In the sense that a lens refracts (bends) light, you could say that using galaxies as a lens is reasonable – if the scale of measurement is nothing less than the age of the universe. It never hurts to have confirmation (in science and a lot of other things). While we know from various studies that the age of the universe is about 13.75 billion years (+/- 170 million), to have this confirmed by another method is…comforting, in a cosmological way. The method in this case is kind of breathtaking.
Researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC, SLAC National Accelerator Laboratory) in cooperation with several other institutions used data gathered by the NASA/ESA Hubble Space Telescope, the Keck Telescope (Mauna Kea, Hawaii) and the Wilkinson Microwave Anisotrophy Probe. The team used a technique called gravitational lensing. Starting with a bright, active galaxy they measured how long it took for light to reach Earth along several paths. This is something like triangulation with radio waves, in that if you have enough distances and angles, you can get an accurate measurement of the distance to the origin. In this case, however, there is another wrinkle – or angle – in that the light pathways were seen through an intermediary galaxy.
What the scientists were looking for was a calculation of Hubble’s constant. This is the figure used in physical cosmology to find the velocity at which various galaxies are receding from Earth. The velocity is proportional to their distance: v = H0D (H0 is the Hubble constant). In this case the Hubble telescope worked with light coming from a galaxy partially blocked by another galaxy in front of it (from Earth’s perspective). In this way, they were able to measure different pathways of light as it comes through different regions of the intervening galaxy. In this case, there were four copies of the source image, which appear as a ring of light around the gravitational lens (of the intermediate galaxy).
This method is not new, but historically it has been plagued by errors. With the improved Hubble, most of these errors have either been eliminated or compensated. For example, the Hubble telescope has infra-red filters that help to eliminate dust effects. The images also contain information about the number of galaxies also in the line of vision; their contribution to the light of the lensing effect needs to be compensated.
By using the multiple images of the background galaxy, the scientists were able to calculate a more precise value for the Hubble constant – 21 kilometers per second per million light years. Put another way, a galaxy a million light years away is moving further away at the rate of about 21 kilometers per second.
An international team is behind this work, which is just out in the Astrophysical Journal. Having made a physical measurement of Hubble’s constant, says Phil Marshall (Stanford University), gravitational lensing “has come of age as a competitive tool in the astrophysicist’s toolkit.” The new value for Hubble’s constant is considered the best estimate of the uncertainty in the constant. Beyond that, however, is the fact that lensing is producing an estimate for the universe’s age that gibes well with other methods of analysis, meaning that we’re learning how to harness this remarkable natural tool for future investigations.
[Source: Centauri Dreams]
This research technique will be extended by other groups finding new systems and developing more detailed information about known ‘lens’ galaxies. Right now, there are about twenty other astronomical systems suitable for analysis with gravitational lensing.