QSO 0957+561 was the first object ever to be identified as a gravitational lens in space. It’s a single quasar, located some 8 billion light-years from Earth. But it looks like two quasars, due to the presence of an gigantic mass located in space between Earth and the quasar. The intervening mass – a distant galaxy – bends light from the quasar, so that we see two quasar images instead of one.
Albert Einstein wrote of gravity as the bending of space and time itself by massive objects.
In the bent space around a star or galaxy, light can’t travel in a straight line. It has to follow a curved line through curved space. EarthSky spoke with Liliya Williams, an astronomer at the University of Minnesota at Minneapolis. She studies massive galaxies that bend light in a big way – a phenomenon known as gravitational lensing.
Liliya Williams: In many respects it is similar to a lens that you can make out of a piece of glass, in the sense that a gravitational lens can focus light from distant objects and make an object that you are looking at appear brighter than it would otherwise. It can also multiply-image a distant background source, meaning you can see more than one image of a distant source than you would otherwise.
In 1979, double images of extremely distant objects called quasars caught the eye of astronomers. The double-images were separated by much more than was predicted. Astronomers realized the separation was caused by a massive intervening galaxy acting as a gravity lens.
Part of the matter in the lensing galaxy might be invisible. It might be what’s called dark matter. By studying many gravity lenses, Williams hopes to map the distribution of dark matter throughout the universe.
Liliya Williams had more to say about how astronomers use gravitational lenses to figure out the distribution of dark matter in the universe. In this investigation, she said, they start with the simple case of a single gravitational lens.
Liliya Williams: Consider the simple case of a distant galaxy causing light to bend from a background source. The source is almost directly behind an intervening galaxy. Astronomers see the light coming from that galaxy, but there is also dark matter associated with it, and the distribution of dark matter is generally somewhat different than the distribution of light. The light, mostly stars, are very centrally concentrated – most of them are piled up close to the very center of the galaxy. But dark matter is more widely distributed. So as you go further away from the center of the galaxy, the amount of light-emitting matter and dark matter falls off with radius. But the amount of light-emitting matter drops off much faster. As you go far enough from the center of the galaxy, all you are left with, pretty much, is just dark matter. So that distribution of dark matter – with respect to the center of that lensing galaxy – can be deduced through gravitational lensing.
After learning about an individual gravitational lens, Williams said, it’s possible to extrapolate that basic picture to other galaxies – and also clusters of galaxies – and also the distribution of dark matter on a much larger scale. Astronomers are now studying a lot of different gravity lenses and action and learning how mass is distributed in those areas. They are using that information to draw general conclusions about the universe.
Our thanks to Research Corporation, a foundation for the advancement of science.
Our thanks to:
Liliya L. R. Williams
University of Minnesota
