View Single Post
  #30  
Old July 5th 17, 05:21 PM posted to sci.astro.research
Steve Willner
external usenet poster
 
Posts: 1,172
Default entropy and gravitation

In article ,
Nicolaas Vroom writes:
Baryonic dark matter? Is there something I'am missing?

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."


As the OP has noticed, there is confusion in the terminology. Many
people use "dark matter" to refer only to non-baryonic dark matter,
but others use the term more generally to include baryonic matter in
any form that's not yet detected. There have been many suggestions
for removing the ambiguity, but none of them has caught on. Until
something does, readers have to understand the context in which the
term is used. Careful authors will define which way they are using
the term.

What is the current main stream opinion about "in part"?


The Concordance Cosmology puts the total of baryonic matter at about
5% of the critical density. That comes most precisely from the CMB
fluctuations, but it's consistent with Big Bang Nucleosynthesis.
About half of that value is well accounted for (stars and gas plus
some other odds and ends). Until the last couple of years, the other
half has been "missing," but recent observations have found very hot
gas associated with galaxy clusters. As far as I can tell, the
weight of opinion is that this gas accounts for all the missing
baryonic matter, but I don't think it's 100% established as yet.

In contrast, non-baryonic dark matter accounts for around 26% of the
critical density according to the Concordance Cosmology.

Web searches should produce more reliable sources than Wikipedia.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA