A Space & astronomy forum. SpaceBanter.com

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » SpaceBanter.com forum » Astronomy and Astrophysics » Research
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

entropy and gravitation



 
 
Thread Tools Display Modes
  #21  
Old June 10th 17, 07:54 AM posted to sci.physics.research,sci.astro.research
Gregor Scholten[_2_]
external usenet poster
 
Posts: 3
Default 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.


Ads
  #22  
Old June 10th 17, 07:54 AM posted to sci.physics.research,sci.astro.research
Martin Brown[_3_]
external usenet poster
 
Posts: 189
Default 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  
Old June 10th 17, 06:25 PM posted to sci.physics.research,sci.astro.research
Richard D. Saam
external usenet poster
 
Posts: 240
Default 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  
Old June 11th 17, 08:46 PM posted to sci.physics.research,sci.astro.research
Gerry Quinn[_3_]
external usenet poster
 
Posts: 3
Default 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  
Old June 11th 17, 08:47 PM posted to sci.physics.research,sci.astro.research
Phillip Helbig
external usenet poster
 
Posts: 38
Default 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  
Old June 15th 17, 04:18 PM posted to sci.physics.research,sci.astro.research
Gregor Scholten[_2_]
external usenet poster
 
Posts: 3
Default 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  
Old June 16th 17, 07:07 AM posted to sci.physics.research,sci.astro.research
Steve Willner
external usenet poster
 
Posts: 1,172
Default 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  
Old June 22nd 17, 02:54 AM posted to sci.physics.research,sci.astro.research
Steve Willner
external usenet poster
 
Posts: 1,172
Default 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  
Old June 30th 17, 06:56 AM posted to sci.astro.research
Nicolaas Vroom
external usenet poster
 
Posts: 216
Default 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  
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
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
sun's entropy RichD Astronomy Misc 6 December 7th 07 05:56 AM
Entropy G=EMC^2 Glazier Misc 5 January 4th 06 04:35 AM
Entropy SeppoP Misc 2 December 30th 05 11:36 PM
Entropy Tax Man Misc 0 December 30th 05 02:48 PM
Entropy Tax Man Misc 0 December 30th 05 02:48 PM


All times are GMT +1. The time now is 10:17 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
Copyright 2004-2022 SpaceBanter.com.
The comments are property of their posters.