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Dark energy may be vacuum

January 17, 2007

Researchers at the University of Copenhagen's Dark Cosmology Centre at the Niels Bohr Institute have brought us one step closer to understanding what the universe is made of. As part of the international collaboration ESSENCE they have observed distant supernovae (exploding stars), some of which emitted the light we now see more than half the age of the universe ago. Using these supernovae they have traced the expansion history of the universe with unprecedented accuracy and sharpened our knowledge of what it might be that is causing the mysterious acceleration of the expansion of the universe.

Background and outline

At the end of last century astronomers discovered the startling fact that the expansion of our universe is not slowing down, as all our previous understanding of gravity had predicted. Rather the expansion is speeding up. Nothing in conventional physics can explain such a result. It means that either the universe is made up of around 70% 'dark energy' (something that has a sort of anti-gravity) or our theory of gravity is flawed.

Now, as part of the international collaboration "ESSENCE", researchers at the Danish Dark Cosmology Centre have added a new piece to the puzzle. In two papers recently released they detail observations of supernovae (exploding stars) that allow them to trace the expansion history of the universe in unprecedented detail. ESSENCE is an extension of the original team that discovered the acceleration of the universe and these results push the limits of technology and knowledge, observing light from dying stars that was emitted almost half the age of the universe ago.

In a third paper, led by the Danish team and released this week, the many new theories that have been proposed to explain the acceleration of the universe are critically assessed in the face of this new data. Dr. Jesper Sollerman and Dr. Tamara Davis lead the team who show that despite the increased sophistication in cosmological models over the last century the best model to explain the acceleration remains one that was proposed by Einstein back in 1917. Although Einstein's reasoning at the time was flawed (he proposed the modification to his theory so it could support a static universe, because in those days everyone 'knew' the universe was not expanding, it may be that he was right all along.

Scientific details:

The results include 60 new type Ia supernovae discovered on the Cerro-Tololo Interamerican Observatory 4m telescope in an ongoing survey that so far has lasted four years. In order to follow up these discoveries the team uses some of the biggest telescopes in the world: the 8.2m VLT (Very Large Telescope) run by the European Southern Observatory and the 6m Magellan telescope (both in Chile), the 8m Keck telescope and the 10m Gemini telescope (both in Hawaii). The ESSENCE team includes 38 top researchers from many different countries on four continents.

The primary aim of the experiment is to measure the 'dark energy'-the thing that is causing the acceleration of the universe-to better than 10%. The feature of this dark energy that we measure is its 'equation of state'. This also allows us to check whether our theory of gravity needs modification. So far it looks like our theory is correct and that the strange acceleration of the expansion of the universe can be explained by Einstein's 'cosmological constant'.

In modern terms the cosmological constant is viewed as a quantum mechanical phenomenon called the 'energy of the vacuum'. In other words, the energy of empty space. It is this energy that is causing the universe to accelerate. The new data shows that none of the fancy new theories that have been proposed in the last decade are necessary to explain the acceleration. Rather, vacuum energy is the most likely cause and the expansion history of the universe can be explained by simply adding this constant background of acceleration into the normal theory of gravity.

University of Copenhagen