Kentucky Windage, For Science: Using Black Holes To Measure The Universe's Rate Of Expansion
    By Hank Campbell | May 22nd 2013 03:01 PM | 5 comments | Print | E-mail | Track Comments
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    Calibration does not always mean fixing a device, it sometimes means adjusting to solve a problem.  In the early years of America, the famous Kentucky longrifles that conquered the frontier (and some British) had fixed sights. Since they couldn't be adjusted, frontiersmen - Kentucky was part of "The West" then - would adjust for wind, elevation and range by experience.  If their shot was hitting low and left, they aimed high and right. Inference helped them get a better result.

    By the early part of the 20th century, "Kentucky Windage" had become part of the American lexicon, to signify the practical, quick approach to solving problems - not always the best one, though. You will still find it in use in today's U.S. military, though they would prefer you sight in the weapons.(1)

    And conceptually it's still used today in research, though most academics have never heard the term. But scientists use inference. In 2011, Saul Perlmutter, Brian P. Schmidt and Adam G. Riess received the Nobel prize in Physics for determining that the universe is expanding at a much faster rate than once believed, "through observations of distant supernovae". That discovery opened up a lot of new science questions. Black holes can't be measured but if they can be used to measure other things, like maybe the rate of expansion, well that's Kentucky Windage - for science.  

    The discovery of accelerating expansion was not trivial stuff. And using black holes to measure that rate is downright arcane, but a paper in Physical Review Letters says it can done. The authors say their new method can measure distances of billions of light years with a high degree of accuracy, using Super-Eddington accreting massive black holes that lie at the center of many galaxies.Their method can gauge distance at even high redshift, they say.(2)

    In the paper they address how to detect these Super-Eddington accreting massive black holes and how their observational uncertainties can be used to deduce cosmological distances. These black holes reach a saturated luminosity as material is drawn into the black hole, becomes hotter and emits fantastic amounts of radiation - 1000X the energy produced by a large galaxy containing 100 billion stars.  Saturated luminosities are well understood and have no 'cosmic evolution' that would have to be factored in to prevent skewing the results.

    Using radiation to measure distances is not new, but using black holes is obviously trickier. It requires deriving the amount of radiation that reaches Earth from the energy being emitted from the vicinity of the black hole. The researchers say the radiation being emitted depends on the properties of the black hole and for Super-Eddington accreting massive black holes the amount of radiation emitted as the object draws matter into itself is actually proportional to its mass.

    Knowing that, they infer the distance to the black hole and the time in the history of the universe when the energy was emitted.They believe their method allows them to measure distances much farther away and with more precision than supernovae.

    What can it tell us? The big cosmological puzzle is the blanket combination of mysteries referred to as 'dark energy' and 'dark matter'.  Dark energy, so far unmeasured, acts as some sort of opponent to gravity and would account for why expansion of the universe is accelerating rather than slowing down and observations of Super-Eddington accreting massive black holes at high redshift may provide some insight.

    They will just need to measure a lot more of them and see if their cosmological model holds up.

    Citation: Jian-Min Wang, Pu Du, David Valls-Gabaud, Chen Hu, and Hagai Netzer, 'Super-Eddington Accreting Massive Black Holes as Long-Lived Cosmological Standards', Physical Review Letters Volume 110 Issue 8, DOI: 10.1103/PhysRevLett.110.081301


    (1) Kentucky windage, US Army style of 1943.

    (1) Want to calculate your own redshift?  Caltech has made it easy


    John Duffield
    Sounds good, Hank. So good that it feels like rock-solid science rather than Kentucky windage. I'd better read up on it I suppose. Re dark energy and cosmological models, there's people like David Wands at Portsmouth and David Wiltshire at Canterbury NZ putting papers up on the arXiv. There's other people too, see the co-authors at least. To date though they don't seem to be getting much publicity. My amateurish simplified reading is that they're in line with Guth's inflation wherein expansion is driven by a positive vacuum energy. It's like large-scale homogeneous space has a kind of innate pressure, and you're only dealing with a diagonal stripe through the stress-energy tensor. That's different to what you usually hear, wherein dark energy is described as being responsible for the increasing expansion, and conservation of energy is broken. Maybe John Webb et al's work on the fine structure constant ( ) will come into play wherein "the strength of space" reduces. Mordehai Milgrom mentioned that when referring to f(R) gravity on page 5 of Play around with silly putty, stretch it and watch it droop. Then play around with a stress ball. Squeeze it down, then open your fist.
    Well, don't take Kentucky Windage to mean more than it means. It obviously won a whole bunch of wars and saved a whole bunch of lives by defending and feeding a whole bunch of people throughout history. Hagai Netzer sent me the PRL paper a month ago so I spent a lot of time mulling it to try and distill it properly.

    If I accept one premise as fact, King Arthur was real. Norma Goodrich did just that in a really wonderful book - but if you didn't accept that one controversial author 700 years after the fact had access to secret documents lost to antiquity all 300 pages derived in the book by her were not valid.  In that sense we have to have some caution on acceptance that the radiation from a black hole is being derived accurately. A 6-sigma superluminal neutrino result was 6-sigma, but not accurate. And so I can't say this method is "rock-solid science", though I certainly hope someone has stumbled on the secret to dark energy.
    King Arthur wasn't a real person ?  Good grief, you'll be telling me next that I should stop believing in Santa Clause. 
    the famous Kentucky longrifles that conquered the frontier (and some British)
    You had to bring the British up, didn't you?  Kentucky windage is an excuse thought up by a shooter who couldn't hit the target on the first attempt.  ;-)
    John Duffield
    Fair enough Hank. I read the paper and followed up on super-Eddington luminosity, mindful of how (Arthur) Eddington got the fine-structure-constant badly wrong. But I have to say it looks good to me. Black holes are like engines regulating their own fuel supply. More fuel in means more radiation pressure which reduces the fuel supply. This isn't in the same league as SUSY, which is built on non-understanding of gamma-gamma pair production. How can you propose a selectron when your understanding of the electron gave up at "the electron is an excitation of the electron field"?

    I reckon if you plied some relativity guys with alcohol and pumped them about dark energy, they'd bang the table and say "What mystery?" Then they'd say "see Doc 30 page 185 where Einstein says the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy. That's dark energy, right under your nose". They'd be stabbing their finger at the stress-energy tensor too. It's like there's a pressure gradient in the space around the Earth. In large-scale homogeneous space there are no pressure gradients, but there's still pressure. So duh, space expands.
    Until it's further defined out of (my) reality, my take on Dark Energy is it behaves like surface tension of water, where matter creates depressions, empty space gets a slight positive curve, and the depressions just move away from open space.
    Never is a long time.