NASA Measure of Universe Questioned
CHICAGO (AP) _ A new method of measuring distances to far galaxies casts doubt about the proclamation last week by NASA astronomers that they had determined the age and expansion rate of the universe.
A technique using a radio-telescope is a ``golden ruler″ for measuring cosmic distances and calls into question the conclusions announced by astronomers using the Hubble Space Telescope who were supported by the National Aeronautics and Space Administration, a researcher said Tuesday.
``Ours is a direct measurement, using geometry, and is independent of all other methods of determining cosmic distances,″ said Jim Herrnstein of the National Radio Astronomy Observatory. ``It is the most precise distance ever measured to a remote galaxy.″
Herrnstein presented his findings Tuesday at the national meeting of the American Astronomical Society.
He said the new technique shows a 15 to 20 percent margin of error in the results announced last week by a NASA team led by Wendy Freedman of the Carnegie Institute of Washington.
The NASA team using the space telescope said last week that it had successfully achieved 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 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.
Herrnstein said this calculation is off by 15 to 20 percent. He based his conclusion on the difference in calculated distance to a specific galaxy using his technique compared with the calculation method used by the NASA group.
Freedman and her group of 27 astronomers funded by NASA 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.″
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 to 29 million light years.
Herrnstein said that 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 can measure visible light.