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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, 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. 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.
“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 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 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 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 theory 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, 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 akin to
Einstein’s cosmological constant — referred to as the energy
of the “quantum vacuum,” a subatomic netherworld pervading
space — 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 made the discovery in collaboration with 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.
Electronic images are available on the Internet at:
http://oposite.stsci.edu/pubinfo/pr/2001/09
