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A Gravitational Explanation for Dark Matter

Because dark matter has never been directly observed, we’re left trying to figure it out using deductions based on its presumed effects on visible matter. Seven dwarf satellite galaxies of the Milky Way — Carina, Draco, Fornax, Leo I, Leo II, Sculptor and Sextans — offer a case in point. Stars in these galaxies do not move more slowly the farther they are from their galaxy’s core. Is dark matter the explanation?

Mario Mateo (University of Michigan) has been studying the velocity of almost 7,000 stars in the seven dwarfs. His observations lead to the same kind of deduction already been made for larger spiral galaxies, that the matter we see does not account for the apparent distribution of mass throughout the galaxy. All that, of course, depends upon subjecting these observations to established theory. Mateo colleague Matthew Walker (now at the University of Cambridge) puts it this way:

“We have more than doubled the amount of data having to do with these galaxies, and that allows us to study them in an unprecedented manner. Our research shows that dwarf galaxies are utterly dominated by dark matter, so long as Newtonian gravity adequately describes these systems.”

But there are those who argue that it does not. You can explain dark matter under other theories as well, but when you do this, you have to start tinkering with some basics of modern physics. While the Michigan team travels to Cambridge MA to present its findings on the 30th, Canadian researchers are pushing a different model, one based on the idea that there is no dark matter. Modified Gravity theory (MOG) takes Newtonian/Einsteinian gravity in a new direction, and if correct, could explain not only the motion of distant galaxies but objects closer to home.

Bullet Cluster

For John Moffat (University of Waterloo, Ontario) and graduate student Joel Brownstein are arguing that they can explain stellar and galactic motion as well as the anomalous deceleration of the Pioneer 10 and 11 space probes by using Modified Gravity theory, which Moffat has been developing for the last thirty years. Their work involves the Bullet Cluster, two merging clusters of galaxies some three billion light years from Earth in the direction of the constellation Carina.

Image: This composite image shows the galaxy cluster 1E 0657-56, also known as the “bullet cluster.” This cluster was formed after the collision of two large clusters of galaxies. Hot gas detected by Chandra in X-rays is seen as two pink clumps in the image and contains most of the “normal,” or baryonic, matter in the two clusters. The bullet-shaped clump on the right is the hot gas from one cluster, which passed through the hot gas from the other larger cluster during the collision. An optical image from Magellan and the Hubble Space Telescope shows the galaxies in orange and white. The blue areas in this image show where astronomers find most of the mass in the clusters.
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

Although previous studies have concluded that gravitational lensing associated with the Bullet Cluster demonstrates the presence of dark matter, Moffat and Brownstein think that normal matter in the cluster can account for the observed lensing. Assuming, of course, that the MOG theory is right. And we should be getting more answers as the search for dark matter continues. Says Moffat:

“If the multi-billion dollar laboratory experiments now underway succeed in directly detecting dark matter, then I will be happy to see Einsteinian and Newtonian gravity retained. However, if dark matter is not detected and we have to conclude that it does not exist, then Einstein and Newtonian gravity must be modified to fit the extensive amount of astronomical and cosmological data, such as the bullet cluster, that cannot otherwise be explained.”

Fair enough, and the beauty of that is that either way we win, with a deeper understanding of dark matter through direct observation, or a growing knowledge of how current gravitational theory can be enhanced. The Michigan study is Walker et al., “Velocity Dispersion Profiles of Seven Dwarf Spheroidal Galaxies,” Astrophysical Journal 667 (September 20, 2007), L53-L56 (abstract). The Modified Gravity paper is Brownstein and Moffat, “The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter,” accepted for November publication in Monthly Notices of the Royal Astronomical Society (abstract).

Comments on this entry are closed.

  • J. R. October 26, 2007, 18:14

    I was reading something about this awhile back. If I remember, the dark matter theory came about because of supposed extremely small error in the gravity calculations that became huge when figuring in the rest of the universe.

    As I understand it, the modified gravitational theory would simply compensate for this supposed error and put everything back in alignment without needing a dark matter theory.

    Is this correct?

  • Administrator October 26, 2007, 19:35

    J.R., the modified gravity theory does imply that dark matter isn’t needed to solve the observational anomalies that first brought the idea of dark matter to our attention. Various things were important in the early study of dark matter, among them the problem of explaining how galaxies behave given that there seems to be more gravitating matter on the galactic scale than what we can observe. This is a major issue, and it’s backed by the anomalous rotational rates of galaxies, which seem to need much more matter in the equation to make them work. Some estimates are that as much as 90 percent of the matter in galaxies is not visible, assuming standard Newtonian/Einsteinian gravitational theory. So it’s a major question, and one way to solve it is indeed to tinker with gravitational theory itself. That’s a bold step and one that will continue to be controversial.

  • andy October 27, 2007, 13:02

    As far as I am aware, pure-MOND theories have been ruled out: there must be some non-baryonic component, (the usual candidate I have seen proposed is neutrinos), to explain the observations. Disagreement is therefore in how much non-baryonic matter is needed, and whether particles beyond the standard model are required for the explanation.

  • Eric James October 27, 2007, 21:31

    How about gravitational effects from virtual pair production?

  • Hamsterbaffle October 30, 2007, 0:04

    God, I hope these guys are onto something here.

    Dark Matter has always seemed like epicycles to me.

  • Ron S October 30, 2007, 11:30

    Epicycles? MOND (or MOG) fits that category far better. Both are examples of mathematical curve-fitting and seem to me to be physically improbable. I’ll stick with dark matter for now.

  • wiseguy November 3, 2007, 23:43

    Dark matter wasn’t theorized until recently. Thus the big bang theory isnt the only gigantic mystery mechanism of the cosmos. If the bible is on target, then energy came first at t=0. “Let there be light..” From this flashpoint, some of this light could have lost momentum or velocity and become both baryonic (the visible stuff)matter and non-baryonic matter (the invisible stuff). Another mystery is why is the ratio of visible (75%)so disparate?

    Ah sweet mysteries of life. SIGH.

  • Jin He November 5, 2007, 0:36

    Gravity Probe B final result will resolve it once for all. Just wait for one more month!

  • ljk November 5, 2007, 9:24

    And how exactly will Gravity Probe B do that?

  • James Collins November 6, 2007, 19:22

    To Centauri dreams

    Your web page is fascinating and the emphasis on dark matter attracted my attention. I published a mathematical based paper that identifies dark matter and its gravitational forces and also addresses both baryonic and non-baryonic matter.
    The paper is titled, ‘The Particle Chamber Theory of Dark Matter’. It is a nine page paper (including two pages of references) which was published April 2007 on the WEB under the domain name of; collinsconsultinggroup.com



    This paper, the “Particle Chamber “ theory, examines and identifies microscopic, dust like, matter as the mechanism that causes the gravitational effect known as dark matter. The initial analysis identified how these particles are distributed around our solar system’. Minute particles of matter are each contained in cubical chambers one kilometer on a side. This paper employs a mathematical analysis to evaluate its potential distribution of the chambers over the universe based on measurements of particulate matter deposited on earth each year. The mathematics indicate that the universe can easily hold both the existing visible matter, and dark matter with a mass ten times greater that the mass of all the visible matter, and the universe still has vast vistas devoid of any matter. A gravitational effect caused by the accumulation of these particle chambers into cubic light-years calculates to reflect the power of stars.
    The paper demonstrates why dark matter can’t be seen (in most cases) and shows that this theory doesn’t conflict with many of the insights found in the papers on this issue.

    To bring up the paper go to Yahoo or Google search and submit “particle chamber theory” (Make sure to add the quotation marks or the search result is several hundred). Please use the PDF file as the other has trouble with the equations.

    Thank you for your time. JIM COLLINS SR.

  • ljk November 8, 2007, 10:58

    Simulating the Bullet Cluster

    Authors: Chiara Mastropietro, Andreas Burkert (USM, Munich)

    (Submitted on 6 Nov 2007)

    Abstract: We present high resolution N-body/SPH simulations of the interacting cluster 1E0657-56. The main and the sub-cluster are modeled using extended cuspy LCDM dark matter halos and isothermal beta-profiles for the collisional component. The hot gas is initially in hydrostatic equilibrium inside the global potential of the clusters. We investigate the X-ray morphology and derive the most likely impact parameters, mass ratios and initial relative velocities. We find that the observed displacement between the X-ray peaks and the associated mass distribution, the morphology of the bow shock, the surface brightness and projected temperature profiles across the shock discontinuity can be well reproduced by offset 1:6 encounters where the sub-cluster has initial velocity (in the rest frame of the main cluster) close to 2 times the virial velocity of the main cluster dark matter halo. A model with the same mass ratio and lower velocity (1.5 times the main cluster virial velocity) matches quite well most of the observations. However, it does not reproduce the morphology of the main cluster peak. Dynamical friction strongly affects the kinematics of the sub-cluster so that the low velocity bullet is actually bound to the main system at the end of the simulation. We find that a relatively high concentration (c=6) of the main cluster dark matter halo is necessary in order to prevent the disruption of the associated X-ray peak. For a selected sub-sample of runs we perform a detailed three dimensional analysis following the past, present and future evolution of the interacting systems. In particular, we investigate the kinematics of the gas and dark matter components as well as the changes in the density profiles and the motion of the system in the L_X-T diagram.

    Comments: 24 pages, 21 figures. Submitted to MNRAS. High resolution images are available at this http URL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.0967v1 [astro-ph]

    Submission history

    From: Chiara Mastropietro [view email]

    [v1] Tue, 6 Nov 2007 22:07:31 GMT (906kb)


  • Claire November 20, 2007, 4:41

    Why does the dark matter theory explain the brightness problem?

  • ljk November 30, 2007, 0:55

    Proto-galaxies tip cold dark matter

    Sighting confirms important stage of galactic evolution


  • ljk November 30, 2007, 11:03

    Multipartite Dark Matter

    Authors: Qing-Hong Cao, Ernest Ma, Jose Wudka, C.-P. Yuan

    (Submitted on 25 Nov 2007)

    Abstract: Dark matter (comprising a quarter of the Universe) is usually assumed to be due to one and only one weakly interacting particle which is neutral and absolutely stable. We consider the possibility that there are several coexisting dark-matter particles, and explore in some detail the generic case where there are two. We discuss how the second dark-matter particle may relax the severe constraints on the parameter space of the Minimal Supersymmetric Standard Model, as well as other verifiable predictions in both direct and indirect search experiments.

    Subjects: High Energy Physics – Phenomenology (hep-ph); Astrophysics (astro-ph)

    Report number: UCRHEP-T443

    Cite as: arXiv:0711.3881v1 [hep-ph]

    Submission history

    From: Qing-Hong Cao [view email]

    [v1] Sun, 25 Nov 2007 05:23:05 GMT (59kb)


  • ljk January 8, 2008, 0:59

    Dark matter and structure formation a review

    Authors: Antonino Del Popolo (Bogazici University and Istanbul Technical University, Istanbul, Turkey)

    (Submitted on 7 Jan 2008)

    Abstract: This paper provides a review of the variants of dark matter which are thought to be fundamental components of the universe and their role in origin and evolution of structures and some new original results concerning improvements to the spherical collapse model. In particular, I show how the spherical collapse model is modified when we take into account dynamical friction and tidal torques.

    Subjects: Astrophysics (astro-ph)

    Journal reference: Astronomy Reports 2007, Vol. 51, No. 3, pp- 169-196

    Cite as: arXiv:0801.1091v1 [astro-ph]

    Submission history

    From: Antonino Del Popolo [view email]

    [v1] Mon, 7 Jan 2008 18:28:45 GMT (463kb)


  • ljk January 18, 2008, 10:39

    Dark matter “scaffolding” of galactic supercluster mapped

    Monday, 14 January 2008 by Ker Than

    Cosmos Online

    NEW YORK: Astronomers are getting their first
    detailed glimpse of how clusters of galaxies and
    dense clouds of dark matter interact to form some
    of the most colossal structures in the universe.

    Dark matter is an invisible form of matter that
    physicists think makes up the bulk of the universe’s
    mass. While it can’t yet be directly detected, they
    have inferred its existence based on the effect its
    gravity has on surrounding normal matter and light.

    First of a kind

    Now, using NASA’s Hubble Space Telescope, an
    international team of astronomers simultaneously
    mapped the invisible dark matter “scaffolding” of
    the galaxy supercluster Abell 901/902 in addition
    to the positions of hundreds of individual galaxies
    contained within.

    Full article here:


  • ljk January 18, 2008, 13:00

    A Decisive test to confirm or rule out existence of dark matter using gravitational wave observations

    Authors: E. O. Kahya

    (Submitted on 13 Jan 2008)

    Abstract: We consider “dark matter emulators,” which are stable modified theories of gravity that reproduce galactic rotation curves and the observed amount of weak lensing without dark matter. In any such model gravity waves follow a different geodesic from that of other massless particles. Over cosmological distances this results in an easily detectable and model-independent difference between the arrival times of the pulse of gravity waves from some cosmic event and those of photons or neutrinos.

    For a repeat of SN 1987a (which took place in the Large Magellanic Cloud) the time lag is in the range of days. For the recent gamma ray burst, GRB 070201 (which seems to have taken place on the edge of the Andromeda galaxy) the time lag would be in the range of about two years.

    Comments: 4 Pages, no figures, Contributed to 12th Annual Gravitational Wave Data Analysis Workshop (GWDAW-12 2007): Connecting Gravitational Waves with Observational Astrophysics, Cambridge, Massachusetts, 13-16 Dec 2007

    Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph); High Energy Physics – Theory (hep-th)

    Report number: UFIFT-QG-08-01

    Cite as: arXiv:0801.1984v1 [gr-qc]

    Submission history

    From: Emre Kahya Mr. [view email]

    [v1] Sun, 13 Jan 2008 21:00:08 GMT (7kb)


  • ljk January 29, 2008, 17:16

    Effects of the interaction between dark energy and dark matter on cosmological parameters

    Authors: Jian-Hua He, Bin Wang

    (Submitted on 28 Jan 2008)

    Abstract: We examine the effects of all possible phenomenological interactions between dark energy and dark matter on cosmological parameters and their efficiency in solving the coincidence problem. We work with two simple parameterizations of the dynamical dark energy equation of state and the constant dark energy equation of state.

    Using observational data coming from the new 182 Gold type Ia supernova samples, the shift parameter of the Cosmic Microwave Background given by the three-year Wilkinson Microwave Anisotropy Probe observations, and the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey, we perform a statistical joint analysis of different forms of phenomenological interactions between dark energy and dark matter.

    Comments: 16 pages, 8 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.4233v1 [astro-ph]

    Submission history

    From: Bin Wang [view email]

    [v1] Mon, 28 Jan 2008 10:38:16 GMT (538kb)


  • ljk January 30, 2008, 13:44

    Heavy Dark Matter Through the Higgs Portal

    Authors: John March-Russell, Stephen M. West, Daniel Cumberbatch, Dan Hooper

    (Submitted on 22 Jan 2008 (v1), last revised 28 Jan 2008 (this version, v2))

    Abstract: Motivated by Higgs Portal and Hidden Valley models, heavy particle dark matter that communicates with the supersymmetric Standard Model via pure Higgs sector interactions is considered. We show that a thermal relic abundance consistent with the measured density of dark matter is possible for masses up to $\sim$ 30 TeV. For dark matter masses above $\sim$ 1 TeV, non-perturbative Sommerfeld corrections to the annihilation rate are large, and have the potential to greatly affect indirect detection signals. For large dark matter masses, the Higgs-dark matter sector couplings are large and we show how such models may be given a UV completion within the context of so-called “Fat-Higgs” models.

    Higgs Portal dark matter provides an example of an attractive alternative to conventional MSSM neutralino dark matter that may evade discovery at the LHC, while still being within the reach of current and upcoming indirect detection experiments.

    Comments: LaTex, 20 pages, 9 figures. Typos fixed and important references added

    Subjects: High Energy Physics – Phenomenology (hep-ph); Astrophysics (astro-ph)

    Report number: OUTP-07-20P, FERMILAB-PUB-08-014-A

    Cite as: arXiv:0801.3440v2 [hep-ph]

    Submission history

    From: John March-Russell [view email]

    [v1] Tue, 22 Jan 2008 18:41:35 GMT (106kb)

    [v2] Mon, 28 Jan 2008 17:43:09 GMT (106kb)


  • ljk January 30, 2008, 13:45

    Sequestered Dark Matter

    Authors: B. v. Harling, A. Hebecker

    (Submitted on 25 Jan 2008)

    Abstract: We show that hidden-sector dark matter is a generic feature of the type IIB string theory landscape and that its lifetime may allow for a discovery through the observation of very energetic gamma-rays produced in the decay. Throats or, equivalently, conformally sequestered hidden sectors are common in flux compactifications and the energy deposited in these sectors can be calculated if the reheating temperature of the standard model sector is known. Assuming that throats with various warp factors are available in the compact manifold, we determine which throats maximize the late-time abundance of sequestered dark matter. For such throats, this abundance agrees with cosmological data if the standard model reheating temperature was 10^10 – 10^11 GeV. In two distinct scenarios, the mass of dark matter particles, i.e. the IR scale of the throat, is either around 10^5 GeV or around 10^10 GeV. The lifetime and the decay channels of our dark matter candidates depend crucially on the fact that the Klebanov-Strassler throat is supersymmetric. Furthermore, the details of supersymmetry breaking both in the throat and in the visible sector play an essential role. We identify a number of scenarios where this type of dark matter can be discovered via gamma-ray observations.

    Comments: 35 pages, 3 figures

    Subjects: High Energy Physics – Phenomenology (hep-ph); Astrophysics (astro-ph); High Energy Physics – Theory (hep-th)

    Cite as: arXiv:0801.4015v1 [hep-ph]

    Submission history

    From: Benedict von Harling [view email]

    [v1] Fri, 25 Jan 2008 20:09:01 GMT (37kb)


  • ljk February 22, 2008, 11:21

    Decaying Majoron Dark Matter and Neutrino Masses

    Authors: Massimiliano Lattanzi

    (Submitted on 21 Feb 2008)

    Abstract: We review the recent proposal by Lattanzi & Valle of the majoron as a suitable warm dark matter candidate. The majoron is the Goldstone boson associated to the spontaneous breaking of ungauged lepton number, one of the mechanisms proposed to give rise to neutrino masses. The majoron can acquire a mass through quantum gravity effects, and can possibly account for the observed dark matter component of the Universe.

    We present constraints on the majoron lifetime, mass and abundance obtained by the analysis of the cosmic microwave background data. We find that, in the case of thermal production, the limits for the majoron mass read 0.12 keV less than m_J less than 0.17 keV, and discuss how these limits are modified in the non-thermal case.

    The majoron lifetime is constrained to be larger than 250 Gyrs. We also apply this results to a given seesaw model for the generation of neutrino masses, and find that this constraints the energy scale for the lepton number breaking phase transition to be above 10^6 GeV. We thus find that the majoron decaying dark matter (DDM) scenario fits nicely in models where neutrino masses arise “a la seesaw” and may lead to other possible cosmological implications.

    Comments: 7 pages, 3 figures. Contribution to proceedings of the 4th Sino-Italian Workshop on Relativistic Astrophysics, Pescara, 20-30 July 2007

    Subjects: Astrophysics (astro-ph)

    Journal reference: AIP Conf. Proc., 966, 163-169, (2007)

    Cite as: arXiv:0802.3155v1 [astro-ph]

    Submission history

    From: Massimiliano Lattanzi [view email]

    [v1] Thu, 21 Feb 2008 16:40:57 GMT (110kb)


  • James M. Essig February 22, 2008, 18:26

    Hi ljk;

    It never ceases to fascinate me that we are at least in theory developing a particle zoo much like that of the 1950s and 1960s before the quark model simplified the description of baryons, mesons, etc.. The newer theoretical additions to the partice zoo: majoron, the broader set of Goldstone Bosons, the multiplicity of Higgs Bosons, neutralinos, inflatons and the list goes on and on; is getting to be very exciting. This should keep the theorists and experimentalists at CERN buzy. Even the technicians who keep the LHC up and running will no doubt feel a sense of personal satisfaction every time a discovery is made.

    More powerful future machines operating at 1,000 TeV Collision energies and above can directly probe the 10 EXP 6 GeV range for the lepton number breaking phase transition.

    I often wonder whether or not we will by chance discover additional quarks, or more leptons, with the increases in collision energy available for the LHC. Sensitivity of current models suggest that there are no additional leptons or quarks, however, the original quantum-chromo-dynamics or QCD models predicted only two or three quarks and proponents were suprised and delighted when additional quarks were discovered.



  • george scaglione February 23, 2008, 9:52

    jim yes sir FANTASTIC stuff! they sure are going to keep the LHC busy! no doubt! i look forward.just hope they don’t get things over complicated i’d sure like some clear answers…lol who would not? ! thank you very much for your thoughts, george

  • James M. Essig February 23, 2008, 18:29

    Hi George;

    Thanks for your response.

    Imagine an accellerator with the diameter of our solar system and accellerator particle steering magnets capable somehow of a whopping 1,000 Tesla. Such a machine might be able to very directly test the Grand Unification Energies according to G.U.T concepts. Building an accellerator with the diameter of the Milky Way Galaxy should enable us to test the Planck Energy levels and beyond. What would we find beyond? God knows what!

    Other proposed accellerators are the plasma beat wave accellerators in which a modulated electrodynamic field is set up in plasma in the form of beat waves. The modulated wave packets have anplitude peaks that can travel faster than light; however, the speed of energy, particle, and information exchange remains at or below C in comformity with special and general relativiity. These accellerators in concept were a popular conjectured technology in the late 70s and Early 80s but I don’t think much has come of them.

    Another option is to accellerate charged particles electrodynamically with super intense tightly focused laser beams, perhaps even someday with gamma ray beams because of their potential sub Angstrom cross-sectional width.

    Particle physics is really cool in my opinion and will help us reach the stars and beyond.


    Your Friend Jim

  • ljk March 4, 2008, 9:57

    Dark matter dynamics in Galactic center

    Authors: Eugene Vasiliev, Maxim Zelnikov

    (Submitted on 29 Feb 2008)

    Abstract: The evolution of dark matter in central areas of galaxies is considered (Milky Way is taken as an example). It is driven by scattering off of dark matter particles by bulge stars, their absorption by the supermassive black hole and self-annihilation. This process is described by diffusion equation in the phase space of energy and angular momentum. The equation was integrated for several different models of initial dark matter distribution and using various assumptions about the dynamical factors. It turns out that because the Milky Way center is rather dynamically old (~5 relaxation times t_r), the difference in initial conditions almost vanishes. The density attains a nearly universal profile, and the gamma-ray flux from dark matter annihilation lies in rather narrow range, which enables more robust determination of the dark matter parameters. By present the mass of dark matter inside the black hole sphere of influence (r less than 2 pc) has been reduced approximately twice, mostly because of heating by stars. It is shown that the dynamics of dark matter for t greater than t_r is determined mainly by stars outside the sphere of influence.

    Comments: 10 pages, 4 figs

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.0002v1 [astro-ph]

    Submission history

    From: Eugene Vasiliev [view email]

    [v1] Fri, 29 Feb 2008 21:00:21 GMT (102kb)