Hubble Clears View Of Early Universe
LOOKING WAY BACK
PEERING back into time, the Hubble Space Telescope is giving astronomers unprecedented views of a cosmic Jurassic Park - primordial clusters of galaxies up to 12 billion years old.
The views, unveiled Dec. 6 at a briefing at the Goddard Space Flight Center in Greenbelt, Md., are helping scientists refine their notions of how the universe formed and what it is made of. The findings also are throwing yet another curve at efforts to determine the age of the universe.
``In the '30s and '40s we made great strides in our understanding of stellar evolution, but we haven't had a clue with galaxies because we couldn't see that far back in time,'' says Anne Kinney, an astronomer at the Hubble Space Telescope Science Institute in Baltimore.
``These images will allow us to understand how galaxies formed,'' she adds.
The latest findings cap ``a spectacular year'' for the repaired Hubble, Ms. Kinney says. Astronomers once were concerned that the instrument might become the orbiting Edsel of observatories. The findings also come at a time when the National Aeronautics and Space Administration and the Hubble program are under tight budget pressures, even as they evaluate proposals for a new generation of instruments to be added in 1999 - when a space shuttle is scheduled to visit the orbiting observatory.
Two teams of researchers studied new images from Hubble of a pair of fully formed elliptical galaxies, one in a cluster that existed when the universe was about a third of its current age, the other when the universe was one-tenth its current age.
That these ``mature'' galaxies, which contain large numbers of very old stars, existed at this early an age ``has immediate cosmological implications,'' says Mark Dickenson, an astronomer at the Space Telescope Institute and leader of one of the teams. Models that give the universe a high rate of expansion since its formation out of the Big Bang, and hence a ``young'' age, ``leave little time for these galaxies to form and evolve to a maturity we're seeing in the Hubble images.''
Moreover, that these galaxies existed in clusters at this early an age may give clues about how the structure of the universe evolved and about the nature of the so-called dark matter, which may account for 90 percent of the universe's mass.
Last month, astronomers using Hubble material concluded that one lead candidate for dark matter - faint red-dwarf stars in the halos of galaxies - was far too scarce to qualify as a significant part of the universe's ``missing mass.'' Other candidates include vast quantities of subatomic particles, grouped into two broad categories: less-massive hot dark matter and more massive cold dark matter.
``A primary question'' in cosmology is ``which size scale formed first - clusters of galaxies or individual galaxies,'' says Dr. Kinney.
``That tells you a lot about initial conditions in the universe,'' he says. ``Models predict that if the early universe was mostly hot dark matter, then the larger scales would have formed first; if the early universe was mostly cold dark matter, then galaxies would have formed first.''
In addition, Dr. Dickenson's team and that of Alan Dressler of the Carnegie Observatories in Pasadena, Calif., appear to have put book ends of time to the rise and the fall of spiral galaxy populations within clusters: the discovery of a veritable zoo of fragments within the early clusters that seem to be the initial parts of spiral galaxies in their early stages of formation, and the subsequent demolition of spirals in clusters ranging in age from 4 billion years ago to the present.