“Dark
energy” an age-old phenomenon, study finds
Nov.
16, 2006
Courtesy Johns Hopkins University
and World Science staff
A
mysterious force making the universe expand faster and
faster—discovered eight years ago—isn’t new, a study
has concluded: it has existed for most of cosmic history.
As
long as nine billion years ago, when the cosmos was about one third its present
age, “the stuff we call ‘dark energy’... was starting
to make its presence felt,” said Adam Riess of The Johns Hopkins
University in Baltimore, Md. He led a research team
conducting the work.
The
finding, the scientists said, is in line with Albert Einstein’s
early suggestion that a repulsive form of gravity
is an intrinsic feature of empty space.
On
the other hand, they added, the result tends to conflict with some competing
views holding that the dark energy’s strength varies
over time.
“Although
dark energy accounts for more than 70 percent of the energy
of the universe, we know very little about it, so each clue is precious,”
said Riess, who is credited with co-discovering the dark energy
in the late 1990s.
Einstein
first conceived of a repulsive force in space in an attempt
to explain how the universe held its size against the inward pull
of its own gravity. If this opposing force didn’t
exist, he reasoned, the cosmos would collapse under the
combined gravity of everything in it.
But
he eventually dropped the idea. It remained an historical
curiosity until 1998, when Riess and others detected
the acceleration expansion of space. Astrophysicists
concluded that Einstein may have been right after all: there
was a repulsive form of gravity. They dubbed it dark energy.
Since
then, they’ve been trying to uncover two of its key properties:
its strength and permanence. These could ultimately help
explain what it is.
For
the new study, Riess and colleagues used NASA’s Hubble Space Telescope
to peer far across the universe. This equates to looking back in time,
because it takes time for the distant objects’ light to reach
us. Thus we see them as they were that much time ago.
The
researchers studied a class of exploding stars, called
supernovae, used to trace the expansion and expansion
rate of the universe at various times. The method, Riess said,
is akin to watching fireflies at night. Because they all glow with
about the same strength, you can judge their distance from their apparent
brightness.
A
similar technique was used to uncover the cosmic
acceleration to begin with. Later work filled in some
details. Hubble sightings of far-off supernovae, reported
in 2004 by Riess and colleagues, indicated that matter
dominated the early cosmos, slowing down its expansion
with its gravity. But the swelling began to speed up some
five billion to six billion years ago, as the eerie repulsion
began to overpower gravity.
The
new findings, Riess said, show that dark energy was obstructing
the gravitational pull of the matter in the universe
even before it began winning that gravitational
“tug of war.”
The
results stem from an analysis of the 24 most distant supernovae
known, most found within the last two years. By measuring the universe’s
relative size over time, astrophysicists tracked its
growth spurts, much as a parent gauges a child’s growth spurts using
marks on a doorframe. Distant supernovae provide the
“marks” for Hubble.
“After
we subtract the gravity from the known matter in the universe,
we can see the dark energy pushing to get out,” said the University
of Western Kentucky’s Lou Strolger, a supernova
hunter on Riess’ team. The findings are to appear in the Feb.
10 issue of The Astrophysical Journal.
