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I am back from a business trip. I bought a special issue of Scientific American just to fight boredom in airports and planes, but I had some unexpected excitement. I showed this issue to my boss, and he borrowed it for a few days. I hope he will return it. Meanwhile, I am anxious to put something in LJ and I decided to search on Internet. This also gives me an advantage: no typing necessary.
Some interesting citations found:
Newton's gravitational theory was inconsistent with relativity; in order to resolve this problem, Einstein replaced Newtonian gravity with the theory of general relativity (GR). GR predicted that gravity could be generated not only by mass but also by pressure - and that negative pressure could produce repulsive gravity. Unlike negative mass, negative pressure is not forbidden by any theory of modern physics, so repulsive gravity became at least a theoretical possibility.
Something that is very elastic (that is, negative pressure) has gravity that repels, rather than attracts.
Quantum mechanics provides a candidate for something that is very elastic: The virtual pairs that fill the vacuum have negative pressure.
Einstein happily abandoned the cosmological constant when, in 1929, Edwin Hubble found the universe was not static but expanding. However, lambda came back strong — albeit 70 years later! — when supernova studies led to the discovery that expansion was accelerating.
"For the cosmological constant, the vacuum — space itself — possesses a certain springiness," says Eric Linder, a cosmologist at Berkeley Lab and director of the Center for Cosmology and Spacetime Physics at Florida Atlantic University. "As you stretch it, you don't lose energy, you store extra energy in it just like a rubber band."
Later, new measurements of cosmic microwave background (CMB) radiation provided strong evidence that the universe is flat (having an overall geometry of space like Euclid's, in which parallel lines never meet or diverge) — and because there is not enough matter in the universe, whether visible or dark, to produce flatness, the difference can be attributed to dark energy, providing a strong confirmation of the supernova measurements.
Some interesting citations found:
Newton's gravitational theory was inconsistent with relativity; in order to resolve this problem, Einstein replaced Newtonian gravity with the theory of general relativity (GR). GR predicted that gravity could be generated not only by mass but also by pressure - and that negative pressure could produce repulsive gravity. Unlike negative mass, negative pressure is not forbidden by any theory of modern physics, so repulsive gravity became at least a theoretical possibility.
Something that is very elastic (that is, negative pressure) has gravity that repels, rather than attracts.
Quantum mechanics provides a candidate for something that is very elastic: The virtual pairs that fill the vacuum have negative pressure.
Einstein happily abandoned the cosmological constant when, in 1929, Edwin Hubble found the universe was not static but expanding. However, lambda came back strong — albeit 70 years later! — when supernova studies led to the discovery that expansion was accelerating.
"For the cosmological constant, the vacuum — space itself — possesses a certain springiness," says Eric Linder, a cosmologist at Berkeley Lab and director of the Center for Cosmology and Spacetime Physics at Florida Atlantic University. "As you stretch it, you don't lose energy, you store extra energy in it just like a rubber band."
Later, new measurements of cosmic microwave background (CMB) radiation provided strong evidence that the universe is flat (having an overall geometry of space like Euclid's, in which parallel lines never meet or diverge) — and because there is not enough matter in the universe, whether visible or dark, to produce flatness, the difference can be attributed to dark energy, providing a strong confirmation of the supernova measurements.
