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Examining the Expected Effects of Dark Matter On the Solar System
First time accepted submitter LiavK writes "Ethan Siegel recently wrote a great post for ScienceBlogs discussing the expected total mass of dark matter in the solar system. As far as we can tell, dark matter only interacts weakly, via gravity, both with itself and normal matter. So, it can't collide with itself, meaning that it has no way of getting hotter and radiating away energy and momentum. This means that it remains a diffuse mess, with a density that is ridiculously low, to the point where detecting its local effects is likely to remain... challenging for the foreseeable future."
The problem with dark matter observation in this case is that science is based on empirical observation. If you can't see it, can't measure it, and can't even draw inferences from what you can see and measure to detect something indirectly... it's not science. What this is saying is that the effects are so miniscule that there is no equipment presently capable of separating an actual effect or observation from systemic inaccuracy in the equipment itself. That is, you can't tell whether it's just random 'noise' or an actual signal.
But we do find it empirically. There is extra mass there, affecting other objects. We can detect it through it's gravitation, just not through light. It's a very strong signal, for example in the rotation velocity of galaxies. A lot of other science is, too, done without directly detecting the object of study, but through indirect effects and inference.
Everyone would like to get rid of Dark Matter. But its effects are clearly there. And we need to explain it. It does not have to be particles, or a kind of matter we know. You can call it something else than Dark Matter if you don't like the name. Anyone is welcome to come up with explanations. But they have to be in agreement with the observations.
NB: The message above might reflect my opinion right now, but not necessarily tomorrow or next year.
Why invent exotic matter when the right combination of dust could be the answer?
Simply put, because baryonic matter (ie. dust) radiates. This article would be titled, "Why our instruments are sensitive enough to detect all that dust that's affecting galaxies and superclusters rotation" if it was dust.
Here's a recent summary paper on the evidence for nonbaryonic dark matter. Dust has, alas, been hypothesized, tested, and rejected.
E pluribus unum
Planets are just clumps of dust.
But clumps of dust with a really low surface area for the mass involved. For example, Jupiter has a density of 1,330 kg per square meter and an average radius of almost 70,000 km (7*10^7 meters), a third more than water at STP. If instead, Jupiter were broken up into many equally sized balls of a smaller radius, then the mass stays the same, but the increase in surface area is inversely proportional to the decrease in radius.
For example a Jupiter-mass cloud of micron sized spheres, each with the density of Jupiter, would have a surface area 7*10^13 larger than Jupiter. That surface area incidentally happens to be roughly a twentieth of a square light year (roughly 4*10^30 square meters by my calculation) meaning at the right densities, such a cloud could intercept and radiate a lot more energy than Jupiter could, perhaps even be visible in small amateur telescopes at a few lightyears.
My point here is that some baryonic matter is a lot more visible, many orders of magnitude more visible, than other baryonic matter. And planet-sized objects are going to interact mostly by gravity as well meeting most of the desired characteristics of dark matter.
My take is having a significantly higher than expected fraction of the mass of your galaxies in rogue planets and similar things would be a way to account for dark matter.
But then there's the early universe observations. For example, the most damning evidence against dark matter hiding in planets and such, is observations of the cosmic microwave background (CMB), which is effectively the study of the period of the universe in which it started to become transparent to photons (about 400k years after the big bang according to the above link). That period of time is not a lot of time in which to create massive objects. And the fluctuations of the CMB yield dark to visible mass of roughly 5 to 1 (again according to claims in the above link).
So that indicates to me that there probably some sort of exotic matter out there which we haven't discovered yet.