The 5th Symposium of Sources and Detection of Dark Matter and Dark Energy in the Universe, held in February at Marina del Rey, California, focused on the current state of detection and theoretical studies of dark matter particles. Meeting organizer David B Cline reports.
The universe around us is nothing like it looks. The stars make up less than 1% of the matter in the universe; while all the gas and other forms of baryonic matter account for less than 5%. We know little about the other 95% except that it is probably divided into 35% cold dark matter and about 60% dark energy.
Dark energy is detected by the recent acceleration of the universe and is observed by the study of type 1a supernova sources. A series of symposia have been organized in Southern California for the past 8 years to hear the latest in the developments in this field of particle cosmology. It was at the 1998 meeting that the two teams that have observed the accelerating universe first made a joint announcement of these important results.
The particle physics of dark matter is perhaps the most advanced in our understanding of these phenomena. Perhaps the best motivated and best understood form of particle dark matter comes from supersymmetry (SUSY).
This theory gives a “semi-natural” explanation of the amount of dark matter in the universe, which would take the form of weakly interacting massive particles (WIMPs) – the parameters are constrained by data from CERN’s LEP experiments and elsewhere. The strong interplay between proposed dark matter detectors and the direct observation of SUSY particles at CERN’s forthcoming Large Hadron Collider (LHC) reveals a strong connection between collider particle physics and astroparticle physics.
There was a complete discussion of the current search for SUSY dark matter and future detectors at the meeting. The DAMA experiment at Italy’s Gran Sasso underground laboratory continues to claim a signal for SUSY due to an observed annual variation. However, there are now three experiments – Edelweiss at Modane in France, ZEPLIN I at Boulby in the UK, and CDMS I at the Stanford Linear Accelerator Center in the US – that cut deeply into the region allowed by DAMA. These experiments all use some form of background discrimination.
A joint analysis of the CDMS I data at DAMA was claimed to exclude the DAMA signal from a WIMP source to 98% confidence level, even assuming all of the CDMS I events are not neutron-induced. The DAMA group disputes this claim, however. The DAMA experiment is being upgraded and hopefully this dispute will be resolved soon. The current predictions for the rate of SUSY WIMP detectors are nearly all well below the DAMA sensitivity, as was discussed extensively at the meeting.
Bigger machines
It was generally agreed that a new generation of much larger detectors will be needed to provide a clean detection of the SUSY WIMP signal. There are several discriminating detectors in the 10-30 kg mass range being constructed or upgraded such as CDMS II, Edelweiss and ZEPLIN II. To provide a clear study of the WIMP signal, detectors of the target mass of 1 tonne will be needed, and there are preliminary studies of possible detectors for this mass range. It is truly remarkable that detectors of this great sensitivity are being developed.
Dark energy
The issue of the origin of dark energy is more complex and possibly much more obscure. After the pioneering work of the two teams working on type 1a supernovae, there are projects for two impressive detectors that will to try to identify the equation of state of the dark energy.
The SNAP satellite would observe type 1a supernovae out to a redshift of around z = 1.5. The other possibility is to study type 1a supernovae from the ground using a large “dark matter” telescope in Chile called the Large Synoptic Survey Telescope (LSST). It may be that both of these methods will be needed to unravel the equation of state and demonstrate that the effect is either due to a cosmological constant or some other elementary particle-like source.
In one of the most interesting talks at the meeting, Paul Stenhardt of Princeton discussed the impact of an accelerating universe on the old question of whether the universe may be cyclic in time. It is possible that an accelerating universe could wipe out the entropy of the universe over a long time and then if the equation of state of the dark energy complies, the universe might contract to a “big crunch”. According to this viewpoint, the accelerating state of the universe is actually required rather than being a bizarre add-on to a Friedmann universe as currently held belief would prefer.
There was considerable discussion of the possibility of self-interacting, warm and hot dark matter (in light of recent claims for the observation of double-beta decay). None of these issues was clarified at the meeting.
During the course of the Southern California meetings, a much clearer picture of the bulk of components of the universe has emerged, but we have yet to find any evidence of what this stuff really is. Hopefully this will change as the new WIMP detectors underground and new detectors in space start taking data and the LHC is turned on. The next symposium will be held in February 2004 in Marina del Rey.
