4.3. Future of SZE/X-ray Distances

The prospects for improving both the statistical and systematic uncertainties in the SZE distances in the near future are promising. Note, from Equation 6, that the error budget in the distance determination is sensitive to the absolute calibration of the X-ray and SZE observations. Currently, the best absolute calibration of SZE observations is ~ 2.5% at 68% confidence, based on observations of the brightness of the planets Mars and Jupiter. Efforts are now underway to reduce this uncertainty to the 1% level (2% in H₀). Uncertainty in the X-ray intensity scale also adds another shared systematic. The accuracy of the ROSAT X-ray intensity scale is debated, but a reasonable estimate is believed to be ~ 10%. It is hoped that the calibration of the Chandra and XMM-Newton X-ray telescopes will greatly reduce this uncertainty.

Possible sources of systematic uncertainty are summarized in Table 1. These values come from 30 GHz interferometric SZE observations and ROSAT data for a sample of 18 galaxy clusters (Reese et al., 2002), but are typical of most SZE distance determinations. The largest systematic uncertainties are due to departures from isothermality, the possibility of clumping, and possible point source contamination of the SZE observations (for detailed discussion of systematics, see, e.g., Reese et al., 2002; Birkinshaw, 1999; Reese et al., 2000). Chandra and XMM-Newton are already providing temperature profiles of galaxy clusters (e.g., Tamura et al., 2001; Markevitch et al., 2000; Nevalainen et al., 2000). The unprecedented angular resolution of Chandra will provide insight into possible small-scale structures in clusters. In addition, multiwavelength studies by existing radio observatories, for example the Very Large Array, can shed light on the residual point source contamination of the radio wavelength SZE measurements. Therefore, though currently at the 30% level, many of the systematics can and will be addressed through both existing X-ray and radio observatories and larger samples of galaxy clusters provided from SZE surveys.

The beauty of the SZE and X-ray technique for measuring distances is that it is completely independent of other techniques, and that it can be used to measure distances at high redshifts directly. Since the method relies on the well-understood physics of highly ionized plasmas, it should be largely independent of cluster evolution. Inspection of Figure 6 already provides confidence that a large survey of SZE distances consisting of perhaps a few hundred clusters with redshifts extending to one and beyond would allow the technique to be used to trace the expansion history of the Universe, providing a valuable independent check of the recent determinations of the geometry of the Universe from Type Ia supernovae (Perlmutter et al., 1999; Riess et al., 1998) and CMB primary anisotropy experiments (Netterfield et al., 2002; Spergel et al., 2003; Stompor et al., 2001; Pryke et al., 2002).