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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.
All the "normal stuff" is thought to have been made in two steps, one occurring when the universe was roughly three minutes old, and the other some 300,000 years later.
:0)
See? Even "God" needs to get it in production, then issue a revision some time later before it's really running right.
(P.S.- My first thought was of "Dork Matter", but then I saw the StarWars DVD ad on the page. *sigh* Too easy...)
Soko
"Depression is merely anger without enthusiasm." - Anonymous
Gack. How do they figure an explosion of spacetime is nuclear? There were no nuclei to fuse or split. My cynicism is telling me that the author just though "nuclear" sounded big and bang-y.
Excellent questions. Only problem is that it has no relevance to the "Creation of Normal Matter" the article is talking about.
The "normal matter" they were talking about are baryons (electrons, protons, neutrons etc and their composites). And the "creation" they talk about is "Big Bang Nucleonsynthesis", which is when protons and neutrons and electrons and stuff fuse together to make H, He and Li. The ratio of the production rates of these stuff implies certain "wiggles" in the CMB spectrum, so gives us a gauge (with lots and lots of caveats the scientists don't tell you) to the so-called "baryonic density". (Dark matter, by definition, do not interact with baryons, so it's hard to measure them since all the tools we have are made out of baryons.)
Big Bang Nucleonsynthesis (despite its name) can occur without a Big Bang : we just need the Universe to be Very Hot and Dense at some point.
Your questions about the origins of Big Bang is a much deeper and harder question. While it seems a philosophical argument, it is recently being attacked by some theorists. Most of the time, they just ask the question : do we need a Big Bang that starts from a singularity? The answer, with our current observations, is a BIG NO. But then they have to figure out a better alternative that can give us a very hot and dense Early Universe (so we can have Big Bang Nucleosynthesis, which is a very very very very well observed and constrained theory : i.e. it's fucking correct.)
Mode (3) smart-aleck mode. Press * to return to main menu.
Dark matter is the packing material the Universe came in...
No theory of everything could ever be complete without allowing for this.
I sure hope you're Ross Tessien, who posted this article to Usenet!
This slashdot comment also looks like this Usenet post.
This slashdot comment also looks like this Usenet post.
They look at the galaxies, and estimate how many stars and stuff there is in the galaxy. Any rotating galaxy. And They figure out how fast the galaxy is rotating.
They notice a problem. For any rotating galaxy there is not enough star stuff to hold the galaxy together. The spiral arms should never be there.
The star stuff in the galaxies do not have have enough gravity to hold galaxies together. Galaxcies should not exist at all. Stars should be all flying about because that is how weak the gravity is.
Just how much too weak? The Star stuff has one tenth the gravity needed to do the job. so something has to be doing the other 90%.
That is what the dark matter is. It is a term to label what the other 90% is. The don't know what it is yet. but they are working on it.
"It is a greater offense to steal men's labor, than their clothes"
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.
We have three methods to calculate the mass of the universe. Two are based on electromagnetic interactions. Those two agree. The other is based on observations of gravitational interactions. It gives a result 10X as large for the total amount of matter. Therefore 90% of the universe is made of particles that interact gravitationally but not electromagnetically. The only way to observe them is to observe their gravitational effects. Like, duh. Why is this such a difficult concept to grasp? It's an empirical observation.
Keep in mind that if something only couples gravitationally, it's going to be extremely hard to see. You're prejudiced by your own experience with the world, which is mostly based on electromagnetism- meaning interactions with photons (real and virtual). Get rid of electrodynamics, and most concepts and phenomena you're familiar with- atomic physics, chemistry, biology, optics, materials science, friction, pressure, radiation, viscosity, resistance, reflection, transparency, iridescence, impenetrability- all this stuff goes out the window! Your ass would sink through your chair, right through the ground, until you reached the center of the earth with everything else. Don't underestimate the importance of photon-mediated interactions. Everything else is gravitation, beta decay, and the strong nuclear force. Of those three, only gravitation operates over non-microscopic distances. And it is very weak. There could be up to several tons of dark matter in the room with you right now. You would never know it's there.
Of course, the mass could be ordinary matter that we're just not seeing. Many people like the idea of lots of Jupiter-sized objects. Lots of black holes might also work (although a black hole can feed off either kind of matter).