SETI [ASTRO] Astronomers dispute NASA gauge of universe's age


Larry Klaes (lklaes@bbn.com)
Mon, 07 Jun 1999 20:55:30 -0400


>X-Authentication-Warning: brickbat12.mindspring.com: majordom set sender to owner-astro using -f >X-Sender: ebert@citrus.ucr.edu >X-Mailer: QUALCOMM Windows Eudora Pro Version 3.0.3 (32) >Date: Mon, 07 Jun 1999 11:56:59 -0700 >To: ASTRO@LISTS.MINDSPRING.COM >From: Ron Ebert <ebert@citrus.ucr.edu> >Subject: [ASTRO] Astronomers dispute NASA gauge of universe's age >Sender: owner-astro@brickbat12.mindspring.com >Reply-To: Ron Ebert <ebert@citrus.ucr.edu> > >CHICAGO (AP) -- A new radio-telescope technique has established a "golden >ruler" for measuring cosmic distances and raises doubts about the claim >last week that NASA astronomers had determined the age and expansion rate >of the universe. > >Jim Herrnstein of the National Radio Astronomy Observatory said his method >produces "the most precise distance ever measured to a remote galaxy" and >suggests a 15 percent to 20 percent margin of error in the technique used >by the astronomers sponsored by the National Aeronautics and Space >Administration. > >"Ours is a direct measurement, using geometry, and is independent of all >other methods of determining cosmic distances," Herrnstein said Tuesday at >a national meeting of the American Astronomical Society. > >The NASA team led by Wendy Freedman of the Carnegie Institute in Washington >said last week that they used the Hubble Space Telescope to successfully >achieve the goal of measuring within an uncertainty of only 10 percent the >speed at which the universe is expanding, a value called the Hubble constant. > >Based on its study, the team concluded that the universe is 12 billion to >13 billion years old and is expanding at an accelerating rate of 70 >kilometers per second for every 3.3 million light-years in distance from >the Earth. > > >'Golder ruler' for measuring cosmic distances > >Herrnstein said this calculation was off by as much as 20 percent. He based >his conclusion on the difference in calculated distance to a specific >galaxy when comparing his technique with the method used by the NASA group. > >Freedman and her team of 27 astronomers spent eight years measuring the >distance to stars that pulsate in brightness at a known rate. Such stars, >called cepheids, are considered "standard candles" whose varying luminosity >gives a gauge for their distance. > >But Herrnstein said the technique using cepheids is far less accurate than >his new method that directly measures the motion of gas around a galaxy. > >Herrnstein used a galaxy called NGC 4258 to establish what he called a >"golden ruler" for measuring cosmic distances. > >NGC 4258 is surrounded by a rotating cloud of gas. Within this gas cloud is >water vapor, which tends to amplify radio signals. This creates radio "hot >spots" called masers. > >The orbital speed of masers between NGC 4258 and Earth was measured in >1994, and again every few months over the following three years. By >determining the speed at which the masers were moving, the astronomers >created a triangle with the first maser position at one angle, the latest >position at a second angle and the galaxy center at the third angle. >Measuring the angles gives the distance. > >"We can use plain old trigonometry to calculate the distance," said Lincoln >Greenhill, a member of the Herrnstein team. > >"It is very simple and direct," said Herrnstein. "Most of us know enough >math from high school to do the calculation." > > >NASA admits uncertainties > >The distance to NGC 4258 was calculated at 23.5 million light-years, with >an accuracy to within 4 percent. This distance has been calculated using >the cepheid star technique at 27 million to 29 million light-years. > >Herrnstein said the distance measure using cepheid stars is based on a >series of assumed values, such as the distance to the Large Magellanic >Cloud, a galaxy neighbor to the Milky Way. These assumed values, said >Herrnstein, have a greater margin of error than does the direct measuring >technique he is using. > >Freedman, who attended a news conference at which Herrnstein announced his >results, said the new study provided "wonderful data," but she would like >other measurements to be taken to confirm its accuracy. Also, she said the >assumed values used in the cepheid star technique have been verified by a >number of researchers and she was "confident" of the results. Freedman >admitted, however, that uncertainties in her team's results still needed to >be checked out. > >Herrnstein conducted his study using the Very Long Baseline Array, which is >a series of radio-telescope antennae that are scattered across the United >States, from the Virgin Islands to Hawaii. The antennae operate as a single >unit and are able to measure natural radio signals to an accuracy 500 times >greater than the Hubble telescope can measure visible light. > >Copyright 1999 The Associated Press. All rights reserved. > > > >



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