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Dark Matter Makes a Comeback

The Milky Way and moonrise over ESO's Paranal observatory (ESO/H.H. Heyer)

Recent reports of dark matter’s demise may be greatly exaggerated, according to a new paper from researchers at the Institute for Advanced Study.

Astronomers with the European Southern Observatory announced in April a surprising lack of dark matter in the galaxy within the vicinity of our solar system.

The ESO team, led by Christian Moni Bidin of the Universidad de Concepción in Chile, mapped over 400 stars near our Sun, spanning a region approximately 13,000 light-years in radius. Their report identified a quantity of material that matched what could be directly observed: stars, gas, and dust… but no dark matter.

“Our calculations show that it should have shown up very clearly in our measurements,” Bidin had stated, “but it was just not there!”

But other scientists were not so sure about some assumptions the ESO team had based their calculations upon.

Researchers Jo Bovy and Scott Tremaine from the Institute for Advanced Study in Princeton, NJ, have submitted a paper claiming that the results reported by Moni Biden et al are “incorrect”, and based on an “invalid assumption” of the motions of stars within — and above — the plane of the galaxy.

(Read: Astronomers Witness a Web of Dark Matter)

“The main error is that they assume that the mean azimuthal (or rotational) velocity of their tracer population is independent of Galactocentric cylindrical radius at all heights,” Bovy and Tremaine state in their paper. “This assumption is not supported by the data, which instead imply only that the circular speed is independent of radius in the mid-plane.”

The researchers point out the stars within the local neighborhood move slower than the average velocity assumed by the ESO team, in a behavior called asymmetric drift. This lag varies with a cluster’s position within the galaxy, but, according to Bovy and Tremaine, “this variation cannot be measured for the sample [used by Moni Biden's team] as the data do not span a large enough range.”

When the IAS researchers took Moni Biden’s observations but replaced the ESO team’s “invalid” assumptions on star movement within and above the galactic plane with their own “data-driven” ones, the dark matter reappeared.

Artist's impression of dark matter surrounding the Milky Way. (ESO/L. Calçada)

“Our analysis shows that the locally measured density of dark matter is consistent with that extrapolated from halo models constrained at Galactocentric distances,” Bovy and Tremaine report.

As such, the dark matter that was thought to be there, is there. (According to the math, that is.)

And, the two researchers add, it’s not only there but it’s there in denser amounts than average — at least in the area around our Sun.

“The halo density at the Sun, which is the relevant quantity for direct dark matter detection experiments, is likely to be larger because of gravitational focusing by the disk,” Bovy and Tremaine note.

When they factored in their data-driven calculations on stellar velocities and the movement of the halo of non-baryonic material that is thought to envelop the Milky Way, they found that “the dark matter density in the mid-plane is enhanced… by about 20%.”

So rather than a “serious blow” to the existence of dark matter, the findings by Bovy and Tremaine — as well as Moni Biden and his team — may have not only found dark matter, but given us 20% more!

Now that’s a good value.

Read the IAS team’s full paper here.

(Tip of the non-baryonic hat to Christopher Savage, post-doctorate researcher at the Oskar Klein Centre for Cosmoparticle Physics at Stockholm University for the heads up on the paper.)

About 

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

Comments on this entry are closed.

  • Olaf2 May 21, 2012, 7:17 PM

    That is science, it is self-correcting. :-)

  • krs33 May 21, 2012, 8:26 PM

    I am nothing even close to a physicist, but I cannot understand how scientists are so sure dark matter exists. Gravitational equations don’t seem work on galactic scales…so the answer has to be that the model really does work and we just need to account for mass that we can’t see? Can’t it be that the fundamental equation may be flawed? Maybe the gravitational constant isn’t as constant as we think? Or something else? Does the answer have to be that there is matter we can’t detect (despite pretty substantial effort)

    Further, if dark matter exists, I can never quite understand how the gravitational constant was even calculated in the 1st place, if it didnt account for dark matter (I am sure there is actually a valid answer to this, but no one has been able to explain it to me)

    Please no one attack me for being stupid, I generally know everything I know about astro- physics from watching shows on TV. But I do find scientists conviction that dark matter exists puzzling.

    • Ed David May 22, 2012, 1:39 AM

      Me too buddy,

      Dark matter was pretty much put into the equation to make the MATHS work for our general understanding of atrophysics on the wide scales of the universe.

      And still, it’s only fitting the MATHS – there’s been no confirmed direct observation of dark matter – so therefore i see it very possible that as you say that the fundamental equation may be flawed.

      • Jason Major May 22, 2012, 1:58 AM

        A lot of things accepted in astrophysics have not been directly observed but fit the “maths”. Much of our understanding of the Universe comes from math.

      • Jason Major May 22, 2012, 1:58 AM

        A lot of things accepted in astrophysics have not been directly observed but fit the “maths”. Much of our understanding of the Universe comes from math.

    • Ed David May 22, 2012, 1:39 AM

      Me too buddy,

      Dark matter was pretty much put into the equation to make the MATHS work for our general understanding of atrophysics on the wide scales of the universe.

      And still, it’s only fitting the MATHS – there’s been no confirmed direct observation of dark matter – so therefore i see it very possible that as you say that the fundamental equation may be flawed.

    • h_v_c May 22, 2012, 8:48 AM

      Dark matter was initially conceived to explain the motion of stars in galaxies. The problem was that the stars did not seem to move the way Newton’s laws predicted. So there were two possible solutions:
      1. Newton’s laws are wrong.
      2. There is a large amount of matter in the galaxies that does not emit any light.

      Many people have tried to modify Newton’s laws in such as way as to explain these observations, and it is in fact possible to do so (the maths get VERY contrived and complicated though; it’s much more difficult than just changing a constant).

      However, today there are many separate observations that point to dark matter, such as colliding galaxy clusters (http://en.wikipedia.org/wiki/Bullet_cluster). Modifications to Newtonian gravity have not been able to explain these. There’s also the fact that we can input the amount of dark matter predicted from other observations into computer simulations of structure formation (e.g. http://en.wikipedia.org/wiki/Millennium_simulation), and reproduce the current statistical distribution of galaxies almost perfectly. Without dark matter, it’s not even close. Again, modified Newtonian dynamics does not work here.

      In short, physicists believe in dark matter because it’s been very well tested in many independent ways and it’s the only theory that matches the observations. That doesn’t mean it’s not a problem that we don’t know what it is. There are many plausible suggestions though, and many ongoing searches to directly detect it.

      • Wezley Jackson May 24, 2012, 2:02 PM

        Thanks for your post…

        (quoting your part of post to krs33):

        However, today there are many separate observations that point to dark matter, such as colliding galaxy clusters (http://en.wikipedia.org/wiki/B… ).

        I think while generally true there are also many unexplained questions and it is apparent to me that Dark matter is at the very least ‘not well understood’

        Abell520 is a good example:
        http://www.universetoday.com/93934/hubble-spots-mysterious-dark-matter-core/
        http://www.universetoday.com/94088/journal-club-aberrant-dark-matter/

        and…
        http://www.universetoday.com/16927/dark-matter-is-missing-from-cosmic-voids/

        A few years ago I was sceptical but have softened and accept that DM/DE is a productive theory and best frames the newest frontiers of our knowledge of our cosmos.

        Best,

        Wezley

        • h_v_c May 24, 2012, 3:55 PM

          I don’t know enough about those galaxy clusters to have an informed opinion, but your third link actually suggests that we *do* understand very well how dark matter works. The fact that we can input what we know about dark matter from other observations into computer simulations and get out density structures that perfectly match the Sloan data is quite remarkable.

          My guess would be that the mysteries regarding the clusters you mention stem not from lack of understanding of dark matter itself but how it couples to baryonic matter (i.e. the stars and galaxies that we can actually see). Simulating dark matter is simple since it only interacts gravitationally, and as your link shows, we can do this very well. However, connecting that to how stars and galaxies form is much more complicated.

          Sean Caroll has a good post about this paper on his blog (http://blogs.discovermagazine.com/cosmicvariance/2012/05/23/dark-matter-still-existing-one-in-a-continuing-series/ )

          He also has a really good post about MOND theories, and just how complicated they tend to become (not to mention the fact that MOND actually needs to include dark matter as well!): http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/dark-matter-just-fine-thanks/

          Finally, I think your last sentence is a bit telling. People tend to mix dark matter and dark energy together, but they have very little to do with each other. Dark matter is an old idea, well supported by observations, and as Sean Caroll puts it, it’s probably “just some new particle we haven’t yet made in the lab, hardly the end of the world”. Dark energy on the other hand is something we know virtually nothing about, and it’s something that tends to make physicists feel a bit uneasy.

          • Wezley Jackson May 25, 2012, 11:11 AM

            Your points are well taken.. Although I am not a physicist (nor do I play one on TV), I have a keen interest in physics/astrophysics and respect bright physicists & enjoy trying to phathom the intricacies and subtleties of modern cosmology..

            You are right that I did associate DM/DE together and I suppose that is a result of my recreational study of Einstein’s theory/equations with Mass/Energy equivalencies.. I guess in the back of my mind somewhere perhaps there is a little goblin thought that Dark Energy = Dark Mass * Acceleration (C2) ;) Probably my own pet misconception…

            Like many within and outside academia, I do find the idea of a “negative energy field” disturbing (ie. DE) since the term itself is inherently counter-intuitive (and possibly non-existent).

            Not wanting to labor the point but how can energy(light/radiation) be “dark” ?
            And how can one describe an energy field with <0 / Less than zero-point energy?? How can one have less than a vacuum ??

            I shall leave these questions to clever, more qualified people than myself…

            I started out reading these forums years ago with a chip on my shoulder that DE in particular was a load of guff…

            I now accept Dark Matter is a valid important theory and there are some interesting suspects such as the Majorana Fermion and it seems that with mankind's new marvelous instruments (LHC, Tevatron, colliders, space and terrestrial instruments that physicists and astrophysicists are getting closer to discovering the elusive particle/s every day (as pointed out below by super_earth))…

            Also I try to keep an open mind that Dark Energy at the very least points to the fact that we haven't accounted for the energy budget of our observable cosmos.

            A big thanks to the people who run UT and the posters in this forum (in particular Tobjorn, LC, Steve Nerlich and others) and people like yourself for the patience to hang in and discuss/share things..

            Thanks for the links above – I shall read the articles tonight.

            All the best,

            Wezley

    • lcrowell May 22, 2012, 4:38 PM

      The first measurement of the gravitational constant was made by Cavendish in the 18th century. Dark matter may constitute most of the matter in the universe (ignoring dark energy here), but its density is very small. As a result measurements of the gravitational constant are very weakly effected by any DM in the vicinity of the Earth.

      Dark matter is just a term for some form of non-light emitting form of matter. This is known to exst because of the general motion of stars in galaxies. These motions deviate far from Kepler’s laws. This means there must be some matter medium in galaxies, but where it is not light emitting.

      LC

      • Dilip G. Banhatti May 22, 2012, 7:32 PM

        Motions of stars in (disk or any other) galaxies should not follow Kepler’s laws. Kepler’s laws are meant for systems where there’s effectively a point attracting mass with bodies neglible mass going around it. A galaxy is far from such a system. A (disk or any other) galaxy has substantial distributed mass, in addition to the concentration at the centre.

        • lcrowell May 22, 2012, 9:16 PM

          Enough luminous matter is concentrated near the center so the orbits should approximately obey Kepler’s laws. This is particularly for matter further out from the center. The observed motions are very far removed from Kepler’s laws. This necessitates some form of DM.

          LC

    • andrew g May 22, 2012, 5:34 PM

      “Can’t it be that the fundamental equation may be flawed? Maybe the gravitational constant isn’t as constant as we think? Or something else?”

      I think you can be confident that all these other alternatives will have been considered (think of the fame if you find evidence) and dark matter will still be the best solution, but I agree the message could be got across a bit better.

  • David Brown May 21, 2012, 8:35 PM

    Yes, it’s self-correcting, and re-correcting, and so on, and so on, etc.

  • super_earth May 22, 2012, 5:46 AM

    What about the observation of a 130 GeV gamma ray emission line by the Fermi gamma ray telescope?

    It could be the first detection of radiation from the annihilation of dark matter!

    The papers about it are:

    A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope
    http://arxiv.org/abs/1204.2797

    Fermi 130 GeV gamma-ray excess and dark matter annihilation in sub-haloes and in the Galactic centre
    http://arxiv.org/abs/1205.1045

    • JonHanford May 22, 2012, 2:21 PM

      Or it “may be a component of the diffuse background and can be of instrumental or astrophysical origin”, thereby making “the dark matter origin of this spectral feature….dubious”: http://arxiv.org/abs/1205.4700

      Seems the jury is still out on this one.

    • lcrowell May 22, 2012, 4:31 PM

      These data support the case that dark matter particles are condensates of supersymmetric fermionic pairs of the photon, Z and higgs particle.

      LC

  • super_earth May 22, 2012, 5:47 AM

    What about the observation of a 130 GeV gamma ray emission line by the Fermi gamma ray telescope?

    It could be the first detection of radiation from the annihilation of dark matter!

    The papers about it are:

    A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope
    http://arxiv.org/abs/1204.2797

    Fermi 130 GeV gamma-ray excess and dark matter annihilation in sub-haloes and in the Galactic centre
    http://arxiv.org/abs/1205.1045

  • Adrian Fartade May 22, 2012, 3:51 PM

    Galactocentric cylindrical radius ?

    wow, that’s nice to say out loud..but what is it exactly ?

    • andrew g May 22, 2012, 5:25 PM

      ““The main error is that they assume that the mean azimuthal (or rotational) velocity of their tracer population is independent of Galactocentric cylindrical radius at all heights,”.

      It’s a stupid mistake I make all the damn time :(

  • Efreet May 22, 2012, 10:16 PM

    it is refreshing to see the “softening” of peoples’ attitudes towards DM in the comments since the days that i used to frequent this blog. the regs even allowed someone to question DM’s existence without attacking them. i’m very pleased. thanks all!

  • The Muss May 23, 2012, 6:21 AM

    I need an alien teacher NOW!!!!

  • thinkingloud May 26, 2012, 9:52 AM

    No, dark matter is not back !

    Many astronomers were feeling uneasy about the Moni Bidin et al. claims, and it is good to see these being rectified. The Moni Bidin et al. result would have ruled out MOND as much as dark matter if it had been correct.

    The MOND _prediction_ for a dark matter effect near the Sun is very close to what Bovy & Tremaine compute by adopting Newtonian dynamics. That is, rather than getting dark matter back, it is indeed not there, and all one is seeing is the MOND “phantom” dark matter. Phantom dark matter is the “dark matter” a Newtonian observer “sees” in a galaxy which obeys MOND. Phantom dark matter is completely different though from the cold or warm dark matter which the standard cosmological model needs.

    Given other recent claims about the non existence of dark matter, this MOND prediction is seen in Fig.12 of this paper: http://adsabs.harvard.edu/abs/2012arXiv1204.2546K (see the description on SciLogs: http://www.scilogs.eu/en/blog/the-dark-matter-crisis/2012-04-19/dark-matter-gone-missing-in-many-places-a-crisis-of-modern-physics). This means that the true (MONDian) rotational velocity must be larger than the one derived just from the normal matter by about 20 per cent.

    A recent (robust) claim that dark matter is missing significantly in the Local Universe can be found here: http://arxiv.org/abs/1204.3377. This appears to be a robust result as it is simply based on adding up the galaxies there are. Further, that the standard model of cosmology fails on massive galaxy clusters is shown here rather convincingly:
    http://arxiv.org/abs/1205.3788 – citing: “For standard LCDM structure formation and observed background field galaxy counts this lens system should not exist.”

    So it seems that while Moni Bidin et al. obtained a wrong result, dark matter is certainly _not_back_. The correct analysis of the excellent data of Moni Bidin et al. by Bovy & Tremaine nicely affirms this by showing beautiful consistency with MOND.

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