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from the Shining-a-light-on-dark-matter dept.
From observations of the Milky Way galaxy, we’ve learned that in any given cubic meter of space, even the particular cubic meter that snugly fits your seated form as you read this article, there’s a small amount of matter—only about 50 proton masses worth—passing through in any given moment. But unlike the particles that make up your seated form, this matter doesn’t interact [electromagnetically]. It doesn’t reflect light, it isn’t repelled by solid objects, it passes right through walls. This mysterious substance is known as dark matter.
Since there’s so little of it in each cubic meter, you would never notice its presence. But over the vast distances of space, there’s a lot of cubic meters, and all that dark matter adds up. It’s only when you zoom out and look at the big picture that dark matter’s gravitational influence becomes apparent. It’s the main source of gravity holding every galaxy together; it binds galaxies to one another in clusters; and it warps space around galaxy clusters, creating a lensing effect.
But despite its importance to the large-scale structure of the Universe, we still don’t know what dark matter really is. Currently, the best candidate is WIMPs, or Weakly Interacting Massive Particles (Which makes sense, now that we know it’s not MAssive Compact Halo Objects, or MACHOs). But WIMPs are not the only option—there are quite a few other possibilities being investigated ( http://arstechnica.com/science/2014/09/exploring-the-monstrous-creatures-at-the-edges-of-the-dark-matter-map/1/ ). Some of them are other kinds of massive particles, which would constitute cold dark matter, while others aren’t particles at all. ( http://arstechnica.com/science/2014/11/looking-for-a-different-sort-of-dark-matter-with-gps-satellites/ )
[Paper]: http://www.pnas.org/content/early/2015/01/07/1308788112.full.pdf+html
[Abstract]: http://www.pnas.org/content/early/2015/01/07/1308788112
(Score: 2) by Non Sequor on Sunday January 25 2015, @10:14PM
I wonder if physics may get stalled by continuing to look for elegant solutions and there may not be an elegant solution. Maybe the real answer is some kind of crackpot MONDesque bastardization of the elegant laws we've been able to construct.
Here's something I came up with to amuse myself. For the cosmological missing matter problem, let's pretend that every particle has a gravitational mass halo that extends well beyond the confinement of its inertial mass. Think of this as adding in some extra gravitational mass around any inertial mass (which we'll continue to treat as doing double duty also serving as gravitational mass).
This gravitational halo would be effectively unobservable at distance scales smaller than the extent of the halo (for the same reason that there is no gravity inside a hollow spherical shell), so this would not contradict experiments setting bounds on any differences in inertial and gravitational mass.
We'll say that the additional gravitational mass created by this halo is sufficient to explain the missing matter in the universe in lieu of dark matter. In this thought experiment, the missing matter is not new dark matter which fits into the Standard Model zoo, but an aspect of existing matter which does not contradict experiments on earth but which throws a wrench in out ability to relate experiments on earth to cosmology.
What if one of the absurd, untestable, speculative theories of a form like this is right? What if the universe is trolling physicists?
Write your congressman. Tell him he sucks.
(Score: 1, Informative) by Anonymous Coward on Monday January 26 2015, @05:31PM
That would observably violate the weak equivalence principle and there are some pretty sensitive experiments testing it.