Ia supernova is a burned-out star that is in a binary orbit around another star,
from which it receives a flow of hydrogen gas that builds up on its surface.
After enough hydrogen has accumulated, it suddenly detonates in a
thermonuclear fusion explosion. The
detonating star shines with extraordinary brilliance, brighter than the rest of
the galaxy, for a period of up to a month and then fades away.
Such supernovas occur in all galaxies and can be observed (during their
period of brilliance) in our galactic neighbors and also in galaxies half way
across the universe. Since the
brightness of a supernova a distance r
away will be diminished by 1/r2,
a measurement of light intensity gives information about distance.
However, a plot of the red shifts of nearby supernovas against the
distances inferred from observed brightness shows considerable scatter around
average straight-line behavior. This
demonstrates that Type Ia supernovas are not
all of identical brightness, and therefore that supernova brightness cannot be
used directly as a distance indicator.
two groups, one led by Australian astronomer Brian P. Schmidt and the other by
Saul Perlmutter of the Lawrence Berkeley Laboratory, found a cure for this
problem. They tracked the
"light curve", the intensity vs. time of nearby Type Ia supernovas as
observed through blue and violet filters, and found significant differences in
falloff times of the light from one object to another, from falloff in about 10
days to over 30 days. They used
these light-curve differences to generate a correction that brought nearby
The groups then extended the plot to more distant supernovas, where the plot was expected to fall below straight line behavior because of the expansion rate of the universe was expected to slow due to the pull of gravity, modifying the red shift. But instead of the distant supernova points falling below the expected straight-line, the points were elevated above the straight line. Conclusion: the expansion rate of the universe is not decreasing with time, it is increasing .
Robert R. Caldwell, Marc Kamionkowski, and Nevin N. Weinberg, Physical Review Letters 91, 071301 (2003).
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