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A. G. Riess (STScI)
Type Ia supernovae (SNe~Ia) provide the only direct evidence for an accelerating universe, an extraordinary result that needs a rigorous test. The case for cosmic acceleration rests on the observation that SNe~Ia at z=0.5 are 0.25 mag fainter than they would be in a universe without acceleration. A powerful and straightforward way to assess the reliability of the SN Ia measurement and the conceptual framework of its interpretation is to look for cosmic deceleration at z>1. This would be a clear signature of a mixed dark-matter and dark-energy universe. Systematic errors in the SN Ia result attributed to grey dust or the simplest evolution of calibrated SN Ia luminosities would not show this change of sign (without additional tuning). A single SN Ia at z~1.7, found and followed by HST, demonstrated proof of this concept. We review current progress in the search for the epoch of decelerated expansion including new SNe Ia at z>1 from the HST GOODs Treasury program and other serendipitous sources.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.