Wednesday, January 08, 2014

Gravitational Lensing - project


Einstein's theory of gravity, General Relativity, made a remarkable prediction. Massive objects, such as stars, would bend the space around them such that passing light rays follow curved paths. Evidence for this revolutionary theory was first obtained by Arthur Eddington in 1919, when during a solar eclipse he observed that stars near the edge of the Sun appeared to be slightly out of position. The Sun was behaving like the lens in a magnifying glass and bending the light from the background stars!
In 1937, Fritz Zwicky realized that massive galaxies (which can contain anywhere from ten million to a hundred trillion stars) or clusters of galaxies could be used to magnify distant galaxies that conventional telescopes couldn't detect. As you can see, not unlike a conventional magnifying glass, these gravitational lenses not only magnify and focus the light of the distant background galaxies but they can, and mostly do, distort them as well.
When one of these gravitational lenses happens to sit right in front of a background galaxy, the magnification factor can be up to x10 or even more, giving us a zoomed-in view of the distant universe, just at that particular point. Lenses can help us investigate young galaxies more than halfway across the universe, as they formed stars and started to take on the familiar shapes we see nearby.
Observations of the distorted background galaxy can also give us useful information about the object that is behaving as a gravitational lens. The separation and distortion of the lensed images can tell astronomers how much mass there is in the object, and how it is arranged. It is one of the few ways we have of mapping out where the dark matter in the universe is, how clumpy it is and how dense it is near the centers of galaxies. Knowing this can provide crucial information about how galaxies evolve.


There is a lot of interesting science to be done with gravitational lenses. The problem is that they are very rare. Only about one in a thousand massive galaxies is aligned with a background object well enough to cause it to appear multiply-imaged. We currently know of about 400 objects that are behaving as gravitational lenses, largely because we have become very good at observing the night sky! Modern optical surveys cover thousands of square degrees, with images sharp and deep enough to resolve about 1 lens per square degree. There should be thousands of lenses that we can detect, but we will need to look at millions of galaxy images to find them!
The ideal solution would be to get a computer to look through all of the images, but unfortunately this is not a straightforward solution. Teaching a computer to recognize the effects of gravitational lensing is not too difficult, but they can be easily confused by galaxies that look very similar to a distorted background galaxy. Also in order for the computer to run fast enough to analyse lots of images quickly, they have to cut a lot of corners, and this makes them less effective.

See video.


Human beings have a remarkable ability to recognise patterns and detect the unusual with only minimal training. With a basic understanding of what the distorted images of galaxies that have passed through a gravitational lens look like, participants in the SpaceWarps project can help discover new examples of this amazing phenomenon, and enable our survey scientists to carry out new investigations of stars and dark matter in the universe. In the current project, we've selected galaxies, and groups of galaxies, that could potentially act as gravitational lenses, and quasars, that are very useful when gravitationally lensed, all from the VICS82 infrared imaging survey. The task is to assess whether or not gravitational lensing is actually going on in each image! There will be confusing objects around - the challenge is to come up with the most plausible explanation for what is going on, in collaboration with the rest of the Space Warps community. Do you think you can spot outer space being warped? We do!

References (accessed 8 Jan 2014)

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