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During the 1990s two competing groups, the Supernova Cosmology Project and the High-z Supernova Search Team, set out to measure "the deceleration parameter": the rate at which the universe's expansion is slowing down. After all, with hundreds of billions of galaxies exerting their gravitational influence on the universe, conventional wisdom clearly dictated that cosmic expansion should be decelerating.
Wrong! To the befuddlement of the scientific community, both teams reported in 1998 that they had found the exact opposite effect of what they had been trying to measure. The universe's expansion is actually speeding up!
Both groups reached this stunning conclusion by monitoring the explosions of white dwarfs, known as a Type Ia supernova. White dwarfs are the dead cores of stars similar to the Sun that have ceased burning nuclear fuel and have shed their outer layers. White dwarfs are roughly the size of Earth and contain a few tenths of a solar mass to 1.4 solar masses. According to calculations performed in 1930 by legendary astrophysicist Subrahmanyan Chandrasekhar, a white dwarf above 1.4 solar masses cannot support itself against the inexorable pull of gravity. Any white dwarf pushed over the 1.4-solar-mass limit, either by accreting material from a binary companion or by colliding with another white dwarf, will blow itself to smithereens as a Type Ia supernova.
Because all Type Ia supernovae share a common origin, they are relatively similar in luminosity and other properties. This fact, coupled with their extremely high luminosity, makes them ideal "standard candles" for measuring cosmic distances. Both teams observed Type Ia supernovae in galaxies at various distances from Earth, and found that faraway supernovae are fainter than expected. The most straightforward interpretation is that the universe was expanding more slowly in the past than it is now. The fact that two teams attained the same result using a reliable method gives scientists great confidence that the conclusion of an accelerating universe is fundamentally correct.
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| In 1572, the Danish astronomer Tycho Brahe observed and studied the explosion of a star that became known as Tycho's supernova. More than four centuries later, Chandra's image of the supernova remnant shows an expanding bubble of multimillion degree debris (green and red) inside a more rapidly moving shell of extremely high energy electrons (filamentary blue). [More...] |
Despite the surprising result, the discovery of the accelerating universe came as a relief to many cosmologists. When they had previously added up all the mass and energy known in the Universe, their inventory contained only about 30% of the mass-energy density that they had expected. But when they calculated the amount of energy needed to cause the observed acceleration, it accounted almost perfectly for the missing 70%. The shortfall had disappeared, and the cosmic accounting books appeared to be balanced.
Since 1998, completely independent methods have bolstered the case for dark energy. Observations of the cosmic microwave background have found subtle temperature fluctuations that match the predictions of a universe now dominated by dark energy. Results from the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey have shown that dark energy must be present to explain how galaxies cluster together on very large scales. X-ray observations of hot gas in galaxy clusters, and measurements of how galaxies are gravitationally lensed, also indicate the existence of dark energy.
Even if some scientists find dark energy conceptually distasteful, and even if we have no idea what dark energy actually is, the evidence is overwhelming that it's for real. Now it’s time to take up the challenge of determining what is causing cosmic acceleration. If it's not dark energy, it's probably something even more bizarre, such as alternative theories of gravity.
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