When two teams of sci­en­tists announced in 1998 that the expan­sion of the Uni­verse was not slow­ing down due to grav­i­ty but was in fact accel­er­at­ing, the world­wide sci­en­tif­ic com­mu­ni­ty was shocked. The dis­cov­ery turned many of the pre­vail­ing assump­tions about the uni­verse upside down. Look­ing back, per­haps the only thing that was­n’t a sur­prise was that the Nobel Prize Com­mit­tee should take notice.

Last Tues­day the Swedish Acad­e­my of Sci­ences announced that the 2011 Nobel Prize in Physics would go to three Amer­i­can-born sci­en­tists from two rival teams: physi­cist Saul Per­mut­ter, head of the Super­no­va Cos­mol­o­gy Project at Lawrence Berke­ley Nation­al Lab­o­ra­to­ry and the Uni­ver­si­ty of Cal­i­for­nia, Berke­ley, would receive half of the prize, while Bri­an P. Schmidt, head of the High‑z Super­no­va Search Team and an astronomer at the Research School of Astron­o­my and Astro­physics at the Aus­tralian Nation­al Uni­ver­si­ty, West­on Creek, would share the oth­er half with a col­league who wrote the orig­i­nal paper announc­ing the team’s find­ings in 1998, astronomer Adam G. Riess of Johns Hop­kins Uni­ver­si­ty and the Space Tele­scope Sci­ence Insti­tute.

Despite pop­u­lar belief, the two teams did not “dis­cov­er” dark ener­gy. As Perl­mut­ter points out in the short film above, “Peo­ple are using the term ‘dark ener­gy’ basi­cal­ly as a place hold­er to describe any expla­na­tion for why it is that we seem to be see­ing the uni­verse’s expan­sion get­ting faster and faster.” What is actu­al­ly known is that the uni­verse has been expand­ing for as far back as we can observe, and about 7 bil­lion years ago–roughly half the esti­mat­ed age of the universe–the expan­sion began to accel­er­ate.

“Why is it speed­ing up?” Perl­mut­ter asked dur­ing a press con­fer­ence on the morn­ing his Nobel Prize was announced. “It could be that most of the uni­verse is dom­i­nat­ed by a dark ener­gy that per­vades all of space and is caus­ing this accel­er­a­tion. It could be, per­haps even more sur­pris­ing, that Ein­stein’s The­o­ry of Gen­er­al Rel­a­tiv­i­ty needs a lit­tle bit of a tweak, per­haps act­ing slight­ly dif­fer­ent­ly on these very large scales of the uni­verse. But at this moment I would say that the ques­tion is wide open.”

The 11-minute doc­u­men­tary above, pro­duced in 2008 by KQED in San Fran­cis­co, gives a good overview of how Perl­mut­ter and his rivals mea­sured the red-shift and bright­ness of light from Type 1a super­novae to plot the uni­verse’s rate of expan­sion across bil­lions of years. For an in-depth his­to­ry of the project, you can read this three-part arti­cle from the Berke­ley Lab. Or, if you only have a minute (1:39 to be exact) you can watch this “Minute Physics” episode nar­rat­ed by Cal­tech physi­cist Sean Car­roll.

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