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The Question

(Submitted February 10, 1999)

Is the Universe's expansion rate slowing down or speeding up? I have always been taught that it was slowing down, but I read something in the New York Times that it was found about a year ago that it was actually speeding up.

The Answer

Up until a year or two ago, it was thought that the Universe's expansion rate was decreasing, due to gravity pulling back on the material exploding form the Big Bang.

However, more recently, scientists have been able to measure how fast the Universe was expanding, and the data indicate that it is actually speeding up.

This measurement was made by looking at distant supernovae. If distant supernovae are different in brightness than nearby supernovae (e.g. if there is more dust dimming the light than we think) then the measurement could be wrong. However, most astronomers think that the measurements are strong.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

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