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Dark Matter Is Even More of a Mystery Than Expected
schwit1 writes: Using the Hubble and Chandra space telescopes astronomers have discovered that dark matter is not only invisible to direct observation, it is invisible to itself! Quoting: "As two galactic clusters collide, the stars, gas and dark matter interact in different ways. The clouds of gas suffer drag, slow down and often stop, whereas the stars zip past one another, unless they collide — which is rare. On studying what happens to dark matter during these collisions, the researchers realized that, like stars, the colliding clouds of dark matter have little effect on one another. Thought to be spread evenly throughout each cluster, it seems logical to assume that the clouds of dark matter would have a strong interaction — much like the colliding clouds of gas as the colliding dark matter particles should come into very close proximity. But rather than creating drag, the dark matter clouds slide through one another seamlessly." The data here is on the very edge of reality, built on too many assumptions. We know that something undetected as yet is influencing the motions of galaxies, but what exactly it is remains completely unknown. These results only make the mystery more mysterious.
If there is a vacuum in space, would their need to be a corresponding antivacuum?
No. Because of Heisenberg's uncertainty principle, there can never be a complete vacuum. Even if there was, you couldn't measure it without destroying it.
Yes.
There are actually many proposed extensions to the standard model that predict dark matter particles that would be classified as WIMPs, and there are some others where the interaction is not through the weak force but through a "hidden sector" force. Some of the possible parameter space of some those hidden sector models predict a cross-section that they would have been able to detect in this experiment. So this is indeed a useful result -- it does rule out some possibilities. But they're not necessarily the possibilities that most people would be betting on anyway, so the headline is overhyped.
[TMB]
What I find more interesting is why stars rarely collide?
Too much empty space.
This.
Consider that for two stars to hit each other, they essentially have to pass within one stellar diameter of each other (absent gravity, but they're moving at over escape speed relative to each other, so gravity won't enlarge that distance a whole hell of a lot).
So, one stellar diameter is ~1.4 Gm for Sol. Nearest star is 40,000,000 Gm away. If that nearest star were headed toward us (it's not), it's course would have to be within 0.01 seconds of arc of our Sun in order to actually hit it.
And stars farther away have an even smaller course window to be in to smack us....
"I do not agree with what you say, but I will defend to the death your right to say it"
You didn't like the Wikipedia article for WIMPs. But since you put the other two as Wikipedia articles, I assume you consider it a valid source.
The answer isn't just "WIMPs", but a special kind of WIMP, or not one at all.
What's new?
I don't have the article in the mail yet, but I'm guessing that's new. At the very least, Weakly Interacting is now Really Weakly Interacting.