With the next generation of ground-based arrays of telescopes now coming on-line, it is expected that the number of extragalactic sources detected above 100 GeV will increase tenfold. Spectral measurements will permit detailed models to be confronted with observations. It may be possible to measure the spectral cut-offs between 10 GeV and 100 GeV and to distinguish between those that are intrinsic to the source and those that are due to the extragalactic infra-red background.
The most exciting aspect of the recent results is the diversity of objects that are now proving to be VHE gamma-ray sources. Hopefully each of these detections is only the tip of the iceberg of each class of source (Figure 2) and, as the flux sensitivity improves, the other members of the class will be detectable. It is noteworthy that although the number of sources in the TeV Catalog (Table 1) is still small, the diversity of objects is large and already exceeds that of the MeV-GeV catalogs.
Figure 2. Tips of the Icebergs in the TeV Universe.