We don't need no stinkin dark energy
Edward Kolb and collaborators have posted a new Paper hep-th/0503117 which suggests that maybe we don't really need dark energy to explain the accelerating universe. About 10 years ago, you may recall, evidence started accumulating that the rate of expansion of the Universe was increasing.
We've known for 70 years or so that the Universe is expanding, that distant galaxies are rushing away from us, and each other, and that the more distant they are, the faster they are rushing away. As it turns out, this was predicted by Einstein's General Relativity, though the prediction was quite a nasty shock to Al when it was first noticed. This story, and everything else I will say except about the new paper, is told very well in Brian Greene's book The Fabric of the Cosmos.
In the General Relativistic picture, the long term fate of the Universe depends very critically on a parameter called Omega. If this parameter is greater than 1, the Universal expansion slows down, halts, and space becomes more and more positively curved while the Universe recollapses in a big crunch. If Omega is less than one, the U expands forever, with space becoming more negatively curved. If Omega is exactly one, though, the expansion continues but at a slower and slower pace and space remains flat. That, at least, was the picture before the acceleration of cosmic expansion was discovered. The evidence now suggests that the Universe is now expanding faster than it was a few billion years ago, but also that the Universe is flat.
Dark energy was invented to explain this apparent conflict. It is supposed to supply just enough energy to make Omega=1, thus making space flat, but also to cause space to expand due to its negative pressure (I'll let Greene explain that part). One important detail, if Omega is exactly one, it stays that way, but if it starts off just a little from from one, the deviation becomes larger and larger with time.
Cosmic inflation, the idea that the Universe underwent an enormous expansion in an extremely short time, is a sort of universal panacea for the diseases of cosmology. In particular, it tends to make space very flat and drives Omega towards one. I refer the reader (if any) to Brian Greene's book again for the details. It does have one peculiar signature: It amplifies tiny quantum fluctuations to gigantic size. Very precise measurements of the cosmic microwave background (CMB)reveal small variations with direction of the CMB with just the right spectrum to be such amplified quantum fluctuations.
The idea of Kolb and his co-authors is that some of these fluctuations could be much larger than the presently observable Universe (and we already know that some are only a little smaller). Suppose we happen to be somewhere in one of those fluctuations that happens to be slightly less than the critical density, so that Omega is slightly less than one. If I understand correctly, this means that our portion of space will expand ever more rapidly, since expansion drives down Omega, which, however, is still quite close to one due to inflation. Very far away galaxies are observed at an earlier time when Omega was closer to one, so they aren't expanding as rapidly. Nearby galaxies are observed at more recent times, so their expansion has been speeded up by the decrease in Omega. The net effect is an apparent acceleration of cosmic expansion. Kolb et.al. point out that the predictions of this idea differ slightly from those of the dark energy hypothesis, so that measurements more refined than those we have at present could distinguish the two theories.
If Kolb's idea is correct, it is a bit disappointing to those who hoped that dark energy implied some new physics.
Of course I'm not a cosmologist, so I might be misinterpreting, or oversimplifying his argument. Actually, pigs, even Capitalistimperialistpigs, aren't even very good physicists, so corrections will be welcomed if due.
We've known for 70 years or so that the Universe is expanding, that distant galaxies are rushing away from us, and each other, and that the more distant they are, the faster they are rushing away. As it turns out, this was predicted by Einstein's General Relativity, though the prediction was quite a nasty shock to Al when it was first noticed. This story, and everything else I will say except about the new paper, is told very well in Brian Greene's book The Fabric of the Cosmos.
In the General Relativistic picture, the long term fate of the Universe depends very critically on a parameter called Omega. If this parameter is greater than 1, the Universal expansion slows down, halts, and space becomes more and more positively curved while the Universe recollapses in a big crunch. If Omega is less than one, the U expands forever, with space becoming more negatively curved. If Omega is exactly one, though, the expansion continues but at a slower and slower pace and space remains flat. That, at least, was the picture before the acceleration of cosmic expansion was discovered. The evidence now suggests that the Universe is now expanding faster than it was a few billion years ago, but also that the Universe is flat.
Dark energy was invented to explain this apparent conflict. It is supposed to supply just enough energy to make Omega=1, thus making space flat, but also to cause space to expand due to its negative pressure (I'll let Greene explain that part). One important detail, if Omega is exactly one, it stays that way, but if it starts off just a little from from one, the deviation becomes larger and larger with time.
Cosmic inflation, the idea that the Universe underwent an enormous expansion in an extremely short time, is a sort of universal panacea for the diseases of cosmology. In particular, it tends to make space very flat and drives Omega towards one. I refer the reader (if any) to Brian Greene's book again for the details. It does have one peculiar signature: It amplifies tiny quantum fluctuations to gigantic size. Very precise measurements of the cosmic microwave background (CMB)reveal small variations with direction of the CMB with just the right spectrum to be such amplified quantum fluctuations.
The idea of Kolb and his co-authors is that some of these fluctuations could be much larger than the presently observable Universe (and we already know that some are only a little smaller). Suppose we happen to be somewhere in one of those fluctuations that happens to be slightly less than the critical density, so that Omega is slightly less than one. If I understand correctly, this means that our portion of space will expand ever more rapidly, since expansion drives down Omega, which, however, is still quite close to one due to inflation. Very far away galaxies are observed at an earlier time when Omega was closer to one, so they aren't expanding as rapidly. Nearby galaxies are observed at more recent times, so their expansion has been speeded up by the decrease in Omega. The net effect is an apparent acceleration of cosmic expansion. Kolb et.al. point out that the predictions of this idea differ slightly from those of the dark energy hypothesis, so that measurements more refined than those we have at present could distinguish the two theories.
If Kolb's idea is correct, it is a bit disappointing to those who hoped that dark energy implied some new physics.
Of course I'm not a cosmologist, so I might be misinterpreting, or oversimplifying his argument. Actually, pigs, even Capitalistimperialistpigs, aren't even very good physicists, so corrections will be welcomed if due.
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