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The dark side of the universe
This article from The Economist is about the placement of a huge digital camera on a telescope in Chile to try to resolve some issues concerning the flatness of the universe. The article discusses the requirements on the size of the cosmological constant for a flat universe and how this relates to relativity, quantum theory, and the expansion of the universe. I found it interesting that in order to get the cosmological constant to work out to its required size of -1, extra spatial dimensions are being proposed to soak up excess vacuum energy. An excerpt:
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It has been known since the late 1920s that the universe is getting bigger. But it was thought that the expansion was slowing. When in 1998 two independent studies reached the opposite conclusion, cosmology was knocked head over heels. Since then, 5,000 papers have been written to try to explain (or explain away) this result. “That’s more than one a day,” marvels Saul Perlmutter, of the Lawrence Berkeley National Laboratory, who led the Supernova Cosmology Project—one of the studies that was responsible for dropping the bombshell. Last October that work earned Dr Perlmutter the Nobel prize for physics, which he shared with Brian Schmidt and Adam Riess, who led the other study, the High-Z Supernova Search.
Many of those 5,000 papers deal with something that has come to be known as dark energy. One reason for its popularity is that, at one fell swoop, it explains another big cosmological find of recent years. In the early 1990s studies of the cosmic microwave background (CMB), an all-pervading sea of microwaves which reveals what the universe looked like when it was just 380,000 years old, showed that the universe, then and now, was “flat”. However big a triangle you draw on it—the corners could be billions of light years apart—the angles in it would add up to 180°, just as they do in a school exercise book.
That might not surprise people whose geometrical endeavours have never gone beyond such books. But it surprised many physicists. At some scales space is not at all flat: the power of Albert Einstein’s theory of general relativity lies in its interpretation of gravity in terms of curved space. Cosmologists were quite prepared for it to be curved at the grandest of scales, and intrigued to discover that it was not.
...
Relativity says that for the universe to be flat, it has to have a very particular density—which in relativity is a measure not just of the mass contained in a certain volume, but also of the energy. The puzzle was that various lines of evidence showed that the universe’s endowment of ordinary matter (the stuff that people, planets and stars are made of) would give it just 4% of that density. Adding in extraordinary matter—“dark matter”, not made of atoms, that interacts with the rest of the universe almost only by means of gravity—gets at most an extra 22%. That left almost three-quarters of the critical density unaccounted for. Theorists such as Michael Turner, of the University of Chicago, became convinced that there was something big missing from their picture of the universe.
...
It has been known since the late 1920s that the universe is getting bigger. But it was thought that the expansion was slowing. When in 1998 two independent studies reached the opposite conclusion, cosmology was knocked head over heels. Since then, 5,000 papers have been written to try to explain (or explain away) this result. “That’s more than one a day,” marvels Saul Perlmutter, of the Lawrence Berkeley National Laboratory, who led the Supernova Cosmology Project—one of the studies that was responsible for dropping the bombshell. Last October that work earned Dr Perlmutter the Nobel prize for physics, which he shared with Brian Schmidt and Adam Riess, who led the other study, the High-Z Supernova Search.
Many of those 5,000 papers deal with something that has come to be known as dark energy. One reason for its popularity is that, at one fell swoop, it explains another big cosmological find of recent years. In the early 1990s studies of the cosmic microwave background (CMB), an all-pervading sea of microwaves which reveals what the universe looked like when it was just 380,000 years old, showed that the universe, then and now, was “flat”. However big a triangle you draw on it—the corners could be billions of light years apart—the angles in it would add up to 180°, just as they do in a school exercise book.
That might not surprise people whose geometrical endeavours have never gone beyond such books. But it surprised many physicists. At some scales space is not at all flat: the power of Albert Einstein’s theory of general relativity lies in its interpretation of gravity in terms of curved space. Cosmologists were quite prepared for it to be curved at the grandest of scales, and intrigued to discover that it was not.
...
Relativity says that for the universe to be flat, it has to have a very particular density—which in relativity is a measure not just of the mass contained in a certain volume, but also of the energy. The puzzle was that various lines of evidence showed that the universe’s endowment of ordinary matter (the stuff that people, planets and stars are made of) would give it just 4% of that density. Adding in extraordinary matter—“dark matter”, not made of atoms, that interacts with the rest of the universe almost only by means of gravity—gets at most an extra 22%. That left almost three-quarters of the critical density unaccounted for. Theorists such as Michael Turner, of the University of Chicago, became convinced that there was something big missing from their picture of the universe.
...
