A New Twist on Dark Energy

The notion that a massive object can literally bend light, what we now call gravitational lensing, has been around for more than 200 years — though Einstein gets the nod for making the effect widely known and getting it right.

Ever since the 1979 discovery of a distant quasar "lensed" into two images by an intervening galaxy, observers have amassed all sorts of gravitationally mangled novelties — none more famous than QSO 2237+0305, the Einstein Cross. By carefully dissecting these cosmic mirages, cosmologists can learn much about the lensing objects in the foreground, such as how much unseen dark matter is fortifying their mass, and about the distances to the farther objects being distorted.

Now a group headed by Eric Jullo (Jet Propulsion Laboratory) and Priyamvada Natarajan (Yale) has used the light-bending power of the massive galaxy cluster Abell 1689 to refine our conceptions about the mysterious, antigravity-like phenomenon called dark energy.

In August 20th's Science, these researchers describe how they used images of Abell 1689 from the Hubble telescope's Advanced Camera for Surveys to identify 114 visible manifestations of 34 different background galaxies. Then Keck and Very Large Telescope spectrographs pinned down distances to 24 of those galaxies.

The team used these observations not only to reconstruct the paths taken by light from each background galaxy, but also to model how dark energy altered the geometry of space along the route. “The precise effects of lensing depend on the mass of the lens, the structure of spacetime, and the relative distance between us, the lens and the distant object behind it,” Natarajan explains in a press release about the results.

The goal was to tighten the constraints on the dark-energy density in the universe. There's much more to the universe than the normal "stuff" (consisting of protons, neutrons, and electrons) that we can see or sense. Cosmologists currently favor a universe in which this baryonic matter is merely 4.5% of all matter, with as-yet-unidentified dark matter adding another 22%.

The remaining 73%, they believe, is accounted for by the "dark energy" that's making the expansion of the universe speed up — and the leading candidate for that is something dreamed up by Einstein in the 1920s as a fudge factor for general relativity, now termed the cosmological constant and abbreviated Λ (Lambda).

(Trust me: no one-liner at a star party will prove your astro-smarts faster than uttering, "Why, yes — I do support a Lambda-cold-dark-matter cosmology.")

Jullo, Natarajan, and their colleagues find that cosmologists' assumptions about the ΛCDM's "equation of state" are reasonably well matched by their observations of Abell 1689. In that sense, nothing's new. Prior results using Type Ia supernovae and other approaches likewise support a ΛCDM cosmology (that is, that the dark energy's equation of state, known as w, is exactly equal to –1). But the Abell 1689 work represents a new and arguably more robust way to get there. "We have to tackle the dark energy problem from all sides,” Jullo notes in a second press release. "It’s important to have several methods, and now we’ve got a new, very powerful one.” They say they've narrowed the value of w to –1 to an uncertainty now of just 7%, including others' work with theirs.

As they conclude in their full paper (warning: not for the fainthearted!), it's a technique that will get even better once observations of ultra-distant objects start rolling in from the forthcoming James Webb Space Telescope.