Science Writing

Gravitational Lensing, seeing the same thing again and again…

Image credit: ESA/Hubble & NASA, D. Coe

Hold a wineglass in front of a light. And empty one, that is, and you will see the image behind the wineglass distort and twist.

In the vastness of space there is a similar effect known as Gravitational Lensing and it has allowed astronomers and astrophysicists to delve deeper into the past of the universe and its composition.

But it’s not as simple as holding a wine glass up to a light.

Hubble has found some incredible objects peering into the abyss but arguably the most startling ones have been like the galaxies in the image below.

Galaxy cluster containing several hundred galaxies
Abell 370. A galaxy cluster containing hundreds of galaxies along with streaks of pale light. These streaks are objects, mostly galaxies, that sit behind Abell 370. The light from the galaxies in the background has been bent by the extreme warping of space due to the incredible gravitational forces of the cluster. In the upper left of the image you can see a galaxy that appears to be smeared in space, this is known as The Dragon, which is several images of a single galaxy. Credit: NASA, ESA, J. Lotz.

As you can see, light doesn’t travel in a straight line like it wants to. Space curves and light bends with it.

This was a seminal discovery that has allowed scientists to map the gravitational influence of clusters and galaxies around the universe. The gravitational influence that can only be accounted for with the existence of dark matter.

The normal matter, or ‘light’ matter, of the galaxies is not enough to account for the huge lensing effects that can be seen. Mathematically, there has to be something else that we cannot yet detect directly increasing the gravitational pull of the galaxy cluster.

In comes dark matter, a mysterious state of matter that cannot be detected directly by any instrument we currently posses. As previously mentioned, however, the effects could not be more obvious.

two Hubble images of galaxy cluster Cl 0024+17 (ZwCl 0024+1652), with right image shaded to illustrate dark matter
Galaxy cluster Cl 0024+17 (ZwCl 0024+1652). The image on the left is the cluster as captured by Hubble. You can see the cluster in the center and around it the bent and smeared blue arcs that are the lensed galaxies and objects that sit behind the cluster. The image on the right is the same with the addition of blue shading which represents the location and intensity of dark matter, required to account for the lensing visible. Credit: NASA.

Dark matter is a difficult concept to wrap your head around, mostly because no one truly understands it, knows what it is made of, or how it is formed. There are theories, but so far, as we can only measure it indirectly, none of those have a solid basis underpinning them.

One thing is for sure though, gravitational lensing is an incredibly powerful tool that will only become more useful as our technology and understanding grows.

Further Reading/References:

Gravitational lensing: a unique probe of dark matter and dark energy | Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences (

Gravitational Lensing (

Discoveries – Highlights | Shining a Light on Dark Matter | NASA

Nature’s Lens: How Gravity Can Bend Light Like a Telescope | Space

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