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Jun. 25th, 2014
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Jun. 25th, 2014 10:27 pm„Both dark matter and dark energy had had a long and controversial history. The simplest candidate for dark energy was the so-called cosmological constant, the above-mentioned fudge factor that Einstein added to his gravity theory and later called his greatest blunder. Fritz Zwicky postulated dark matter in 1934 to explain the extra gravitational pull that kept galaxy clusters from flying apart, and Vera Rubin discovered in the 1960s that spiral galaxies rotated so fast that they, too, would fly apart unless they contained invisible mass gravitating enough to hold them together. These ideas met with considerable skepticism: if we’re willing to blame unexplained phenomena on entities that are both invisible and can pass through walls, shouldn’t we also start believing in ghosts while we’re at it? Moreover, there was a disturbing precedent: in ancient Greece, when Ptolemy realized that planetary orbits weren’t perfect circles, he cooked up a complicated theory in which they moved on smaller circles (called epicycles) that in turn moved in circles. As we saw earlier, the subsequent discovery of a more accurate law of gravity killed the epicycles, predicting that the orbits aren’t circles but ellipses. Perhaps the need for dark matter and dark energy could be eliminated just as the epicycles were, by discovering a still more accurate law of gravity? Could modern cosmology really be taken seriously?
These were the sorts of questions we asked while I was a grad student. Answering them would require much more accurate measurements, to transform cosmology from the data-starved and speculative field that it was into a precision science. Fortunately, that’s exactly what happened.
From these measurements, they made the most accurate reconstruction to date of how fast our Universe has expanded at various times in the past, and in 1998, they announced a startling discovery that earned them the 2011 Nobel Prize in physics: after spending its first 7 billion years slowing down, the cosmic expansion started speeding up again and has accelerated ever since! If you throw a rock up in the air, Earth’s gravitational pull will decelerate its motion away from Earth, so the cosmic acceleration revealed a strange gravitational force that’s repulsive rather than attractive. As I’ll explain in the next chapter, Einstein’s gravity theory predicts that dark energy has exactly this antigravity effect, and the supernova teams found that a cosmic-matter budget with 70% dark energy beautifully explained what they saw.“
These were the sorts of questions we asked while I was a grad student. Answering them would require much more accurate measurements, to transform cosmology from the data-starved and speculative field that it was into a precision science. Fortunately, that’s exactly what happened.
From these measurements, they made the most accurate reconstruction to date of how fast our Universe has expanded at various times in the past, and in 1998, they announced a startling discovery that earned them the 2011 Nobel Prize in physics: after spending its first 7 billion years slowing down, the cosmic expansion started speeding up again and has accelerated ever since! If you throw a rock up in the air, Earth’s gravitational pull will decelerate its motion away from Earth, so the cosmic acceleration revealed a strange gravitational force that’s repulsive rather than attractive. As I’ll explain in the next chapter, Einstein’s gravity theory predicts that dark energy has exactly this antigravity effect, and the supernova teams found that a cosmic-matter budget with 70% dark energy beautifully explained what they saw.“