(Originally published by Kavli IPMU)
January 27, 2017
An international team of astronomers, including Kavli IPMU (Kavli Institute for the Physics and Mathematics of the Universe) Project Researcher Alessandro Sonnenfeld, has used galaxies as giant gravitational lenses to make an independent measurement of how fast the Universe is expanding. Led by Sherry Suyu, the Max Planck @TUM professor at the Technical University Munich and Max Planck Institute for Astrophysics, Germany, the team used the NASA/ESA Hubble Space Telescope, the Subaru Telescope, and other telescopes in space and on the ground to study five galaxies .
The study arrived at an independent measurement of the Hubble constant—a fundamental quantity that describes the rate at which the Universe is expanding . The new results are entirely independent and agree with other measurements of the Hubble constant in the local Universe, measurements that used as their reference points Cepheid variable stars and supernovae.
However, the findings from Suyu and her team, as well as those from the Cepheids and supernovae, differed from those made previously by the ESA Planck satellite—in the latter case, measurement of the Hubble constant was made for the early Universe by observing the cosmic microwave background. While the Planck measurements agree with our current understanding of the cosmos, the results emanating from measurements for the local Universe disagree with currently accepted theoretical models of the Universe.
“The expansion rate of the Universe is now starting to be measured in different ways with such high precision that actual discrepancies may possibly point towards new physics beyond our current knowledge of the Universe,” explained Suyu, who carried out much of her research when she was affiliated to Academia Sinica Intitute of Astronomy and Astrophysics, Taiwan .
The study looked at massive galaxies located between Earth and distant quasars, which are very luminous galaxy cores. Due to strong gravitational lensing, huge masses of galaxies bend light emerging from more distance quasars, thereby creating multiple images of the background quasars, with some of them smeared into extended arcs . Light from the images formed of background quasars follow paths that are of different length because the distortions caused by galaxies to the fabric of space are not perfectly spherical. What is more, the lensing of galaxies and quasars are not perfectly aligned.
As the brightness of quasars changes over time, different images can be seen at different times, the lag between them depending on the lengths of the paths the light has taken. Such delays are directly related to the value of the Hubble constant. The team was able to measure the Hubble constant with precision (3.8 percent)  by using accurate measurements of the time delays between the multiple images, in addition to utilizing computer models.
Kavli IPMU’s Sonnenfeld said: “The idea of measuring the Hubble constant using time delays between lensed images of quasars has been around for over fifty years, but it is only recently that such measurements have become possible, thanks to the efforts of our collaboration. The next goal will be to increase the number of lenses used for the analysis. The Subaru Telescope is playing an important role in the hunt for new gravitational lenses.”