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Charismatic Megaparticles Might Hint at Dark Matter, and Much Besides


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At a lecture I went to some years ago, astrophysicist Trevor Weekes compared garden-variety elementary particles to mosquitoes. They are plentiful and easy to find—indeed, they find you. But ultra-high-energy gamma rays, he said, are like elephants. They are fairly rare, but among the greatest of creatures. They often roam in spectacular habitats. Their sheer heft tests the limits of the laws of nature.

I naturally wanted to invite an article for Sci Am about these charismatic megaparticles, but for years I struggled with what the article would say. Although they may be the most powerful electromagnetic radiation known to science—photons with an energy of around a teraelectron-volts (TeV), the kinetic energy of a mosquito concentrated into a single quantum—once you use up all the superlatives in your thesaurus, what was there to say, really? At the time I saw Weekes speak, astronomers had found a grand total of about a dozen celestial sources of TeV gamma rays, and they were the usual suspects: giant black holes and suchlike. Teragammas had revealed nothing about the ecology of the universe which astronomers didn’t already know. They were like animals in a zoo rather than out in the wild: fun to look at before you move onto the baby penguins.

This has all changed in the past couple of years. Observatories have catalogued 136 TeV sources, which is enough to start doing systematic astronomy rather than freak-show physics. They have turned up some striking results, questioning conventional wisdom about pulsars and shedding some light on dark matter.

Blazars, giant black holes that just so happen to be oriented that we are looking down the barrel of the jets they spray out (see picture above), are the largest single category of TeV gamma source outside our galaxy. They are pretty extreme to begin with, but some go all out. They blaze with the intensity of a thousand Milky Way galaxies and can vary in brightness by a factor of five within an hour—a puzzlingly rapid time, too fast even for light to cross from one side of the black hole to the other. “They’re some of the wildest animals in the whole astronomical zoo,” says astrophysicist Chuck Dermer. “The luminosities are just incredible.”

Superlatives aside, last year Christoph Pfrommer, Philip Chang, and Avery Broderick proposed that TeV gammas from blazars play an unappreciated role in heating up intergalactic gas. The injection of thermal energy would prevent the gas from settling into galaxies—especially into small galaxies, whose gravitational fields are too weak to overcome the tendency to dissipate. This may solve one of the most perplexing puzzles in modern cosmology: the fact that dark matter should nucleate lots of miniature galaxies, yet doesn’t seem to do so.

The blazars listed in the TeV catalog are only a small fraction of the ones out there. To our instruments, all the others blur together, forming a diffuse glow spread over the entire sky. In the 1990s, the Compton satellite measured this gamma-ray background up to an energy of 0.1 TeV. Yet when Compton’s successor, the Fermi satellite, went to take a look, the background glow looked so different that it was as if astronomers were seeing it for the first time. The earlier observatory appears to have been miscalibrated at the highest energies.

The upshot is that blazars are not the only things bathing our sky in a diffuse glow of high-energy gammas. Dermer says they account for only about a sixth of the background. The rest must come from pulsars, collisions of cosmic rays produced by supernovae, and maybe the decay or annihilation of dark-matter particles. “We still cannot explain the intensity of the isotropic flux,” says physicist Steve Ritz, one of the leaders of the Fermi project. Astrophysicists gathered to discuss this mystery during a special session of the American Astronomical Society meeting in Anchorage last week.

Pulsars are another example of how recent measurements have forced theorists back to the drawing board. By rights, these hyperdense neutron stars should be denuded of very-high-energy gammas. Although the stars might well produce such gammas near their surface, the surrounding magnetosphere should snuff them out, while gammas produced at higher altitudes should be comparatively wimpy. “A lot of people discouraged us from looking at pulsed emissions from pulsars,” recalls gamma-ray astronomer Nepomuk Otte.

So when the MAGIC observatory saw hints of high-energy pulses from the pulsar at the heart of the Crab Nebula, Otte says few paid any attention. But he and his colleagues kept at it and, last year, Fermi and the VERITAS observatory confirmed photons with up to 0.4 TeV. “This has changed the picture that we have of how gamma rays are produced in the Crab pulsar,” Otte says. A new idea is that streams of electrons and positrons are carrying energy into the outer magnetosphere and converting into gammas there. Astrophysicists had known that neutron stars were complicated, but not this complicated.

 

The biggest wildcards in teragamma astrophysics are so-called dark accelerators. These are TeV gamma sources that astronomers have yet to see any other way; they do not seem to correspond to any star, nebula, or other discernible object. They are tantalizingly marked “UNID” in the database. They might turn out to be known systems such as pulsar nebulae, but there’s always the hope they are dark matter or some other never-before-seen species. “There’s a lot of speculation about them,” Otte says.

To know for sure what’s going on, astronomers need even more than 136 TeV sources. A thousand would be more like it. So they are now planning the next generation of observatory with telescopes scattered over a square kilometer of land. Like the animals of Madagascar, gammas have broken out of their zoo and returned to the wild—with emphasis on the word “wild.”

Blazar image credit: copyright: ESA/NASA, the AVO project and Paolo Padovani; Telescope image credit: G. Perez, SMM, IAC

George Musser About the Author: is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 10:02 am 06/18/2012

    OK, I’ll bite: what property of disperse dark matter would now allow it to condense to form a compact, dense, ultra high energy emission source for TeV gamma rays?

    Link to this
  2. 2. rloldershaw 11:00 am 06/18/2012

    The recent discovery of an estimated 200 billion unbound planetary-mass objects (UPMOs) in the Galaxy, and previous evidence for stellar-mass microlenses, prompt the following comments.

    If the Galaxy has huge populations of unbound ultracompact objects with masses in the 0.1 to 1.0 solar mass range (MACHOs) and Neptune/Jupiter mass range (UPMOs), then several key astrophysical enigmas might be solved by their existence. A partial list of those mysteries that MACHOs and UPMOs might explain would include the following.

    The composition of the galactic dark matter.
    The ubiquitous 1-100 day variability of quasars.
    The factor of 6 excess in the ARCADE-2 radio background observations.
    The origin of cosmic rays.
    The composition of the recently discovered infrared-faint radio transients.
    A significant unidentified component of the X-ray background.
    A significant unidentified component of the Gamma-ray background.
    Ubiquitous Gamma-ray burst phenomena.
    The origin of ubiquitous unidentified optical transients.

    It would seem prudent at this point to put as much effort into the dark matter search for “primordial” Kerr-Newman ultracompact objects as has been expended in the unsuccessful 35-year effort to find any form of “WIMP” or exotic particle dark matter.

    Robert L. Oldershaw
    http://www3.amherst.edu/~rloldershaw
    Discrete Scale Relativity; Fractal Cosmology

    Link to this
  3. 3. julianpenrod 8:36 pm 06/18/2012

    So often, it is a case that simple repetition, withpout explanation, is sufficient to convince many that something is a “fact”. As with so many articles, anymore, black holes are spoken of as if they definitely exist. in fact, no black hole has been opbserved. Phenomena that supposedly can be sattirbuted to them can be caused by other items, as well. To so diffidently ascribe various phenomena to black holes is, actually, very unprincipled, but so many do that, these days, and so few upbraid them on it. The same is true of “dark matter” and “dark energy”. And, the fact of the matter is that, frankly, no one has really seen such things as “relativity” or “evolution” to occur. Their “existence” has only been asserted by “scientists” behind “laboratory” doors, and devotees of “science” have insisted that what they say cannot be wrong.

    Link to this
  4. 4. jtdwyer 8:10 am 06/19/2012

    Perhaps I should better explain. I fail to see how dark matter could form a dense object capable of emitting TeV gamma rays, since the implied requirement for its producing an enormous halo surrounding galaxies is that it _not_condense_ at the center of galaxies. This property is required so that the envisioned vast dark matter halo can extend galaxies’ mass distribution far beyond their visible periphery, to fit their characteristically flat rotational curves to the Keplerian model for planetary systems.

    BTW, the fundamental error in logic that led to the perceived requirements for galactic dark matter is that the already vast distribution of galactic mass should comply with the rotational characteristics described by Kepler’s laws of _planetary_ motion – this presumption has never been formally established.

    Link to this
  5. 5. jtdwyer 5:12 pm 06/19/2012

    rloldershaw – you may well be correct that astronomers’ methods of estimating galactic mass omits a significant number of actual ‘ordinary’ objects of mass.

    However, the fundamental issue that led to the establishment of the perceived requirement for galactic dark matter was that (especially spiral) galaxies’ rotation curves (plots of rotational velocity as a function of radial distance) did not comply with Kepler’s characteristic rotation curves derived from the laws of planetary motion. The rotational velocity of planets in the Solar system are known to diminish with their distance from the Sun.

    A seminal research report concluding 10 years of observations proved that galactic rotation curves definitely did not comply with Keplerian rotation curves. Please see Rubin, et al., (1980), “Rotational properties of 21 SC galaxies with a large range of luminosities and radii, from NGC 4605 /R = 4kpc/ to UGC 2885 /R = 122 kpc/ “, http://dx.doi.org/10.1086/158003
    http://adsabs.harvard.edu/abs/1980ApJ…238..471R
    As stated in section VIII. “DISCUSSION AND CONCLUSIONS”:
    “1. Most galaxies exhibit rising rotational velocities at the last measured velocity; only for the very largest galaxies are the rotation curves flat. Thus the smallest Sc’s (i.e., lowest luminosity) exhibit the same lack of a Keplerian velocity decrease at large R as do the high-luminosity spirals. This form for the rotation curves implies that the mass is not centrally condensed, but that significant mass is located at large R. The integral mass is increasing at least as fast as R. The mass is not converging to a limiting mass at the edge of the optical image. The conclusion is inescapable that non-luminous matter exists beyond the optical galaxy.”

    The fundamental requirement of a dark matter halo is that it vastly extend the peripheral distribution of galactic mass. In that way, it was demonstrated using galaxy models that the flat rotation curve of the visible galaxy merely represents the inner region of the galactic mass distribution, and that if the rotational velocities of the peripheral dark matter halo were plotted, they would diminish with radial distance in compliance with Kepler’s laws of planetary motion.

    Simply increasing the estimated mass within the visible galaxy would not produce compliance with Kepler’s characteristic rotation curves. As a result, MACHOs and non-luminous objects of mass within the visible galaxy periphery, even within the observed galaxy halo, would not produce the expected diminishing rotation curve.

    Only recognition that the simple approximations provided by the empirical laws of planetary motion (derived solely from observations of the Solar system, where the Sun contains 99.86% of total system mass) simply cannot not apply to vast spatial distributions of massive objects comprising galaxies – invalidates the perceived ‘evidence’ for galactic dark matter. In no galaxy is the distribution of mass comparable to that of our Solar system – it should never have been presumed that they should rotate identically.

    Link to this
  6. 6. rloldershaw 6:47 pm 06/19/2012

    In Discrete Scale Relativity the predominant Kerr-Newman ultracompact objects at 8 x 10^-5, 0.145 and 0.580 solar masses are in every way as fundamental as the electron, proton and alpha particle, of which they are exact self-similar analogues.

    We do not ask how electrons or protons “form”, do we?

    The NuSTAR X-ray telescope launched last week, and due to start scientific observations in about 3 weeks, should have the ability to detect the definitively predicted planetary-mass and stellar mass ultracompacts that comprise the dark matter, since NuSTAR’s resolution and sensitivity are a factor of 10 and 100, respectively, better than any previous X-ray telescope.

    If DSR is right about the dark matter, then we have a radically new and amazingly unified new paradigm for understanding nature. Moreover, theoretical physics will finally be put back on the tracks to progress.

    Robert L. Oldershaw
    http://www3.amherst.edu/~rloldershaw
    Discrete Scale Relativity
    Fractal Cosmology

    Link to this
  7. 7. vinodsehgal1957@yahoo.com 1:40 am 06/20/2012

    julionpenord

    Yes, I agree with you that when a statement is repeated thousand, million and billion of times, without adequate explanation that after some period it appears a reality — a truth so sacrosanct that normally no is supposed to question its validity. For centuries, solar system was perceived with earth as center and it was non-sense to question this concept during those days. I think in the modern cosmology, expansion of space/universe is the holy grail. Not even an inch of space expansion has ever been measured, no one knows what is in the space which expands, no one also knows from where the expanded space emerges out, no one knows where space expands out and so one knows what is that which expands space, yet in the hierarchy of cosmological concepts, space expansion occupies the top order. In the area of space expansion, either direct and leading questions are avoided OR such irrational explanations are extended for which even commonsense will be belittled. Reason? same as that when a statement is repeated billion of times, it appears a truth sacrosanct.

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  8. 8. jtdwyer 3:34 am 06/20/2012

    BTW, “charismatic megaparticles” is not only hyperbole but inaccurate. If you must invent terminology, “teragamma” is the most descriptive, IMO.

    Link to this
  9. 9. Laird Wilcox 9:56 pm 06/20/2012

    Particals are charismatic? OK. I’m waiting for the punch line.

    Link to this
  10. 10. jack.123 6:25 pm 06/21/2012

    As for no one having seen a black hole,we have seen stars orbiting what appears to be one at the center of our galaxy.

    Link to this
  11. 11. christinaak 12:19 pm 06/22/2012

    Mr. Penrod I am sure you are sincere in your beliefs, but why bother to visit science websites if you neither understand or appreciate science or its findings? Biological evolution is well established scientific fact that is supported by an immense amount of overwhelming evidence from multiple scientific fields (you can do the research if you are serious about learning). Relativity has been tested over and over again and the theory still stands. GPS satellites must take both special and general relativity into account to remain accurate. The evidence for the existence of black holes is getting increasingly stronger, and it is hoped that the existence of dark matter and dark energy will either be supported or disproven eventually. Science is not perfect but it eventually gets things right because unlike religion it is always subject to revision when new evidence requires it to do so.

    Link to this
  12. 12. kfreels 1:02 pm 06/22/2012

    @julianpenrod – are you kidding? Using your statements I can say that no one has ever seen a radio wave. It is just inferred by scientists behind lab doors.

    Link to this
  13. 13. Strangel 1:25 pm 06/22/2012

    The image referred to above is not a blazar.

    @Julian -What? Science is not a religion buddy, it’s a verb. It’s something you do and anyone can do it. If you think that someone’s science is wrong you’re more than welcome to review their assertions and prove them wrong (or right, if you actually do your science correctly). Ever seen gravity?

    Link to this
  14. 14. vinodsehgal1957@yahoo.com 6:41 am 06/23/2012

    A galaxy is a radically different system than our solar system.There is no comparison at all. Solar system being highly centralized two part mass system(99.86% mass at center) having some 8 planets. In contrast, a galaxy a very very complex system having billions of stars, planets both bounded and unbounded), black holes, neutron stars, brown dwarfs, gas, dust, radiations and host of other unknown objects of all type and that too spanning into 100000 Lys (MW). In view of this applicability of Kepler’s Law to stellar movement in a galaxy is not only naive but also misleading

    Correct approach shall be to devise some strategy to ascertain gravitational mass of some part of galaxy or universe correctly and then to have comparison with the luminous mass. This comparison will only lead to correct finding if there is need for extra gravitation, over and above gravitation from luminous mass and energy. If need for extra gravitation is established with clinching evidence, then dark matter may be one suspect candidate apart from other probable explanations of modified Newtonian gravitation or quantum level gravitation.

    In any area of galaxy, strong condensation of visible mass may be due to diverse factors — presence of dark matter, density of condensed mass, density of mass and radiation in vicinity, presence of MACHO providing strong gravitation. Condensation of visible mass in any area should not lead to naive conclusion that dark matter may also be condensed there. Further dark matter may not necessarily be localized in any area of galaxy.

    Since dark matter does not interacts with visible mass except gravitationally, dark matter may be present even in voids but it will not be detectable since presence of dark matter in any area is detectable by its gravitational pull upon visible matter only. It may cast its influence upon visible matter within galaxies in peripherial regions. Dark matter being more fundamental than normal matter, its dispersal will not follow dispersal of normal matter. However, normal matter may follow the the pattern of dispersal of dark matter

    Link to this
  15. 15. m allworth 12:50 pm 06/28/2012

    Supposing – just supposing that ‘dark matter’ could somehow transmute into matter i.e. into a lower energy state.

    Might that give some answers ?

    Best wishes!

    Link to this

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