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: 1) by khallow on Saturday January 24 2015, @07:27PM
Which makes sense, now that we know it’s not MAssive Compact Halo Objects, or MACHOs
Unless what we know about MACHO, isn't so.
I'd like actual observation of the WIMPs and their densities in local space before we get to ruling out other possibilities like the MACHOs.
(Score: 2) by mtrycz on Saturday January 24 2015, @08:13PM
If this "dark matter" *doesn't interact*, how does it hold galaxies toghether?
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(Score: 2) by maxwell demon on Saturday January 24 2015, @08:27PM
Journalism fail. It doesn't interact electromagetically but it certainly interacts gravitationally, and if it is WIMPs, it also interacts weakly (it's right in its name: weakly interacting massive particles). Oh, and as massive particles, it certainly interacts with the Higgs field.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by frojack on Saturday January 24 2015, @09:16PM
Actually the ARS article does a good job of leading you by the hand through the theory.
And it also explains, unlike TFS, why we might find out soon if Dark Mater is really Axions. Worth the read.
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(Score: 4, Interesting) by kaszz on Saturday January 24 2015, @08:58PM
Other scientists says that the black hole in the middle of our galaxy (Milky Way) which is 4 million solar masses is what keeps the galaxy together. Perhaps it's wrong or incomplete then?
(Score: 4, Interesting) by frojack on Saturday January 24 2015, @09:18PM
I often wondered if the Giant Black Hole wasn't yet another equation balancing invention.
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(Score: 2) by kaszz on Saturday January 24 2015, @09:39PM
The light from its orbiting stars should show what there is.
(Score: 2) by aristarchus on Sunday January 25 2015, @09:18AM
I love speculation as much as the next natural philosopher, but why would it be that some matter does not react electromagnetically but still does gravitationally? Especially since we are verifying the existence of the Higg's Boson in terms of (presumeably electromagnetic) energy levels? Damn, guess I will have to RTFA.
(Score: 2) by boristhespider on Sunday January 25 2015, @08:02PM
Because there isn't such a thing as a gravitational "force", nor is there a gravitational "charge". Electromagnetism is a genuine force, in that you can impart the same force on two objects of different (inertial) mass, and they will experience a different acceleration. Centrifugal and centripetal forces are the best known examples of fictional forces, in which you impart the same force on two objects of different (inertial) mass and they experience the *same* acceleration. Fictional forces arise in every accelerating frame of reference, and the acceleration due to the frame itself is experienced as a force. Each object naturally feels the same acceleration so it looks as though they experience a different force. Now take a look at the video of an astronaut on the moon dropping a hammer and a feather and seeing them hit the ground at the same time. One of those things weighs a few kilograms and the other weighs the sum total of fuck all. The conclusion is basically inescapable -- to some manner of speaking, gravity acts exactly as a fictional force or, to rephrase it, the effects of gravity actually look suspiciously like the effects of an acceleration and if we change our frame of reference we might expect to (at least locally) see no gravity.
Even in Newtonian physics you can see this. Newton's second law states that F = m_i * a where m_i is the inertial mass governing the response of a body to a force. Newton's gravitational law states that F = A * m_g / r^2, where A is a constant (depending on the mass of the other body you're interacting with, and I've bundled in the Newton gravitational constant). m_g is the gravitational mass, governing the amount of gravity a body "produces". Setting these equal gives us m_i * a = A * m_g / r^2, or the acceleration a = (A/r^2) * (m_g/m_i). Now, there's absolutely no a priori reason for the inertial and the gravitational masses to be the same; that's nothing more than an implicit assumption. However, they have to be (or at least proportional, but then any constant left over can be absorbed into the gravitational constant) so that every body feels the same acceleration. This is the "weak equivalence principle" and viewed one way it says that the gravitational and inertial masses are equivalent, and viewed another way states that the effects of gravity can be removed (at least locally) with an appropriate acceleration. Exploring the consequences of this principle leads pretty naturally to general relativity in which gravity is not a force but is merely a fictional force caused by a non-trivial geometry of spacetime -- moving along the path of least resistance, or what we'd think of as a "straight line", in spacetime yields curved paths through space.
The point of this is that as a result it's very natural for a particle to interact gravitationally; if it moves, it's going to experience it. (A nice way of seeing this is the action principle. Physical theories can be written as "actions", the sum of individual terms that describe the physics. The standard model of particle physics plus gravity is S = S_gravity + S_matter. "Minimal coupling" means that the interactions with gravity are governed by the immediate consequences of that equation, which are that the equations of motion for matter necessarily include gravity. It's meaningless to write down the system without gravity, or to attempt to write a system where gravity couples with some matter but not others. We can of course couple gravity non-minimally, but that goes in the other direction. Note that this is vanilla GR; in things like bimetric theories we don't have one geometry of spacetime but two, and couple, say, photons to one of those geometries and matter to another. But those are speculative, if interesting.)
It isn't, however, necessary for a particle to interact electromagnetically since it does not necessarily hold charge. (In the action, that's seen by the need to explicitly write a term like phi * F^2, where phi is your type of matter and F^2 the (square of the) Faraday tensor -- this would be something like E^2 - B^2 in flat spacetime -- to ensure that there's an interaction. The difference with gravity is that it makes no sense to write an action that somehow links gravity with only one type of matter; the interaction is automatic unless you try very hard to remove it. With electromagnetism, the interaction has to be explicitly added in. That's just the nature of the theories. Don't be fooled by the classical descriptions of the force looking identical, since a closer analysis reveals that these are two extremely different forces. Entertainingly, if you make spacetime five dimensional you can unify them neatly and geometrically so that actually EM doesn't exist either, but you do suffer some ugly consequences from this, such as the presence of otherwise unobserved fields known as dilatons.)
(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?
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(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.