My first Magazine Sky & Telescope - 01.2019

stant using a very distant source (since the universe emitted the CMB when it was only 370,000 years old). There are still some dis- crepancies between data sets: The Hubble constant may be 68 kilometers per second per megaparsec or 73 km/s/Mpc, depend- ing whether you measure it from nearby sources or from the CMB. But there’s simply no way that current measurements would allow a value of 10 km/s/Mpc, as Harrison proposed, and ultimately a scientiﬁ c theory must be able to predict our observed reality. Gravity Doesn’t Discriminate In a sidebar (S&T: Sept. 2018, p. 25), Camille Carlisle mentions black holes undergoing the EKL mechanism. Is it also possible for the Hills mechanism to occur, ﬂ inging a black hole away from the center of our (or any other) galaxy to travel through intergalactic space? I don’t recall seeing this possibility men- tioned as a source of dark matter. Ed Evans Seneca, South Carolina “ Camille Carlisle replies: Any kind of binary can undergo the Hills mechanism, since in these situations gravity doesn’t discriminate among stars, black holes, and other stellar remnants. Unfortu- nately, a hypervelocity black hole would be immensely difﬁ cult to detect. A star emits light, so we can see it and calculate both its proper motion across the sky and its radial velocity along our line of sight, based on the redshift of its spectrum. We couldn’t do that with a black hole. I’m afraid hypervelocity black holes wouldn’t work as a potential explanation for dark matter. For one thing, a hyperveloc- ity black hole would not be bound to the galaxy, so it wouldn’t create a big, deep well of gravitationally bound matter for the galaxy to sit in. For another, the Milky Way has roughly 10 times more mass in dark matter than it does in all its stars combined. There just haven’t been enough massive stars to 1969 1994 Here Goes Nothing “The Void Next Door” (S&T: Oct. 2018, p. 12) helps a lot in visualizing our cos- mic neighborhood. Over and above the numerous successful large simulations, is there an intuitive picture of why voids, sheets, and ﬁ laments are what gravity produces out of Gaussian quan- tum ﬂ uctuations in the early universe? Ken Wachter The Sea Ranch, California “ Ken Croswell replies: Voids have a lower density, so they expand faster than the overall universe. I like to think of voids as pushing against the rest of the universe, evicting their galaxies and herding them into the ﬁ laments and sheets that crisscross the cosmos. SUBMISSIONS: Write to Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, U.S.A. or email: letters@ skyandtelescope.com. Please limit your comments to 250 words; letters may be edited for brevity and clarity. 75, 50 & 25 YEARS AGO by Roger W. Sinnott 1944 make enough black holes to explain all that dark-matter mass. º January 1944 Director’s Rebuke “[Long ago, on occasion, George Airy would tell an assistant at the Greenwich Obser- vatory,] ‘The Royal Observatory was founded for observation of the moon. We get about 300 observa- tions of the moon during the year in all; and the Observatory costs the nation 6,000 pounds a year. Hence each observation of the moon is worth 20 pounds; and by losing one last night you have cost the nation 20 pounds! . . .’ “[That sum] seems pretty high for one observation of the moon’s position, whereas Dr. Harlow Shapley igured a few years ago that it cost only a few cents . . . to discover a galaxy!” This gem is from Roy K. Mar- shall’s ever-popular Astronomical Anecdotes column. rapidly pulsating radio sources and supernova explosions. Two cases of such association have recently been announced. . . . “The irst of these pulsars [lies] near the midpoint of the extended radio source Vela X [,] believed to be the result of a prehistoric super- nova explosion. [Secondly,] R. B. E. Lovelace and his associates observed [Tau- rus pulsar] NP 0532 with the 1,000- foot dish at Arecibo, Puerto Rico. Several scans on November 15th placed the pulsar within 10 minutes of arc of the Crab nebula’s center. . . . The distance of NP 0532 [from us] agrees fairly well with that of the Crab nebula, [so perhaps] this pulsar is, like the Crab nebula, a product of the supernova observed in A.D. 1054.” The pulsar-supernova link, now amply conﬁrmed, was made just a year after pulsars were discovered. º January 1969 Growing Evidence “[T]here may be a close connection between º January 1994 Distant Blinks “Among the excit- ing astronomical news of last year was David Jewitt and Jane Luu’s discovery of objects orbit- ing the Sun beyond Pluto. . . . They may lie within the Kuiper Belt, the hypothetical reservoir of comets postulated [to exist] just outside the region occupied by planets. . . . “Is it possible to see [such remote comets] where they live before making their suicidal dives toward the Sun? [Using] relected sunlight to detect them is hopeless. But there is another way to ind comets; it involves looking for dark objects by observing occultations of more distant, bright objects. . . . If we adopt Kuiper’s optimistic estimate of the Belt population, 10 13 bodies, we can calculate that a star is occulted . . . once every two days or so.” Princeton physicist Freeman Dyson, reviving an earlier proposal by Mark E. Bailey, lamented it had been largely ignored. In recent years at least ﬁve major observa- tories have launched occultation surveys of this type, but so far the results are inconclusive. sk yandtele scope.com • JA N UA RY 2 019 7

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