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#21
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entropy and gravitation
"Richard D. Saam" wrote:
The solution is that it is a matter of temperature. That a lumpy distribution has higher entropy than a smooth distribution as soon as gravity is involved is only true for low temperatures. For high temperatures, the smooth distribution still has the higher entropy. That's why the universe has to be cold enough before galaxies and stars can form. In as much as galaxy and star planetary system size distributions are different, are two different formation temperatures required within the concept of Jeans' length? As you can read he https://en.wikipedia.org/wiki/Jeans_...eans.27_length Jeans' length depends on T^(1/2) for constant mass density and constant G. So, for high temperatures, the length is very big, allowing only for big clouds to collaps, e.g. a proto-galactic cloud to form a galaxy, whereas for low temperatures, also smaller clouds can collaps, e.g. a proto-stellar cloud to a star. |
#22
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entropy and gravitation
On 08/06/2017 20:26, Phillip Helbig (undress to reply) wrote:
In as much as galaxy and star planetary system size distributions are different, are two different formation temperatures required within the concept of Jeans' length? The Jeans length is important for star formation, but the stuff which forms (rocky) planets is only a small fraction of a larger cloud which collapsed (as described by Jeans) to form a star. There doesn't seem to be a lower limit on the size of "planets". There is an obvious upper limit for (gaseous) planets---stars. The sizes of planets are determined more by accretion, where gravitation is only one factor. That suggests an interesting question. Is it possible to compute either by simulation or from observations what percentage of ordinary matter is tightly bound together (either gravitationally or electromagnetically) as a function of length scale (or mass). There is clearly everything from ionised hydrogen, neutral hydrogen (which must be a fair chunk in itself) dust grains and upto ~300Msun. Does it obey some power law or are the preferred mass/length scales? -- Regards, Martin Brown |
#23
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entropy and gravitation
On 6/10/17 1:54 AM, Gregor Scholten wrote:
"Richard D. Saam" wrote: In as much as galaxy and star planetary system size distributions are different, are two different formation temperatures required within the concept of Jeans' length? As you can read he https://en.wikipedia.org/wiki/Jeans_...eans.27_length Jeans' length depends on T^(1/2) for constant mass density and constant G. So, for high temperatures, the length is very big, allowing only for big clouds to collaps, e.g. a proto-galactic cloud to form a galaxy, whereas for low temperatures, also smaller clouds can collaps, e.g. a proto-stellar cloud to a star. What is the origin of these different proto-galactic or proto-stellar cloud formation temperatures in the context of the accepted ubiquitous present CMBR 2.7 K temperature observation that can be redshifted to any proto-galactic or proto-stellar cloud era by (1+z)? |
#24
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entropy and gravitation
In article ,
says... On 08/06/2017 20:26, Phillip Helbig (undress to reply) wrote: In as much as galaxy and star planetary system size distributions are different, are two different formation temperatures required within the concept of Jeans' length? The Jeans length is important for star formation, but the stuff which forms (rocky) planets is only a small fraction of a larger cloud which collapsed (as described by Jeans) to form a star. There doesn't seem to be a lower limit on the size of "planets". There is an obvious upper limit for (gaseous) planets---stars. The sizes of planets are determined more by accretion, where gravitation is only one factor. That suggests an interesting question. Is it possible to compute either by simulation or from observations what percentage of ordinary matter is tightly bound together (either gravitationally or electromagnetically) as a function of length scale (or mass). There is clearly everything from ionised hydrogen, neutral hydrogen (which must be a fair chunk in itself) dust grains and upto ~300Msun. Does it obey some power law or are the preferred mass/length scales? I would imagine that there has been a lot of work done in this area in conjunction with studies of dark matter. Obviously the density and distribution of baryonic dark matter (i.e. ordinary matter that's not in stars) is a basic starting point for this research. In fact, googling 'baryonic dark matter distribution' gives links which will probably be in the ballpark of what you are interested in. - Gerry Quinn --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
#25
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entropy and gravitation
In article , Martin Brown
writes: The Jeans length is important for star formation, but the stuff which forms (rocky) planets is only a small fraction of a larger cloud which collapsed (as described by Jeans) to form a star. There doesn't seem to be a lower limit on the size of "planets". There is an obvious upper limit for (gaseous) planets---stars. The sizes of planets are determined more by accretion, where gravitation is only one factor. That suggests an interesting question. Is it possible to compute either by simulation or from observations what percentage of ordinary matter is tightly bound together (either gravitationally or electromagnetically) as a function of length scale (or mass). At larger scales, dark matter is important, but we don't know what it is. In particular, we don't know whether it is self-interacting (other than via gravity) and even if it isn't, it might not be in the form of isolated particles (though that is what many people assume); the was a paper by Bernard Carr and co-authors recently which pointed out that there is still a mass range where it could be in primordial black holes. At smaller scales, the last I heard, the IMF (initial mass function) for stars was not computable from first principles. From observations, we have a pretty good idea what it is locally, but it was probably different at high redshift. With certain assumptions, the Press-Schechter formalism allows one to calculate a mass function, and, not surprisingly (but a good consistency test and sanity check), this also comes out of simulations with the same assumptions. |
#26
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entropy and gravitation
"Richard D. Saam" wrote:
As you can read he https://en.wikipedia.org/wiki/Jeans_...eans.27_length Jeans' length depends on T^(1/2) for constant mass density and constant G. So, for high temperatures, the length is very big, allowing only for big clouds to collaps, e.g. a proto-galactic cloud to form a galaxy, whereas for low temperatures, also smaller clouds can collaps, e.g. a proto-stellar cloud to a star. What is the origin of these different proto-galactic or proto-stellar cloud formation temperatures in the context of the accepted ubiquitous present CMBR 2.7 K temperature observation that can be redshifted to any proto-galactic or proto-stellar cloud era by (1+z)? In today's universe, the average temperature is 2.7 K, which is low enough for proto-stellar clouds to collaps. In the early universe, the average temperature was higher, allowing only for proto-galactic clouds to collaps. That's why star formation started some time later than galaxy formation. |
#27
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entropy and gravitation
In article ,
Steven Carlip writes: I don't know of anywhere this has been worked out, but I suspect that if you found a measure of the amount of lumpiness in the maximum entropy state you'd find that it varies smoothly with G. More generally, I think one has to consider velocities as well as positions. The initial state with random positions _but all zero velocities_ (in co-moving coordinates) has low entropy. In equilibrium, the velocities would follow a Maxwell distribution. With gravity, you can raise the entropy of velocities at the expense of introducing clumpiness, which lowers entropy of the positions. As Martin (I think) indicated, the tradeoff between the two depends on temperature. I'm not at all sure I have all the details right, but this looks like at least one way to think about the problem. For Martin in another message, the baryon census of the universe is a subject of active research. Until recently, about half the baryons were unaccounted for, but it now seems they are located in very hot gas associated with galaxy clusters. The fraction of baryons in stars increases with cosmic time but is only of order 10% now. One recent paper, which apparently still finds baryons to be "missing" is http://adsabs.harvard.edu/abs/2016A%26A...592A..12E I have not researched this question in any detail, but the Introduction of the above paper has lots of relevant references. For another poster, 'entropy' is a defined physical quantity, not some general synonym for disorder. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#28
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entropy and gravitation
[[Mod. note -- This article was originally submitted on 2017-07-15
(about a week ago), but I mistakenly misfiled it and have only just (re)discovered it. I apologise to the author and to readers for the mixup & long delay. -- jt]] In article , Steven Carlip writes: I don't know of anywhere this has been worked out, but I suspect that if you found a measure of the amount of lumpiness in the maximum entropy state you'd find that it varies smoothly with G. More generally, I think one has to consider velocities as well as positions. The initial state with random positions _but all zero velocities_ (in co-moving coordinates) has low entropy. In equilibrium, the velocities would follow a Maxwell distribution. With gravity, you can raise the entropy of velocities at the expense of introducing clumpiness, which lowers entropy of the positions. As Martin (I think) indicated, the tradeoff between the two depends on temperature. I'm not at all sure I have all the details right, but this looks like at least one way to think about the problem. For Martin in another message, the baryon census of the universe is a subject of active research. Until recently, about half the baryons were unaccounted for, but it now seems they are located in very hot gas associated with galaxy clusters. The fraction of baryons in stars increases with cosmic time but is only of order 10% now. One recent paper, which apparently still finds baryons to be "missing" is http://adsabs.harvard.edu/abs/2016A%26A...592A..12E I have not researched this question in any detail, but the Introduction of the above paper has lots of relevant references. For another poster, 'entropy' is a defined physical quantity, not some general synonym for disorder. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#29
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entropy and gravitation
On Sunday, 11 June 2017 21:46:50 UTC+2, Gerry Quinn wrote:
I would imagine that there has been a lot of work done in this area in conjunction with studies of dark matter. Obviously the density and distribution of baryonic dark matter (i.e. ordinary matter that's not in stars) is a basic starting point for this research. In fact, googling 'baryonic dark matter distribution' gives links which will probably be in the ballpark of what you are interested in. 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." What is the current main stream opinion about "in part"? IMO darkmatter is (was?) always considered as non-baryonic as compared to normal matter which is considered as baryonic. 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. 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. Nicolaas Vroom. |
#30
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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 |
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