Abstract
When the predecoupling plasma is thermodynamically reversible its fluctuations are classified in terms of the adiabatic and entropic modes. A different category of physical solutions, so far unexplored, arises when the inhomogeneities of the viscosity coefficients induce computable curvature perturbations. The viscous modes are explicitly illustrated and compared with the conventional isocurvature solutions.
Highlights
The adiabatic and the entropic modes have been introduced long ago [1] in connection with the pioneering analyses of the temperature and polarization anisotropies of the cosmic microwave background (CMB in what follows)
While the conventional distinction between the adiabatic and the entropic solutions posits that the ambient fluid is thermodynamically reversible, it seems plausible to relax this hypothesis with the aim of complementing and extending the physical initial conditions of the predecoupling plasma
According to the current data [3,4], anticorrelated nonadiabatic modes in the presence of a dominant adiabatic mode may even improve the fit of the temperature autocorrelations accounting for potential large-scale suppressions of the corresponding angular power spectra
Summary
The relative position of the first anticorrelation peak of the cross-spectrum between the temperature and the polarization demonstrated (already from the first releases of the WMAP data [3]) that the initial conditions of the Einstein-Boltzmann hierarchy are predominantly adiabatic. This evidence has been further scrutinized and quantitatively refined by the subsequent. Across the matter-radiation transition the entropic solutions are determined by the total sound speed of the plasma and by the specific form of δpnad; for the CDM-radiation mode we have: c2st.
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