By holes in the microwave sky, we hope to invoke an image of the Sunyaev--Zel'dovich (SZ) effect at low frequencies: a region of lower than average sky brightness, or a hole in the cosmic microwave background (CMB). The image presented by B. Partridge at these proceedings is an excellent example and, along with a similar detection by the Ryle Telescope, is the motivation for this presentation; because if the two radio decrements are indeed due to SZ effect, this can have powerful implications for the value of .
In what has yielded the deepest radio map to date, the VLA discovered a radio decrement -- characteristic of the SZ effect below 1.4 mm -- during an observation of one of the HST Medium Deep Survey fields (Richards et al. (1996)). The object is just resolved, extending over an area of about . The other object (Jones et al. (1996)) was found by the RYLE Telescope (RT) during an ongoing program of double quasar observations (Saunders (1997)). They find a radio decrement covering an area of about . In both cases, subsequent follow-up in the optical and in the X-ray band has not revealed the supposed clusters (Richards et al. (1996), Jones et al. (1996), Saunders et al. (1997)). Definite confirmation of the SZ nature of the two decrements will thus come from efforts to measure the effect at different frequencies, to see if the spectra are consistent with the SZ effect. If they are indeed clusters, then the lack of optical or X-ray counterparts may be interpreted as evidence that they lie at large redshift. It is in this way that we may obtain very strong constraints on : The number of massive, high-redshift clusters depends sensitively on , so much so that the observation of even a small number of such clusters can eliminate the critical model (Oukbir & Blanchard (1992), Barbosa et al. (1996), Eke et al. (1996), Oukbir & Blanchard (1997)).