more than a decade, astrophysicists have grappled with evidence of a
baffling force that seems to be pushing the universe apart at an
ever-increasing rate. Exactly what constitutes the dark energy
responsible for this cosmic speed-up is unknown, says Michael Turner at
the University of Chicago. "The simplest question we can ask is 'does
the dark energy change with time?'"
So far, the evidence has suggested that dark energy is constant,
though its effect on the universe has become stronger as the universe
has expanded and the gravitational force between objects weakens with
Now an analysis of supernovae suggests dark energy may actually be on the wane. In a paper on the physics preprint website,
a team led by Arman Shafieloo at the University of Oxford examined a
newly released catalogue of supernova explosions, including a number of
relatively recent blasts nearby (www.arxiv.org/abs/0903.5141).
They found that the new data made the best fit with a universe in which
dark energy is losing strength. "It seems acceleration is slowing
down," says Shafieloo.
The first evidence of dark energy emerged in 1998, when two teams of astronomers spotted distant supernova explosions that appeared dimmer than expected, and so further away.
The find suggested the exploding stars were receding from Earth faster
than anticipated, and therefore so was the rest of the universe. "Dark
energy" was invoked to explain the apparent anomaly. Since then more
supernovae have been catalogued to help build up a picture of how the
universe has expanded over time.
biggest set of supernova data was released earlier this year by the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts. It includes data on 147 supernovae that exploded in the
last billion years, more than half of them newly discovered (www.arxiv.org/abs/0901.4787). The Harvard team analysed the new supernovae assuming that dark energy has remained unchanged.
however, dropped the requirement that dark energy be constant over the
universe's history. Together with Varun Sahni of the Inter-University
Centre for Astronomy and Astrophysics in Pune, India, and Alexei
Starobinsky of the Landau Institute for Theoretical Physics in
Chernogolovka, Russia, Shafieloo used an approach he says is
particularly sensitive to rapid changes in the universe's rate of
with factors like red shift - a measure of how much the expansion of
space has stretched the light from each explosion - they calculated a
representative number for the epoch in which each supernova occurred.
After plotting all of these numbers, they found that the best fit was a
scenario in which dark energy has weakened over the last 2 billion
years, causing cosmic acceleration to slow down. Shafieloo cautions
that their result is preliminary, but adds that it could be time to
begin revisiting other models of dark energy.
approach is reasonable," though the effect is slight, says cosmologist
Dragan Huterer of the University of Michigan, Ann Arbor. "If that is
really the case it would be a tremendous discovery."
Indeed, it would change our ideas about the source of dark energy. Until now, all signs have pointed to the cosmological constant
as the simplest explanation for the accelerating expansion of the
universe. This constant is an unchanging energy that arises from
quantum fluctuations in the vacuum of space. "The cosmological constant
is the only thing that makes any sense to particle physicists right
now," says Huterer.
if dark energy is changing, the cosmological constant could not be the
driver. Instead, it would suggest far more exotic physics at work. It
might even mean dark energy does not exist at all (see "We don't need the stuff"). One example of an exotic origin is "quintessence",
a theoretical quantum field that permeates space like the
as-yet-unidentified field thought to have driven inflation right after
the big bang. This field could be dissipating and losing energy,
eventually causing the universe to decelerate and collapse back on
more likely explanation for the team's result is a slight bias in the
new supernova data, Huterer says. Robert Kirshner, a member of the
Harvard team, agrees. "I think these are serious people whose analysis
should be taken seriously, but there can be more than one cause for the
apparent effect," he says.
example, a potential bias could have been introduced thanks to dimmer
objects being easier to see if they are nearby. It is possible that the
Harvard team happened to catalogue a disproportionate number of nearby
supernovae that were faint or obscured by dust. Astronomers must
correct for the dimming effect of dust and other subtleties in order to
estimate a supernova's true peak brightness. But the team may have
overcompensated in this correction, producing a catalogue of nearby
supernovae that are slightly too bright for their distance. That would
create the illusion that the universe's acceleration has been slowing.
observations from other groups need to be examined to look for the same
effect, Kirshner says, though determining whether dark energy really is
changing could take a while. The fine details of so many supernovae
have been recorded that the so-called "systematic floor" has been hit -
a scenario in which everything from subtle differences between
supernova explosions to the warp of a telescope mirror can skew
results, Huterer says.
Upcoming "precision projects" like the Dark Energy Survey,
which will mount a supersensitive 500-megapixel camera on a 4-metre
telescope at the Cerro Tololo Inter-American Observatory in Chile, aim
to reduce some of the sources of uncertainty. One of the project's aims
is to measure some of the universe's most recent history, by recording
about 2000 supernovae that have exploded in the last 7 billion years.
Other probes that will push the limit in sensitivity are still in early planning, including two space probes - the US's Joint Dark Energy Mission and Europe's Euclid. Some astronomers suspect a partnership will be forged between these missions to send up a single international probe instead.
is practically impossible to definitively discover if dark energy is
constant. "There isn't a target to shoot for," says cosmologist Sean
Carroll of Caltech. "As we narrow down the error bars and get closer
and closer to perfectly constant, there's no point at which you say
'OK. We're done. Dark energy is constant.'"
the next burst of effort could reveal in glowing detail if dark energy
has been changing. "It would be a surprise if we found that dark energy
were varying with time," says Carroll, "but it would be so hugely
important that it's still worth looking."