Gravitational lenses: spy glasses in the universe


This illustration depicts a phenomenon known as gravitational lensing, which is used by astronomers to study very distant, very faint galaxies. Note that the scale has been greatly exaggerated in this diagram. In reality, the galaxy far away is much further away and much smaller.
Lens clusters are clusters of elliptical galaxies whose gravity is so strong that they deflect light from the galaxies behind them. This produces distorted and often multiple images of the background galaxy. But despite this distortion, gravitational lenses allow for greatly improved observations because gravity bends the light’s path to Hubble, amplifying the light and making otherwise invisible objects observable.
Credit: NASA, ESA & L. Calçada

Light does not always travel in a straight line. As Einstein predicted in his theory of general relativity, massive objects will warp the fabric of space itself. When light passes in front of one of these massive objects, such as a cluster of galaxies, its trajectory is slightly altered.

This effect, called gravitational lensis visible only in rare cases and only the best telescopes can observe the associated phenomena.

Different Types of Gravitational Lenses

Gravitational lenses produce images of different shapes depending on the shape of the lens body. If the lens is spherical then the image appears as an Einstein ring (ie a ring of light) (top); if the lens is elongated, then the image is an Einstein cross (it appears split into four separate images) (middle), and if the lens is a cluster of galaxies, such as Abell 2218, then arcs and arclets (banana-shaped images) of light are formed (bottom). Credit: European Space Agency

Hubble’s sensitivity and high resolution allow it to see weak and distant gravitational lenses which cannot be detected with ground-based telescopes whose images are blurred by the Earth’s atmosphere. Gravitational lensing yields multiple images of the original galaxy, each with a characteristic distorted banana shape or even in rings.

Abell 383 lens cluster

The cluster of giant galaxies at the center of this image contains so much mass of dark matter that its gravity deflects light from more distant objects. This means that for galaxies far away in the background, the cluster’s gravitational field acts as a kind of magnifying glass, bending and focusing light from the distant object towards Hubble. These gravitational lenses are a tool astronomers can use to extend Hubble’s vision beyond what it would normally be able to observe. In this way, some of the earliest galaxies in the Universe can be studied by astronomers. Lensing can also be used to determine the distribution of matter – both ordinary and dark – within the cluster. Credit: NASA, ESA, J. Richard (CRAL) and J.-P. Kneib (LAM), Acknowledgements: Marc Postman (STScI)

Hubble was the first telescope to resolve the details of these multiples banana shaped bows. With its sharp vision, it can directly reveal the shape and internal structure of lensed background galaxies. This way one can easily match the different arcs coming from the same background object – be it a galaxy or even a supernova – at the eye.

The gravitational lens can be used to ‘weigh’ the grapes because the quantity of lenses depends on the total mass of the cluster. This has greatly improved our understanding of the distribution of “hidden” dark matter in galaxy clusters and therefore in the Universe as a whole. The gravitational lensing effect has also allowed a first step towards reveal the mystery of dark energy.

As gravitational lenses work like magnifying glasses it is possible to use them for study distant galaxies of the early Universe, which would otherwise be incredibly faint to see due to their great distance from Earth.

The article on the composition of the Universe has more details on Hubble’s work on black matter.

“When we first observed the galaxy cluster Abell 2218 with Hubble in 1995, our main objective was to study the cluster and its galaxies. But we had a surprise. The images showed dozens and dozens of gravitationally lensed arcs. When we showed these ultra-sharp images to our colleagues, they immediately understood the importance of using gravitational lensing as a cosmological tool.

Richard Ellis, Astronomer, University of Cambridge and California Institute of Technology


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