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Dark Matter Measurements
ksp0704 writes: "According to this article at space.com, scientists have finally measured the approximately 90% of the universe we can't see (the dark matter)." I'm sure it will continue to be a topic of debate for years, but two independent measurements agreeing is a good sign.
What caused the big bang? How was it initiated? What were the bounds of the "universe" as it were before the big bang?
Sometimes bored physicists do try to give serious thought to this. Being a physicist, I've sometimes gotten to listen to what others consider to be serious thoughts on the matter.
Basically there are too camps, people that want the universe to be timeless and exist forever and people that want the Big Bang to be the ultimate start of things. People in the first group will given you various stories about the cyclical nature of the universe (usually expand, collapse, repeat), or some notion of universes spawning other universes, ad infinitum.
People who believe that the Big Bang was THE START of things tend to either believe it to be uncaused, caused by God, or unknowable and irrelevant. There are a few however in this camp that try to posit explanations of what did cause the universe out of nothing. Some bring in exotic theories (such as string theory) to try and construct physical laws that can hold before, during, and after a big bang event. Of course these people also have to change the nature of a big bang away from that strictly based on general relativity (which implicitly prevents any meaningful reference to a "before" the big bang).
One of the most interesting stories I've heard is that the fabric of space has the property of being unstable in a total absence of energy, and at any moment and any location, there is infinitesimal but non zero probablity that it will transition to a different state which has energy, which then billows out into the rest of the universe. So basically the vaccuum has certain properties that exist forever and are timeless, and the big bang has a chance of spontaneously erupting simply because it has never happened. Hence the universe, as we expereince it, has a single well defined start within a larger timeless existence.
As absurd as this might sound, this is quite serious, and as reasonable as many other things people say about "before" the big bang.
Ultimately though, it only transfers the problem of first cause to the "fabric of the universe" and the basic physical laws governing everything. While science may be able to tell you that something is NOT the first cause, it can never say with certainty that something IS the first cause. As far as I'm concerned, whether you choose to believe that the chain of causation goes infinitely backward or has some definably beggining, is a matter of faith.
If bright matter truly makes 10% of the universe, then by definition the remaining 90% of the universe must be dark matter.
The reason is that neither the terms "bright matter" nor "dark matter" specify a single type of matter. Rather, they define two values of a single common characteristic of all matter. The characteristic in question is how the matter interacts with photons. If you shine a light on something and you can see it, then it's bright matter. If you heat something up and you can see it, then it's bright matter. If you energize something then let its energy level drop and you can see it, then it's bright matter. Otherwise it's dark matter.
Therefore we can't measure dark matter directly merely because we can't see it. All astronomical observations depend on photons. Radio. Light. X-Ray. Gamma. Just different frequencies of photons. Since dark matter neither reflects nor emits photons, astrophysicists can't observe it. Or perhaps it does emit photons, but then immediately reabsorbs them (as in the case of black holes). Either mechanism comes down to the same thing. They can observe its effects indirectly by watching, for instance, the effect that its gravity has on surrounding bright matter, but no direct observation is even theoretically possible.
But there really aren't any theories about the nature of dark matter, because it's fundamentally impossible to observe remotely. Maybe it's some truly strange substance. Maybe its just a whole bunch of black holes. No one knows. The only reason that we know about black holes is that some brilliant physicist who'd been downing a few too many beers one night did a thought experiment about the implications of gravity's inverse square strength. So we had a theoretical phenomenon that astrophysicists could later go and look for. But that's not true of other forms of dark matter.
All that's important is that "dark" matter is every piece of matter that isn't "bright" matter. It's still matter, and will still behave exactly the same as bright matter behaves. But it may come to be discovered that some characteristic that we thought was endemic to all matter is, in fact, only endemic to bright matter. We have no comparison yet, so we can't make that determination.
I don't think that anyone believes that all dark matter is in the form of black holes. Who knows, maybe so. I'm certainly not an astrophysicist (though I know a number of them who are on the bleeding edge), so someone can easily have come up with some theories about all this of which I'm unaware.
But this is my current understanding, and with the rate that astrophysics moves, I'm probably at least 5 years out of date.
Oh, explaining this caused me to remember a theory about dark matter that I heard from my undergraduate adviser back in my college days (Dr. Douglas Lin: he was and is a big shot in the astrophysics circles). The idea is that there actually isn't any special dark matter. It's all bright matter. But some matter might be in locations where so few photons fall on them that we just never get a chance to observe that matter. For instance, it's known that all the galaxies of the universe exist on the surfaces of voids in the universe (that observation is what gave rise to superstring theory). Think of soap suds. We've got complex surfaces, where all the soap is, each surrounding a small void with no soap. Small from our perspective, but from the point of view of a technological civilization living in one of the "galaxies" within the soap film, those voids are huge. The universe has the same structure. And these voids are just monstrously huge. In the center of one of these voids, there would be very little light, because all the light sources are very far away. So you could stick a whole lot of matter there and no one would ever see it. These voids are so huge that you could easily fit 90% of the universe's mass in them and still have a very low density of matter. It's normal "bright" matter, but insufficient light reaches it for us to observe it. The problem with the theory is that if you have 90% of the universe stuck in these voids, then the voids should collapse from gravity and make the galaxy distribution homogenous. And we don't see that. Perhaps this problem has been resolved by now. I don't know. And, of course, there are other locations where matter can be hidden, where we wouldn't be able to observe it. Those voids are just a single example.
-- Nolite audere delere orbiculum rigidum meum.
A further problem not mentioned in the above is that the angular motion of spiral arms is such that the speed of star motion is much more consistent from the center of the spriral to the outside of the spiral then they should be. It is almost as if they were a solidf or semi-solid disc. which is silly, but that is how they behave.
This may be less consistent with a high energy point source of gravity, and more consistent with mass spread out for a large distance. but I haven't kept up and my math sucks [smile]
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