Dark Matter is Back!

A few weeks ago a paper claiming strong bounds on the local dark matter density made a news, hitting also particle physics blogs. Currently, the most solid evidence of dark matter comes from  analyzing the Cosmic Microwave Background, and from the observed flatness of the galactic rotation curves. It is less known than in our galaxy the support for dark matter comes from studying the rotation curves at distances of 20 kpc or more from the galactic center. In the immediate neighborhood of the Sun (8 kpc from ground zero), the presence of dark matter is more difficult to deduce. The value of the local dark matter typically quoted, ρ = 0.4 GeV/cm^3, is based on extrapolations using particular models of the dark matter halo.

The recent paper by Moni Bidin et al. attempted a direct measurement of the local dark matter density.  Studying the kinematics of a population of stars drifting a few kpc above the galactic plane, they were able to estimate the so-called surface density, that is the integral of the mass density in the vertical (wrt to the galactic plane) direction. If there is only the visible matter concentrated in the disc then the surface density should be constant above the disc. Conversely, if there is dark matter in the form of a spherical halo then the surface density should continue growing above the disc.  The paper finds the data are well fit by a constant surface density for z > 1.5 kpc above the disc, setting the limit ρ < 0.04 GeV/cm^3, that is 10 times smaller than what is usually assumed. 

The authors went as far to saying that  "our results may indicate that any direct DM detection experiment is doomed to fail". This is of course a sheer nonsense, even if their limits were true. The event rate in direct detection experiment depends, among other things, on the product of the  local dark matter density ρ and the scattering cross section of dark matter on protons and neutrons σ.  The latter has no obviously preferred value, and in concrete particle physics models it may span many orders of magnitude. Thus, the constraint on ρ doesn't tell us anything about the subjective chances of detecting dark matter; it only changes the interpretation of direct detection experiments in terms of the limits on σ. A smaller ρ would mean weaker limits on σ, thus weaker limits on the parameter space of dark matter models, which would actually make many of us happy.    

Nevertheless, the claim that there is no indication of the presence of dark matter in the solar neighborhood was somewhat disturbing to these experimentalists who spend entire lives in underground caverns, preparing and running dark matter detection experiments. Those can now utter a sigh of relief. A new paper by Bovy and Tremaine that has just appeared on arXiv says that the paper of Moni Bidin et al is "flawed". The strong limits on the dark matter density were obtained as a consequence of an observationally unsupported assumption about the velocities of the studied population of stars. As can be seen in the plot, correcting the wrong assumption leads to a perfect consistency between the data and the predictions assuming the presence of dark matter. pwned.

One lesson, supported by large statistics, is that papers who come with aggressive press releases are, more often than not, wrong. The present case is however more interesting than that: it seems that although some crucial assumptions were wrong, the proposed method of constraining ρ is promising. Using same data and similar methods, Bovy and Tremaine obtained the best estimate of the local dark matter density to date, ρ = 0.3 ± 0.1 GeV/cm^3; very close to what is usually assumed but with a smaller error. Moreover, the error may be further reduced in the near future when data from other astronomical surveys are analyzed.  This will eliminate one important source of error in interpreting  results of dark matter detection experiments, leading to more reliable constraints on particle physics models. So the second lesson here is that even wrong papers may be for the better...