Dark energy, even more mysterious than dark matter, is pushing the universe to expand more and more quickly. Science magazine's Breakthrough of the Year in 1998 by the Supernova Cosmology Project (SCP) and the High-Z SN Search teams leads to more questions than it answers. In particular, is dark energy a constant vacuum energy or something even more exotic that has changed or will change in the future? Measurements and characterizations of its properties are the focus of work of the SCP, High-Z search, and include research on baryon oscillations, weak lensing, and cluster searches such as South Pole Telescope and APEX.

March 2008 - Science@Berkeley Lab, Dark Energy's 10th Anniversary Part III, The aftermath: confirmation and exploration by Paul Preuss

Saul Perlmutter announced the Supernova Cosmology Project's evidence for a cosmological constant at the annual meeting of the American Astronomical Society in Washington, D.C., on January 8, 1998. On February 18 of that year, Gerson Goldhaber and Perlmutter discussed the SCP evidence at the UCLA conference on Dark Matter in Los Angeles, where Alexei Fillipenko announced similar results from the High-Z Supernova Search Team.

What they had observed was the accelerating expansion of the universe, presumably caused by Einstein's cosmological constant (lambda). Initially a purely mathematical term in the equations of General Relativity — which Einstein later dropped — theorists by the end of the 20th century had come to regard the cosmological constant as a manifestation of the vacuum energy described by quantum mechanics.

The proposed SuperNova/Acceleration Probe (SNAP) satellite (pictured) inspired DOE and NASA's Joint Dark Energy Mission. It will find and measure thousands of Type Ia supernovae and will measure the distribution of matter in the universe through weak gravitational lensing. Read more

April 2008 - Scientific American, Dark Forces at Work  Ten years ago two teams discovered that the universe will expand forever at an ever faster rate, thanks to an unseen energy. The leader of one of the groups, Saul Perlmutter, expects that new observations will soon illuminate the universe's dark side. Read more by David Appell

Scientific American Q & A with Saul Perlmutter

Dark energy: The Decade Ahead physicsworld.com

"Ten years after astrophysicists discovered that the expansion of the universe is accelerating, further measurements have given us few clues into the nature of the dark energy that drives it. But, as Eric Linder and Saul Perlmutter describe, advances in observational techniques promise to shed light on this revolutionary physics in the decade ahead." Read more from physicsworld.com here.

Saul Perlmutter's "Supernovae, Dark Energy, and the Accelerating Universe" examines the history of cosmic expansion. Physics Today, April 2003  PDF

On the Trail of Dark Energy One of the most remarkable discoveries of recent years is that the universe appears to be dominated by some form of "dark energy." Eric Linder, Cern Courier  PDF

Cosmic Sound Waves Rule In the microwave background and the distribution of galaxies, relic imprints of primordial soundwaves have contributed to an extraordinarily detailed history of the cosmos. And they provide yardsticks for solving a great mystery. By Daniel Eisenstein and Charles Bennet PDF

Strung Out on the Universe: Scientific American interviews Raphael Bousso about dark energy and string theory

New Frontiers from Dark Energy

Beyond Einstein: What happens when gravity is no longer an attractive force?

Image: Scientific American

Discovery (SCP, High-Z 1998): 70% of the universe acts this way!

This represents fundamentally new physics - Cosmology is the key.

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The Nature of Acceleration

How much dark energy is there? energy density Ω_Λ

How springy is it? equation of state w, w'

Does it clump on subhorizon scales? Couple to matter? Couple to gravity non-minimally?

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BCCP associates researching dark energy: Saul Perlmutter, Martin White, David Schlegel, Nikhil Padmanabhan, George Smoot, Eric Linder, Joanne Cohn, Adrian Lee, and William Holzapfel.

Theoretical understandings of its presence are the target of investigations by Hitoshi Murayama and Raphael Bousso.

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