The Gravitational Field of the Cluster Acts like a Zoom Lens
Gravitational Lenses - Cosmic Mirages
Just as a wanderer in the desert can experience mirages, when light from remote objects is bent by the warm air hovering just above the sand, we may also see mirages in the Universe. The mirages we see with modern telescope like the Hubble Space Telescope do not arise from oases, but instead from remote clusters of galaxies - huge concentrations of mass.
The light rays (the grey arrows) from the distant galaxy (to the right in the image) are bent when passing a large gathering of mass - such as the galaxy cluster symbolised by the ball with blue glow in the centre. When the light finally arrives at the Earth (to the left), Hubble observes it as coming from a slightly different direction (the red arrow). Note that the shape of the normal-looking spiral galaxy has changed. After passing the large galaxy cluster, there is more than one image, and they are all elongated and bent like bananas. One might say that the cluster has acted like a giant magnifying glass, or gravitational lens, in space - focusing, magnifying and distorting the images of the galaxy. In addition the images of some of the lensed galaxies appear red - the large distance to these galaxies introduce redshift, which shifts their light to the red part of the spectrum.
Warping of space
Long ago people thought the Earth was flat. Which in some way is understandable - In our daily life we can't see the curvature of our planet. Space itself is also actually curved, even though we can't see this with our eyes on a starry night. But the curvature of space does create phenomena that we can observe with the best telescopes on Earth, and naturally with the sharp-sighted Hubble.
The curvature - or warping - of space was originally proposed by Einstein as early as 1915 in his theory of General Relativity. In 1919 his calculations were indeed proved to be correct. During a solar eclipse expedition to Principe Island near the west coast of Africa led by the renowned British astronomer Arthur Eddington, positions of stars around the darkened solar disk were observed. It was found that the stars had moved almost 2 arcseconds (1/1800 of a degree) outwards on the sky, compared to when the Sun was not in the vicinity.
It takes rather massive objects, like clusters of galaxies, to make space curve so much that the effect is observable in deep images of the distant Universe - even with Hubble's astonishing resolution. And so far gravitational lenses have mainly been observed around clusters of galaxies. They are collections of hundreds or thousands of galaxies and are thought to be the largest gravitationally bound structures in the Universe.
Hubble's sensitivity and high resolution allow it to see faint and distant gravitational lenses that cannot be detected with ground- based telescopes whose images are blurred by the Earth's atmosphere. Observations of lensing, such as these, can be used to "weigh" clusters. This will considerably improve our understanding of the distribution of the "hidden" dark matter in the clusters, and in the Universe as a whole.