What Is A Complex Dark Matter?
There is no denying the fact that science has solved many mysteries of nature, from demonstrating that the Sun and not the Earth is at the center of the solar system, to discrediting the long-held stork theory of where babies came from. This success gives scientists tremendous pride in their accomplishments and is the reason that a recent poll by the National Academy of Sciences showed that scientists are generally highly respected by the American public.
However, there is also no doubt that there are unsolved mysteries and one of the biggest mysteries in modern physics originates from the fact that we scientists can’t conclusively identify 85% of the matter of the universe. So here’s the deal. The most modern research says that the kind of matter that makes up you and me constitutes only about 5% of the mass and energy of the universe.
It seems that about 25% is made of a substance called dark matter and 70% is made of an even more elusive substance called dark energy. Earlier videos of mine describe both of these possible dark substances. However, today I want to talk about dark matter, which seems to be about five times more prevalent than ordinary matter.
The simplest picture that scientists have of dark matter is a subatomic particle with a mass between one and ten thousand times the mass of a proton. This article has no electrical charge and interacts with ordinary matter via gravity and possibly involving forces weaker than we’ve discovered so far. For this reason, scientists call this particle a WIMP, for weakly-interacting massive particles.
Using the WIMP idea, scientists can explain many mysteries, from why galaxies rotate faster than can be explained from the known laws of physics and visible matter in the universe to the motion of vast clusters of galaxies. Because of the success of the theory, the WIMP idea is very well regarded in the scientific community. However, while the WIMP theory solves many mysteries in astronomy, it doesn’t solve all of them.
For instance, when scientists use WIMP theory to simulate the formation of large galaxies like the Milky Way, the simulations predict many, perhaps as many as 500, small satellite galaxies buzzing around the periphery of the Milky Way. When astronomers look, they find only about twenty.
Also, the simulations predict that the small galaxies should be arrayed uniformly around the Milky Way, while the observed satellite galaxies are preferentially located in a plane perpendicular to the Milky Way’s orbital plane.
These discrepancies and a few others have pointed to possible weaknesses of the WIMP theory. These weaknesses are not enough to entirely throw away the idea of the WIMP dark matter and, truth be known, scientists debate just how important these discrepancies really are. It‘s entirely possible that with improved simulations the disagreement between the model and observations will be smaller. Further, astronomers may still find more satellite galaxies.
This recently happened when the researchers using data taken by the Dark Energy Survey collaboration found several additional satellite galaxies at the Milky Way. This discovery demonstrates just how fluid our understanding of the situation really is. Still, it may be that some tweaks to the WIMP theory might be needed. While there are a number of proposed solutions, one idea is particularly cool.
One of the key properties of dark matter is that it does not absorb, nor emit, electromagnetic radiation. Basically, visible light, radio waves, infrared, ultraviolet, none of them interact with dark matter. This is because dark matter carries no electric charge. But what if the dark matter had a different kind of charge- a dark charge so to speak?
If it did, we could imagine that, in analogy to familiar electromagnetism, that particles carrying dark charge could emit dark photons and feel the force of dark electric fields. Because ordinary matter doesn’t carry this hypothetical dark charge, these dark photons zip by ordinary matter without interacting at all.
We just wouldn’t see them. Now, this sounds like it might be a kind of silly proposal, but is it? I mean, after all, ordinary matter is quite complex, with its quarks and leptons and at least five forces that govern its behavior. Why couldn’t dark matter be equally complicated? While we admit that we have no real reason to believe this dark charge idea, we can ask ourselves “Given existing data, is there any chance this new theory could be true? “It turns out that measurements of ordinary matter can tell us both what is possible and impossible for this new idea.
For instance, observations of the rotation of galaxies tell us that dark matter is not concentrated in a disk like the matter of the Milky Way. Instead, most of it is located in a big sphere surrounding most galaxies. This sets limits on both the strength of the dark matter charge and how dense the dark matter is. The reason is relatively simple.
If the dark charge were the same as an ordinary electric charge, it could radiate away energy like familiar matter and the dark matter distribution would be disk-like and not sphere-like. Using ideas like this, scientists are able to rule out some possible properties of dark matter that interact strongly with itself. However, there is one proposed idea of self-interacting dark matter that is very interesting. This model suggests that dark matter is even more complicated.
In this model, in addition to the regular WIMP particle, there are two forms of charged dark matter, with one of the dark matter particles being heavy and the other being light. This is kind of like imagining a dark matter proton and a dark matter electron that could conceivably bind together to make dark matter atoms. I should caution you against making too much of that analogy, but it helps give you a mental image of the idea.
The reason I caution you against this analogy is that if one postulated dark atoms, it’s easy to let your imagination get away from you and start thinking about dark molecules and then dark planets, dark stars, and an entire dark matter Milky Way living alongside ours, forever invisible except through gravitational interactions between the dark and the ordinary worlds. However, we know this can’t be true. In the past, before the scientific community settled on the dark matter hypothesis, astronomers looked for other explanations for why galaxies rotate so fast.
The simplest hypothesis was that rogue planets, brown dwarfs, black holes, and similar dense bodies of ordinary matter were to be found in the Milky Way. The idea was that scientists had simply underestimated the amount of matter in the galaxy. However, these experiments set pretty strict limits on the existence of these compact objects.
Because of the way that these measurements were done, we can rule out the existence of stars made solely of dark matter. The reason is simple. If dark matter stars existed, they’d have been detected in these searches for dark objects made of ordinary matter. However, it’s still possible that smaller compact dark objects could exist- say dark asteroids, for instance. So what do existing measurements still allow? The result is actually surprising. Under the idea that dark matter consists of a WIMP and two particles carrying dark charge, dark matter could consist of a big cloud of WIMPs with a smaller disk of dark matter held together with dark electromagnetism.
In fact, given existing measurements, the disk of strongly interacting dark matter could contain as much mass as we see in ordinary matter. This is a surprising conclusion and gives us a very different picture of the structure of galaxies. Now while I have told you about this interesting hypothesis, I must add a word of caution.
No matter how interesting this idea is, it’s just that- an idea- and most ideas in science turn out to be wrong. However, for the moment, it’s a cool possibility that lives in the gap between ideas that are still possible and those that have been ruled out completely. Only time, and experiments, will tell us in what category this idea will finally end up.
Dark Matter And Dark Energy Explained Simply
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