Differentiating CDM and baryon isocurvature models with 21 cm fluctuations

Abstract

We discuss how one can discriminate models with cold dark matter (CDM) and baryon isocurvature fluctuations. Although current observations such as cosmic microwave background (CMB) can severely constrain the fraction of such isocurvature modes in the total density fluctuations, CMB cannot differentiate CDM and baryon ones by the shapes of their power spectra. However, the evolution of CDM and baryon density fluctuations are different for each model, thus it would be possible to discriminate those isocurvature modes by extracting information on the fluctuations of CDM/baryon itself. We discuss that observations of 21 cm fluctuations can in principle differentiate these modes and demonstrate to what extent we can distinguish them with future 21 cm surveys. We show that, when the isocurvature mode has a large blue-tilted initial spectrum, 21 cm surveys can clearly probe the difference.

Constraining isocurvature perturbations with the 21cm emission from minihaloes

“…Although the evolution of the metric perturbations is almost same between the CDM and baryon isocurvature modes, the difference in the initial fluctuations between the CDM and baryon isocurvature modes can lead to observable effects, as will be shown in this work (see also Kawasaki et al (2011)). …”

Baryons do trace dark matter 380,000 years after the big bang: Search for compensated isocurvature perturbations with WMAP 9-year data

“…We use the approximate analytical normalization of Eq. (48). This neglects the secondary CIP trispectrum contractions, which we see in Fig.…”

“…The resulting anisotropies would be imprinted on the baryonic sound horizon, at l $ 10 6 [33,37,46]. Existing measurements at low redshift constrain the CIP perturbation amplitude to be & 10% [33,34], while more sensitive proposed measurements of 21-cm absorption during the cosmic dark ages are a ways off in the future [33,[46][47][48].…”

Isocurvature modes in the CMB bispectrum

“…The only exception are the CDM and baryon isocurvature modes which give exactly the same pattern, up to the rescaling: 11) where the parameters Ω b and Ω c denote, as usual, the present energy density fractions, respectively for baryons and CDM (note however that these two modes can in principle be discriminated via other effects, see e.g. [25,[39][40][41]). More generally, when we also allow for possible correlations between the modes and include E-polarization, the angular power spectra are given by 12) where I and J label the isocurvature mode and α and α the polarization (i.e.…”

“…[21][22][23][24][25][26][27][28]). However, isocurvature modes could also contribute to non-Gaussianities as discussed in several works [29][30][31][32][33][34][35][36][37].…”