Starship Asterisk*

Don't shoot the messenger. I am only reporting the news, not creating it. Here is a direct quote from an article by Jerome Drexler (former Research Professor in physics at New Jersey Institute of Technology, founder and former Chairman and chief scientist of LaserCard Corp):

"Recently, Science magazine published three papers or articles questioning the Cold Dark Matter hypothesis, namely on May 25,"Missing Mass in Collisional Debris from Galaxies", on August 3, "Seeing Through Dark Matter" and on September 14, "Lighting the Universe with Filaments".

Hopefully, Science magazine will be considered for a Pulitzer Prize in journalism for exposing the apparent mainstream Cold Dark Matter hypothesis that has evolved into a scientific mistake. Unfortunately this scientific mistake continues to retard progress in cosmology, demoralize cosmology researchers, and ill-prepares future cosmology researchers."

He goes on to quote Michael Disney, an Emeritus Professor in the School of Physics and Astronomy at Cardiff University in the UK, who has this to say in his recent article in the September-October 2007 issue, Volume 95, of the American Scientist magazine:

"The currently fashionable concordance model of cosmology (also known to the cognoscenti as 'Lambda-Cold Dark Matter,' or 'LambdaCDM') has 18 parameters, 17 of which are independent. Thirteen of these parameters are well fitted to the observational data; the other four remain floating. This situation is very far from healthy. Any theory with more free parameters [hypotheses] than relevant [astronomical] observations has little to recommend it. Cosmology has always had such a negative significance, in the sense that it has always had fewer [astronomical] observations than free parameters [hypotheses] (as is illustrated on page TK), though cosmologists are strangely reluctant to admit it. While it is true that we presently have no alternative to the Big Bang in sight, that is no reason to accept it. Thus it was that witchcraft took hold."

"The three successful predictions of the concordance model (the apparent flatness of space, the abundances of the light elements and the maximum ages of the oldest star clusters) are overwhelmed by at least half a dozen unpredicted surprises, including dark matter and dark energy. Worse still, there is no sign of a systematic improvement in the net significance of cosmological theories over time."

So we should vigorously discuss the limitations of current theory, in the hopes that ideas for better theories will come forward.

goredsox wrote:Don't shoot the messenger. I am only reporting the news, not creating it. Here is a direct quote from an article by Jerome Drexler (former Research Professor in physics at New Jersey Institute of Technology, founder and former Chairman and chief scientist of LaserCard Corp):

"Recently, Science magazine published three papers or articles questioning the Cold Dark Matter hypothesis, namely on May 25,"Missing Mass in Collisional Debris from Galaxies", on August 3, "Seeing Through Dark Matter" and on September 14, "Lighting the Universe with Filaments".

Hopefully, Science magazine will be considered for a Pulitzer Prize in journalism for exposing the apparent mainstream Cold Dark Matter hypothesis that has evolved into a scientific mistake. Unfortunately this scientific mistake continues to retard progress in cosmology, demoralize cosmology researchers, and ill-prepares future cosmology researchers."

He goes on to quote Michael Disney, an Emeritus Professor in the School of Physics and Astronomy at Cardiff University in the UK, who has this to say in his recent article in the September-October 2007 issue, Volume 95, of the American Scientist magazine:

"The currently fashionable concordance model of cosmology (also known to the cognoscenti as 'Lambda-Cold Dark Matter,' or 'LambdaCDM') has 18 parameters, 17 of which are independent. Thirteen of these parameters are well fitted to the observational data; the other four remain floating. This situation is very far from healthy. Any theory with more free parameters [hypotheses] than relevant [astronomical] observations has little to recommend it. Cosmology has always had such a negative significance, in the sense that it has always had fewer [astronomical] observations than free parameters [hypotheses] (as is illustrated on page TK), though cosmologists are strangely reluctant to admit it. While it is true that we presently have no alternative to the Big Bang in sight, that is no reason to accept it. Thus it was that witchcraft took hold."

"The three successful predictions of the concordance model (the apparent flatness of space, the abundances of the light elements and the maximum ages of the oldest star clusters) are overwhelmed by at least half a dozen unpredicted surprises, including dark matter and dark energy. Worse still, there is no sign of a systematic improvement in the net significance of cosmological theories over time."

So we should vigorously discuss the limitations of current theory, in the hopes that ideas for better theories will come forward.

Do you have a reference to the issue of Science article? It would be interesting to read it.

ADS has five entries for J. Drexler, two of which seem to have little to do with DM, and the remaining three basically just cite the earlier J. Drexler material. The only one of the five which seems relevant is this one. I'm sure if you search persistently you will find dozens of 'wild card' papers like this; no doubt most go nowhere, being cited by only the author her/himself, or perhaps a student or two.

Mike Disney has been a persistent critic of mainstream cosmology for many years now; perhaps the most interesting of his recent papers dealing with cosmology is this one. Note that his stance is quite radical, and would make good study material for an HPS student ('examine competing claims concerning whether modern cosmology is a science', for example, or 'Disney's critique of modern mainstream cosmology: not Lakatosian, not Kuhnian, not even Popperian?').

The limitations of current understandings of DM are indeed vigourously discussed and debated; more importantly, from the POV of astronomy as a science, hundreds of hours of time on the best telescopes are devoted exclusively to testing 'current theory', and millions of dollars are being spent searching for DM particles, in labs on, above, and below ground.

To take just one example (from about a dozen or three) in last week's preprints on astro-ph: "Angular Signatures of Annihilating Dark Matter in the Cosmic Gamma-Ray Background":

The cosmic gamma-ray background (CGB) is a very promising channel to look for signatures of dark matter annihilation. Indeed, if gamma-rays from dark matter constitute a relevant fraction of the CGB, then, together with an imprint in the energy spectrum, peculiar angular signatures are also expected. In particular, the expected anisotropies differ significantly compared to the anisotropies from emission of astrophysical sources only, provided that annihilation in sub-galactic clumps and/or in cuspy haloes gives only a moderate enhancement in the dark matter signal. In this scenario the differences are at a level detectable with the forthcoming GLAST observatory. As complementary observables we further introduce the cross-correlation between surveys of galaxies and the CGB and the cross-correlation between different energy bands of the CGB and we study their sensitivity to the dark matter angular signatures. We find that a combined analysis of all the anisotropy observables provides a powerful tool to investigate the dark matter signal.

The Cosmic Gamma Background (CGB) Radiation article you linked to is a good example of what I am after. I think that scientists in the field of cosmology are sometimes unduly quick to draw unilateral conclusions from their observations, and are prone to use what I call "circular links" to link observations with theories. Perhaps my standards for science are too high; but in other fields of science such as biology and engineering, the links between observations and theories are more linear.

For instance, Cuoco asserts in the last article you cited that

"if gamma-rays from dark matter constitute a relevant fraction of the CGB, then, together with an imprint in the energy spectrum, peculiar angular signatures are also expected. In particular, the expected anisotropies differ significantly compared to the anisotropies from emission of astrophysical sources only, provided that annihilation in sub-galactic clumps and/or in cuspy haloes gives only a moderate enhancement in the dark matter signal."

I think it is extraordinary to draw conclusions about dark matter from CGB when we do not know the composition of dark matter. Of course, you will say, "Well, how else are we supposed to find out anything about what constitutes dark matter? We have to look at CGB to find out!!" But the problem is, any interpretation of CGB data to prove a property of dark matter is dependent upon the assumptions made about the properties of dark matter that allow us to use CGB as a probe. In other words, it is a circular construct which cannot be independently verified.

goredsox wrote:The Cosmic Gamma Background (CGB) Radiation article you linked to is a good example of what I am after. I think that scientists in the field of cosmology are sometimes unduly quick to draw unilateral conclusions from their observations, and are prone to use what I call "circular links" to link observations with theories. Perhaps my standards for science are too high; but in other fields of science such as biology and engineering, the links between observations and theories are more linear.

For instance, Cuoco asserts in the last article you cited that

"if gamma-rays from dark matter constitute a relevant fraction of the CGB, then, together with an imprint in the energy spectrum, peculiar angular signatures are also expected. In particular, the expected anisotropies differ significantly compared to the anisotropies from emission of astrophysical sources only, provided that annihilation in sub-galactic clumps and/or in cuspy haloes gives only a moderate enhancement in the dark matter signal."

I think it is extraordinary to draw conclusions about dark matter from CGB when we do not know the composition of dark matter. Of course, you will say, "Well, how else are we supposed to find out anything about what constitutes dark matter? We have to look at CGB to find out!!" But the problem is, any interpretation of CGB data to prove a property of dark matter is dependent upon the assumptions made about the properties of dark matter that allow us to use CGB as a probe. In other words, it is a circular construct which cannot be independently verified.

The question of 'independent verification' is an interesting one, in the fields of astronomy, astrophysics, and (above all) cosmology, for those who do scientific research in these fields.

Let's start with something that is both obvious and quite profound: what is the observational basis for astronomy beyond the solar system?

With some very few exceptions*, it is the detection of photons, and analyses of those detections.

So:

Question: how do we get from detecting lots of photons from 'the Crab nebula' (example) to 'expanding debris from the death explosion of a massive star'?
Answer: by a 'cosmological principle' and textbook physics.

There are, no doubt, many ways to write cosmological principles; for our purposes here and now something as simple as 'the physics we can work out in our Earth-bound labs applies universally' will likely do.

This enables us to say that certain, quite prominent, lines in the spectra of many astronomical objects are [OIII] or [NII] (for example), despite the fact that no such lines have ever been seen in any earthly lab. Why? Because textbook physics predicts that such atomic transitions (called 'forbidden lines') will occur in very low density gases, even though we cannot create such low densities in our best lab vacuums.

Similarly, we cannot test our understanding of stars by creating gravitationally bound objects of mass 2x10^30 kg and composition 74% H, 25% He in our labs.

And so on, all the way to 'dark (non-baryonic) matter', with white dwarfs (electron degeneracy), neutron stars (nuclear degeneracy), black holes (general relativity, particle physics), and much more, along the way.

In other words, once you accept the foundation (a cosmological principle and textbook physics), you cannot escape the conclusion (e.g. a lot of non-baryonic dark matter).

Oh, there is one other core component: as with biology as a science, theories rule in the engine room, and you only abandon a theory when a better one comes along, even if this means having a shelf-full of 'anomalies' in the meantime. Or, with specific reference to dark (non-baryonic) matter, how else do you account for the millions of excellent, relevant observations?

*The exceptions are: ~20 neutrinos (from SN 1987a), an isotropic rain of ('galactic') cosmic rays, a few picograms of inter-stellar medium dust, and a few atoms of neutral helium. And for all these, except the cosmic rays, an origin beyond the solar system is a deduction, an inference, or an assumption (depends how you choose to view science).

We actually agree on this issue a lot more than I realized. I think it would be a very useful exercise to literally go step by step like you just did, from detecting light-wavelength photons, detecting x-rays, detecting radio waves, detecting microwave radiation, detecting gamma, detecting spectra and red shifts, detecting lensing, detecting pulsating signals and bursts and so on. Really get down to the nitty gritty observations. Try to list them in chronologic order and SEPARATE THEM from all the theories that we use to interpret them. List all of the theories and the mathematics too, but go through it chronologically, and include alternative theories, side theories, and theories that have been discarded, and why. Then link the theories to the observations, but assess how firm (non-circular) each link is. Once in a while I think you get 3 or 4 weak links in a chain of theories without an independent observation bolster it. Also once in a while there will be a weak chain of links but suddenly, at the end, there WILL be an independent observation to boster it after the 4th link.

This would be a gargantuan task, but probably could be undertaken by a group of cosmologists, perhaps even in a forum like this one. I've done a lot of reading, but I haven't found anything quite like this out there. It would lay out a better foundation for discussion for all of us, and I think if it were all layed out (spoon fed, if you will) scientists of all persuations would be more likely to not only develop productive conversations in these forums but would probably come up with some really exciting, but useful, theories. The closest thing I have found so far is APOD, which is like a catalog of 4,300 observations. But an impartial assessment of the theories I find lacking.

goredsox wrote:We actually agree on this issue a lot more than I realized. I think it would be a very useful exercise to literally go step by step like you just did, from detecting light-wavelength photons, detecting x-rays, detecting radio waves, detecting microwave radiation, detecting gamma, detecting spectra and red shifts, detecting lensing, detecting pulsating signals and bursts and so on. Really get down to the nitty gritty observations. Try to list them in chronologic order and SEPARATE THEM from all the theories that we use to interpret them. List all of the theories and the mathematics too, but go through it chronologically, and include alternative theories, side theories, and theories that have been discarded, and why. Then link the theories to the observations, but assess how firm (non-circular) each link is. Once in a while I think you get 3 or 4 weak links in a chain of theories without an independent observation bolster it. Also once in a while there will be a weak chain of links but suddenly, at the end, there WILL be an independent observation to boster it after the 4th link.

This would be a gargantuan task, but probably could be undertaken by a group of cosmologists, perhaps even in a forum like this one. I've done a lot of reading, but I haven't found anything quite like this out there. It would lay out a better foundation for discussion for all of us, and I think if it were all layed out (spoon fed, if you will) scientists of all persuations would be more likely to not only develop productive conversations in these forums but would probably come up with some really exciting, but useful, theories. The closest thing I have found so far is APOD, which is like a catalog of 4,300 observations. But an impartial assessment of the theories I find lacking.

Sounds like a plan, I'll put some thoughts together.

In the meantime, here is something you may find interesting: What is the observational basis for (cold, non-baryonic) dark matter?

Hello Neried

You said in another forum

Two classes of 'dark matter' are somewhat tangential to this thread - 'hot dark matter' ('dark matter' which is, or was, moving at relativistic speeds; neutrinos are an example), and 'baryonic dark matter' (or 'ordinary matter' - atoms and molecules and ions of H, He, ..., whether in the form of plasma, gas, dust, or bigger clumps that does not emit detectable electromagnetic radiation; this also includes baryonic matter in the form of white dwarfs or neutron stars).

I agree with you.


Smile,,,,,but not limited to it.

goredsox wrote:We actually agree on this issue a lot more than I realized. I think it would be a very useful exercise to literally go step by step like you just did, from detecting light-wavelength photons, detecting x-rays, detecting radio waves, detecting microwave radiation, detecting gamma, detecting spectra and red shifts, detecting lensing, detecting pulsating signals and bursts and so on. Really get down to the nitty gritty observations. Try to list them in chronologic order and SEPARATE THEM from all the theories that we use to interpret them. List all of the theories and the mathematics too, but go through it chronologically, and include alternative theories, side theories, and theories that have been discarded, and why. Then link the theories to the observations, but assess how firm (non-circular) each link is. Once in a while I think you get 3 or 4 weak links in a chain of theories without an independent observation bolster it. Also once in a while there will be a weak chain of links but suddenly, at the end, there WILL be an independent observation to boster it after the 4th link.

This would be a gargantuan task, but probably could be undertaken by a group of cosmologists, perhaps even in a forum like this one. I've done a lot of reading, but I haven't found anything quite like this out there. It would lay out a better foundation for discussion for all of us, and I think if it were all layed out (spoon fed, if you will) scientists of all persuations would be more likely to not only develop productive conversations in these forums but would probably come up with some really exciting, but useful, theories. The closest thing I have found so far is APOD, which is like a catalog of 4,300 observations. But an impartial assessment of the theories I find lacking.

A few random thoughts of what to look into ...

Scope: 'beyond the solar system' astronomy

Astronomy in any waveband other than the visual* didn't even start until after the two great (20th century) revolutions in physics were well established (quantum mechanics and relativity) ... radio astronomy began in the 1930s.

By the 1930s, the idea of finding something first in 'the heavens', and later 'on Earth' was well established; it's a history that goes back at least to Newton (Cavendish didn't do his famous - lab - experiment until the closing years of the 18th century), and includes helium; the discovery of the positron (around the same time as radio astronomy began) in cosmic rays a nice historical coincidence.

By the 1930s, the nature of stars as 'distant suns' was well established, though the details of what powered them were not worked out for another decade or so.

The 'distance ladder' was in place, albeit with some considerable inaccuracies in rungs above direct ('geometric') parallax.

The bifurcation of 'nebulae' into 'island universes' and {the rest} had become established only a decade earlier.

For our purposes, perhaps the most powerful legacy of astronomy, up until the 1930s, is General Relativity (GR), and the strong equivalence principle, which is one aspect of a cosmological principle. The great success of GR, in terms of passing specifically designed tests, has lead to a reluctance to develop ad hoc alternative theories. This is particularly pertinent wrt (non-baryonic) dark matter (DM): there are alternative theories (to GR), but even the best (MOND, and its relativistic successors such as TeVeS) still requires some (non-baryonic) DM to account for relevant observations.

With a speed that is astonishing when compared to the pace of astronomy over the previous centuries, much of the rest of the electromagnetic spectrum became available to astronomers. Today, only very low frequency radio, some (small) parts of the far infrared (perhaps), and the gamma ray spectrum above a few TeV remain entirely unexplored.

For cosmology, the most exciting thing, these last 70 or so years, has been the CMB (cosmic microwave background). Scientifically, perhaps the most amazing thing has been how well modern LCDM (lambda cold dark matter) models can account for all relevant observations!

Back to this:

Try to list them [the nitty gritty observations] in chronologic order and SEPARATE THEM from all the theories that we use to interpret them.

It's actually very easy to do, and doing so makes you realise just how poor astronomy would be without the relevant theories!

There's almost nothing you say, cosmologically speaking, if all you have is your unaided vision ... Olbers' paradox.

Telescopes may be considered (empirical) extensions of eyes, but their construction requires at least Newtonian theories (gravity and optics, for example).

Photographic plates may be considered (empirical) extensions of eyes, but you need at least some theory of light to interpret what they show (consider the UV, filters, and so on).

Spectra are meaningless without the quantum theory of the atom, especially all those prominent 'nebular lines' (initially thought to be an unknown element, nebulium).

And so on ... you even need GR to do at least one part of modern astronomy (VLBI radio astronomy; consider locations and clock synchronisation)!

In a nutshell, you need the Standard Model (of particle physics; SM), which incorporates the relevant quantum bits, and GR to do modern astronomy; not surprisingly, these are what's used to interpret the observations.

Finally (for now):

alternative theories, side theories, and theories that have been discarded, and why

To SM and GR? There aren't any ... at least, none that have survived testing in earthly labs.

What parts of these random notes would you like to dig deeper into goredsox?

*This includes the portion of the UV between where the atmosphere becomes opaque and the blue limit of human vision; it also includes a portion of the near infrared, though this was not much used, by astronomers, up to then (IIRC).

I would like to dig deeper into the specific question of evidence for cold dark matter theory (CDMT), which we infer exists due to unsolved problems with observations. In the bautforum thread you referred to above, a detailed list of reasons why science needs CDMT is given, including the following:

1) to get the gravity right in spiral galaxies
2) to get the gravity right in clusters of galaxies, as inferred from their motions
3) to get enough gravity without messing up the observed fractions of light nuclei that were formed early in the Big Bang
4) to explain how galaxies were able to form so quickly from a nearly homogeneous matter distribution
5) to avoid altering the theory of gravity, a very successful theory with excellent axiomatic underpinnings, with something completely ad hoc that so far has not proved to be promising.

I couldn't help but notice that these particular five top reasons given for CDMT have to do with problems with gravity. So CDMT was developed, and it conveniently corrects all of these discrepancies.

Here is my proposal. Maybe there is something wrong with gravity theory. It has happened before. Classic Newtonian gravity theory worked for masses up to 10^24 kg, but then obsersations did not match for larger systems. Then GR gravity theory came along and it worked better for masses up to 10^30 kg. Now we have at least 5 observations, all for masses above 10^40 kg that don't work with either Newtonian or GR. We already know that GR gravity theory is incompatible with quantum mechanics. So we have a choice. We can either come up with a theory that expands on GR, or we can introduce a fudge factor (CDMT) and keep GR the same.

I have no data that says one way is better than the other. But I would like skeptics out there to think seriously about both paths equally.

goredsox wrote:I would like to dig deeper into the specific question of evidence for cold dark matter theory (CDMT), which we infer exists due to unsolved problems with observations. In the bautforum thread you referred to above, a detailed list of reasons why science needs CDMT is given, including the following:

1) to get the gravity right in spiral galaxies
2) to get the gravity right in clusters of galaxies, as inferred from their motions
3) to get enough gravity without messing up the observed fractions of light nuclei that were formed early in the Big Bang
4) to explain how galaxies were able to form so quickly from a nearly homogeneous matter distribution
5) to avoid altering the theory of gravity, a very successful theory with excellent axiomatic underpinnings, with something completely ad hoc that so far has not proved to be promising.

I couldn't help but notice that these particular five top reasons given for CDMT have to do with problems with gravity. So CDMT was developed, and it conveniently corrects all of these discrepancies.

Here is my proposal. Maybe there is something wrong with gravity theory. It has happened before. Classic Newtonian gravity theory worked for masses up to 10^24 kg, but then obsersations did not match for larger systems. Then GR gravity theory came along and it worked better for masses up to 10^30 kg. Now we have at least 5 observations, all for masses above 10^40 kg that don't work with either Newtonian or GR. We already know that GR gravity theory is incompatible with quantum mechanics. So we have a choice. We can either come up with a theory that expands on GR, or we can introduce a fudge factor (CDMT) and keep GR the same.

I have no data that says one way is better than the other. But I would like skeptics out there to think seriously about both paths equally.

Spot on!

However, scientists are, by nature, very conservative. GR has been extensively tested, and has passed all the tests, to date. A good many research programmes are on-going, or proposed, that will test GR even more thoroughly.

There's also, as I mentioned, the cosmological principle - hard to see how abandoning that would lead very far, in terms of science.

Further, no one has (yet) come up with even a clue how to extend, modify GR, to produce even a hypothesis that could be tested*.

Of course, the next Einstein may very well be, right now, working on just that theory ... or that person may not be born yet for another 1000 years.

But here's one thing about CDMT that helps keep it alive and well within the relevant science communities: a very simple hypothesis can explain a huge range of observations (from spiral galaxy rotation curves, through X-ray and microwave observations of rich clusters, to gravitational lensing, to the CMB!); observations which, on the face of it, have nothing to do with one another.

Putting this another way: the observations fully make sense if there is this (non-baryonic) cold dark matter; it's as if the universe is behaving just like it should, if there were such CDM.

Further, there are many precedents for this, in the history of astronomy and physics; think of Neptune, exo-solar planets discovered by one means and confirmed by another (doppler and transits, for example), the neutrino, ...

*This is an exaggeration; there have been dozens and dozens of such, but AFAIK none has even got to the 'free of serious internal inconsistencies' stage, except MOND and its generalisations (e.g. TeVeS).

If you're interested in keeping abreast of research into gravity, the arXiv.org sub-directory General Relativity and Quantum Cosmology may interest you.

Just in the last week there are some 60 new preprints.

Of course, not all are on CDM! Nor are all of them devoted to new theories of gravity with the specific intent of addressing astronomical CDM observations.

===============

Also, there's another reason why CDM is of particular interest to physicists: the shortcomings of the Standard Model (of particle physics). Many extensions have been proposed, including whole classes of super-symmetry (SUSY) models. In many of these, the lightest super-symmetry particle is stable, and has properties that make it an ideal candidate for the cold dark matter which astronomers observe (or infer). And that's just one example ...

IIRC, there are quite a few planned LHC experiments which involve searching for footprints of one or other new theory that may contain a suitable CDM particle ...

Well if there are that many independent threads (no pun intended) to CDMT I am encouraged that something will come of it that will elevate its status. Thanks for all the links and the comments.

I am wondering if anyone has considered the possibility that the massive dust lanes that are seen in virtually every image of spiral galaxies are actually composed of CDM. I understand that CDM does not emit light, nor does it reflect light. I would think it would at least interfer with light passage, particularly if concentrated and in the massive quantities proposed. Or is it supposed to be entirely transparent?

From what i read, CDM does interfere with light by gravitation. They tought of dark matter with the CBM result and then test it by looking at cluster of galaxy and they saw that the light was deviate more then it should if there was only the gravity of a cluster of galaxy. The intensity of the light deviation correspond to what the dark matter mass estimate should be in the univers (Total gravity). Many hypothesis will come and go for sure but none are better for now.

"The dark connection between the Canis Major dwarf, the Monoceros ring, the gas flaring, the rotation curve and the EGRET excess of diffuse Galactic Gamma Rays" (preprint available here) is one example of a (very!) recent paper looking for the 'particle' footprint of (local) dark matter ... in this case, in terms of diffuse GeV gamma emission as a signal of dark matter particle annihilation.

It may be that another decade or so of these (and similar) observations will result in quite strong constraints on a major particle component of CDM, at least in the MW halo ...

Two interesting (back to back!) preprints, dated 6 November, 2007:

Solar System tests of some models of modified gravity proposed to explain galactic rotation curves without dark matter

In this paper we consider the recently estimated corrections to the non-Newtonian/Einsteinian secular precessions of the perihelia of several planets of the Solar System in order to evaluate whether they are compatible with the predicted precessions due to models of modified gravity put forth to account for certain features of the rotation curves of galaxies without resorting to dark matter. In particular, we consider a logarithmic-type correction and a f(R) inspired power-law modification of the Newtonian gravitational potential. The results obtained by taking the ratio of the apsidal rates for different pairs of planets show that the modifications of the Newtonian potentials examined in this paper are not compatible with the secular extra-precessions of the perihelia of the Solar System's planets estimated by E.V. Pitjeva as solve-for parameters processing almost one century of data with the latest EPM ephemerides. However, we would advise caution because, at present, it is not yet possible to make comparisons with the planetary apsidal extra-precessions independently estimated by other teams of astronomers.

Quantum Physics Exploring Gravity in the Outer Solar System: The Sagas Project

We summarise the scientific and technological aspects of the SAGAS (Search for Anomalous Gravitation using Atomic Sensors) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements.

We certainly do live in interesting times!

Here is another solid critique in the journal Nature (Nov 2):

http://www.nature.com/news/2007/071102/ ... 7.215.html

Theorist suggests mysterious force could be an 'artefact' of a void in space.

Here's another question for all the Dark Matter enthusiasts out there. Has anyone ventured to calculate how close the closest Dark Matter to earth should be? Galactic core? Oort Cloud? Or closer?? Kuiper Belt? Asteroid Belt? Also, if Dark Matter is permeating our solar system (undetectable except by gravitational effects) shouldn't the solar wind interact with it?

goredsox wrote:Here is another solid critique in the journal Nature (Nov 2):

http://www.nature.com/news/2007/071102/ ... 7.215.html

Theorist suggests mysterious force could be an 'artefact' of a void in space.

It may be a solid critique of dark energy, but it seems to have nothing to do with dark matter!

And I'm not sure it's even the former ... all that link gives you is a request for payment, in order to get an article written by Geoff Brumfiel, a journalist. Do you know what the actual paper is? For instance, who the author(s) is (are)?

Here's another question for all the Dark Matter enthusiasts out there. Has anyone ventured to calculate how close the closest Dark Matter to earth should be? Galactic core? Oort Cloud? Or closer?? Kuiper Belt? Asteroid Belt? Also, if Dark Matter is permeating our solar system (undetectable except by gravitational effects) shouldn't the solar wind interact with it?

There are quite a few papers on just this topic!

IIRC, the over-simplified summary is that the solar system likely contains dark matter whose combined mass is less than that of the Earth.

Depending on what its nature is, we should be able to detect DM with equipment in labs here on Earth ... or, more likely, deep under the Earth. Here is one such search.

No, DM is not expected to interact with the solar wind, except via gravitation. One way to think of it: DM is like neutrinos only more massive and more ghostly; i.e. it is 'cold' (only moves at ~km/s, not some appreciable fraction of c) and 'dark' (does not interact with normal matter, like neutrinos, only more so).

The article is an editorial by Brumfiel in Nature which is based on an original article by Subir Sarkar, Prof of Astronomy at Oxford, and yes (oops) I accidently changed the subject from Dark Matter to Dark Energy. Here is the abstract from Sarkar, as posted on arXiv.org:

Is the evidence for dark energy secure?
Authors: Subir Sarkar (Oxford U.)
(Submitted on 28 Oct 2007)
Abstract: Several kinds of astronomical observations, interpreted in the framework of the standard Friedmann-Robertson-Walker cosmology, have indicated that our universe is dominated by a Cosmological Constant. The dimming of distant Type Ia supernovae suggests that the expansion rate is accelerating, as if driven by vacuum energy, and this has been indirectly substantiated through studies of angular anisotropies in the cosmic microwave background (CMB) and of spatial correlations in the large-scale structure (LSS) of galaxies. However there is no compelling direct evidence yet for (the dynamical effects of) dark energy. The precision CMB data can be equally well fitted without dark energy if the spectrum of primordial density fluctuations is not quite scale-free and if the Hubble constant is lower globally than its locally measured value. The LSS data can also be satisfactorily fitted if there is a small component of hot dark matter, as would be provided by neutrinos of mass 0.5 eV. Although such an Einstein-de Sitter model cannot explain the SNe Ia Hubble diagram or the position of the `baryon acoustic oscillation' peak in the autocorrelation function of galaxies, it may be possible to do so e.g. in an inhomogeneous Lemaitre-Tolman-Bondi cosmology where we are located in a void which is expanding faster than the average. Such alternatives may seem contrived but this must be weighed against our lack of any fundamental understanding of the inferred tiny energy scale of the dark energy. It may well be an artifact of an oversimplified cosmological model, rather than having physical reality.

But we digress. I really did want to stick with Dark Matter and gain a better understanding of Current Thinking. It is very interesting what you said about the solar system containing enough dark matter to equal something less than the mass of Earth. Obviously it is spread very thin. Would it be fair to say that it is spread homogeneously throughout space in the solar system? If so, would it then be fair to say that there is Dark Matter actually embedded within Baryonic (Normal Matter) in the earth's atmosphere or oceans or interior as it moves through at a snails pace of a few km/second? If so it must be a small amount, perhaps amounting to the equivalent of a tennis ball dispersed dilutely throughout earth in our midst. I find this fascinating to contemplate.

goredsox wrote:Obviously it [dark matter in the solar system] is spread very thin.

It's worth spending a little time on the estimated density of DM in the solar system.

Start with some assumptions: that the average mass-energy density of the universe is 'critical', and that it is 1 x 10^-29 grams/cubic cm*; that CDM comprises 20% of the universe's total mass-energy. Thus, on average, the density of CDM is ~2 x 10^-30 g/cc.

If a 50 au sphere centred on the Sun contains CDM at the universal average density, how much mass is that? I get ~4 x 10^12 kg (please check the calculation!), which is less than one billionth of the mass of the Moon.

According to Iorio, observations of the motion of solar system planets (etc) set an upper bound on the density of CDM in the solar system, assumed distributed spherically symmetrically and with uniform density, of ~10^-19 g/cc, which is (very approximately!) about the same as the mass of the Earth smeared out over that volume.

Assuming uniform distribution, what is the upper limit on the amount of CDM is within the Earth, at any given time?

One variety of CDM may have settled into the bottoms of local gravitational wells, such as the Sun's core, or the Earth's core. Such CDM cannot comprise more than a tiny fraction of universe's CDM, if only because so much of that CDM clearly does not reside in galaxy nuclei, or even galaxies, much less stars and planets. Nonetheless, it is an interesting exercise to estimate upper bounds on this kind of CDM. For example, perhaps there's a signal of such CDM lurking in the combined SOHO instruments, MDI/SOI and VIRGO, data+solar model visually represented here?

*These are not intended to be precise; for the purposes of illustration, order-of-magnitude (OOM) estimates are more than good enough.

Nereid, you said

One variety of CDM may have settled into the bottoms of local gravitational wells, such as the Sun's core, or the Earth's core. Such CDM cannot comprise more than a tiny fraction of universe's CDM, if only because so much of that CDM clearly does not reside in galaxy nuclei, or even galaxies, much less stars and planets.

Don't you think that that is a rather serious paradox??

CDM is supposed to follow gravitational laws, which means it should be captured by massive bodies such as galaxies. CDM should reside for the most part in galaxy nuclei, star cores, and planet cores, because it can pass right through normal matter and settle in the core. Or failing that, it should follow orbital pathways around masses. It sounds like you are saying that CDM does not follow gravitational laws.

Goredsox,

I see the paradox too? Or is it one?

From what i read so far no one know what the dark matter is exactly. It is an hypothesis to explain what we can see in the universe. If someone can define dark matter i wonder what it would be.

May be something like this.

- Non barionic matter(No electron or proton or neutron in it!!?)
- It has a gravitational effect (On barionic matter and light!!?)
- It can not agglomerate with itself.(It repel itself some how?)
then explaining why it is dissipated in the univers and why the expansion of the univers is in acceleration (Repelling itself).

I dont know but i try to understand or guess. If there was 11 or 26 or any number of dimensions just before the Big Bang and that there are 3 of space, one of time, a few in the nuclear force (weak and strong) and a few in gravitation and in electromagnetism and ... then at the end of the begining of the univers some of those 26 dimensions left might be dark matter and dark energy and they are in differents dimensions and are related to our 4 dimensions that we see and live trough the gravity those 2 matters have in common. (Both form of matter being in touch with gravity.).

So we can see the effect of that dark matter trough the gravity effect it has but not what it is since it is in another dimension. Ouf!!! That was hard to think. LOL "The univers on a single page by Doum."

Please i think that many would like ( me include) to have the best definition of what Dark matter is and is not. My attemp upthere is a start.

So if a scientist here can define "Dark matter" it will help. That way i will be less encline to go wild on a definition of matters and the entire univers.

Just found Nereid address of Astrophysic data system. Nice one.

http://adsabs.harvard.edu/

By typing Dark matter i found many publication on the subject. So i will try to get more info on dark matter before making wild guess on the univers . Thanks for the link.

goredsox wrote:Nereid, you said

One variety of CDM may have settled into the bottoms of local gravitational wells, such as the Sun's core, or the Earth's core. Such CDM cannot comprise more than a tiny fraction of universe's CDM, if only because so much of that CDM clearly does not reside in galaxy nuclei, or even galaxies, much less stars and planets.

Don't you think that that is a rather serious paradox??

CDM is supposed to follow gravitational laws, which means it should be captured by massive bodies such as galaxies. CDM should reside for the most part in galaxy nuclei, star cores, and planet cores, because it can pass right through normal matter and settle in the core. Or failing that, it should follow orbital pathways around masses. It sounds like you are saying that CDM does not follow gravitational laws.

A good example of how one's intuitions can mislead one!

A system of particles with mass which interact only gravitationally takes a very long time to reach an equilibrium state ('relax' is the technical term). And, somewhat counter-intuitively, that relaxed state is a spherical blob, with the mass density increasing smoothly, radially ... rather like a globular cluster.

But the bigger counter-intuitive part comes from reversing the importance of CDM and baryonic matter (intuitively) ... if CDM dominates over baryonic matter, in terms of mass, then the baryonic matter will merely go along for the CDM ride - the distribution of baryonic mass would reflect the distribution of CDM. So you would find galaxies at the centre of approximately spherical blobs of CDM, and clusters of galaxies at the centre of even bigger blobs ... like this.

As baryonic matter can interact electromagnetically, and via the strong and the weak forces, its distribution can be somewhat different from the dominant CDM in which it is embedded, so planets, stars, and even (small) galaxies are not necessarily the cores of CDM blobs.

Doum, I like the idea that you have, that dark matter and/or dark energy might be artifacts of other dimensions in our universe. I can certainly imagine that we would have difficulty working with other dimensions, and would find ways to bend physical laws so that we can collapse the other dimensions into our favorite four dimensions.

Nereid, let's accept the premise that CDM dominates over baryonic matter. Let's also accept the premise that the distributions of CDM and baryonic matter overlap but do not fully correspond with each other, but both have formed very large "clumpy" structures, interspersed with voids, stretching over billions of light years. There are some questions that could lend support for the CDM model, but could also lead to refutation of the CDM model, because the questions lead to testable hypotheses:

1. Is our solar system embedded in a CDM-dense-blob or a CDM-void, relative to the average density? If you think about it, we really have no way of knowing. Whatever the density of CDM around here is, we have developed our gravitational models to calibrate to our local environment. On the one hand, we could be in a CDM-poor region, which forces us recognize that CDM is necessary to explain gravitational movements of very large structures elsewhere. On the other hand, we could be in a CDM-rich region, which would cause our gravitational models to OVER-estimate the force of gravity universally, which would then force us to posit the existence of dark energy as a repulsive force. Mathematicians out there would perhaps be able to test sets of models to see which one fits the data best?????

2. Would a planetary system elsewhere in our galaxy, embedded in either a much denser CDM region, or conversely, in a much less dense CDM region, behave differently, in terms of local orbital mechanics or local gravitational forces? We are observing other planetary systems in our galaxy now.

3. We have observed gravitational lensing due to a galactic cluster embedded in CDM. (http://antwrp.gsfc.nasa.gov/apod/ap060824.html). Since CDM dominates, why haven't we observed gravitational lensing due to CDM in the absence of a galactic cluster? Or have we????

4. A Doomsday Question, asked somewhat tongue-in-cheek. Very large clumps of CDM, arranged loosely, can theoretically move through space without much regard for the presence or absence of baryonic matter. What would happen to our solar system if a very large, dense clump of CDM moved through? Wouldn't orbits be disrupted? Could non-homogenous CDM over the 4.5 billion year history of our solar system explain various eccentricities of our planetary system?

First, may I suggest that if you are interested in exploring this further goredsox that you join an internet discussion forum such as BAUT (http://www.bautforum.com/) or Physics Forums (http://www.physicsforums.com/index.php). Both have much higher traffic in astronomy and cosmology than the Asterisk Café has, and, more importantly, many more regulars who 'know their stuff'.

goredsox wrote:Doum, I like the idea that you have, that dark matter and/or dark energy might be artifacts of other dimensions in our universe. I can certainly imagine that we would have difficulty working with other dimensions, and would find ways to bend physical laws so that we can collapse the other dimensions into our favorite four dimensions.

Nereid, let's accept the premise that CDM dominates over baryonic matter. Let's also accept the premise that the distributions of CDM and baryonic matter overlap but do not fully correspond with each other, but both have formed very large "clumpy" structures, interspersed with voids, stretching over billions of light years. There are some questions that could lend support for the CDM model, but could also lead to refutation of the CDM model, because the questions lead to testable hypotheses:

1. Is our solar system embedded in a CDM-dense-blob or a CDM-void, relative to the average density? If you think about it, we really have no way of knowing. Whatever the density of CDM around here is, we have developed our gravitational models to calibrate to our local environment. On the one hand, we could be in a CDM-poor region, which forces us recognize that CDM is necessary to explain gravitational movements of very large structures elsewhere. On the other hand, we could be in a CDM-rich region, which would cause our gravitational models to OVER-estimate the force of gravity universally, which would then force us to posit the existence of dark energy as a repulsive force. Mathematicians out there would perhaps be able to test sets of models to see which one fits the data best?????

Something like this has been done; in fact, in one sense the history of theories of gravity provides a good summary of why the density of CDM in the solar system is below the limits of the best efforts (to date) to measure it.

Off the top of my head, I'd guess that the Hulse-Taylor (binary) pulsar would provide a quite straight-forward demonstration of this - the observed orbital decay is consistent with GR, and the object is well away from the solar system (thank goodness!)

2. Would a planetary system elsewhere in our galaxy, embedded in either a much denser CDM region, or conversely, in a much less dense CDM region, behave differently, in terms of local orbital mechanics or local gravitational forces? We are observing other planetary systems in our galaxy now.

Yes; the planets would have orbital parameters that were (very obviously) inconsistent with Kepler's laws (of planetary motion) ... unless the CDM were distributed in a highly unusual way (or its density in the system were quite low).

While there would be only a small minority of such planetary systems we could robustly test like this (those which have both doppler footprints and transits, for example), it's something that would stand out like the proverbial sore thumb.

However, no need to rely upon such hard-to-even-detect objects; there are plenty of binary stars that would do just as well ... and such binaries are both very numerous and have a very wide range of mass differences and orbits.

3. We have observed gravitational lensing due to a galactic cluster embedded in CDM. (http://antwrp.gsfc.nasa.gov/apod/ap060824.html). Since CDM dominates, why haven't we observed gravitational lensing due to CDM in the absence of a galactic cluster? Or have we????

Not sure I understand this fully, but such lensing has been observed due to individual galaxies, whether in clusters or not.

Further, there are now some leading edge 'blind' observations of gravitational lensing due to CDM - i.e. surveys done to measure the lensing independently of whether there is an obvious rich cluster in the foreground.

4. A Doomsday Question, asked somewhat tongue-in-cheek. Very large clumps of CDM, arranged loosely, can theoretically move through space without much regard for the presence or absence of baryonic matter. What would happen to our solar system if a very large, dense clump of CDM moved through? Wouldn't orbits be disrupted? Could non-homogenous CDM over the 4.5 billion year history of our solar system explain various eccentricities of our planetary system?

A question for later!