Distant type Ia supernovae (SNe Ia) appear fainter than their local counterparts. Independent of what explanation will eventually be found to be correct, this implies a significant change in how we see the distant universe and what we understand of these stellar explosions. The observational characteristics of nearby SNe Ia show some differences from event to event. Despite their considerable range in observed peak luminosity, they can be normalized by their light-curve shape. Through this normalization, SNe Ia can be used as exquisite distance indicators. The Hubble diagram of nearby SNe Ia, demonstrating the linear cosmic expansion at small scales, is the simplest observational proof for the standard character of these objects. Compared with Friedmann models of the universe, the distant SNe are too faint even for a freely coasting, “empty” universe, barring other influences that could dim the events. This result is independent of the absolute calibration of the peak luminosity, which is needed to derive the Hubble constant. Possible noncosmological explanations could be gray dust, with properties that do not change the color of the objects significantly, evolution of the explosions, or deamplification by gravitational lensing. Current indications are that none of these alternatives alone can explain the dimness of the distant SNe. The intrinsic colors of the distant SNe Ia are typically bluer when compared with the local sample. This in itself makes the dust hypothesis less likely. On the other hand, it could be a signature of evolutionary trends that could influence the peak luminosity. This trend is contrary to the observations in the local sample, where bluer objects typically are more luminous. However, current lack of understanding of the explosion physics and the radiation transport of SNe Ia encumbers any investigation of evolutionary changes. Any change in the peak luminosity of SNe Ia must be inferred from indirect observations, such as light-curve shape, colors, and spectral evolution. At the moment, many of the distant SNe do not have the required data set for a detailed investigation of these parameters. The near-uniform light-curve and spectral evolution of SNe Ia can be used as accurate cosmic clocks to demonstrate the time dilation as predicted from expanding world models. The test has been performed through both photometry and spectroscopy, and is fully consistent with the predictions. The supernova (SN) results can be reconciled only with cosmological models that provide some form of acceleration. The simplest such models either include the cosmological constant or refer to a decaying particle field (“quintessence”). Combined with recent measurements of the cosmic microwave background that indicate a flat geometry of the universe, and low-matter density, as derived from bulk flows and the evolution of galaxy clusters, the SNe define a fairly narrow likelihood region for Ω and Ω. With these new values for the cosmological parameters, the long-standing problem of the dynamical age of the universe appears to be solved. On the other hand, the size of the acceleration, if interpreted as a cosmological constant, is in clear contradiction to predictions from particle theories. In addition, we live in a very privileged period when matter density and the cosmological constant are equal contributors to the cosmic expansion.


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  • Article Type: Review Article
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