Astrophysics

Explosions Near and Far

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Science  13 Mar 2009:
Vol. 323, Issue 5920, pp. 1406
DOI: 10.1126/science.323.5920.1406a

Our universe appears to be undergoing a period of accelerated expansion driven by dark energy. This phenomenon was brought to the fore in 1998, when distant type Ia supernovae—bright transient events that mark the explosion of an accreting white dwarf star—were observed to be dimmer than they would be in the absence of acceleration. The conclusion rests on the assumption, actively debated over the past decade, that the properties of type Ia supernovae are independent of their distance from the observation point; the dimming in that context may be attributed reliably to cosmic acceleration and not to systematic differences characterizing distant supernovae. Sullivan et al. contribute to this discussion by compiling spectra of type Ia supernovae spanning the past 9 gigayears. They constructed and compared the mean ultraviolet spectra, which are sensitive to the composition of the progenitor stars and the physics of the explosion, for supernovae at short, intermediate, and long distances. Although the strength of some spectral features varies with distance, the variation can be explained by a relative increase in explosions of younger stars in the distant, younger universe, a trend that has been seen in previous studies and for which it should be possible to correct. Thus, type Ia supernovae may be useful as tools to study the expansion history of the universe. — MJC

Astrophys. J. 693, L76 (2009).

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