The Nature of the Dark Matter

The nature of the dark matter that dominates the present mean mass density of the Universe remains enigmatic. Searches for microlensing in the direction of the Large Magellanic Cloud have set strong constraints on the possibility that the halo of our Galaxy is composed of low mass stars (e.g. Alcock et al. 1995). It is unlikely, therefore, that the bulk of the dark matter is composed of baryonic material. The most popular candidate for the dark matter is a weakly interacting massive supersymmetric particle (see e.g. Ellis 1990). Many groups around the world have initiated experiments to search for such cold dark matter by the laboratory detection of nuclear recoil (see e.g. Smith & Lewin 1990).

Figure 1.10: The solid and dashed lines show the fractional difference in the CMB power spectrum for mixed dark matter models compared to a scale-invariant cold dark matter model with $\Omega_0=1$, $\Omega_b = 0.05$ and h=0.5 (adapted from Dodelson et al. 1996). In the mixed dark matter models, light neutrinos contribute $\Omega_\nu = 0.3$ (dashed line) and $\Omega_\nu = 0.2$ (solid line); in both cases $\Omega_0=1$ and $\Omega_b = 0.05$. The dotted lines show th fractional error in the CMB power spectrum attainable by an experiment that surveys one-third of the sky with resolutions $\theta_{FWHM}$ of $30^\prime$ and $10^\prime$ at a sensitivity of $\Delta T/T = 2 \times 10^{-6}$ per resolution element.

However, studies of large-scale structure in the Universe are inconsistent with the simplest versions of the CDM model (see e.g. Maddox et al. 1990, Park et al. 1994). One way of resolving this discrepancy is to postulate that the dark matter consists of a mixture of cold dark matter and light neutrinos ( e.g. Davis et al. 1992, Klypin et al. 1993), a possibility that is extremely difficult to test directly in laboratory experiments. However, even a small admixture of light neutrinos leads to systematic differences of typically $10 \%$ in the spectrum of CMB anisotropies compared to a model consisting only of cold dark matter and baryons (e.g. Dodelson et al. 1996). The differences in the CMB anisotropies arise primarily from differences in the equation of state of the Universe at the time of recombination. An example is shown in Figure 1.10. These small differences in the CMB power spectrum can be detected by an experiment with sufficient angular resolution, sky coverage and sensitivity. High angular resolution is particularly crucial, however. The dashed lines in the figure show the fractional error in the power spectrum attainable by a CMB experiment with 1/3 sky coverage, sensitivity of $\Delta T/T = 2 \times 10^{-6}$ per resolution element and angular resolutions of $\theta_{FWHM}= 30^\prime$ and $10^\prime$. An experiment with the high angular resolution of PLANCK is therefore capable of detecting the small difference in the power spectrum at multipoles $\ell \lower.5ex\hbox{$\; \buildrel \gt \over \sim \;$}400$ and hence of setting constraints on the nature of the dark matter, in addition to its contribution to the total mean mass density.

[last update: 1 August 1999 by P. Fosalba]