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entropy and gravitation



 
 
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Old November 13th 17, 06:30 AM posted to sci.astro.research
Phillip Helbig
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Posts: 38
Default 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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