Gravity gathers together and hews apart. The powerful gravity of a galaxy embedded in a massive cluster of galaxies called MACS J1149.6+2223 has split the light of distant supernova into four separate images, a phenomenon predicted by Einstein called gravitational lensing.
Although astronomers have discovered dozens of multiply imaged galaxies, they’ve never seen a lensed supernova explosion until last November. “It really threw me for a loop when I spotted the four images surrounding the galaxy – it was a complete surprise,” said Patrick Kelly of the University of California, Berkeley, a member of the Grism Lens Amplified Survey from Space (GLASS) collaboration. The GLASS team was studying deep Hubble images of 10 massive clusters when Kelly came across the scene.
Named “Refsdal” for Sjur Refsdal, a Norwegian astrophysicist who did early work in the field of gravitational lensing, the supernova is located 9.3 billion light years from Earth, far beyond the galaxy cluster, which sits between us and the explosion 5 billion light years away. The images are arranged around the galaxy in a cross-shaped pattern called an Einstein Cross. The blue streaks wrapping around the galaxy are the stretched images of the supernova’s home spiral galaxy, which has been distorted by the warping of space.
As Einstein wrote in his General Theory of Relativity, massive objects warp the fabric of spacetime, a fusion of the three familiar dimensions with time. Light rays traveling across the spacetime landscape follow its invisible curves. Gravity from the Sun for instance deflects the light of a star passing near it from its original straight-line path. In effect, the Sun acts like a lens, bending and redirecting light from a distant source.
When light from a background object like Refsdal encounters a massive galaxy cluster, the cluster’s gravity not only bends the light but distorts and multiplies the single beam into multiple images. In the case of the supernova, both the cluster and a massive elliptical galaxy within the cluster, which happens to be precisely positioned directly between us and the exploding star, combine their gravitational chops to neatly cleave Refsdal into four symmetrically-arranged images.
While the galaxy cluster’s loaded with good, old-fashioned matter, it’s even richer in invisible dark matter, an unknown material that comprises 96% of all the matter in the universe. Studying changes in the multiple images over time will help astronomers refine their estimate of the cluster’s dark matter content.
As exotic as gravitational lensing is, it gets even weirder. When the four images fade away, astronomers predict they’ll have a rare opportunity to watch a rerun of the supernova. This is because the current four-image pattern is only one part of the lensing display. The 4-leaf-clover grouping we see here appeared within a few days or weeks of each other. The supernova may have appeared as a single image some 20 years ago elsewhere in the cluster field, and it is expected to reappear once more within the next five years.
Measuring the time delays between images gives astronomers clues to the type of warped-space terrain the supernova’s light had to cover and will help them fine-tune the models that map out the cluster’s mass.
Thank you Albert.