Gravitational lens helps solve mystery of dark energy

Using a galaxy cluster as a magnifying lens, an international team of astronomers has used the NASA/ESA Hubble Space Telescope to examine distant regions of the universe.

The aim is  to try and solve the riddle of dark energy, which somehow appears to power the Universe’s accelerating expansion.

“Dark energy is characterised by the relationship between its pressure and its density: this is known as its equation of state. Our goal was to try to quantify this relationship,” says Eric Jullo, lead author of a new paper in Science.

“It teaches us about the properties of dark energy and how it has affected the development of the Universe.”

The team used the galaxy cluster Abell 1689 as its gravitational lens. At cosmic distances, a huge cluster of galaxies in the foreground has so much mass that its gravitational pull bends beams of light from very distant galaxies, producing distorted images.

The distortion induced by the lens depends partly on the distances to the objects, which have been precisely measured with large ground-based telescopes.

Looking at the distorted images helps astronomers reconstruct the path that the light from distant galaxies takes to Earth. It also lets them study the effect of dark energy on the geometry of space in the light path from the distant objects to the lensing cluster and then from the cluster to us.

As dark energy pushes the Universe to expand ever faster, the precise path that the light beams follow as they travel through space and are bent by the lens is subtly altered. This means that the distorted images from the lens can provide information about the underlying cosmology, as well as about the lens itself.

“The geometry, the content and the fate of the Universe are all intricately linked,” says Priyamvada Natarajan, a co-author of the paper.

“If you know two, you can deduce the third. We already have a pretty good knowledge of the Universe’s mass-energy content, so if we can get a handle on its geometry then we will be able to work out exactly what the fate of the Universe will be.”