In 1917, a year after his general theory of relativity was published, Einstein tried to extend his field equation of gravitation to the universe as a whole. The universe as known at the time was simply our galaxy—the neighboring Andromeda, visible to the naked eye from very dark locations, was thought to be a nebula within our own Milky Way home. Einstein's equation told him that the universe was expanding, but astronomers assured him otherwise (even today, no expansion is evident within the 2-million-light-year range to Andromeda; in fact, that galaxy is moving toward us). So Einstein inserted into his equation a constant now known as "lambda," for the Greek letter that denoted it. Lambda, also called "the cosmological constant," supplied a kind of force to hold the universe from expanding and keep it stable within its range. Then in 1929, Hubble, Humason, and Slipher made their monumental discovery using the 100-inch Mount Wilson telescope in California of very distant galaxies and the fact that they were receding from us—implying that the universe was indeed expanding, just as Einstein's original equation had indicated! When Einstein visited California some time later, Hubble showed him his findings and Einstein famously exclaimed "Then away with the cosmological constant!" and never mentioned it again, considering lambda his greatest "blunder"—it had, after all, prevented him from theoretically predicting the expansion of the universe. Fast forward six decades to the 1990s. Saul Perlmutter, a young astrophysicist at the Lawrence Berkeley Laboratory in California had a brilliant idea. He knew that Hubble's results were derived using the Doppler shift in light. Light from a galaxy that is receding from us is shifted to the red end of the visible spectrum, while a galaxy that is approaching us has its light shifted to the blue end of the spectrum, from our vantage point. The degree of the shift is measured by a quantity astronomers call Z, which is then used to determines a galaxy's speed of recession away from us (when Z is positive and shift is to the red). But Perlmutter knew much more than that. As an astrophysicist he had studied the light curves (the way the intensity of a light source changes through time) that characterize immensely powerful celestial explosions called a Type Ia supernova. This kind of explosion is so powerful---six times more so than the more common Type II supernova, such as the one that created the Crab Nebula--that its light can be as intense as that of an entire galaxy. This allowed him to detect such mammoth, yet rare explosions in very faraway galaxies. Using telescopes in Hawaii, Chile, the Canary Islands, and space, his research team took pictures of hundreds of distant galaxies at a time, repeating the process at intervals of three weeks. In an entire galaxy, a Type Ia supernova will occur only roughly once a century---but once such an explosion is captured, it yields extremely important information. Since the light curve of such a supernova is the same, regardless of where it takes place, the intensity of the light from the explosion can be used as a "standard candle" for measuring the distance to the galaxy in which it takes place (in the same way that the size of the flame of a candle could be used to estimate how far a candle is from the observer since all candle flames are essentially of the same size). Thus Perlmutter's team, the Supernova Cosmology Project at Berkeley, was able to establish for each galaxy in which they were fortunate to observe a Type Ia supernova, both a distance estimate (through analysis of the light curve), and a speed of recession (from the redshift, Z). An analysis of the data revealed a stunningly unexpected result: the universe is accelerating its expansion! The reason no one had expected such a finding was that the widely held assumption in cosmology had been that the mutual gravitational attraction among galaxies would eventually win out against the expansion, slow it down to a stop, and lead to a re-collapse of the universe on itself (and then perhaps a new big bang, a rebirth)---similarly to how a stone thrown up in the air will slow down, stop, and fall back to Earth. The study's result led to a major rethinking in cosmology, and it was then that physicists rediscovered Einstein's lambda. In the same way that the cosmological constant had originally been used to hold down a universe that wants to expand, the same mathematical device, lambda---acting in the opposite way---can now be used to accelerate the universal expansion. The cosmological constant, Einstein's "blunder," was back with a vengeance! The energy that is believed to cause the accelerated expansion is called "dark energy," and also "quintessence." Dark energy is now believed to comprise as much as 73% of the entire mass-energy of the universe. What it actually is, nobody knows; mathematically, its action is performed by Einstein's old lambda term. "Imagine a lattice in three dimensions," Saul Perlmutter told me when I interviewed him about his groundbreaking work completed in 1998, "At each corner of the lattice there is a galaxy. Now imagine that the lattice itself is growing in size---the distances from our corner, our galaxy, to all other corners of the lattice keep increasing." These distances increase at a rate that is increasing all the time. Eventually, therefore, the universe will likely become very large and very diffuse---something that no one had expected. There will probably never be a recollapse and a rebirth. Our present universe seems to be a one-time event. Saul Perlmutter shared this year's Nobel Prize in physics for his discovery with Brian P. Schmidt of the Australian National University and Adam G. Riess of Johns Hopkins University and the Space Telescope Science Institute, who headed a competing research team, the High-Z Supernova Search, which obtained similar results at the same time. Their research changed the way we view the universe.
Amir D. Aczel is a researcher at the Center for the Philosophy and History of Science at Boston University and the author of 18 books about mathematics and physics, as well as numerous research articles. He is a Guggenheim Fellow and a frequent commentator on science in the media. See more at his website or follow him on Twitter: @adaczel
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