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When Perlmutter found an Ia supernova, he measured its brightness to determine its
distance from Earth. (The brighter it is, the closer it is.) Perlmutter also measured the red
shift of the supernova’s light. This is a technique based on Doppler shifts. If a star is moving
toward Earth, its light is compressed and its color shifts a little toward blue. If the star is
moving away, its light is stretched and the color shifts toward red. The faster the star is moving, the greater its color shift. By measuring the supernova’s red shift, Perlmutter could calculate the star’s velocity away from Earth.
Now came the hard part. Other factors could account for a red shift, and Perlmutter had
to prove that the red shifts he measured were the result of only the star’s motion away from
Earth. Space dust can absorb some light and shift its color. Some galaxies have an overall
color hue that could distort the color of the light coming from a supernova. Each of a dozen
possible sources of error had to be explored, tested, and eliminated.
Finally, in early 1998, Perlmutter had collected reliable distance and velocity data for a
dozen Ia supernovae spread across the heavens. All were moving at tremendous speeds
away from Earth.
Perlmutter used mathematical models to show that these galaxies couldn’t have been
traveling at their current speeds ever since the Big Bang. If they had, they would be much
farther away than they really are. The only way Perlmutter’s data could be correct was for
these galaxies to now be traveling outward faster than they had in the past.
The galaxies were speeding up, not slowing down. The universe had to be expanding
at an accelerating rate!
Perlmutter’s discovery showed that some new and unknown force (named “dark energy” by Michael Turner in 2000) must be pushing matter (stars, galaxies, etc.) outward.
More recent research using new specially designed satellites has shown that the universe is
filled with this “dark energy.” (Some estimates say that two-thirds of all energy in the universe is dark energy.) Over the next few years this new discovery will rewrite human theories of the origin and structure of the uni ????)?????????????????????????????????????????????????M????A???)??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????Q???????????????????????????????????????()5??????????)?????I?????M?????????%?????????????????U?????????????M?????????Q????????)????????????????U??????????????????
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