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NOVA follows cosmologists trying to understand why the universe may be expanding at an accelerating rate.
- reviews data from two teams of astronomers that seem to indicate some unknown force that is causing the universe to expand faster and faster.
- explains that the astronomers are basing their theories on observations of type 1A supernovae, which are believed to be uniformly bright and thus can be used as reference points or "mileage markers" to measure the expansion of the universe.
- reviews the technical and logistical challenges involved in finding the right kind of supernova for further study.
- shows how astronomers painstakingly compare recent and months-old images of thousands of galaxies looking for minute changes in brightness that signal a supernova.
- notes that preliminary data indicate that the universe is accelerating in its expansion, not slowing down due to gravity as previously believed.
Main article: Ultimate fate of the universe
Depending on the average density of matter and energy in the universe, it will either keep on expanding forever or it will be gravitationally slowed down and will eventually collapse back on itself in a "Big Crunch". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for eternity (see accelerating universe). Other ideas of the fate of our universe include the Big Rip, the Big Freeze, and Heat death of the universe theory. For a more detailed discussion of other theories, see the ultimate fate of the universe.
The main attraction of the cosmological constant term is that it significantly improves the agreement between theory and observation. The most spectacular example of this is the recent effort to measure how much the expansion of the universe has changed in the last few billion years. Generically, the gravitational pull exerted by the matter in the universe slows the expansion imparted by the Big Bang. Very recently it has become practical for astronomers to observe very bright rare stars called supernova in an effort to measure how much the universal expansion has slowed over the last few billion years. Surprisingly, the results of these observations indicate that the universal expansion is speeding up, or accelerating! While these results should be considered preliminary, they raise the possibility that the universe contains a bizarre form of matter or energy that is, in effect, gravitationally repulsive. The cosmological constant is an example of this type of energy. Much work remains to elucidate this mystery!