Structure Formation with Mirror Dark Matter: CMB and LSS

TL;DR

The paper addresses whether mirror baryons can serve as dark matter and how they modify CMB anisotropies and large-scale structure. It employs a quantitative linear perturbation analysis in a flat Universe, doubling the equations to include the mirror sector and exploring the parameter space defined by $x$ and $\beta$. It identifies the mirror Jeans length $\lambda'_{\rm J}$ and Silk scale $\lambda'_{\rm S}$ and shows how mirror decoupling occurs at $1+z'_{\rm dec}\simeq x^{-1}(1+z_{\rm dec})$, with a critical threshold $x_{\rm eq}\simeq 0.34$ that shapes the observable signatures in LSS and CMB. By computing spectra and comparing with data, the work constrains the mirror parameter space, finding that high $x$ and large $\beta$ are disfavored and that $x<0.3$ renders MBDM effectively CDM-like. These results provide a linear-regime framework for future nonlinear investigations into reionization, mirror star formation, and MACHO phenomenology.

Abstract

In the mirror world hypothesis the mirror baryonic component emerges as a possible dark matter candidate. An immediate question arises: how the mirror baryons behave and what are the differences from the more familiar dark matter candidates as e.g. cold dark matter? In this paper we answer quantitatively to this question. First we discuss the dependence of the relevant scales for the structure formation (Jeans and Silk scales) on the two macroscopic parameters necessary to define the model: the temperature of the mirror plasma (limited by the Big Bang Nucleosynthesis) and the amount of mirror baryonic matter. Then we perform a complete quantitative calculation of the implications of mirror dark matter on the cosmic microwave background and large scale structure power spectrum. Finally, confronting with the present observational data, we obtain some bounds on the mirror parameter space.

Figures (4)

• Figure 1: Evolution of perturbations at the scale $k = 0.1$ Mpc$^{-1}$ in the case when dark matter is entirely due to mirror baryons $(\Omega_{\rm CDM}=0)$. Dot-dashed and dotted lines correspond to ordinary baryons and photons, while long dashed and dashed lines are for mirror baryons and photons. All cosmological parameters are taken as in (\ref{['ref-model']}). Left panel (a) corresponds to the case $x=0.6$ and the right panel (b) to the case $x=0.3$.
• Figure 2: The same as in Fig. \ref{['evolmirr']} but in the case when the MBDM is a sub-dominant dark matter component with $\beta=2$, i.e. $\Omega'_b=0.09$ and $\Omega_{\rm CDM}=0.12$.
• Figure 3: LSS power spectrum in the linear regime for different values of $x$ and $\omega_{\rm b}' \equiv \Omega_{\rm b}' h^2$, as compared with a standard model prediction (solid line). In order to remove the dependences of units on the Hubble constant, we plot on the x-axis the wave number in units of $h$ and on the y-axis the power spectrum in units of $h^{-3}$. All parameters are taken as in (\ref{['ref-model']}). We also show the binned data of 2dF observations 2df-teg. Top panel. Models where dark matter is entirely due to MBDM (no CDM, i.e. $\beta=5$) for different values $x = 0.3, 0.5, 0.7$. Bottom panel. Models with mixed CDM+MBDM components, $\beta=1,2,3,4$ for a value of $x=0.7$.
• Figure 4: The CMB angular power spectrum for different values of $x$ and $\omega_{\rm b}'$, compared with a standard model (solid line). We also show the WMAP wmap-data and ACBAR acbar-data data. The choices of the parameters for both top panel and bottom panel exactly correspond to those of Fig. 3.