of spherically symmetric overdensities is nonlinear and implies very rapid growth even in a low-density background, particularly at the epoch when the putative cosmological constant begins to dominate the Hubble expansion. Small comoving scales enter the MOND regime earlier than larger scales and therefore evolve to large overdensities sooner. Taking the initial COBE-normalized power spectrum provided by Seljak and Zeldarriaga's CMBFAST, I find that the final power spectrum resembles that of the standard ΛCDM universe and thus retains the empirical successes of that model.

Abstract: I consider the growth of inhomogeneities in a low-density baryonic, vacuum energy-dominated universe in the context of modified Newtonian dynamics (MOND). I first write down a two-field Langrangian-based theory of MOND (non-relativistic), which embodies several assumptions such as constancy of the MOND acceleration parameter, association of a MOND force with peculiar accelerations only, and the deceleration of the Hubble flow as a background field which influences the dynamics of a finite size region. In the context of this theory, the equation for the evolution of spherically symmetric over-densities is non-linear and implies very rapid growth even in a low-density background, particularly at the epoch when the putative cosmological constant begins to dominate the Hubble expansion. Small comoving scales enter the MOND regime earlier than larger scales and therefore evolve to large over-densities sooner. Taking the initial COBE-normalized power spectrum provided by CMBFAST (Seljak & Zeldarriaga 1996), I find that the final power-spectrum resembles that of the standard LCDM universe and thus retains the empirical successes of that model.

Abstract: We compute the imprints left on the CMB by two cosmic reionization models consistent with current observations but characterized by alternative radiative feedback prescriptions (suppression and filtering) resulting in a different suppression of star formation in low-mass halos. The models imply different ionization and thermal histories and 21 cm background signals. The derived Comptonization, u, and free-free distortion, y_B, parameters are below current observational limits for both models. However, the value of u = 1.69 * 10^-7 (9.65 * 10^-8) for the suppression (filtering) model is in the detectability range of the next generation of CMB spectrum experiments. Through the dedicated Boltzmann code CMBFAST, modified to include the above ionization histories, we compute the CMB angular power spectrum (APS) of the TT, TE, and EE modes. For the EE mode the differences between these models are significantly larger than the cosmic and sampling variance over the multipole range l = 5-15, leaving a good chance of discriminating between these feedback mechanisms with forthcoming/future CMB polarization experiments. The main limitations come from foreground contamination: it should be subtracted at per cent level in terms of APS, a result potentially achievable by novel component separation techniques and mapping of Galactic foreground.

Abstract: We study the effect of a prolonged epoch of reionization on the angular power spectrum of the Cosmic Microwave Background. Typically reionization studies assume a sudden phase transition, with the intergalactic gas moving from a fully neutral to a fully ionized state at a fixed redshift. Such models are at odds, however, with detailed investigations of reionization, which favor a more extended transition. We have modified the code CMBFAST to allow the treatment of more realistic reionization histories and applied it to data obtained from numerical simulations of reionization. We show that the prompt reionization assumed by CMBFAST in its original form heavily contaminates any constraint derived on the reionization redshift. We find, however, that prompt reionization models give a reasonable estimate of the epoch at which the mean cosmic ionization fraction was ~50%, and provide a very good measure of the overall Thomson optical depth. The overall differences in the temperature (polarization) angular power spectra between prompt and extended models with equal optical depths are less than 1% (10%).