CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)

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1 Jonathan Thornburg formation of dwarf galaxies in CDM cosmology vrijdag 15 januari 2010 22:30
2 Nicolaas Vroom Re: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology) donderdag 28 januari 2010 11:34
3 Kent Paul Dolan Re: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology) vrijdag 29 januari 2010 1:23
4 Oh No Re: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology) vrijdag 29 januari 2010 22:19
5 Nicolaas Vroom Re: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology) woensdag 3 februari 2010 1:53


1 formation of dwarf galaxies in CDM cosmology

Van: Jonathan Thornburg
Onderwerp: formation of dwarf galaxies in CDM cosmology
Datum: vrijdag 15 januari 2010 22:30

From: "Jonathan Thornburg [remove -animal to reply]" Subject: formation of dwarf galaxies in CDM cosmology (was: Re: Kepler's Recent Results in Contradiction to Standard Planet Formation Theory) Newsgroups: sci.astro.research References:

[[I've changed the subject line, as this discussion no longer has much of anything to do with Kepler's results or planetary formation theory]]

Robert L. Oldershaw wrote:
> In the Jan. 14th issue of Nature is a paper that claims to resolve a serious problem that has plagued the CDM cosmology for a long time.
[[...]]

For the benefit of others, the paper in question is this one:

F. Governato, C. Brook, L. Mayer, A. Brooks, G. Rhee, J. Wadsley, P. Jonsson, B. Willman, G. Stinson, T. Quinn & P. Madau "Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows" Abstract: For almost two decades the properties of `dwarf' galaxies have challenged the cold dark matter (CDM) model of galaxy formation1. Most observed dwarf galaxies consist of a rotating stellar disk2 embedded in a massive dark-matter halo with a near-constant-density core3. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles4, 5, 6, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers7. Processes that decrease the central density of CDM halos8 have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate9. Here we report hydrodynamical simulations (in a framework10 assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies-bulgeless and with shallow central dark-matter profiles-arise naturally in these simulations. Nature volume 463, number 7278, pages 203-206 (14 Jan 2010) http://www.nature.com/nature/journal/v463/n7278/full/nature08640.html doi:10.1038/nature08640 preprint (open-access!) at http://arxiv.org/abs/0911.2237

Nature also has an "Editor's summary" and a "News and Views" article about this work, Marla Geha "Galaxy formation: Gone with the wind?" Nature volume 463, number 7278, pages 167-168 (14 Jan 2010) http://www.nature.com/nature/journal/v463/n7278/full/463167a.html doi:10.1038/463167a

> So now international teams of theorists using "millions of hours on supercomputers" have run SIMULATIONS that reproduce the desired phenomenon. Just so! Break out the champagne! Mission Accomplished!

But does anyone else see some reasons for very serious misgivings here?

(1) The "correct" answer was assumed to be known from the start.

(2) Theorists were not going to give up until they got the "right" answer.

I think there may be some misunderstanding here of the role that computers and computer simulations play in theoretical astrophysics. The usual way things get done is this:

  1. Someone suggests a physical model (which usually has a bunch of parameters). This might be based purely on speculation, and/or it might be based on analysis of past observatious. In fact, it might be the same as some other model which is already well-known, but this time it's elaborated a bit more.
  2. Are people able to solve the model's equations analytically?
    1. 2a: Yes --> Goto step 3 to explore the analytical solution
    2. 2b: No --> (i) Computational people figure out how to solve the equations numerically (or, if this has already been done, how to solve them more efficiently, accurately, and/or robustly than has been done in the past). If this is a lot of work, write a paper on just this and submit it to arxiv and a journal. (ii) Goto step 3 to explore the numerical solution.
  3. Try varying the model parameters to see if we can get reasonable agreement between the model equations' solution (either analytical or numerical) and past observations (if there are any).
  4. Try varying the model parameters some more to see if any interesting predictions can be made about what future observations might show.
  5. Write papers on steps 3 and 4 & submit them to arxiv and journals. (Papers on step 4 also serve as hints to our observational colleagues that they might try making said observations.)

Notice that very little of this process (only steps 2a vs 2b) depends on whether computers are used. As Forman Acton has famously said (in his classic (though now rather dated) book "Numerical Methods that Work"), "the purpose of computing is insight, not numbers".

Notice also that the process is actually very similar regardless of whether the observations predate or postdate the theoretical calculation.

> (3) Using computers and many adustable parameters you can get whatever you want.

I'm sorry, but that's simply not true. It may be that for *some* physical systems (the models for which might have analytical solutions, or they might not) with many adjustable parameters, you can get a wide range of qualitative behavior. But that's certainly not the case for all physical systems, or even for all physical systems whose equations need a computer to solve.

A case in point: Consider a bound binary black hole system in an otherwise empty asymptotically flat spacetime, with dynamics given by general relativity. This system has around 15 parameters, and simulating its outcome takes a lot of computer time (on the order of 10,000 - 100,000 CPU-hours for a medium-resolution simulation). But despite that, (I hereby assert) *no* combination of those parameters will result in the binary becoming unbound. Indeed, *no* combination of those parameters will result in any other final end state than (the two black holes merging to form) a single black hole.

> (2) Theorists were not going to give up until they got the "right" answer.

What the paper in question actually says is that

(a) Here's a well-known problem (a bunch of observations which weren't
    well explained by past theoretical predictions).
(b) Here's a particular theoretical model (which the authors argue is
    better than past models, in that it more accurately approximates
    what we believe to be the underlying physics than past models did).
(c) Here are some (numerical) calculations of that model's outcomes
    for certain (given) sets of parameters which (the authors assert)
    are plausible.
(d) The calculated outcomes agree pretty well with past observations.

Could you be more specific about which of (a), (b), (c), and/or (d) in this paper you consider to be not "right"?

Or, if you don't find fault with any of (a), (b), (c), or (d) in this paper, could you explain more clearly just why it is that we should disbelieve the author's conclusions?

Do you have any specific evidence that "you can get whatever you want" by adjusting the paremeters in the authors' model? Is there some other reason why we should distrust their model?

ciao,

-- -- "Jonathan Thornburg " Dept of Astronomy, Indiana University, Bloomington, Indiana, USA "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral." -- quote by Freire / poster by Oxfam

next posting Mesg2


2 CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)

Van: Nicolaas Vroom
Onderwerp: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)
Datum: donderdag 28 januari 2010 11:34

"Jonathan Thornburg" schreef in bericht posting Mesg1
> From: "Jonathan Thornburg [remove -animal to reply]" Subject: formation of dwarf galaxies in CDM cosmology Newsgroups: sci.astro.research

Robert L. Oldershaw wrote:

>> In the Jan. 14th issue of Nature is a paper that claims to resolve a serious problem that has plagued the CDM cosmology for a long time.
>

For the benefit of others, the paper in question is this one:

F. Governato, C. Brook, L. Mayer, A. Brooks, G. Rhee, J. Wadsley, P. Jonsson, B. Willman, G. Stinson, T. Quinn & P. Madau "Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows" Abstract: For almost two decades the properties of `dwarf' galaxies have challenged the cold dark matter (CDM) model of galaxy formation1. Most observed dwarf galaxies consist of a rotating stellar disk2 embedded in a massive dark-matter halo with a near-constant-density core3. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles4, 5, 6, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers7. Processes that decrease the central density of CDM halos8 have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate9. Here we report hydrodynamical simulations (in a framework10 assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies-bulgeless and with shallow central dark-matter profiles-arise naturally in these simulations. Nature volume 463, number 7278, pages 203-206 (14 Jan 2010) http://www.nature.com/nature/journal/v463/n7278/full/nature08640.html doi:10.1038/nature08640 preprint (open-access!) at http://arxiv.org/abs/0911.2237

Nature also has an "Editor's summary" and a "News and Views" article about this work, Marla Geha "Galaxy formation: Gone with the wind?" Nature volume 463, number 7278, pages 167-168 (14 Jan 2010) http://www.nature.com/nature/journal/v463/n7278/full/463167a.html doi:10.1038/463167a

What is CDM and why do we need this ? Accordingly to the Book "The Big Bang" by Joseph Silk At page 182 we read: "The dark matter responds to gravity, and initial density fluctuations grow in contrast, just as they do with ordinary matter" That means the behaviour dark matter can be described by Newton's Law, but it is different from ordinary matter i.e. dark matter is not described by the chemical elements of the periodic table, which are the building blocks of the Sun and the Earth. If you combine those chemical elements in small quantities you get small pieces (grains) of ordinary matter, which are all dark and cold. Those pieces only become visible if they grow in size and become as large as the size of the Earth and if the temperature increases to more than 2000 degrees C at the outside! (There are exceptions) What important is that those small pieces (objects) of cool ordinary matter are impossible to detect.

This raises for me the question: Why introducing CDM which consists of nonbaryonic particles. See: http://en.wikipedia.org/wiki/Dark_matter The answer is partly because the observed rotation curves of galaxies don't match the calculated rotation curves based on the observed(visible) values of ordinary matter. One specific discrepancy is on the right side where the rotation curve assumes high speeds of something while outside that region (away from the bulge) no (visible) mass can be detected. The solution is to invent the concept of (invisible) dark matter (in a halo) which behaves mathematical the same as ordinary matter but physical not. There exists no inter exchange. I 'am astonished by this reasoning. Why not introducing ordinary matter in small quantities (inside the disc) ? This is a much simpler concept which allows for a free mixing (colliding) environment.

At page 167 at the above mentioned document we can read: "Yet despite this uncertainty (hypothesized particle) the prevailing model of galaxy formation based on CDM is tremendously successful, predicting a vast range of observational data" What I would like to know which data CDM predicts correct and why a model only based on ordinary matter would fail based on the same circumstances. I think to describe the evolution in galaxy formation only using ordinary matter is much simpler. CDM models require supernova in order to move dark matter outwards. Ordinary matter models are less strict. The whole issue is how fast small objects form larger objects which form stars at the same time converting invisible matter into visible matter. This whole process depends about the initial density of the original grain clouds.

Page 167 shows one image of a real galaxy (right) and one simulation image of a galaxy. The right image shows ordinary visible matter (stars). However this is only a part of the total ordinary mass of that galaxy. What is not shown is a type of small objects in the plane of the disc (like the Kuiper Belt) and a type of Oord cloud within and surrounding the galaxy

Page 205 shows two galaxy rotation curves: The real one and a simulation using CDM. IMO simulating the same galaxy rotation using only ordinary matter is not more difficult. In fact the total galaxy rotation curve is much larger and stretches far into the Kuiper Belt.

Nicolaas Vroom http://users.pandora.be/nicvroom

next posting Mesg3


3 CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)

Van: Kent Paul Dolan
Onderwerp: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)
Datum: vrijdag 29 januari 2010 1:23

Oh No a écrit in posting Mesg2:

[Somewhat sorry for chopping unneeded attributions of the top of this.]

>>> Robert L. Oldershaw wrote:

>>>> In the Jan. 14th issue of Nature is a paper that >>>> claims to resolve a serious problem that has >>>> plagued the CDM cosmology for a long time.

I emphasize here the word "claims". As far as I know, the situation is "resolves", full stop.

People who can't or won't do the math before commenting shouldn't be casting aspersions on the writings of those who can and do.

This is the tip of the iceberg of a larger problem.

This unaccepted resolution of a problem in an existing theory brings to mind a failure (within my knowledge) of the system of scientific journals in general, which affects discussions here.

To show first how an alternative system works...

The Request for Comments series of Internet Engineering Task Force papers,

http://rfc-editor.org/rfc-index.html

which are in many cases pretty much the law, despite the mildness of the series title, for how the Internet will work, are comprised of papers submitted by interested participants and reviewed and maintained available by the IETF working body.

Each, in the above index, carries with it, among other possible annotations, "updates", "is updated by", "obsoletes", and "is obsoleted by".

This makes it possible to navigate easily to the most current information.

Thus, one is not tempted to cite papers overcome by events as if they were the current understanding of matters.

This avoids the endless rounds of rediscussions of already visited subjects based on superseded information that is so prevalent in sci.astro.research, one among several myriad of similarly afflicted discussion forums, and many instances of which have come on display here in the weeks recently passed.

In part, this problem exists simply because there is not a mechanism such as the one above for RFCs to annotate a list of "all the journal articles since the inventtion of writing", that update or obsolete earlier articles, despite, say, that the later article may be published in some obscure journal with little reputation and the former article may have been published in a journal with great prestige.

It can far too often be the case that an article suggesting, correctly, that some deeply entrenched folk wisdom of science is just flatly wrong has to go begging to the most mediocre of journals for page space, at best. [In times past, the situation was certainly even worse, as persecutions of scientists for correctly opposing church dogma document.]

Is there any hope for a way to be put forward by someone participating here (not by me, my knowledge is insufficient, as are my political skills), or adopted from elsewhere, to shed the endless rounds of bickering, based entirely on superseded information, which eat up so much of the meme-space here?

I'd really like to read much more about interesting new research here, and much less from long simmering interpersonal feuds, science repudiated by convincing evidence (even if not convincing to its proponent) or kook science unsupported by evidence convincingly (beyond the mind of the originator) differentiating its validity from accepted science and in its favor.

The internet already has endless venues for kook science, one of them founded by a participant here, and for interminable arguments (29 of them on Usenet attributable to me).

IMWTK

xanthian.

A similar situation prevails here where persons introducing themselves as having little knowledge, ask questions and then spend endless rounds of postings quarreling with or ignoring in their responses answers furnished to their questions by experts, while making no attempt to consult recommended reference material.

Sigh.

[Mod. note: however accurate it may be, further meta-discussion of the standards of posting to the group is discouraged. I can only say that if s.a.r. was filled with professional astronomers talking to each other about genuine research issues, nobody would be happier than your humble servant -- mjh]

next posting Mesg4


4 CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)

Van: Oh No
Onderwerp: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)
Datum: vrijdag 29 januari 2010 22:19

Thus spake Kent Paul Dolan in posting Mesg3
> [Somewhat sorry for chopping unneeded attributions of the top of this.]
> >>>

Robert L. Oldershaw wrote:

>
> >>>>

In the Jan. 14th issue of Nature is a paper that claims to resolve a serious problem that has plagued the CDM cosmology for a long time.

>

I emphasize here the word "claims". As far as I know, the situation is "resolves", full stop.

People who can't or won't do the math before commenting shouldn't be casting aspersions on the writings of those who can and do.

In this case you are wrong. The math has been done, in the form of simulations on supercomputers (don't ask individuals to do that kind of math), and this inconsistency in CDM is well known). It is you who should understand the situation before making claims that it has been resolved.

[Mod. note: perhaps references would prevent this discussion from degenerating into contradiction -- mjh]

>

This is the tip of the iceberg of a larger problem.

It is indeed. A hypothesis is presented, and becomes accepted as de facto scientific knowledge siimply because it is the first idea which gains currency. Then problems with that hypothesis are ignored, and alternative hypothesis are dismissed as overspeculative, although in fact they may be no more speculative and less problematic than the first hypothesis.

Your post is a case in point. You want to suppress discussion, although the situation with CDM is very far from resolved.

> Is there any hope for a way to be put forward by someone participating here (not by me, my knowledge is insufficient, as are my political skills), or adopted from elsewhere, to shed the endless rounds of bickering, based entirely on superseded information, which eat up so much of the meme-space here?

>

A similar situation prevails here where persons introducing themselves as having little knowledge, ask questions and then spend endless rounds of postings quarreling with or ignoring in their responses answers furnished to their questions by experts, while making no attempt to consult recommended reference material.

I cannot help but remark on the conflict between these two paragraphs. You yourself profess insufficient knowledge in the one, and yet you then take the very role which you criticise in the second.

Regards

-- Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and braces)

http://www.rqgravity.net

next posting Mesg5


5 CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)

Van: Nicolaas Vroom
Onderwerp: CDM Cosmology (was Re: formation of dwarf galaxies in CDM cosmology)
Datum: woensdag 3 februari 2010 1:53

"Oh No" schreef in bericht posting Mesg4
>

In this case you are wrong. The math has been done, in the form of simulations on supercomputers.

That is correct. However all of this is very complicated and very difficult to evaluate if it is done correctly.

To get an idea about the software being used read this: http://www.mpa-garching.mpg.de/galform/gadget/gadget2-paper.pdf "The cosmological simulation code GADGET-2" by Volker Springel The above software also takes supernovae into account. The details are at page 1109. SPH = Smoothed Particle Hydrodynamics

For a general document about SPH simulations read this: http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.3115v1.pdf "Gas cooling in semi-analytic models and SPH simulations: are results consistent?" Specific read the introduction. Here we read: "In fact, lacking a 'complete theory' of star formation (as well as of almost all the physical processes at play), we are currently not in the position to model galaxy formation from first principles"

For a detailed document using Gadget 2 read this: http://arxiv.org/PS_cache/arxiv/pdf/0909/0909.0664v1.pdf "Simulating the effect of AGN feedback on the metal enrichment of galaxy clusters"

IMO there are two types of simulations:
1. N body simulations. A typical application is the merger of two galaxies.
2. Physical simulations. A typical application is one supernova. Of course you can combine those two. The problem is they are extremely complex. For example: to study metal enrichment in a galaxy. The easy part is that you can test your simulation because this simulation belongs to the visible realm. What IMO is also important that this is some sort of mixing process i.e. the leftovers from the supernova are the building blocks for the next generation of "heavier" stars.

In the simulation under discussion: "Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows" the physics involved is totally different: the underlying concept is gas removal.
The question now becomes: How does CDM responds to a supernova ? Will it stay within or will it be removed from ? IMO the simplest is a mixing process. A removal process which results in a halo (NFW profile) is much more complex.
See also: http://en.wikipedia.org/wiki/Navarro%E2%80%93Frenk%E2%80%93White_profile The extra complication is that this whole process which involves dark matter is invisible.

For more comments see: http://users.telenet.be/nicvroom/nature%2014%20Jan%202010.htm

> Regards

-- Charles Francis

http://www.rqgravity.net

Nicolaas Vroom


Created: 13 March 2010

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