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A turn to the dark side

Searching the universe for a different kind of energy

It’s a lot easier to disprove a theory than to prove it. Many scientific models rest on educated guesses – sometimes little more than large assumptions – that can explain phenomena in the real world, but these invisible mechanisms are only good as long as experimental observation cannot contradict them. Liberal economics has the invisible hand, evolutionary biology has natural selection, and, for astrophysics, there is dark energy.

Dark energy is a hypothetical type of energy that, according to the current space-time model, makes up approximately 74 per cent of the total mass-energy in the universe. The existence of dark energy is a key component of the theories that make up our current understanding of the universe. Astrophysicists know from observing supernovae (bright stellar explosions) not only that the universe is expanding, but that it is doing so at an increasing rate.

When this was first established in 1998, scientists were baffled – the observations seemed to contradict gravity. All objects with mass attract one another, so assuming our universe is still expanding from the Big Bang, the rate of expansion should be decreasing in the absence of outside forces. Gravity would be trying to pull all mass together.

There are two competing theories that would account for this accelerating expansion, and both hinge on the presence of dark energy. The first theory, the cosmological constant, comes from Albert Einstein’s theory of relativity. Stricken from Einstein’s universe-balancing equation when Edwin Hubble – after whom the NASA telescope is named – confirmed in 1929 that neighbouring galaxies are moving away from us, the cosmological constant has since come into vogue again. It represents a kind of energy reservoir. So a positive value for the constant would, mathematically speaking, result in an accelerated expansion of empty space.

One of the biggest problems with the cosmological constant theory, however, is that the amount of energy required to drive the expansion is not consistent with observations gathered by NASA’s Voyager probes. Moreover, some scientists argue that if this kind of acceleration had been present at the beginning of the universe, stars and galaxies could not have formed.

A competing theory takes a different approach. It posits that the universe is filled with a fluid-like substance, named quintessence, which has a negative gravitational mass. This theory has been thus far unsuccessful in working out some of the simpler models.

Dark energy is not only central to explaining the shape and behaviour of the universe – it may also eventually be the cause of its demise. Some astrophysicists theorize that, when the density of the dark energy present in the universe becomes great enough, all matter will eventually be reduced to its elementary particles. This theory, dubbed the Big Rip, holds that the outward acceleration caused by this dense “phantom energy” will be strong enough to tear apart everything in the universe – down to the atomic level.

Not all scientists agree with this prediction. Anton Baushev, from the Joint Institute for Nuclear Research in Russia, argues that current models can be interpreted to show that the aging of the universe will not necessarily produce this gravitational effect.

“Even if the phantom energy prevails in the universe, the Big Rip does not necessarily occur,” writes Baushev in a recent article in the journal Physics Letters B. “A more probable outcome of the cosmological evolution is the decay of the phantom field into ‘normal’ matter.”

Scientists hope to learn more about the nature of dark energy through an experiment that is taking place at the Cerro Tololo Inter-American Observatory in Chile. The Dark Energy Survey Project, an international consortium of universities and government institutions, is building a massive camera that will allow the observatory’s telescope to take very precise measurements of the redshift of distant stellar bodies – that is, the change in the wavelength of light that occurs when the light-emitter and the light-observer are at motion in respect to one another.

The data gathered by this $35-million apparatus will allow them to calculate the dark energy density to a precision margin of five to 15 per cent, bringing physicists one step closer to figuring out how well this theory represents reality.