Re: Particles in the Universe

["Paul Camp" <pjcamp@coastal.edu>]

Jim Green waxes cosmic again. 8-)

Most of these questions aren't really answerable and may not even 
have meanings.


> Some time ago we thought there were something like 1 proton/m3 in
> interstellar space.  Is that correct?  
Pretty much, at least based on the visible matter.
 
> How does this proton mass compare to the total mass of the Universe?
>
> How many total particles are there in the Universe??
>
> How many of these are protons??  Neutrons???

Here it starts to be problematic. First "total mass of the universe" 
is somewhat ill-defined and partly depends on your model of the 
universe's geometry. If it is an open geometry, then it is infinitely 
big and so the total mass is infinite. It would make more sense to 
talk about the densities of various constituents and this is what is 
usually done. Even if we discuss the density of protons versus the 
density of everything, that still isn't a very easily answerable 
question as it depends on how much dark matter there is, of which 
there seems to be a great deal but nobody has any idea how much. In 
principle, you could determine this by study of the apparent 
recession of distant galaxies (i.e. by how much does the Hubble 
constant fail to be constant -- what is the deceleration parameter) 
but since the value of H itself is not well known, how it changes 
over time is pretty murky. However, the results seem to give values 
corresponding to an overall density of the universe somewhere in the 
neighborhood of that required to produce a geometrically flat 
universe (neither open nor closed).

The current best figures are: (a) the luminous component of the universe 
(basically 70% Hydrogen, 20% Helium) provides less than 1% of the 
density required to produce flatness. This figure is determined from 
the number density of galaxies and the average mass of typical 
galaxies. (b) The rotation of spiral galaxies is inconsistent with 
gravitational physics unless there is a great deal of dark matter 
largely beyond the visible edge of the galaxy. The measurements are 
weak but this dark matter seems to contribute 3 to 10 times the mass 
of the luminous matter. Much of it may be in the form of atomic 
matter, largely protons. (c) local aggregations of matter, such as 
the relatively nearby Virgo supercluster, distort the Hubble flow. By 
modelling this distortion, one can determine the mass associated with 
typical galaxies in the supercluster, which presumably has 
concentrated the dark matter associated with them as well as 
concentrating the galaxies. This method gives mass associated with 
galaxies of about 10 to 20% of the critical density for flatness. It 
is unclear how much of the dark matter associated with galaxies is 
atomic in nature and in any case it only measures matter that tends 
to cluster around bright galaxies which may be anomalous. Locally 
smooth distributions of matter would make negligible contributions to 
the mass associated with aggregations like superclusters but may well 
contribute a great deal to the overall density of the universe. If 
one believes the iffy results from studying the variation of H, then 
that smooth distribution of *something* would account for about 80% 
of the density of the universe.

Having said all that, it is worth noting that there is a theoretical 
range on the baryon density of the universe based upon study of 
primordial nucleosynthesis -- about 1.5% to 16% of the critical 
density for flatness. Since visible matter only accounts for about 
1% as previously noted, at least *some* of the dark matter is 
baryonic, probably largely protons.

So the questions you ask are probably not answerable with certainty. 
If the density of the universe is < ~16% of the critical density, 
then all of the dark matter may be baryonic. On the other hand, if it 
is > ~16%, then much of it may be exotic matter (WIMPS, massive 
neutrinos and the like). The data from dynamical studies indicate 
that the material which clusters with galaxies on scales less than 
about 30 Mpc contributes about 20% + or - 10%. So you could go either 
way.


> TX for yur help.

I'm not sure that was help. 8-)

> Jim Green
> JMGreen@sisna.com
>                                      

Paul J. Camp                   "The Beauty of the Universe
Assistant Professor of Physics  consists not only of unity
Coastal Carolina University     in variety but also of 
Conway, SC   29528              variety in unity.
pjcamp@coastal.edu                    --Umberto Eco
pjcamp@postoffice.worldnet.att.net    The Name of the Rose
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