Dark energy enigma won't go away

Nature of dark energy, the mysterious force that is causing the expansion of the universe to accelerate. "Cosmic acceleration is the biggest mystery in all of science," says cosmologist Michael Turner of the University of Chicago.

This month, at a meeting of the American Astronomical Society (AAS) in Washington DC, the mystery deepened when Brad Schaefer of Louisiana State University in Baton Rouge reported that dark energy appears to be changing C rapidly. Though his experimental method left most cosmologists unconvinced, the result stressed how little we know about dark energy and the need for different approaches.

Within months, a dark energy task force will recommend projects to study this cosmic enigma. But there is uncertainty over what the approach should be. Many believe that both ground telescopes and space probes are needed, while some think that space probes are too expensive. Meanwhile, NASA remains noncommittal about a flagship space mission to investigate dark energy, and there is concern that the project could be abandoned.

The reason for this fuss is that dark energy is fundamentally important and baffling. Its existence became clear only eight years ago when astronomers discovered that the universe's expansion is speeding up; till then everyone had expected gravity to be slowing it down. "We have to get our heads and telescopes around that," says Turner.

But what is the origin of dark energy? There are several theories, including a hypothetical energy field called quintessence, changes to Einstein's theory of gravity, the existence of extra spatial dimensions and the cosmological constant theory. The first step towards ruling out any of these theories will be to find out whether dark energy changes over time.

The leading theory involves a parameter from Einstein's equations of relativity known as the cosmological constant, which represents the inherent energy of empty space from which dark energy arises. In this favoured scenario, the density of dark energy is fixed: a litre of space always holds the same amount of energy, so as space keeps expanding the amount of dark energy keeps increasing. But the continuing expansion causes matter and its gravitational influence to thin out, so dark energy will eventually dominate.

However, the density of dark energy can itself change with time. This is represented by a parameter called w C the repulsiveness of dark energy. If w is -1, dark energy is the energy represented by the cosmological constant. If it is more than -1 (less repulsive), then the density of dark energy falls slowly over time; if it is less than -1 the density increases.

And then there is the possibility that w might also change with time. In rather complex ways, the fate of the universe depends on the value and constancy of w.

The standard technique for measuring changes in dark energy makes use of exploding stars known as type 1a supernovae. All type 1a supernovae are roughly the same brightness. So if astronomers know the distance to at least one supernova, this can act as a "standard candle". Researchers can measure how far away any other supernova is by comparing its apparent brightness with that of the standard.

In addition, the red shift of a supernova's spectrum provides a measure of how much space has expanded since it detonated. Data from hundreds of supernovae can be used to plot the expansion history of the universe and potentially the change in the effect of dark energy over time.

In December, the SuperNova Legacy Survey (SNLS), which uses just such a strategy, reported that dark energy acts like the cosmological constant, with w roughly between -0.9 and -1.1 (New Scientist, 3 December 2005, p 18). But some scientists criticised the analysis, arguing that the SNLS team did not allow for the possibility that w itself is changing.

Now Schaefer's observations suggest that w is indeed changing fast. They seem to show that although w is about -1 today, in the early universe w was positive, which means dark energy was attractive, like ordinary gravity, not repulsive. "It would be absolutely extraordinary if correct," Turner says.

Schaefer used gamma-ray bursts (GRBs) as standard candles. Space probes can spot extremely distant GRBs as they are far more powerful than supernovae, giving us vital clues about expansion in the early universe. Schaefer claims that the most distant of the 52 bursts he studied, which were up to 12.8 billion light years away, are much brighter than would be expected if dark energy were constant. But he admits that more observations are needed to eliminate statistical error.

But other researchers have concerns about Schaefer's use of GRBs as standard candles. "We don't know much about bursts so there is a risk this is misleading," says Robert Kirshner of Harvard University. "It is a blunt tool to measure a delicate effect."

Co-discoverer of dark energy Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, is even more sceptical. No GRBs have gone off nearby, he says, so there are no reliable baseline measurements of their distance and brightness. Schaefer's analysis might be unsound, says Riess. "Stay tuned, come back a few weeks after his paper is refereed, and you'll see he's wrong."

We could soon find out if GRBs can serve as standard candles. NASA's Swift gamma-ray probe is seeing bursts at the rate of about two a week C one was detected during Schaefer's 10-minute talk at the AAS meeting -- and Schaefer is hoping that a new set of GRBs will produce the same result. But cosmologists want independent confirmation. "Other methods will catch up, and then we'll know whether it is a statistical fluke," Turner says.

To corroborate these early results, several teams have put forward designs for the joint dark energy mission (JDEM), a spacebased project that was expected to be funded by NASA and the US Department of Energy. Space probes have an advantage: they can detect light from distant supernovae that cannot be observed from the ground because their light is red shifted into an infrared band that is blocked by Earth's atmosphere.

But NASA may not be able to afford JDEM, and some scientists question its worth. The James Webb Space Telescope, which will replace Hubble, and the planned ground-based Large Synoptic Survey Telescope, will be powerful tools for probing dark energy, but without a dedicated space probe, we may never know the fate of the universe.

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Author: Stephen Battersby

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