Apr. 3, 2001 NASA's Hubble Space Telescope has seen a burst of light from an exploding star located much farther from Earth than any previously seen - a supernova blast in the early universe that is casting light on a mystery of truly cosmic scale.
This stellar explosion is extraordinary not only because of its tremendous distance, 10 billion light-years from Earth, but also because its discovery greatly bolsters the case for the existence of a mysterious form of "dark energy" pervading the universe. The concept of dark energy, which shoves galaxies away from each other at an ever-increasing speed, was first proposed, and then discarded, by Albert Einstein early in the last century.
The Hubble discovery also reinforces the startling idea that the universe only recently began speeding up, a discovery made about three years ago when the unusually dim light of several distant supernovas suggested the universe is expanding more quickly than in the past, but there were alternate explanations. The more distant supernova (redshift z=1.7) refutes these alternatives and offers the first tantalizing observational evidence that gravity began slowing down the expansion of the universe after the big bang. Only later did the repulsive force of dark energy win out over gravity's attractive grip.
"The supernova appears to be one of a special class of explosions that allows astronomers to understand how the universe's expansion has changed over time, much as the way a parent follows a child's growth spurts by marking a doorway," said Adam Riess of the Space Telescope Science Institute (STScI), Baltimore, MD. "This supernova shows us the universe is behaving like a driver who slows down approaching a red stoplight and then hits the accelerator when the light turns green." The team of astronomers, led by Riess, made the discovery by analyzing hundreds of images taken by Hubble in infrared and visible light to study how galaxies formed. Fortuitously, one of those galaxies contained a supernova previously discovered by astronomers Ron Gilliland, STScI, and Mark Phillips, Carnegie Institutions of Washington.
The record-breaking supernova appears brighter than it should if the universe had been expanding at a steady rate. The reason is that a decelerating universe holds galaxies relatively close together and objects in them would have appeared brighter because they would be closer. "Long ago, when the light left this distant supernova, the universe may have been slowing down due to the mutual tug of all the mass in the universe," said Riess. "Billions of years later, when the light left more recent supernovas, the universe had begun accelerating, stretching the expanse between galaxies and making objects in them appear dimmer."
"Hubble's ability to find titanic stellar explosions at these extreme distances is what it takes to confirm this theory that the universe must have been slowing down before it switched into high gear,'" said Dr. Anne Kinney, director of NASA's Origins program at NASA Headquarters, Washington, DC. "Later this year Astronauts will install a new camera on Hubble that will give us 10 times better resolution than the current camera, which will give us an even better capability to find answers to grand cosmic questions like this."
Observations of several distant supernovas by two teams of astronomers in 1998 led to the prediction that the universe got the "green light" to accelerate when it was half its present age. Astronomers say the new Hubble findings rule out other explanations.
Nearly a century ago, Albert Einstein's Law of General Relativity concluded the universe must collapse under the relentless pull of gravity. However, like many scientists of his time, he assumed the universe to be static and unchanging. To make his equations fit those observations, Einstein added something he called the "cosmological constant" whose gravity is repulsive, though he had no idea if it was real.
Shortly afterwards, astronomer Edwin Hubble made the celebrated discovery that the universe was expanding. He assumed that the universe must be slowing down under gravity and might even come to a halt, leading Einstein later to say that his cosmological constant was the biggest blunder of his career. Now it appears Einstein was on the right track after all.
The source of the repulsive gravity may be something akin to Einstein's cosmological constant, referred to as the energy of the "quantum vacuum," a subatomic netherworld pervading space that provides a source of energy, or it may be something entirely new and unexpected. "While we don't know what dark energy is, we are certain that understanding it will provide crucial clues in the quest to unify the forces and particles in the universe, and that the route to this understanding involves telescopes, not accelerators," said astrophysicist Michael Turner of the University of Chicago.
Riess’ collaborators include Peter Nugent (Lawrence Berkeley National Laboratory), Brian Schmidt (Mount Stromlo Observatory), and John Tonry (Institute for Astronomy). NASA’s Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
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