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A Dearth of Dark Matter

April 23, 2012

One of the astronomical mysteries of the last century was the missing mass of the Universe. In the 1930s, through measurements of the orbital velocities of stars about the galactic center, it was discovered that our Milky Way Galaxy contained more mass than the sum of its stars. Further evidence came from the orbital velocities of galaxies in clusters, the rotation rate of galaxies; and, more directly, by the gravitational lensing of background objects by galactic clusters.

This nagging problem of missing mass went on for decades, and it occupied the research programs of quite a few astronomers. Many years ago, the American Physical Society had a Physics Limerick Contest, and I submitted this entry summarizing the missing mass problem.[1]
The accountant was ranting and hissing!
Such an audit was not of our wishing!
But such was our state,
An astronomer's fate,
Since some of our mass was found missing!

Our missing mass now has a name, dark matter, but we still have no idea what it is. Dark matter can't be observed with telescopes, since it neither emits nor absorbs electromagnetic radiation, so its presence is still inferred indirectly. Results from the Wilkinson Microwave Anisotropy Probe show that 83% of the matter in our present universe is dark matter, and this proportion hasn't changed from 13.7 billion years ago (see figure).

Composition of the UniverseThe more we know, the more we're in the dark.

Not only do we have dark matter, there's dark energy, the energy that causes the universal expansion to accelerate.

(NASA Illustration / WMAP Science Team).[2]

So, dark matter is pervasive, right? Pervasive, perhaps, but maybe not uniformly distributed. A recent study by astronomers at the European Southern Observatory shows that dark matter is remarkably absent from the region of space around our solar system.[3-4]

C. Moni Bidin (Departamento de Astronomía, Universidad de Concepción, Chile), G. Carraro (European Southern Observatory, Santiago, Chile), R. A. Méndez (Departamento de Astronomía, Universidad de Chile, Santiago, Chile) and R. Smith (Departamento de Astronomía, Universidad de Concepción, Chile) made this discovery by a precise measurement of the motion of stars distant from the plane of the galaxy.

These observations, which covered a large volume around the Sun, as shown in the figure, show no evidence for dark matter. The team used several different telescopes to record the motions of more than 400 red giant stars in a volume out to 13,000 light years. This work extended such measurements to four times a larger volume than previously observed.[3] One important aspect of this discovery is that attempts by particle physicists to detect dark matter here, on Earth, would be fruitless.

Figure captionThe volume of the ESO survey was quite large, even by galactic standards.

(Based on an ESO image by L. Calçada).

Said team leader, Christian Moni Bidin,
"The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun... But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there! ...The mystery of dark matter has just become even more mysterious."[3]

The bottom line of the study is this - Theory predicts that there should be 400-1000 grams of dark matter in the Sun's vicinity in a volume the size of the Earth, whereas the observations indicate no dark matter within an error of 70 grams.[3]

A paper on this work, "Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood," will appear in The Astrophysical Journal. A preprint of the paper appears on the arXiv web site.[4]

References:

  1. Physics Limericks-Devlin Gualtieri, American Physical Society Web Site. A local copy can be found here.
  2. WMAP- Content of the Universe, NASA Web Site.
  3. Serious Blow to Dark Matter Theories? New study finds mysterious lack of dark matter in Sun's neighbourhood, European Southern Observatory Press Release eso1217, April 18, 2012.
  4. C. Moni Bidin, G. Carraro, R. A. Mendez, R. Smith, "Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood," arXiv Preprint Server, April 17, 2012.