Constant Controversy Bedevils Hubble’s
Magic Number
(From the November/December 2006 issue of StarDate magazine)
At the close of the Roaring Twenties, American astronomer Edwin
Hubble discovered that the universe is expanding. Ever since, scientists
have debated how fast or slow the expansion is taking place. That
value, called the “Hubble constant,” has bearing on the
size and age of the universe. Two recent announcement are at odds
concerning Hubble’s magic number.
Astronomers combining radio
and X-ray observations say they have confirmed the current value
used for the Hubble constant. A team led by Max Bonamente of NASA’s
Marshall Space Flight Center, calculated the distances to 38 galaxy
clusters using two radio telescope arrays in California and the
orbiting Chandra X-ray Observatory. The result takes advantage
of the “Sunyaev-Zeldovich
effect,” in which photons of microwave background radiation
interact with the hot gas in galaxy clusters. This interaction
distorts the microwaves, and radio telescopes measure the distortion.
Bonamente’s
team combined the radio data with X-ray observations of the clusters’ hot
gas, enabling them to measure the clusters’ sizes and distances.
Their resulting calculation of the Hubble constant is consistent
with the value determined several years ago by a Hubble Space Telescope
team.
Not so fast, says Ohio State University astronomer Kris Stanek.
His team has developed yet another method of measuring distances,
which the team says is more precise and simpler than previous methods.
Over 10 years, the astronomers have made careful observations of
both members of an eclipsing binary star in the Triangulum galaxy,
allowing them to measure the exact difference in the stars’ apparent
and intrinsic brightnesses. Once these two values are known, the
distance can be calculated easily. They found that the binary,
and thus the Triangulum galaxy, is 15 percent farther away than
previously thought. They conclude that the use of a faulty value
for the Hubble constant in previous distance measurements is the
likely culprit. If they are correct, the true value of the Hubble
constant may be 15 percent smaller than the currently accepted
value, so the universe is 15 percent bigger and older than thought.
— Rebecca Johnson
|
Contact StarDate | About StarDate | Friends of McDonald | Sign up for Sky Tips
