SPACE.com -- Farthest Supernova Detected, 'Dark Energy' Suspected

Astronomers found this supernova in 1997 during a second look at the northern Hubble Deep Field, a tiny region of sky first explored by the Hubble telescope in 1995. The image shows the myriad of galaxies Hubble spied when it peered across more than 10 bi

This is a close-up view of the Hubble Deep Field image containing the farthest supernova. The white arrow points to the exploding star's home galaxy, a faint elliptical. Its redness is due to the billions of old stars residing there.

This picture shows the supernova itself, distinguished by the white dot in the center. Although this stellar explosion is among the brightest beacons in the universe, it could not be seen directly in the Hubble images. The stellar blast is so distant from

This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe beg

Understanding Dark Matter and Light Energy

Density of Universe, Superclusters Revealed in Sky Survey

Universe at Least 12.5 Billion Years Old

Most of Universe"s Matter Still MIA


	Farthest Supernova Detected, 'Dark Energy' Suspected By Robin Lloyd Science Editor posted: 01:00 pm ET 02 April 2001

FARTHEST SUPERNOVA EVER SEEN SHEDS LIGHT ON DARK UNIVERSE

The Hubble Space Telescope recently captured light from a supernova located farther from Earth than any previously seen.

The finding has bolstered cosmological models for the universe's expansion and a mysterious "dark energy" pervading it. The supernova, or exploding star, is located 10 billion light-years from Earth.

The more distant supernova, called SN1997ff, 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," Adam Riess of the Space Telescope Science Institute (STScI) in Maryland said in a prepared statement.

"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 the galaxies contained a supernova in Ursa Major previously discovered by astronomers Ron Gilliland of STScI and Mark Phillips of the Carnegie Institutions of Washington.

"The problem was there was only a single measurement of the supernova made when it was discovered," Riess said. "This was not enough to know if this supernova was old, young, at its brightest -- just the information we need to know how bright it is intrinsically and figure out how far away it is."

"The serendipity part was that other astronomers observed the same region of the sky at the same time for other purposes. I obtained those observations and found all the data we needed -- hundreds of hours of observations by the Hubble, which provided this information and allowed us to reach the cosmological conclusions."

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."

Observations of several distant supernovae 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.

Astronauts are set to install a new camera on the Hubble Space Telescope later this year that will provide 10 times better resolution than the observatory's current camera and help to answer more cosmological questions.

Dark energy

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.

His 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 discovered 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). NASAs Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.