Scientists Get Closer to Determining the Age of the Universe

views updated

Scientists Get Closer to Determining the Age of the Universe

Overview

In the first half of the twentieth century, astronomers demonstrated that the universe was expanding, thus providing support for the idea that it had arisen from a tremendous explosion billions of years ago. This explosion came to be known as the Big Bang. Depending on the methods used and the assumptions that are made, different values are obtained for the age of the universe. Today, scientists are coming closer to agreement on an age of 12-16 billion years.

Background

The nineteenth century invention of spectroscopy allowed astronomers to break up the light coming from distant stars and galaxies into its constituent wavelengths. Different types of stars and galaxies have characteristic patterns, or spectra. According to the Doppler effect, the spectra are shifted towards longer wavelengths, or red-shifted, when the object is receding. The greater the red shift, the faster the object is moving away from us. This corresponds to the lowered pitch you hear as a train whistle or ambulance siren fades into the distance.

During the 1920s scientists began using spectroscopy to compile data on the velocities of galaxies. Meanwhile, American astronomer Edwin Powell Hubble (1889-1953) was studying their distances. The powerful new telescope on Mt. Wilson in California, with its 100-inch (2.5 m ) mirror, allowed him to observe individual stars in many galaxies. In particular, Hubble was interested in Cepheids.

Cepheids are a type of variable star; that is, their brightness changes over time in a regular way. The special feature that makes Cepheids so valuable for determining distances is that the period of their variability is related to their brightness. The longer the period, the brighter the star. Knowing how bright the star actually was, and comparing this to how bright it appeared from Earth, enabled Hubble to calculate its distance. He employed this method for the galaxies in which he could observe Cepheids. For other galaxies he used the brightest star he could find and assumed its actual brightness was comparable to the brightest stars in our own galaxy. If a galaxy was too faint for Hubble to measure any individual stars, he measured the brightness of the galaxy as a whole to estimate its distance.

In 1929, Hubble announced that the farther away a galaxy was from Earth, the faster it was receding. This result, since called Hubble's law, is written v = H₀d, where v is the velocity, d is the distance, and H₀ is the Hubble constant, pronounced "H nought."

At first it might seem odd that everything appears to be moving away from us. The Solar System does not occupy any special place in the cosmos; we circle an average star in a sort of celestial suburb, out on one of the spiral arms of the Milky Way. What is actually happening is that on a large scale everything is moving away from everything else, like the raisins in a loaf of rising bread. The entire universe is expanding, and the galaxies are receding into the distance at rates of hundreds of millions of miles per hour.

Such an expansion could have been caused by an enormous explosion at the beginning of time. Russian-born American physicist George Gamow (1904-1968) coined the term Big Bang to describe this explosion, and the name stuck. The Big Bang implied an actual beginning to the universe. Many scientists of the time resisted the idea. They preferred the steady state theory, which held that the universe had no beginning or end, and any expansion was simply a temporary phase, perhaps one stage of a periodic oscillation.

However, in 1948 Gamow realized that a Big Bang would have generated extremely high energy radiation that would have gradually cooled down over the eons. It should still exist as cosmic background radiation, coming equally from every direction. In 1964 Arno Penzias (1933- ) and Robert Wilson (1936- ) detected this cosmic background radiation, an achievement for which they received the 1978 Nobel Prize in physics. At this point almost all scientists agreed that the Big Bang had actually happened. But there was still the problem of determining when it had occurred.

Impact

Looking at the equation for Hubble's law, it is easy to see that if the rate of the expansion of the universe is constant, its age is simply the inverse of H₀, often called the Hubble time. Based on Hubble's data, the calculated age was about 2 billion years. This presented an immediate problem, because by the 1930s geologists using radioactive dating methods had found rocks on Earth that were almost twice as old.

Of course, it was by no means a foregone conclusion that the expansion speed was actually constant. If the expansion is slowing down, then the universe would be younger than the Hubble time; if it is speeding up, the universe would be older. Furthermore, H₀ was determined by estimating distances. These estimates could very well be off, and the true value for H₀ has been vigorously debated for decades.

In 1952 German-American astronomer Walter Baade (1893-1960) discovered that there were two varieties of Cepheid variables. Hubble had used the equations for the wrong type in some of his measurements. In distant galaxies he had occasionally mistaken glowing hydrogen gas clouds for especially bright stars. Correcting these errors pushed the date of origin back far enough to satisfy the geologists, to something on the order of 10 billion years. Data taken in the 1960s and 1970s supported various ages ranging from about 10 to 20 billion years.

A problem with the smallest estimates for the age of the universe is that some of the oldest stars in our own galaxy, seen in globular clusters, have been tentatively dated at 14 to 18 billion years old. Obviously, the universe itself can not be younger than its oldest stars. However, measurements in the late 1990s seemed to indicate that the clusters were further away than had been previously thought, implying that their stars were more luminous. This would put them at a different stage in their life cycle, with ages closer to 11 or 12 billion years.

For some time there have been two main camps in the age debate, with Allan Sandage (1926- ) of the Carnegie Institution supporting an older universe, and other astronomers proposing a younger one. At one point Sandage's expansion rate was half the value considered accurate by some of his colleagues, and there were many estimates in between.

The Hubble Space Telescope, launched by NASA in 1990, has provided new measurements that have narrowed the gap. The ability to see distant clusters of galaxies is important for accuracy. For relatively nearby objects, local motion, such as two galaxies moving toward one another due to gravitational attraction, may be more obvious than the overall expansion of the Hubble flow. Most scientists now believe the universe is between 12 and 16 billion years old; Sandage continues to maintain that ages up to about 18 billion years old are possible.

Accurate values for the universe's age, rate of expansion, and how that rate changes will provide clues to its future. The ultimate fate of the universe depends on whether its expansion can counteract the gravitational forces tending to pull it in upon itself. At present the evidence seems to favor a universe that expands forever, rather than collapsing in what has been called the "Big Crunch."

SHERRI CHASIN CALVO