Mirror dark matter and large scale structure

TL;DR

The paper develops a linear perturbation framework to study mirror dark matter (MDM) in a two-sector cosmology with parameters $x= T'_0/T_0$ and $y=0.14/(\,\Omega_m h^2)$, deriving a processed matter power spectrum that exhibits a peak at $\\lambda_{max}$ and a Silk-scale damping at $\\lambda'_S$. It shows that, depending on $x$, MDM can be CDM-like ($x \lesssim 0.2$) or exhibit warmer/dissipative features at small scales, with $\\lambda_{max}$ decreasing as $x$ decreases; the linear regime thus predicts bottom-up structure formation similar to CDM for small $x$, but with distinctive small-scale oscillations and damping unique to MDM. The work discusses implications for Lyman-$\\alpha$ forest data, CMBR acoustic peaks, and non-linear evolution, suggesting that precise observations of small-scale structure and early reionisation could distinguish MDM from WIMP/CDM scenarios. The findings motivate future studies of the non-linear evolution of mirror baryons, including potential observational signatures such as compact mirror objects and altered star formation histories, and highlight the role of small-scale power in constraining $x$ and the viability of MDM as the dominant dark matter component.

Abstract

Mirror matter is a dark matter candidate. In this paper, we re-examine the linear regime of density perturbation growth in a universe containing mirror dark matter. Taking adiabatic scale-invariant perturbations as the input, we confirm that the resulting processed power spectrum is richer than for the more familiar cases of cold, warm and hot dark matter. The new features include a maximum at a certain scale $λ_{max}$, collisional damping below a smaller characteristic scale $λ'_S$, with oscillatory perturbations between the two. These scales are functions of the fundamental parameters of the theory. In particular, they decrease for decreasing $x$, the ratio of the mirror plasma temperature to that of the ordinary. For $x \sim 0.2$, the scale $λ_{max}$ becomes galactic. Mirror dark matter therefore leads to bottom-up large scale structure formation, similar to conventional cold dark matter, for $x \stackrel{<}{\sim} 0.2$. Indeed, the smaller the value of $x$, the closer mirror dark matter resembles standard cold dark matter during the linear regime. The differences pertain to scales smaller than $λ'_S$ in the linear regime, and generally in the non-linear regime because mirror dark matter is chemically complex and to some extent dissipative. Lyman-$α$ forest data and the early reionisation epoch established by WMAP may hold the key to distinguishing mirror dark matter from WIMP-style cold dark matter.

Figures (4)

• Figure 1: Schematic form of the processed power spectrum (not to scale) at $z=z_{eq}$ for case I ($x > x_{eq}$). The curve depicts the mirror dark matter density perturbation $\log(\delta \rho'_B/\rho'_B)_{\lambda}$ as a function of the logarithm of the scale, $\log\lambda$. The dashed section represents oscillatory evolution, while the solid line section shows non-oscillatory evolution. See the text for a full discussion and for the defintion of the important scales indicated along the horizontal axis.
• Figure 2: As for Fig. \ref{['fig1']}, except that $z = z'_{dec}$.
• Figure 3: As for Fig. \ref{['fig1']}, except that it refers to case II ($x < x_{eq}$).
• Figure 4: As for Fig. \ref{['fig3']}, except that $z = z_{eq}$.