Has the dark matter mystery been solved?
Figure 1: Edge on spiral galaxy and a rotation curve. Speeds of stars measured from the centre of a galaxy like this, as a function of distance in light-years. Using carbon-monoxide (CO) as a tracer gas the speeds of gas in the rotating disk can be also measured where there are no visible stars (labelled “No Stars”).
Published: 6 April 2017 (GMT+10)
Unseen dark matter has been invoked several times to solve problems in astrophysics and cosmology. Historically the most significant problem has been the rotation curves of galaxies, particularly spiral galaxies. Using the Doppler Effect the speeds of the stars and gases in the disk regions of spiral galaxies can be measured. See Figure 1.
By now hundreds of thousands of galaxies have been measured this way. What is observed is that the speeds of the stars, and the gases beyond where the stars are observed, are much greater than it would appear Newtonian physics allows for.
As a result it has been suggested that there is an invisible halo of cold non-interacting matter. This putative invisible halo has the needed gravitational effect on the stars and gases but it cannot be seen, hence it is called dark matter. Dark matter is alleged not to be normal atomic matter, made from protons and neutrons (which are known as baryons), but some sort of slowly moving (cold) exotic non-baryonic matter. Weakly Interacting Massive Particles (WIMPs) were suggested.
And until recently it was believed that WIMPs might be the lowest mass stable supersymmetric particle, called a neutralino, but 10 years of experiments at the Large Hadron Collider (LHC) has disproven the theory upon which such a speculation was based.1 Other laboratory investigations with supersensitive detectors, deep underground, have failed to detect any type of particle that could be considered a candidate for dark matter.2
On the cosmological scale
When we put this all together we can generally say that dark matter (and other dark sector entities) are really only needed in astrophysics and cosmology.6,7 But the lack of detection of any type of dark matter in laboratory experiments has driven some to speculate and develop new physical theories to explain what we observe astronomically.8,9
And there are two cosmological reasons why cosmologists need non-baryonic (non-normal atomic) cold dark matter (CDM). One is that the measured density of the gravitating mass in the universe as determined by astronomical measurements10 appears to considerably exceed that of normal matter as constrained by big bang nucleosynthesis (BBN). BBN does present other problems but we won’t go into them here.3
The amount of matter needed in the universe is six times the observed amount of baryonic (normal atomic) matter. Therefore it is believed that the density of particles that don’t comprise normal atomic matter, i.e. the non-baryonic matter, is about five times the amount of the baryonic matter.
Dark matter in the form of black holes, supermassive or otherwise, has implications for BBN. If dark matter was real and, as now suggested, comprised of stellar size black holes (i.e. non-baryonic matter) in the early universe, then the black holes would have had to have formed in the first minute after the beginning of the universe. Otherwise the black holes would have had an observable effect on BBN. The details of this are now also being debated and recalculations being made.12
A second cosmological reason is that gravity itself is too weak to grow, on the available cosmic timescales, the presently observed structures (e.g., galaxies, clusters, superclusters and long galactic filaments) from the alleged smooth initial condition observed in the cosmic microwave background (CMB), unless something was available to accelerate the growth process.
Additional matter with a density much higher than the observed matter density could have done it. But if it was normal atomic matter, i.e. baryons, it would be observable and would have shown up in the observations of the CMB radiation. Therefore this new non-baryonic matter must not interact with the photons of the CMB like ordinary atomic matter allegedly does.
Another look at rotation curves
When we measure the speed of planets in the solar system, the measurement of a planet’s constant speed (v) in orbit around the sun and its distance from the sun (r) allows us to determine the mass of the sun (Ms). Considering only the sun and an orbiting planet this is a two-body problem and all that is needed is Newton’s equation for this two-body problem to solve for the mass of the sun. The only unknown is Ms, the mass of the sun. With near circular orbits (a planet’s acceleration around the sun is very close to zero, i.e. r′′≈0). It follows that11
where G is Newton’s gravitational constant.
Similarly, we can write down an almost identical equation for the enclosed mass, M(r), within the orbit of any star at a distance r from the centre of a galaxy.
In this case M(r) gives you the mass enclosed inside the orbit of the star you are measuring. It is not constant but depends on the distance r the star (or circulating gas) is from the centre of the galaxy under investigation. But this is where the assumption of dark matter comes from.
Figure 2. Typical rotation curve (red dashed, labelled Observed) for a spiral galaxy. The solid blue curve, labelled Newtonian, is what is expected from an application of Eq. (2) to the visible matter distributed through a galaxy.
If we plot the speed of the stars, v (and the same for gases outside the radius of visible stars), as a function of distance, r, from the centre of a galaxy we typically get the result shown in Figure 2. But as can be seen, stars (and gases) move “too fast” when they are outside the central nucleus of the galaxy (indicated by the hump in the Newtonian curve).
The observed speeds are much faster than what is expected from standard Newtonian physics. But Newtonian physics works very well in the solar system. The more distant a planet is from the sun the slower it moves. Why then doesn’t the same physics work for galaxies?
- The galaxy is dissipating, or
- Newtonian physics is incorrect, or
- Massive amounts of unobserved halo dark matter exist.
On bullet point 1, the implication is that the galaxy under observation is not in equilibrium, which possibly could be argued because we have not observed galaxies over millions of years to see any structural evolution. But let us assume here (as is generally assumed) that all observations are consistent with stable equilibrium conditions. In fact Eq. (2), which is the basis of the problem, implicitly assumes an equilibrium condition.
On bullet point 2, since under weak gravity, which is the case in most regions of a galaxy, apart from maybe inside or near a supermassive black hole at the core of the galaxy under consideration, Einstein’s general relativity theory produces the same equations of motion as Newton’s law of gravitation. Therefore, for circular motion of bodies in orbit we only need consider Newtonian physics.
Figure 3. Artist’s illustration of a binary stellar size black hole merger detected by LIGO in 2016.
On bullet point 3, it is obligatory on those who profess to believe the existence of some unseen exotic type of matter that does not respond to or is not detectable by electromagnetic radiation to produce evidence of the existence of even one such particle in a laboratory experiment, especially considering that they require the matter of this galaxy to be about 85% dark matter. This is the typical percentage of additional unseen matter if one were to accept the answer is dark matter in the form of a spherical halo around not only our galaxy but also most spiral galaxies of which we are aware.
However, due to the lack of any laboratory evidence in favour of the existence of any new particles that might comprise halo dark matter in hundreds of thousands of galaxies, it has now been suggested that dark matter could be comprised of millions, even billions, of stellar-size black holes.12 See Figure 3. This suggestion has come about after the detection of stellar-size black holes by the LIGO gravitational wave detectors.13
The notion that stellar mass (10 to 100 times the mass of the sun) black holes could be dark matter in halos of galaxies is currently being debated. In the 1990s microlensing all but ruled out the possibility of swarms of such black holes but now it is said that the surveys were too short and longer surveys are needed. It is also argued that black holes in excess of 10 solar masses would have long ago settled in the centres of galaxies and any in the regions surrounding galaxies would leave the galaxies ruffled up, but observations indicate the opposite—compact and unruffled.12
Stars have drag
It could be said that there is one aspect that needs to be added to bullet point 2. It may not be new physics that is needed but the correct application of the known physics. If an incorrect boundary condition was assumed or an incomplete application was made of the known physics where one aspect was not properly modelled, then this might just solve the problem of the rotation curves without the need to “invent” exotic dark matter.
This is what has now been proposed.11 The application of Eq. (2) as described above neglects the billion-body problem and therefore neglects drag between the individual members of the system. Donald Saari, a mathematician14 who has worked on this problem, writes in the online Scientific American SciAm News,11 that the
… general properties of billion-body Newtonian systems are unknown, so astronomers developed creative approximations to study galactic systems. Intuition for a standard approach comes from star-soup type pictures of galaxies, with bodies pulling others along (as in Figure 1). This star-soup appearance suggests approximating actual N-body systems with a continuum …
And that continuum approximation results in Eq. (2), which then leads to the dark matter conundrum.
The illustration often used in reference to Einstein’s general relativity theory (and remember Newtonian physics can be derived from GR) is that “masses tell space how to curve and the curved space tells the masses how to move”. This in effect assumes the masses in a galaxy are a continuum and thus curve the space in the galaxy in a smooth fashion.
The individual stars are neglected in favour of a smooth continuum of matter density. And in the modelling the matter density is modelled in this fashion. But by approximating the billion-body system of a galaxy as a continuum of smoothed-out matter it appears that something is neglected in the application of the standard Newtonian physics.
Saari did the billion-body system calculations and found that11
… a star’s Newtonian rotational velocity is the M(r) gravitational effect plus dragging terms; these larger v values exaggerate M(r) values of [Eq.] (2). As dragging is more pronounced at larger distances, expect this error to incorrectly predict halos. …. No solution has halos, yet [Eq.] (2) incorrectly predicts massive ones! These egregious errors manifest differences between systems of discrete bodies and approximations; actual systems involve dragging effects, while continuum approximations ignore this crucial dynamic. (emphases added)
Saari ends the online SciAm News article with his own conclusion.11
“This casts doubt about a standard argument claiming massive amounts of dark matter.”
There you have it! I have not personally checked his maths but his work is published in good journals including The Astronomical Journal.15 I have also published in that journal and I know it takes some effort to get published there.
The relevance for biblical creationists is just as important as it is for cosmic evolutionists. The problem of galaxy rotation curves has been an issue for a very long time and no matter to which cosmogony you subscribe, you still have to explain how galaxies hold together over billions of years if the standard application of Newtonian physics is correct.16
Of course, a biblical creationist might argue that the galaxies are not in equilibrium, because the universe is not billions of years old. That is a possibility but one I believe not very likely. It is more likely that God created all galaxies in a stable configuration, just as he did the sun and the solar system, and all other systems in the galaxy. Those stable systems exhibit operational physics, which, in principle, we can investigate. Now it seems we can add one more thing, the operational physics of galaxies is standard Newtonian when all factors are correctly included.
For the big bang cosmologist this is a big problem. Up to this point the 80% of all galaxy matter being dark matter is consistent with what they need in big bang cosmology. Without this five times the amount of non-baryonic matter to baryonic matter in the universe the standard big bang cosmology does not agree with the cosmological observations, such as from high-redshift-supernova measurements and even the fluctuations in CMB radiation. In my opinion, it was brave for Scientific American to publish Saari’s news item. Let’s see if his work gains any traction.
If this pans out, for me it is a relief. We can continue to use standard physics in the cosmos and no longer need to look for alternative theories9 to explain what we could not up until now.
References and notes
- Hartnett, J.G., SUSY is not the solution to the dark matter crisis, J. Creation 31(1):6–7, April 2017. Return to text.
- Hartnett, J.G., Dark matter search comes up empty, biblescienceforum.com, July 2016. Return to text.
- Hartnett, J.G., Dark matter and the standard model of particle physics—A search in the ‘dark’, September 2014; creation.com/search-in-the-dark. Return to text.
- Hartnett, J.G., Dark radiation in big bang cosmology, creation.com, November 2014; creation.com/dark-radiation. Return to text.
- Hartnett, J.G., A missing neutrino—dark radiation, ARJ 7:357–361, September 2014. Return to text.
- Hartnett, J.G., Hairy dark matter is still dark matter, which is still a fudge, biblescienceforum.com, February 2016. Return to text.
- Hartnett, J.G., ‘Dark photons’: another cosmic fudge factor, creation.com, August 2015; creation.com/dark-photons. Return to text.
- Ananthaswamy, A., Maybe Dark Matter Is All Just a Big Mistake, cosmos.nautil.us, February 2017. Return to text.
- Hartnett, J.G., Why look for a new theory of gravity if the big bang cosmology is correct?, creation.com, February 2017; creation.com/gravity-theory-search. Return to text.
- See Table I and Figure 1 in Ref. 5. Two methods using high redshift supernovae and the CMB radiation (from the Planck satellite) give significantly different masses for the universe. Of course these methods both depend on assumptions about the choice of cosmology applied to interpret the observations. Return to text.
- Saari, D.G. , Dynamics and the Dark Matter Mystery, sinews.siam.org, December 2016. Return to text.
- Cho, A., Debate heats up over black holes as dark matter, Science 355(6325):560, 2017. Return to text.
- Hartnett, J.G., What impact does the detection of gravitational waves on biblical creation?, creation.com, February 2016; creation.com/gravitational-waves. Hartnett, J.G., A second gravitational wave has been detected by LIGO, biblescienceforum.com, June 2016. Return to text.
- Donald G. Saari is a distinguished professor and director of the Institute for Mathematical Behavioral Sciences at the University of California, Irvine. His research interests range from the Newtonian N-body problem to voting theory and evolutionary properties of the social and behavioral sciences. Return to text.
- Saari, D.G., N-body solutions and computing galactic masses, AJ 149:174–180, 2015; Saari, D.G., Mathematics and the ‘Dark Matter’ puzzle, Am. Math Monthly 122(5):407–423, 2015. Return to text.
- Creationist cosmologies involve millions and billions of years of time in the cosmos even if, like with time dilation cosmologies, only 6,000 years pass on Earth. Return to text.
Here is an argument for dark matter (and I am not a dark matter proponent). We know that if something is to have measurable properties, it must actually exist, that is it must have substance of some type – even if we do not understand it. Given that, how do we account for the fact that ‘empty space’ has properties such as permeability and permittivity? If we consider the dipole based ‘empirical’ dispersion equation, we believe that ‘empty space’ is the only substance where the [relative] refractive index is unity at all wavelengths. Would not that lead to the conclusion that empty space is in fact something, something special?
I do not like the term dark matter and the proposal that space is not empty does not solve the physics problem, but I am certain that there is more here than we can get our arms around. The math for the many body problem is one of those quandaries that increases exponentially as N increases. Perhaps there is a non-statistical mathematical approach that could work (I have acquired Saari’s N-body paper and requested the other). However, I am of the opinion that our prescriptive models tend to be linear and this is clearly a non-linear problem. I believe that we do not necessarily need new physics; we just have to figure out how to apply the physics we have without making so many assumptions and simplifications.
Keep up the good work!
My thoughts on 'empty' space are found in this article Expansion of Space--A Dark Science.
On your last point I agree. It would be so much better and more elegant to solve this problem with known physics but I suspect that among both secularists and creationists alike where is resistance. Dark matter has been recently embraced by one creationist, Dr Faulkner in his "The Case for Dark Matter". But, as I have pointed out many times before, there are problems upon problems for not just dark dark matter but all the entities of the dark sector. Read Dark Photons.
By me, here is crucial problem - with "gravity." As wrote Newton, bodiy don´t attract other one. He treated this idea as absurdity. My proposal is explication by like-Casimir´s effect. Vacuum isn´t empty, but entity with great energy density.
If dark matter has existed since the Big Bang, and it is gravitationally attracted to ordinary matter, and there is a significant quantity, then can we assume that stars and our sun and planets and humans are made up of mostly dark matter ? Given 14 billion years of gravity. When I get on the bathroom scale, can I blame some excess on dark matter ?
From the location of our sun in the Galaxy dark matter particles should be streaming through our location constantly. But none are detected. Our solar system is located 28,000 light-years from the centre and this is exactly the region where those who believe in halo dark matter say it should be found because of the anomalously high speed of the solar system around the galactic centre. But alas the conjecture now appears to be no longer necessary.
Great article! I was wondering about your thoughts on Dr. Danny R. Faulkner "case for dark matter" article in ARJ? I think this could be a great forum between the two of you.
I don't agree with Dr Faulkner's position. There may have once been a case to argue for dark unseen matter in galaxies, but that can no longer be supported. Dark matter is assumed everywhere as a 'god of the gaps' to fill in where the standard cosmology fails. Yet all efforts over 50 years have failed to find any laboratory evidence for it. Recently it was admitted that 10 years of LHC experiments have ruled out the lowest mass stable supersymmetric particle as a candidate. And why look for a new theory of gravity if the standard model is correct? Because as time goes by the research problem of the non-detection of dark matter, dark energy, dark radiation (a sterile neutrino), and many more dark entities starts to indicate that the whole paradigm itself is in doubt. And that paradigm needs enormous amounts of dark matter as well as dark energy, which is even stranger than dark matter. Along with that even dark photons are proposed. This has led to the crazy notion of dark intelligent life on dark matter planets.
If dark matter exists locally and in any significant amount within our sun, then would it increase the internal gravitational pressure and therefore increase the nuclear reaction rate? If so, the solar lifetime would be reduced?
Correct. There are many ad hoc corrollaries that must be applied with dark matter. The biggest is that it does not gravitate to the centre of galaxies, just where it should if it gravitates at all.
For planets like Mercury Newtonian and Einsteinian physics give different orbital values. Have planetary and possible galactic drag effects been applied here as well? Are they significant?
The precession of the orbit of Mercury involves a general relativistic effect that comes from a stronger gravitational field than Newtonian physics could account for. In the case of these drag effects on stars and gasses, it is where the gravitational forces are very weak. We do see such an effect in the Solar System though. In the orbit of Jupiter, 60 degrees before and after the planet, there are many bodies co-orbiting with Jupiter at two Lagrangian points, points of stability. These are called trojans and there may be millions of them greater than 1 km in diameter. They are locked into the same heliocentric orbit as Jupiter. This situation has resulted from a drag effect. Neptune and Mars are known to have a few also.
Interesting article. It's good to hear that a solution consistent with Newtonian physics for the rotation of galaxies may have been found.
I have wondered if the solution might be in part a non-uniform distribution of charge throughout galaxies, with the center being strongly positive charged (for instance), the innermost portion that follows the Newtonian gravity curve being neutrally charged, and the outer portion of the galaxy might be gradually more negatively charged (for instance) as you get farther from the center. Sort of like an atom but on a galaxy-wide scale. I think this would produce a similar rotational velocity to what is observed because then both the force of gravity and the electromagnetic force would be acting inward on the visible matter, causing a faster rotation of the outer portion of the galaxy. Has this possibility been discussed in scientific literature?
There are proponents of the electric universe who propose that electric fields (with their associated long range forces) are responsible. The problem to overcome there is that plasma in the universe is generally neutral. There are positive and negative electrically charged particles (positive ions, protons and negative electrons) but they cancel each other in a plasma. Whereas there are no negative and positive gravitons. Hence the gravitational force is never quenched, and acts over long ranges.
Very interesting, as always! Especially after reading an article just some weeks ago from AiG's Dr. Faulkner about dark matter not being just a fudge factor to save the Big Bang, but something that is a reasonable deduction from the observations, like Fritz Zwicky's measurements on the Coma Cluster of galaxies in 1937. Could Zwicky's measurements also have been influenced by "continuum approximations ignoring this crucial dynamic" that Saari found?
One other thing: shouldn't there be some relativistic component in the equations, to account for the fact that a change on one side of the galaxy only has an effect on the other side of the galaxy some 100,000 years later? Or do these factors cancel each other out once you assume a stable galaxy in equilibrium? Or should laypeople like me not even try to understand, because it makes us sound stupid? ;)
Anyways, I'm looking forward to see where this is going. Keep us posted, Dr Hartnett!
It may have been a reasonable deduction back in Zwicky's time but it is not a reasonable deduction now. The reason for that is that all evidence for the existence of dark matter comes from the cosmos. No lab experiment has detected any sort of dark matter, and they have been looking now for at least 50 years. Recently the best hoped for particle was disproved, the neutralino, which I mentioned in the article. Zwicky's interpretation of higher than expected dispersion within clusters, or x-ray emission from clusters, was probably the result of a misapplication of the known physics, due to the continuum approximation.
The idea of delayed time effects is an interesting idea and worth looking into. But they do assume long ages for the galaxies, or put it another way, they assume equilibrium conditions with the galaxies. So yes, an equilibrium condition within the galaxy is important.
Can't claim to follow all the details (not a physicist, though I always loved physics in high school and university) but this is fascinating. Thank you. Sometimes we make things more complicated than we need to.
Well, well, John!
After reading a few paragraphs of this, I had to check to see if it was written on April 1.
I agree. The whole idea of dark matter, and unseen exotic type of matter, not detectable by any form of light or radiation, does seem like an April fools prank. But no, it is real alright. The problem as I have pointed out before, is that there are so many unknowns that have to be accepted by 'faith'. Astrophysicists and cosmologists use ‘unknowns’ to explain ‘unknowns.’ Dark matter is just one of those, and dark energy is another. See Cosmology is not even astrophysics.