Rarely has so much been accomplished by staring into empty space. But that’s exactly what astronomers have been doing with the Hubble Space Telescope in the Frontier Fields program since 2012. It works like this: while Hubble keeps one eye (camera) on a distant galaxy cluster, it aims another at an adjacent but empty field of view. In the first, the massive cluster acts as a “gravitational lens”. Its immense gravity warps and magnifies the light of far more distant galaxies directly behind the cluster, coaxing objects into view what would otherwise be invisible.
Gravitational lensing was predicted by Einstein in his General Theory of Relativity Hubble 101 years ago, but it took big telescopes and sensitive detectors to finally prove Einstein’s assertion. The first gravitational lens was discovered only in 1979. Now, thanks to lensing, Hubble can capture photos of galaxies so far away, they fired up not long after the universe made its debut in the Big Bang.
At the same time as the cluster and lensed galaxies are being recorded, a second Hubble camera soaks up light in a “parallel field”, an empty bit of sky next door to the cluster. Much like trick o’ treat on Halloween, you never know what might fall into Hubble’s bag in these adjacent views. Astronomers want to find out if the dense carpeting of galaxies seen in earlier deep images varies across the sky. Just look at the wealth of galaxies in that bit of sky! Nearly every disk and speck you see is a galaxy, not a foreground Milky Way star.
Hubble has probed these cosmic lenses before, but this concentrated effort has devoted 560 orbits around the Earth — 630 hours of camera time — to just six clusters of galaxies and their parallel fields. They were chosen based on features like the strength of their lensing, their ability to support complementary observations by telescopes like the Spitzer Space Telescope, the upcoming James Webb Space Telescope, and the avoidance of bright stars whose light would obstruct the faint light from distant galaxies. The photo featured at the top of this article was made at the same time as the f camera focused in the galaxy cluster Abell 2744, nicknamed Pandora’s Cluster, in the southern constellation of Sculptor and located some 3.5 billion light years from Earth.
Every time Hubble takes a step backward in time, we take a step forward in our understanding of the early universe.