Metric
05-22-2006, 01:02 AM
In cosmology, there is a well-known problem known as the "cosmological constant problem" -- it's been touched on a few times here, but basically the problem is that apparently no theory is any good at predicting the observed energy density of the vacuum, from quantum field theory zero point energy, to spontaneous symmetry breaking, up to superstring theory -- it's all horribly wrong, by something like 100 orders of magnitude (ten years ago the superstring theorists would have said "we're pretty sure it's zero, but if it isn't zero it would have to be negative" -- turns out that it's measured to be non-zero and positive).
Another, lesser-known problem (not quite as severe or embarassing, but still oddly compelling) is called the "cosmological coincidence problem." When the universe was young, up until very recently, gravity on the largest scale was dominated by the energy density of matter. But as the universe expands, because galaxies are moving apart, the energy density of matter in the universe becomes less and less, while the energy density of the vacuum remains constant. So in the future, the energy density of the vacuum will dominate gravity on the largest scale.
The interesting thing, though, is that this transiton from "matter dominated universe" to "vacuum dominated universe" happens rather abruptly. See the following figure:
http://nedwww.ipac.caltech.edu/level5/Carroll2/Figures/fig11.jpeg
Here, the vertical axis represents the percent of energy in the universe due to the vacuum. The horizontal axis is the "cosmological scale factor" which you can think of simply as "time." As you can see, in the past the vacuum was totally inconsequential, but in the future, it will be everything. By now you must have noticed, however, that "now" seems to occupy a strangely special point on the graph -- the point at which the two contributions to the vacuum energy are roughly equal.
For another point of view, look at its derivative:
http://nedwww.ipac.caltech.edu/level5/March01/Carroll/Figures/figure1.jpg
As I have heard one professor remark, the transition happens exactly on September 24th, at 3:19 pm, eastern standard time -- which, lo and behold as I look at my watch, happens to be RIGHT NOW!
Now, this seems to violate the Copernican principle, which asserts that we occupy no "special" place in the universe. Yet -- there sits the data. Obviously, if you're inclined to believe that "there are no coincidences," you're very apt to think that this is a serious problem. If you don't mind occupying a special place in the universe -- well, maybe you don't think it's such a big problem.
This is one of those facts that could be pointing to some fundamental new kind of understanding -- or maybe not. Unlike the cosmological constant problem, it is irritatingly vague. If it really is "just a coincidence," then we're probably doomed to be asking the same question forever without a satisfying answer.
(Sean Carroll has a great article on this stuff here: http://nedwww.ipac.caltech.edu/level5/March01/Carroll/frames.html )
Another, lesser-known problem (not quite as severe or embarassing, but still oddly compelling) is called the "cosmological coincidence problem." When the universe was young, up until very recently, gravity on the largest scale was dominated by the energy density of matter. But as the universe expands, because galaxies are moving apart, the energy density of matter in the universe becomes less and less, while the energy density of the vacuum remains constant. So in the future, the energy density of the vacuum will dominate gravity on the largest scale.
The interesting thing, though, is that this transiton from "matter dominated universe" to "vacuum dominated universe" happens rather abruptly. See the following figure:
http://nedwww.ipac.caltech.edu/level5/Carroll2/Figures/fig11.jpeg
Here, the vertical axis represents the percent of energy in the universe due to the vacuum. The horizontal axis is the "cosmological scale factor" which you can think of simply as "time." As you can see, in the past the vacuum was totally inconsequential, but in the future, it will be everything. By now you must have noticed, however, that "now" seems to occupy a strangely special point on the graph -- the point at which the two contributions to the vacuum energy are roughly equal.
For another point of view, look at its derivative:
http://nedwww.ipac.caltech.edu/level5/March01/Carroll/Figures/figure1.jpg
As I have heard one professor remark, the transition happens exactly on September 24th, at 3:19 pm, eastern standard time -- which, lo and behold as I look at my watch, happens to be RIGHT NOW!
Now, this seems to violate the Copernican principle, which asserts that we occupy no "special" place in the universe. Yet -- there sits the data. Obviously, if you're inclined to believe that "there are no coincidences," you're very apt to think that this is a serious problem. If you don't mind occupying a special place in the universe -- well, maybe you don't think it's such a big problem.
This is one of those facts that could be pointing to some fundamental new kind of understanding -- or maybe not. Unlike the cosmological constant problem, it is irritatingly vague. If it really is "just a coincidence," then we're probably doomed to be asking the same question forever without a satisfying answer.
(Sean Carroll has a great article on this stuff here: http://nedwww.ipac.caltech.edu/level5/March01/Carroll/frames.html )