|Annu. Rev. Astron. Astrophys. 1994. 32:
Copyright © 1994 by . All rights reserved
10.3. Future Prospects
Some of the most exciting developments in this field have come from gravitational lensing studies and we can expect a proliferation of these efforts in the next few years. Macrolensing searches are placing ever more stringent limits of the number of high mass baryonic objects; microlensing searches may have already provided evidence for low mass ones. It is surely significant that microlensing evidence from both quasars and stars all point towards lens masses in the range 0.001-0.1 M. Admittedly, the masses indicated by the MACHO and EROS results are marginally too high; the most likely values are all in the M-dwarf range, whereas light constraints require the halo objects to be smaller than 0.08 M. Nevertheless, there is a fairly broad probability distribution for the lensing masses, depending on the assumed velocity and spatial distribution of the halo objects (Kerins 1994), so this puzzle may yet be resolved.
Although we have not reviewed here the plethora of theoretical papers that have appeared since the microlensing results were announced, it should be stressed that, even if the lensing results are genuine, they do not preclude WIMPs from providing some or even most of the dark halo. This is because the microlensing searches only probe the part of the halo at Galactocentric radii from 10-20 kpc, whereas the halo itself could extend much further than this There could therefore be plenty of WIMPs further out, especially if the dark baryons are preferentially concentrated as a result of dissipation. Even within the 10-20 kpc region, the number of MACHO and EROS events observed merely suggests that the fraction of halo mass in MACHOs must exceed 10% (Gates & Turner 1994). It is therefore important that WIMP searchers should not be too discouraged by the success of the MACHO searchers. It still seems a fair bet that the world needs both MACHOs and WIMPs.
Another source of exciting developments in this field has been COBE. We have seen that the FIRAS constraints on the microwave spectral distortions and the DIRRE measurements of the infrared background density already severely restrict any scenario in which the dark matter is in the relics of massive stars. This is especially true if the radiation has been reprocessed into the far-IR by dust. Indeed, the only hope for these scenarios may be that the radiation remains in the near-IR where it may be hidden by interplanetary dust emission. The FIRAS constraints on the Compton y-parameter may also exclude the supermasive accreting black hole scenario. The CORE DMR constraints on the microwave anisotropies (though not treated in detail here) are also highly pertinent to the baryonic dark matter scenarios. Some people claim that these already rule out baryon-dominated models, but this conclusion is sensitive to assumptions about the form of the initial density fluctuations, so this has not been stressed here.
Looking further to the future, two more developments will have an important impact. If the halo dark matter is in brown dwarfs, then the next generation of infrared space satellites will either detect these or push their mass down to below 0.001 M. In this case, we have stressed the importance of knowing whether the brown dwarfs are clustered because this determines whether one is seeking discrete or extended sources. If the halo dark matter is in black holes, then the next generation of gravitational wave detectors (either ground-based or space-based interferometers) will have an excellent chance of detecting the associated gravitational radiation - the period and amplitude of the waves will indicate the mass and formation redshift of the black holes. It seems likely that MACHOS will have been identified or excluded by the end of the millenium!
Much of my own work over the past decade has focused on the topic of this review, so I hope my own prejudices have not shown too glaringly. I would like to thank all my baryonic-dark-matter collaborators over this period for many enjoyable and stimulating interactions: Dave Arnett, Keith Ashman, Bruno Bertotti, Dick Bond, Wolfgang Glatzel, Craig Hogan, Satoru Ikeuchi, Eamonn Kerins, Cedric Lacey, Jonathan McDowell, Joel Primack, Martin Rees, Michael Rowan-Robinson, Mary Sakellariadou, Humitako Sato, Joe Silk, and Mike Turner. I would also like to thank Cathy Clarke, Andy Fabian, Gerry Gilmore. Mike Hawkins, George Lake, Ben Moore, Adi Nusser, Bernard Pagel, and Sasha Polnarev for helpful discussions. Finally, I would like to thank Eamonn Kerins for helping me in my literature search and Jonathan Gilbert for his assistance in producing the figures.