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entropy and gravitation
In article ,
Nicolaas Vroom writes: Baryonic dark matter? Is there something I'am missing? Maybe. Today, "dark matter" is often used as an abbreviation for "non-baryonic matter apart from neutrinos, the identity of which is unknown". Technically, electrons are also non-baryonic matter, but a) they are not dark and b) their mass is negligible in cosmology. Also, keep in mind that "dark" really means "does not interact electromagnetically". Not only does it not glow, neither does it reflect and is transparent. Of course, not all baryonic matter emits light so, in this sense, it is baryonic dark matter. We have a good idea from the CMB and big-bang nucleosynthesis how many baryons there are. Known baryonic matter in stars, gas etc is actually substantially less. When you go directly to the link: https://en.wikipedia.org/wiki/Dark_m...s._nonbaryonic Or https://en.wikipedia.org/wiki/Dark_matter and select paragraph 4 You will read: "Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g. stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least IN PART, be made up of standard baryonic matter, such as protons or electrons." Yes, that's one definition. What is the current main stream opinion about "in part"? This means that we know that most dark matter cannot be baryonic, because cosmological observations imply that there is much more than is compatible with the CMB and big-bang nucleosynthesis. IMO darkmatter is (was?) always considered as non-baryonic as compared to normal matter which is considered as baryonic. It depends on the definition. Again, this is an area where different notation can be confusing but learning the different schemes gives some insight into the matter. The problem is that the name dark matter is linked to the human sense: see. visible versus invisible. And as such it is a very unlucky name. A much better way is to make a distinction solely between baryonic and non baryonic matter. Not necessarily; it depends on the question one is asking. The problem is that in order to explain a galaxy rotation curve you can assume a certain amount baryonic matter which density is so low that it becomes invisible. The question is here what is this limit and how much baryonic matter is involved. The question here is whether the dark matter needed to explain galaxy rotation curves could a) be baryonic and b) escape detection by other means. |
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