Cosmological implications from observations of type Ia supernovae

Distant type la supernovae (SNe la) appear fainter than their local counter parts. Independent of what explanation will eventually be found to be corre ct, this implies a significant change in how we see the distant universe an d what we understand of these stellar explosions. The observational charact eristics of nearby SNe Ia show some differences from event to event. Despit e their considerable range in observed peak luminosity, they can be normali zed by their light-curve shape. Through this normalization, SNe Ia can be u sed as exquisite distance indicators. The Hubble diagram of nearby SNe la, demonstrating the linear cosmic expansion at small scales, is the simplest observational proof for the standard character of these objects. Compared w ith Friedmann models of the universe, the distant SNe are too faint even fo r a freely coasting, "empty" universe, barring other influences that could dim the events. This result is independent of the absolute calibration of t he peak luminosity, which is needed to derive the Hubble constant. Possible noncosmological explanations could be gray dust, with properties that do n ot change the color of the objects significantly, evolution of the explosio ns, or deamplification by gravitational lensing. Current indications are th at none of these alternatives alone can explain the dimness of the distant SNe. The intrinsic colors of the distant SNe la are typically bluer when co mpared 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 obs ervations in the local sample, where bluer objects typically are more lumin ous. However, current lack of understanding of the explosion physics and th e radiation transport of SNe la encumbers any investigation of evolutionary changes. Any change in the peak luminosity of SNe la must be inferred from indirect observations, such as light-curve shape, colors, and spectral evo lution. At the moment, many of the distant SNe do not have the required dat a set for a detailed investigation of these parameters. The near-uniform li ght-curve and spectral evolution of SNe la can be used as accurate cosmic c locks to demonstrate the time dilation as predicted from expanding world mo dels. The test has been performed through both photometry and spectroscopy, and is fully consistent with the predictions. The supernova (SN) results c an be reconciled only with cosmological models that provide some form of ac celeration. The simplest such models either include the cosmological consta nt or refer to a decaying particle field ("quintessence"). Combined with re cent measurements of the cosmic microwave background that indicate a flat g eometry of the universe, and low-matter density, as derived from bulk flows and the evolution of galaxy clusters, the SNe define a fairly narrow likel ihood region for Omega (M) and Omega (Lambda). With these new values for th e cosmological parameters, the long-standing problem of the dynamical age o f the universe appears to be solved. On the other hand, the size of the acc eleration, if interpreted as a cosmological constant, is in clear contradic tion to predictions from particle theories. In addition, we live in a very privileged period when matter density and the cosmological constant are equ al contributors to the cosmic expansion.