|MadSci Network: Astronomy|
An interesting question that has several aspects. Let's deal with the easiest one first. In a big, complicated system, like the Universe, the motions can become quite complex. Just look at the swirls when you mix milk into coffee for a microcosmic example. The swirls go every-which-way. Similarly, the "random" motions of the stars and gasses will result in both large and small eddies that will mask any underlying angular momentum left over from the Big Bang. You would need to add up all the little pluses and minuses due to these eddies on "small" scales (like our solar system, or even our whole galaxy) before you could find out anything about the underlying "universal" angular momentum distribution. No one has done this yet (it wouldn't be easy!). Another, very interesting question is this: when an object spins, what is it spinning with respect to? The standard example is a pail of water in an elevator. Without looking outside, the fact that the water sits down in the bottom of the pail tells you that either 1) you are in a gravtitational field, such as that of the Earth, or 2) you are accelerating in the "up" direction. Turns out that there is NO WAY to distinguish these two explanations, and the theory of General Relativity basically says that they are, in FACT exactly the same thing, despite appearances. Going back to the pail: If you now spin the pail, you will observe the water starting to spin as the pail drags on it, and then that the surface of the water will rise up at the edges of the pail. So you can tell, just from soemthing you can see INSIDE the elevator, that the pail is spinning, just from the shape of the surface of the water. Suppose we spin the whole elevator. Although it would not be obvious, then, that the pail was spinning, since the pail, and the elevator, and you would all be moving together, yet you could still see the curved surface of the water, and deduce that the elevator was spinning -- without having to look outside. But how can that be? What is the connection between the outside world and the pail? There is a mystery here. A famous conjecture was made by Mach, that if you could spin the ENTIRE UNIVERSE around the pail, rather than spinning the pail, the surface of the water would behave in the same way. But it is demonstrably NOT true that just spinning some of the local stuff will do it. Is this conjecture true? If it were, what does it say about the connection between something local, like the angular momentum of the water in the pail, and the rest of the universe? The relevance is this: what does it mean to even SAY that the entire Universe is spinning? With respect to what? Finally, the "standard" models of the Big Bang have a total angular momentum of zero. We use these models because they are the simplest. It is possible to make "twisted" models with non-zero angular momentum, but they are harder to deal with. I don't think there are any very stringent observational limits, however, on the total angular momentum of the universe. These days, it is very "fashionable" to use a variant of the standard Big Bang model called "inflation." I have no idea if "inflation" would force the angular momentum of the universe towards zero or not. But then I am of the opinion that we just don't have enough data to have much assurance that ANY of our basic cosmological theories is really correct. I anticipate that over the next couple of decades, however, we will get enough new information from new observatories on the ground and in space, that these cosomological theories can be put to the test. Should be exciting!
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