Radio telescope network helps answer cosmic questions
How old is the universe? How big is it? How distant are remote cosmic objects? These classic, interrelated questions continue to taunt astronomers who wrestle with fundamental cosmic issues. But while experts have yet to pin down reliable answers, new observations promise to refine their estimates. Right now, those ``how old?'' estimates are an embarrassment. They range from a youthful 10 billion years to a respectable maturity twice that figure. If the low estimate were correct, one might well wonder what the concept of ``age'' really means when applied to the cosmos. It's far less than the 18 billion-year figure attributed to globular clusters of stars, which orbit galaxies like swarms of bees around a honey pot.
Rather than speculate on how the universe could be younger than some of its components, astronomers accept that they don't have a good grasp on the cosmic size or age and go on trying to get better estimates.
Recently, Norbert H. Bartel of the Harvard-Smithsonian Center for Astrophysics and a team of American and European colleagues reported a new distance-measuring technique in the journal Nature, one that may eventually help narrow the uncertainties.
Bartel and his colleagues have given a new, sophisticated twist to the old technique of finding distances by triangulation. Using a network of nine radio telescope observatories located in Western Europe and the United States, they had the equivalent of an instrument nearly the size of the Earth. This enabled them to measure the angular diameter of the debris from an exploding star in the moderately distant galaxy M100. (Angular diameter is the angle a celestial object subtends at the eye when see n against the sky. The full moon is about half a degree in those terms.)
The astronomers know, from studies with optical telescopes, how fast the star debris is spreading out through space. The debris comes from a supernova in M100 that was observed to explode in 1979 and is known by its astronomical catalog number SN1979c. Because the researchers know how fast the debris has been spreading since then, they know how great a distance it has covered, assuming it started out as a compact, starlike mass. This enables them to translate the angular width of the de bris cloud, as measured by the radio telescopes, into a width in light-years or, to use the usual astronomical unit, a width in parsecs. (A parsec is 3.26 light-years, or nearly 20 million million miles.) Once they know the object's width in parsecs and the angle it subtends at Earth, astronomers can calculate its distance by triangulation.
Astronomers like triangulation because it tends to give them a firmer grasp on distances than do other methods, which often involve what are called ``standard candles'' -- objects such as certain variable stars whose intrinsic brightness astronomers think they know. Distance is estimated by how dim these ``standard'' lights appear, making allowance for additional dimming by intervening cosmic dust. Such estimates are always uncertain, partly because no one can be sure that the ``standard candles'' are r eally as bright as they're supposed to be. Triangulation, when it can be applied, gives a good firm distance.
Until now, it has been a short-range technique, good only for nearby stars. Bartel and his co-workers have extended it to truly cosmic distances. Unfortunately, their method is in its early stages. They are hampered by not knowing exactly where the edge of the debris cloud, as detected by the radio telescopes, really is. They aren't even sure what shape it has. It makes a difference in their calculations whether the cloud is ring shaped, disklike, or at least roughly spherical. So the best they could do
at this time is to give a distance to the exploding star -- and hence to Galaxy M100 -- of 19 megaparsecs (61.9 million light-years) as their best estimate, give or take 8 Mpc on the high side and 6 Mpc on the low side.
Such distance measurements are important because they give astronomers insight into the size and age of the universe. The technique is linked to the universe's expansion. The late Edwin Hubble showed some 60 years ago that the universe is expanding, with major groups of galaxies moving away from one another at speeds that depend on their distances. The farther away a galaxy is, the faster it is receding. The ratio of the overall expansion rate to the size of the universe is a numerical value known as th e ``Hubble constant.'' Its inverse is a rough estimate of the age of the cosmos -- that is, the time it has taken the universe to expand from a small, primordial fireball to its present size.
Actually, the inverse of this constant gives an upper limit on the true age. The calculation assumes that the expansion has gone at its present rate since the beginning, whereas the rate was probably faster earlier on.
Most estimates of the Hubble constant cluster around two figures: a low value of 50 km per second per megaparsec, and a high value of 100. That corresponds to ages of around 10 billion years and 20 billion years, and would mean the farthest galaxy is between 10 billion and 20 billion light-years away. The estimate of Bartel and his colleagues, based on their triangulation work, falls ambiguously in the middle, with rather large uncertainties. It's 65 km per second per Mpc, give or take 35 on the high si de and 25 on the low side.
That lies smack in the middle of the cosmological debate. It is compatible either with the low estimates of around 10 billion years or the high range of around 20 billion years. But it holds the promise of substantial refinement.
More-precise radio studies, both from the ground and from Earth orbit, will pin down the distances to SN1979c and other distant supernovae much more securely. Observations made by the Hubble Space Telescope, to be launched from the shuttle next year, will also refine distance --and hence age -- estimates.
It may still be many years before astronomers can agree on the size and age of the cosmos in which we live. But new technology and clever work are bringing them much closer to this goal.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.