Hubble Sheds Light on Age of Universe
New evidence from the troubled Hubble Space Telescope suggests that the universe may be much older than many scientists believe.
The results, to be presented today at a workshop in Sardinia, Italy, by a team headed by astronomer Allan R. Sandage of the Carnegie Institution observatories in Pasadena, may spark increased controversy in a cosmological community that is heatedly divided by the dispute over the universe’s birth date.
But the new telescope images should provide relief to theorists who faced the prospect of having to revise standard theories on the evolution of stars to mesh with recent experimental results that are in conflict with that theory.
Perhaps more important, experts said, the new findings suggest that the orbiting telescope can produce much better experimental results than scientists had predicted after they discovered that the Hubble’s huge mirror had been incorrectly ground, causing distant images to blur.
“A lot of us were fairly pessimistic about what (the space telescope) could do,” said Carnegie astronomer Wendy Freedman, a member of the group whose experiments favor a younger universe. “But it looks like we’ll be able to push out to more distant stars than we had thought initially. . . . This is really a big step forward.”
“This is a very nice piece of work,” said astronomer Garth Illingworth of UC Santa Cruz, “but there will be an intense amount of discussion about whether (Sandage’s approach) will give the right answer.”
The high quality of the new images suggests that once the telescope is repaired by a shuttle mission next year, it may provide a definitive answer to one of the most disputed questions in cosmology--how old the universe is.
The question has bothered astronomers for more than 70 years, and the search for its answer was the driving force in the construction of the 200-inch telescope at Mt. Palomar and the Hubble Space Telescope.
It is a crucial question because it provides insight not only to the birth of the universe, but also to its evolution and eventual fate.
The new data suggests that the universe was formed 18 to 20 billion years ago. That is in marked contrast to several other recent experiments, some of which suggest that the universe is only 13 billion years old and others that suggest an age less than 10 billion years.
Those results were shocking to many cosmologists because independent evidence about dense pockets of stars called globular clusters indicates that the clusters are 15 billion years old. That age, which is calculated from the elemental composition of the stars, is predicted by standard theories of stellar evolution.
“But how do you fit 15-billion-year-old stars into a 10-billion-year-old universe?” asked cosmologist Mark Birkinshaw of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. If the universe is only 10 billion years old, he said, “a lot of people are going to have to go back to their computers and develop new theories of stellar evolution, which is something we think we understand quite well.”
The researchers are not measuring the age of the universe directly. Instead, they are finding values for the Hubble constant, which is a measure of how fast the universe is expanding. It and the Hubble telescope are named after Edwin D. Hubble, who discovered in 1929 that the universe is expanding, with all other objects in the universe moving away from Earth at high speed.
Hubble discovered that light from distant stars is shifted toward the red part of the wavelength spectrum, evidence that all objects in the universe are speeding away from us. The farther away from Earth a galaxy seemed to be, the greater the red shift. In fact, if a galaxy is twice as far from Earth as another galaxy, it is moving away at twice the speed.
The Hubble constant tells astronomers what that speed is. Typical values for the Hubble constant range from 50 to 100 kilometers per second per megaparsec. A parsec is 3.26 light-years.
But to calculate the Hubble constant, astronomers need to know the precise distance of at least some stars from Earth. That can only be measured indirectly by analyzing a star’s brightness. If astronomers know how much light a distant star is giving off, then measurements of its brightness at Earth indicate its distance. Such stars are called “standard candles.”
Sandage and his colleagues at the Space Telescope Science Institute in Baltimore, Md., approached the problem by focusing on a class of stars known as cepheids (because the prototype is a star in the constellation Cepheus).
The brightness of cepheids oscillates over intervals of a few days. Early in the century, astronomers found a link between the oscillation period of a cepheid star and its inherent brightness, making them useful standard candles.
Ground-based telescopes can see only very near cepheids, and the use of nearby objects produces a large margin of error. Using the Hubble telescope, Sandage and his colleagues focused on 27 much more distant cepheid stars in a faint spiral galaxy called IC 4182. By measuring the periods of their oscillations, the scientists determined that the stars are 16 million light-years from Earth, providing a value for the Hubble constant of about 45.
A Hubble value of 100 indicates that the universe is about 10 billion years old, while a value of 50 indicates that it is 20 billion years old.
More precise measurements of the Hubble constant can be made by looking at stars farther from Earth, but that must await repair of the telescope’s mirror.
Many questions remain unanswered by the new results, critics say. The IC 4182 cepheids are in a region with a lot of background light that could interfere with the measurements. There might also be dust between them and Earth, causing unexpected dimming of the stars’ light.
Because of such complications, astronomers will be more certain about the value of the Hubble constant only after they have looked at a much larger number of standard candle stars spread throughout the heavens.
