Death of stars sheds light on dark Universe

Slow expansion of young universe confirms early Einstein theory

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 supports 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, then discarded, by Albert Einstein early in the last century (see below).

The record-breaking supernova appears relatively bright, consequence of the Universe slowing down in the past (when the supernova exploded) and accelerating only recently. 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 appears to have been slowing down due to the mutual tug of all the mass in the universe," said astronomer Adam Riess from the Space Telescope Science Institute. "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."

The Hubble discovery therefore reinforces the startling idea that the universe only recently began speeding up. The discovery was 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. At the time, there were several explanations as to why this might be so, including "dark energy". The more distant supernova refutes the other alternatives and offers the first tantalizing observational evidence that gravity began slowing down the expansion of the Universe after the Big Bang, and only later did the repulsive force of dark energy win out over gravity's grip.

Nearly a century ago, 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 assumptions, 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 similar to Einstein's cosmological constant -- referred to as the energy of the "quantum vacuum," a subatomic netherworld pervading space. Or possibly, it may be something entirely new and unexpected.

Picture from http://oposite.stsci.edu/pubinfo/pr/2001/09

Mostly taken from http://ftp.hq.nasa.gov/pub/pao/pressrel/2001/01-058.txt

Yves

May 2001

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