Section 8: The Search Continues
Dark matter remains an active area of research, with the results of current and planned experiments eagerly anticipated by the entire physics community. In coming years, large-scale studies of galaxies like the continuing Sloan Digital Sky Survey and the Anglo-Australian 2dF Galaxy Redshift Survey, supported by numerical simulations, will continue to develop our picture of the way in which dark matter is distributed over galactic and larger distances. Better cosmological measurements of supernovae and the cosmic microwave background will sharpen our knowledge of cosmological parameters, in particular the total amount of normal and dark matter. Detailed measurements of galaxies using gravitational lensing will tell us more about the distribution of dark matter within a galaxy. New space probes, nuclear recoil, and axion experiments will continue to hunt for evidence that dark matter interacts with normal matter in ways other than gravity. In addition, colliding beam accelerators, particularly the LHC, will try to make dark matter particles in the laboratory.
Figure 20: Galactic studies such as the "2dF Galaxy Redshift Survey" will improve our understanding of the distribution of dark and normal matter.
Source: © The 2dF Galaxy Redshift Team, 30 June 2003. Reproduced by permission of Commonwealth of Australia. More info
If some or all of the dark matter consists of WIMPs, the final picture will not emerge from any single endeavor. Rather, physicists will combine evidence produced by many different measurements to understand just what these new particles are. Even though the sensitivity of searches for dark matter on Earth has improved by about a factor of ten every few years over the past two decades, it might still take some time before the first convincing laboratory evidence for dark matter appears. Following first indications, further measurements using different targets will sharpen the picture. But conclusive evidence will require a directional signal as well as consistency with cosmic ray experiments, astronomical observations, and exploration of the Terascale from collider experiments.
What if dark matter consists of axions? In that case, ADMX may make an observation in the next 10 years—if dark matter conforms to theory.
Of course, dark matter may be something completely new and unexpected. It may be a different manifestation of dark energy or it may be that we never find out. Dark matter raises the question of what it means to discover something. We already know what dark matter does: how it regulates the structure of galaxies and clusters of galaxies. This knowledge will certainly improve steadily as we make more astronomical observations. Learning what dark matter actually is, however, will take a big jump—one that we may never make. What does it mean for science if we find that we can't make this jump? Most likely, we will never have to answer that question. Physicists will continue to probe the universe in expectation of eventually unearthing its deepest secrets.