A chance alignment between two massive galaxies could help astronomers better measure the expansion of the universe and shed light on the mysterious nature of dark energy.
While working on his general theory of relativity, Albert Einstein predicted that light from distant galaxies and stars should be bent by the gravity of very massive objects, similar to how a glass lens bends and magnifies light. Astronomers have since observed hundreds of galaxies, or clusters of galaxies, acting as gravitational lenses, warping and sometimes duplicating the images of galaxies that are sitting behind them.
Now, at Stanford University in California and his colleagues have found an incredibly rare case of two galaxies, 13.4 billion light years apart, both bending the light of a quasar, an incredibly bright supermassive black hole at the centre of a third galaxy.
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鈥淭his is the only [double gravitational lens] that has been found so far,鈥� says Millon. 鈥淚t鈥檚 probably a 1-in-50,000 gravitational lens, but we haven鈥檛 found 50,000 lensed quasars. There鈥檚 only 300, so it鈥檚 super strange that we found one.鈥�
Astronomers have described this configuration as an 鈥淓instein zigzag lens鈥�. The resulting image contains six copies of the original, some of them flipped on opposite sides due to the complicated path that the quasar鈥檚 light took between the two gravitational lenses.
鈥淲e鈥檝e never seen anything like this before,鈥� says at the University of Portsmouth, UK. While there have been alignments of more than two galaxies, the lensing from the second galaxy has normally been so small as to be irrelevant, he says.
The lens was first spotted in 2021, but astronomers concluded that it was a single lens distorting two quasars behind it. 鈥淵ou can see there is this big bright yellow galaxy in the centre 鈥� that鈥檚 the main lensing galaxy 鈥� and then there is this red arc that was interpreted as another galaxy at the same distance as the source,鈥� says team member at the Swiss Federal Institute of Technology in Lausanne.
However, new observations with the Nordic Optical Telescope in the Canary Islands and the James Webb Space Telescope revealed that this red arc was in fact a closer galaxy acting as a second lens, says Dux.
One advantage of seeing a lensed object duplicated six times, rather than four times, which is the standard for a single-source lens, is that it can be used to calculate a more accurate value of the Hubble constant, which measures the rate of expansion of the universe. This could help settle a longstanding cosmological debate over two very different values for the Hubble constant given by two different measurement techniques.
The same pattern of four images can be produced by a single lens under different values for the Hubble constant depending on how spread out the mass in the lensing galaxy is, but astronomers can鈥檛 separate these scenarios. With two lenses, however, you can differentiate between them and measure the Hubble constant more precisely, says Millon.
One of the lensing galaxies is also the most distant lens we know of, at 15.8 billion light years. This presents a unique opportunity to accurately measure the mass of very early galaxies, says Collett. 鈥淭he really exciting thing here is learning about that second lens, this galaxy that鈥檚 halfway to the edge of the universe.鈥�
Gravitationally lensed systems are one of the most accurate methods we have of measuring the mass of galaxies, but because galaxies in the early universe often haven鈥檛 had time to grow to a size where they can act as gravitational lenses, there is a gap in our knowledge. 鈥淭his is the first real opportunity to understand the makeup and contents of galaxies at such a high [distance from Earth], and that鈥檚 very exciting,鈥� says Collett.
The lens system could also tell us about the expansion rate of the universe in the past, at the age of the more distant galaxy. This is closely tied to the nature of dark energy, the mysterious force that appears to be accelerating the expansion of the universe, says Millon. 鈥淚t constrains how dark energy accelerated the expansion [of the universe] in the past billion years, and this is something you can do only if you have [multiple lenses].鈥�
arXiv
Article amended on 15 November 2024
Changes were made to clarify that the red arc in the image corresponds to the second lensing galaxy.
