"Motions of the stars tell you how much matter there is. They don’t care what form the matter is, they just tell you that it’s there." -Pieter van Dokkum
Everything in the Universe was born with the same ratios of dark matter to normal matter: approximately 5-to-1. It shows up in everything from the cosmic microwave background to galaxy clustering to internal motions of spiral and elliptical galaxies, including the Milky Way.
From simulations and inferred maps, dark matter (blue) may form some clumps, but overall exists in a massive, diffuse halo around the luminous, disk-like part of galaxies we're familiar with. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).
But some galaxies, particularly dwarf galaxies and rapidly moving galaxies within clusters, show a significant enhancement of dark matter over what’s expected. Ratios of even hundreds-to-1 aren’t unheard of. This is a difficulty for theorists to explain, but it doesn’t mean that dark matter is wrong.
Dwarf galaxies, like the one imaged here, have a much greater than 5-to-1 dark matter to normal matter ratio, as bursts of star formation have expelled much of the normal matter. Image credit: ESO / Digitized Sky Survey 2.
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If the normal matter was stripped away by (gas) ram pressure, it must have happened before many stars were formed, otherwise a decent faction of the mass would have been in the forms of stars,which aren't stripped out by fast travel through the intergalactic medium. So I think this new one is still an enigma.
Re: Your article on the three scarce light elements.
Your description of the origin of Boron by spallation got me to thinking about the two main sources of boron on earth, Turkey and California.
If the early earth was struck by a cosmic ray stream big enough to penetrate and have an entry and exit point, on opposite sides of the earth, would that be a likely scenario for borons relative abundance and geographical localization ?
I cannot get credible scientific authorities to even examine this idea heuristically. You seem to be open to outside ideas and have credentials which would give the elaboration of this concept some heft.
What about it ?
Please let me know. Otherwise direct me to another font. I really don't want to ask this question on the "Quora" web site.
Bill Murphy
Retired Thoracic Surgeon
Physics major who studied under Clyde Cowan
@William Murphy #2: No, it wouldn't work, and you should be able to work out for yourself why not. The most important word is "flux," the rate of incoming particles per unit area per second. The second most important word is "product," the number of atoms of boron (in this case) you want to produce.
For simplicity, you can assume perfect (100%) efficiency: every single incoming cosmic ray produces one boron atom. In reality, of course, the efficiency for an single species is tiny, as spallation produces a broad spectrum.
Start with an estimate of the total number of atoms of boron in each region, the approximate area of each region, and from those, work out what the flux would have to be. Also, estimate how _fast_ that flux would have to be delivered in order to produce all the boron in a limited area, before the Earth's rotation spread that area out too much.
This is an excellent homework problem requiring nothing but arithmetic, and an ability to do (or to look up) reasonable order of magnitude estimates. It's called a "Fermi problem," and I assign such problems to physics students all the time.
Once upon a time I had the notion that 5 additional branes, each with a Universe matching our own in mass-energy, (each with a unique set of particles/fields, but equivalent to ours from the perspective of observers made of the particular matter), was the source of Dark Matter. The hypothesis had a simple permutation principle to account for six, and only six, brane-Universes, thus explaining the 5 to 1 ratio of Dark Matter to Normal Matter in our Universe - the same applying to all the other 5 Universes.
But, alas, I read someplace that matter sequestered on another brane, or branes, was ruled out for some reason or another, as the source of Dark Matter.