Mind Over Dark Matter

Conference Report

Mario Livio, mlivio@stsci.edu

bannerAs its name implies, dark matter does not emit any light and therefore it cannot be “seen” in the common sense of the word. Yet, this invisible stuff constitutes most—more than 80%—of the universe’s mass. No wonder, then, that astronomers and particle physicists worldwide have been trying for decades to detect the presence of the elusive particles of dark matter, from laboratories underground and from both ground- and space-based observatories. More than 130 participants attended the Institute’s 2011 May Symposium, which provided a wonderful opportunity to review the latest results of the searches for dark matter.

Astronomers have known since the 1970s that the combined mass of stars, gas, and dust in spiral galaxies is insufficient to gravitationally account for the way stars revolve around the galactic centers. Much more mass is needed for the outer parts of galaxies not to be flung out. The conundrum is further exacerbated in clusters of galaxies, where it has been clear since the 1930s that ordinary, visible matter falls far short of being able to hold clusters together. To account for the necessary gravitational pull, astronomers hypothesize that most of the mass in galaxies and clusters of galaxies is invisible. Most theorists believe that the constituents of dark matter are massive particles that barely interact with ordinary matter. They assume that these particles interact only gravitationally and through the weak nuclear interaction. Consequently, the particles are generally known as Weakly Interacting Massive Particles, or WIMPS. Some theories even suggest the existence of super-WIMPS, which interact only gravitationally. The dark-matter hypothesis has received strong support from observations of gravitational lensing and of structure formation.

General relativity predicts that the gravitational fields of mass concentrations deflect light. As a result, when a cluster of galaxies lies along the line of sight between the Earth and a distant galaxy, the cluster acts as a lens, magnifying and distorting the image of the galaxy. By precisely measuring the distortion geometry, the mass of the cluster and the distribution of the mass within it can be determined. Similarly, one can statistically study the tiny distortions observed in extensive galaxy surveys. The collective gravitational effects of foreground objects introduce these distortions. The latter technique is known as “weak lensing.” About half a dozen of the talks at the symposium demonstrated the use of the various lensing measurements to reconstruct the detailed, three-dimensional distribution of dark matter. In particular, combined optical (by Hubble) and X-ray (by Chandra) observations of the system known as the “bullet cluster,” in which two clusters of galaxies are colliding, appear to show that the baryonic hot gas in these clusters has been separated from the dark matter. This is to be expected, since while the baryonic gas collides and shocks, the dark matter is essentially non-interacting.

In the current cosmological paradigm, dark matter is essential for the formation of the large-scale structure of the universe. Dark matter is supposed to have acted as the scaffolding, creating the initial potential wells into which the ordinary (baryonic) matter later flowed, forming structure hierarchically, from small to large scales. Several talks at the symposium discussed the agreement (or not) between this scenario and detailed observations of the cosmic microwave background, of the halos of galaxies, of baryon acoustic oscillations, of satellites of large galaxies, and also of the abundances of light elements. Generally, large computer simulations (employing billions of dark matter particles) produce predictions of the so-called Lambda-Cold-Dark-Matter (ΛCDM) model that agree with observations. Nevertheless, some difficulties persist. In particular, the simulations produce more satellite galaxies than are typically observed. Also, the simulations predict more “cuspy” halos—that is, halos with density more sharply rising toward the center than observations seem to indicate.

A few theorists have examined the possibility that dark matter does not exist at all, but rather we have to change our theory of gravity. Along these lines, theories such as the Tensor-Vector-Scalar theory (TeVeS), and the Bimetric Modified Newtonian Dynamics (Bimetric MOND) have been proposed. Constantinos Skordis of the University of Nottingham reviewed these theories extensively at the symposium, and showed that all encounter significant difficulties.


More heat than light?

As intriguing as some of the new astronomical results on dark matter have been, there is no question that the real drama is now on the particle physics side. An Italian underground experiment called DAMA (for DArk MAtter) has been claiming detection of a dark matter signal for a few years. This experiment is based on the idea that, as the Earth revolves around the Sun, it may be moving through a halo of WIMPS, and one might expect to observe an annual modulation due to changes in the direction of the Earth’s motion. Pierluigi Belli from the DAMA collaboration presented their latest results, which show—at a very significant level (9σ)—a periodic modulation with a rise in June and a decline in December. The question remains, however, of whether this truly represents a WIMPS signal. This question becomes particularly acute in view of the presentation by Elena Aprile, who heads the XENON 100 experiment. She reported that her team has not detected any WIMPS signal. Aprile claimed that XENON 100 should have seen the DAMA signal, had it been there—and had it been due to WIMPS. The Cryogenic Dark Matter Search (CDMS), which runs in the Soudan mine in Minnesota, also presented results. This experiment also fails to see any WIMPS. Most independent researchers have seen the negative results of XENON 100 and CDMS as two strikes against the DAMA claims. This was the situation until the last day of the symposium, when Juan Collar from the Coherent Germanium Neutrino Technology (CoGeNT) experiment surprised everyone at the meeting.

Collar presented preliminary results that (at the 2.4σ level) appear to corroborate the DAMA signal! In particular, the results show a tentative modulation that agrees in phase with the DAMA curve. While he did emphasize that the results are preliminary, Collar suggested that the failure of XENON 100 and CDMS to detect the WIMPS results from an insufficient understanding of the uncertainties and poor sensitivity to low-energy events. Collar emphasized that he originally thought that the CoGeNT results would refute the DAMA claims, but instead they ended up tentatively supporting them.

It will probably be at least a few years before we will know the final answer about the nature of dark matter. Nevertheless, one could hardly have hoped for a more exciting, cutting-edge symposium on a hot scientific topic. The race to shed light on dark matter may be entering the home stretch.