Is dark matter just an illusion?

[Mk]

http://www.physorg.com/news77190620.html

In a paper in the August 3 online edition of the Institute of Physics' peer-reviewed Journal of Cosmology and Astroparticle Physics, they put forth the idea that scientists were forced to propose the existence of dark energy and dark matter because they were, and still are, working with incorrect gravitational theory.

The group suggests an alternative theory of gravity in which dark energy and dark matter are effects – illusions, in a sense – created by the curvature of spacetime (the bending of space and time caused by extremely massive objects, like galaxies). Their theory does not require the existence of dark energy and dark matter.

“Our proposal implies that the 'correct' theory of gravity may be one based solely on directly observed astronomical data,” said lead author Salvatore Capozziello, a theoretical physicist at the University of Naples, to PhysOrg.com.

Dark energy and dark matter were originally conceived to explain, respectively, the accelerating expansion of the universe (despite the tendency of gravity to push matter together) and the discrepancy between the amount of matter scientists expect to observe in the universe but have not yet found. Astronomers suggested the existence of dark matter when they noticed something odd about spiral galaxies: Stars at the middle and edge of a spiral galaxy rotate just as fast as stars near the very center. But according to Newtonian mechanics (the physics of bodies in motion), stars further away from the galactic center should rotate more slowly. Scientists thus assumed that some sort of “dark” matter, not observable by emitted light, must be boosting the total gravity of the galaxy, giving the stars extra rotational speed.
[...]
In their paper, he and his co-authors demonstrate this using data from 15 well-studied galaxies. Among this data was each galaxy's “rotation curve,” a graph that plots the rotational speed of the stars in the galaxy as a function of their distance from the galaxy's center. These curves were successfully fit to curves produced using the new theory. Since these 15 galaxies are believed to be dominated by dark matter, fitting their rotation curves using this new gravity model is strong evidence to support an alternative theory of gravity.

Despite this, the notion that dark matter and dark energy are “wrong” is potentially very unpopular. Capozziello and his colleagues are aware that a new theory of gravity impacts the dynamics of the universe as scientists now understand them.

 

[franznietzsche]

I've personally never liked the idea of either dark matter or dark energy. Very kludgy. What someone needs to do is do a proper set of GR calculations on galactic rotation. There was the Cooperstock & Tien paper a while back, but there simulations were heavily critiqued and as I understand, not accepted, but I haven't heard of anyone doing the job properly, as it were. Anyone hear of such a paper?

 

[Chronos]

This appears to be an extension, or rehash of an earlier arxiv paper by Capozziello:

http://arxiv.org/abs/astro-ph/0602349
Dark energy and dark matter as curvature effects

The idea is a bit muddy to me and does appear to answer more questions than the LCDM model. The recent press release by NASA:

NASA Finds Direct Proof of Dark Matter
http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

appears to largely rule out most alternative models. While this finding does not identify the responsible particle, it is compelling evidence that cold dark matter does exist. The are many other indicators as well, but this is the strongest to date.

The case for dark energy is not as strong, which primarily relies upon the Perlmuter supernova study for observational support.

 

[franznietzsche]

This appears to be an extension, or rehash of an earlier arxiv paper by Capozziello:

http://arxiv.org/abs/astro-ph/0602349
Dark energy and dark matter as curvature effects

The idea is a bit muddy to me and does appear to answer more questions than the LCDM model. The recent press release by NASA:

NASA Finds Direct Proof of Dark Matter
http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

appears to largely rule out most alternative models. While this finding does not identify the responsible particle, it is compelling evidence that cold dark matter does exist. The are many other indicators as well, but this is the strongest to date.

The case for dark energy is not as strong, which primarily relies upon the Perlmuter supernova study for observational support.

The thing with astrophysics is that it is a game of error bars. As Frank Timmes point out to some of the summer students this year, the error bars on the supernovae measurements are large enough that the result could change with new measurements. However, as I understand it there were several other unrelated measurements (one of them of the CMB) backing up the same conclusion conclusion. This game of error bars makes me suspicious of the study finding proof of dark matter, until I have time to actually read the paper, which I can't seem to find on arxiv. It may be on ADS though, have to check there. And of course, I'm inherently suspicious of any press story claiming that scientists have found proof of anything. Proof is a very big word. Kinda like never, in its full meaning. Most people don't realize the full extent of its meaning.

So I take it no one has done the GR rotational calculations since C&T then?

 

[Gokul43201]

In a paper in the August 3 online edition of the Institute of Physics' peer-reviewed Journal of Cosmology and Astroparticle Physics, they put forth the idea that scientists were forced to propose the existence of dark energy and dark matter because they were, and still are, working with incorrect gravitational theory.

Maybe...but that's almost ironic timing given this announcement, hardly days after!

 

[EL]

Maybe...but that's almost ironic timing given this announcement, hardly days after!

Exactly!
MOND is (almost?) dead, face it!

 

[ZapperZ]

The thing with astrophysics is that it is a game of error bars. As Frank Timmes point out to some of the summer students this year, the error bars on the supernovae measurements are large enough that the result could change with new measurements. However, as I understand it there were several other unrelated measurements (one of them of the CMB) backing up the same conclusion conclusion. This game of error bars makes me suspicious of the study finding proof of dark matter, until I have time to actually read the paper, which I can't seem to find on arxiv.

Then you should see some of the error bars on high energy physics experiments!

While the degree of certainty in a field like this is certainly lower than, let's say, condensed matter, one needs to look at it in the broader sense. People in this field know the strength and weakness of the "evidence" or the observation - it is the general public and the quacks who don't. This is why the presence (or absence) of dark matter and dark energy continues to be studied, leading up to the most recent result from Chandra that Chronos has mentioned. Furthermore, the evidence does not come from just one source and one type of observation - the same way the presence of things like the top quark did not come from just one detector using the same methodology (CDF and D0 at Fermilab are two very different detectors looking at different characteristics).

For those of us who are not experts in this field, I'd suggest that we let those who know hash it out before we draw our own ignorant conclusion. This is, after all, a research-front area, and by definition, it continues to evolve as more is known. It is highly premature at this stage, in light of the current evidence, to make a conclusion of any kind.

Zz.

 

[franznietzsche]

Then you should see some of the error bars on high energy physics experiments!

While the degree of certainty in a field like this is certainly lower than, let's say, condensed matter, one needs to look at it in the broader sense. People in this field know the strength and weakness of the "evidence" or the observation - it is the general public and the quacks who don't. This is why the presence (or absence) of dark matter and dark energy continues to be studied, leading up to the most recent result from Chandra that Chronos has mentioned. Furthermore, the evidence does not come from just one source and one type of observation - the same way the presence of things like the top quark did not come from just one detector using the same methodology (CDF and D0 at Fermilab are two very different detectors looking at different characteristics).

For those of us who are not experts in this field, I'd suggest that we let those who know hash it out before we draw our own ignorant conclusion. This is, after all, a research-front area, and by definition, it continues to evolve as more is known. It is highly premature at this stage, in light of the current evidence, to make a conclusion of any kind.

Zz.

All I was saying is that I won't take a press release at face value, I want to see the published article.

 

[SpaceTiger]

All I was saying is that I won't take a press release at face value, I want to see the published article.

Then don't you think you should read it before speaking up?

http://adsabs.harvard.edu/cgi-bin/n...pe=HTML&format=&high=445b02ffde21812"

I do agree with you about the use of the word "proof". It's a bit too heavy a word for a scientific article about recent results.

 

[MeJennifer]

Then don't you think you should read it before speaking up?

http://adsabs.harvard.edu/cgi-bin/n...pe=HTML&format=&high=445b02ffde21812"

I do agree with you about the use of the word "proof". It's a bit too heavy a word for a scientific article about recent results.

The abstract states (highlight is mine):

We present new weak-lensing observations of 1E 0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray-emitting plasma are spatially segregated. By using both wide-field ground-based images and HST/ACS images of the cluster cores, we create gravitational lensing maps showing that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8 sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law and thus proves that the majority of the matter in the system is unseen.

Did anybody read the article to understand how they concluded that an alteration of the gravitational force law cannot possibly explain that?

 

[franznietzsche]

Then don't you think you should read it before speaking up?

http://adsabs.harvard.edu/cgi-bin/n...pe=HTML&format=&high=445b02ffde21812"

I do agree with you about the use of the word "proof". It's a bit too heavy a word for a scientific article about recent results.

Thank you very much for the link. I hadn't had time to check ADS (I've got this presentation for LA Astro tomorrow, and I've been working at it 12+ hours/day since friday). I'll try and get to it tomorrow afternoon.

 

[Chaos' lil bro Order]

I have never taken MOND seriously anyways, it seems like a theory born of necessity and little else. CDM is the way to go IMO.

 

[xAbsoluteZerox]

What someone needs to do is do a proper set of GR calculations on galactic rotation. There was the Cooperstock & Tien paper a while back, but there simulations were heavily critiqued and as I understand, not accepted, but I haven't heard of anyone doing the job properly, as it were. Anyone hear of such a paper?

So I take it no one has done the GR rotational calculations since C&T then?

Heres the problem with what you are proposing, that someone do GR rotation curves. The problem lies within the application of each; GR is what happens as v approaches c, while MOND is what happens as a approaches 0. From an emperical standpoint, these two topics coundn't be further removed. However, with this being said, the future of MOND is questionable, but to me, it seems like MOND is not flawed in its emperical sense, and I still believe that right now, it is a much better idea than CDM.

 

[Chronos]

The NASA release is the fluffy version and should be taken with a grain of salt. For technical discussion see:

A direct empirical proof of the existence of dark matter
http://arxiv.org/abs/astro-ph/0608407

 

[franznietzsche]

Heres the problem with what you are proposing, that someone do GR rotation curves. The problem lies within the application of each; GR is what happens as v approaches c, while MOND is what happens as a approaches 0. From an emperical standpoint, these two topics coundn't be further removed. However, with this being said, the future of MOND is questionable, but to me, it seems like MOND is not flawed in its emperical sense, and I still believe that right now, it is a much better idea than CDM.

I fail to see your point, unless you are trying to argue that GR will not change the rotation curves so why bother, which if that is your whole argument, I don't buy it. I wouldn't be surprised to see the rotation curves not change significantly, but that doesn't mean it shouldn't be checked. The basic assumption that the results would be the same is not one that should be just accepted a priori, and it is IMO unacceptable to knowingly base these theories on what we KNOW is a less accurate theory of gravity--Newton's theory--solely because it is easier to work with (please correct me if I am wrong on this characterization of why Newtonian gravity has been used over GR). I was shocked to find out when the C&T paper came out that the previous analyses hadn't been done with GR.

At any rate, now that I've actually had to read the paper, I'd like to make some direct comments:

First off, the title. The referee shouldn't have allowed them to use the term 'Proof', see my statement above. This is mostly a semantic issue, not anything to do with the content of the paper.

Can someone explain to my why it is 'not feasible' to measure the redshifts of the all the background galaxies used to calculate the lensing? I'm not an astronomer, so I'm not familiar with observational issues, but their method of selecting background galaxies to calculate the lensing seems like a significant weak point to me.

High uncertainty in the mass-to-light ratio for the stellar component of the clusters' mass. Granted, they state that even in the case of extreme deviation that the X-ray plasma is the dominant mass component, but I think it would have been better to go with the highest possible stellar mass component. It probably wouldn't have changed the results, but would have strengthened the argument somewhat (although, with 8 sigma significance, I'll grant that the argument is pretty strong).

The last sentence also strikes me as being overly strong and assertive. This is again a semantic issue, not a substantive one.

I'll certainly grant that the case they present is strong, having had the chance to read the paper. The second to last paragraph is particularly interesting, where they point out that alternative gravity theories would require even more dark matter to match the observations. Having actually read the paper now, my main concern is how they selected which galaxies to use in calculating the lensing effects.

 

[Nereid]

The abstract states (highlight is mine):

We present new weak-lensing observations of 1E 0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray-emitting plasma are spatially segregated. By using both wide-field ground-based images and HST/ACS images of the cluster cores, we create gravitational lensing maps showing that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8 sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law and thus proves that the majority of the matter in the system is unseen.

Did anybody read the article to understand how they concluded that an alteration of the gravitational force law cannot possibly explain that?

Yes (me, for one).

Would you like a summary? Or perhaps we could go through the paper, paragraph by paragraph?

Of course, the part of the abstract which you highlighted is terse, perhaps to the point of oversimplification.

 

[Chronos]

I prefer 'compelling evidence' as opposed to 'proof'. The 'p' word is, IMO, inappropriate in a seminal paper such as this. I share their enthusiasm. It is a fabulous find and a paper that will be heavily cited for decades, but, calm restraint is in order and, again IMO, it would benefit from a more critical discussion of the error bars. The Perlmutter paper sets a better example of how a sensational result should be introduced to the scientific community - let the evidence speak for itself.

 

[xAbsoluteZerox]

I fail to see your point, unless you are trying to argue that GR will not change the rotation curves so why bother, which if that is your whole argument, I don't buy it. I wouldn't be surprised to see the rotation curves not change significantly, but that doesn't mean it shouldn't be checked. The basic assumption that the results would be the same is not one that should be just accepted a priori, and it is IMO unacceptable to knowingly base these theories on what we KNOW is a less accurate theory of gravity--Newton's theory--solely because it is easier to work with (please correct me if I am wrong on this characterization of why Newtonian gravity has been used over GR). I was shocked to find out when the C&T paper came out that the previous analyses hadn't been done with GR.

Sorry I haven't been around much to answer you. Because I am in a rush, I will reference this publication: http://arxiv.org/abs/astro-ph/0604022" which basically states that Cooperstock and Tieu's theory involving GR is doubtfully true. Take a look at it, it is a very quick read and I agree with it.

 

[Garth]

Sorry I haven't been around much to answer you. Because I am in a rush, I will reference this publication: http://arxiv.org/abs/astro-ph/0604022" which basically states that Cooperstock and Tieu's theory involving GR is doubtfully true. Take a look at it, it is a very quick read and I agree with it.

? Did you mean that?

It is shown that the model of Cooperstock & Tieu, which was so constructed that it gives an excellent fit of the observed rotation curve, singularly fails to reproduce the observed local mass density and the vertical density profile of the Milky Way.

Garth

 

[xAbsoluteZerox]

? Did you mean that?


Garth

Yes, I meant that, I don't know if you read the entire thing, not just the abstract, but you will notice his closing remark: "This one counter example casts, in our view, severe doubts on the viability of Cooperstock & Tieu’s theory of the dynamics of galactic disks in general." and this is what I agree with.

 

[Garth]

Okay, I misunderstood what you were saying.

I was questionning your use of the phrase "doubtfully true", which means to me that you thought the theory was more or less 'true' but that questions could be raised.

The paper quite categorically claims that it cannot be true as it "singularly fails to reproduce the observed local mass density and the vertical density profile of the Milky Way."

Garth

 

[Jimmy Snyder]

Being made of dark matter myself, I'm not sure I like where this thread is going.