Saturday, December 10, 2005

More evidence for dark energy

Back here we provided an overview of the topic of dark energy and some of the evidence for its existence.

Recently announced research results from a project known as the Supernovae Legacy Survey (SNLS) have now provided additional evidence for dark energy, and suggested that it is more likely that the dark energy is due to Einstein's cosmological constant than to an even more exotic possibility known as quintessence.

Was Einstein's 'biggest blunder' a stellar success?
The genius of Albert Einstein, who added a “cosmological constant” to his equation for the expansion of the universe but later retracted it, may be vindicated by new research.

The enigmatic dark energy that drives the accelerating expansion of the universe behaves just like Einstein's famed cosmological constant, according to the Supernova Legacy Survey (SNLS), an international team of researchers in France and Canada that collaborated with large telescope observers at Oxford, Caltech and Berkeley. Their observations reveal that the dark energy behaves like Einstein’s cosmological constant to a precision of 10 per cent.

“The significance is huge,” said Professor Ray Carlberg of the Department of Astronomy and Astrophysics at U of T. “Our observation is at odds with a number of theoretical ideas about the nature of dark energy that predict that it should change as the universe expands, and as far as we can see, it doesn’t.”

The basic difference between dark energy in the form of a cosmological constant or of quintessence is that the former yields a constant acceleration, while the latter produces ever increasing acceleration. The latter scenario is sometimes called the "big rip" because eventually it would cause even atoms, neutrons, and protons to burst apart.

By measuring the spectra of light from distant supernovae of a special type (known as Type 1a), it is possible to determine both the distance to a supernova and its velocity relative to Earth. Whether the acceleration of the universe's expansion is constant or increasing can be deduced from this data. The evidence is that the acceleration is constant, which rules out quintessence.

Further, when the acceleration data is combined with measurements of the cosmic microwave background (called "baryon acoustic oscillation"), it is possible to determine that the dark energy makes up about 75% of the total matter and energy in the universe.

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Additional resources:

Einstein's Dark Energy Accelerates the Universe -- Particle Physics and Astronomy Research Council press release

First results describing the nature of dark energy -- press release from the journal Astronomy & Astrophysics

Gemini's Nod-&-Shuffle Provides Critical Deep Spectroscopic Data for Supernova Legacy Survey -- contains a more technical description of the SNLS results

New Study of Supernovae May Absolve Einstein of Self-confessed "Biggest Blunder" -- Keck Observatory press release

The Supernova Legacy Survey: measurement of ΩM, ΩΛ and w from the first year data set -- original paper (PDF)

Canada-France-Hawaii Telescope Legacy Survey

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