What is Gravitational Lensing?

Cosmology is the branch of astronomy which asks the biggest questions of all – what is the Universe made of? How did it form? How old is it? What will happen to our Universe in the distant future? How and why do the biggest structures in the Universe come about?

Humanity has been asking questions like this for millennia, but it is only in the past century that modern telescopes have been powerful enough to start providing meaningful answers. Our understanding of the Universe today can be summarised in one simple pie chart:

Image: physicsforme.wordpress.com

This chart shows the total mass-energy content of the Universe. Mass-energy equivalence means that we can equate the two; mass is just a measure of the internal energy content of an object. All the 'regular' matter in the Universe – the stuff that makes up galaxies, planets, stars, nebulae, dust, rocks and gas – is known as baryonic matter, and only makes up 4% of the mass-energy content of the Universe. The other two pieces of the pie are dark matter and dark energy, and together they make up almost all of the known Universe. Dark energy and dark matter are so named because cosmologists don't know what they actually are – we can't see them directly and can only infer their existence by the effect they have on the regular matter that we can see. It might seem strange to claim that most of the Universe is invisible and unknown to us, but the evidence for these mysterious entities is compelling. To learn more about dark matter and dark energy, follow the links on the left hand side.

When astronomers refer to lensing, they are talking about an effect called gravitational lensing. Normal lenses such as the ones in a magnifying glass or a pair of spectacles work by bending light rays that pass through them in a process known as refraction, in order to focus the light somewhere (such as in your eye).

Gravitational lensing works in an analogous way and is an effect of Einstein's theory of general relativity – simply put, mass bends light. The gravitational field of a massive object will extend far into space, and cause light rays passing close to that object (and thus through its gravitational field) to be bent and refocused somewhere else. The more massive the object, the stronger its gravitational field and hence the greater the bending of light rays - just like using denser materials to make optical lenses results in a greater amount of refraction.

Image: NASA/ESA

Gravitational lensing happens on all scales – the gravitational field of galaxies and clusters of galaxies can lens light, but so can smaller objects such as stars and planets. Even the mass of our own bodies will lens light passing near us a tiny bit, although the effect is too small to ever measure.

So what are the effects of lensing? The kind of lensing that cosmologists are interested in is apparent only on the largest scales – by looking at galaxies and clusters of galaxies. When astronomers take a telescope image of a part of the night sky, we can see many galaxies on that image. However, in between the Earth and those galaxies is a mysterious entity called dark matter. Dark matter is invisible, but it does have mass, making up around 85% of the mass of the Universe. This means that light rays coming towards us from distant galaxies will pass through the gravitational field of dark matter and hence will be bent by the lensing effect.

Dark matter is found wherever 'normal' matter, such as the stuff that makes up galaxies, is found. For example, a large galaxy cluster will contain a very great amount of dark matter, which exists within and around the galaxies that make up that cluster. Light coming from more distant galaxies that passes close to a cluster may be distorted – lensed – by its mass. It is the dark matter in the cluster that does almost all of the lensing as it outweighs regular matter by a factor of six or so. The effects can be very strong and very strange; the images of the distant, lensed galaxies are stretched and pulled into arcs as the light passes close to the foreground cluster. This can be seen in the image below of the famous Abell 2218 cluster. The real galaxies are not this shape – they are usually elliptical or spiral shaped – they just appear this way because of lensing.

Image: NASA/ESA

This strange shape distortion comes from the fact that galaxies are large objects, and the light rays leaving one side of the galaxy (e.g. the left hand side from our point of view) will pass through a different part of space than the light rays leaving the other side (e.g. the right hand side). The light rays will therefore pass through different parts of the dark matter's gravitational field and will be bent in slightly different ways. The net effect of this is a distortion to the shape of the galaxy image, which can in some cases be very severe. Another interesting effect that can occur due to lensing is the formation of multiple images of the same galaxy. This occurs because light rays from a distant galaxy that would otherwise diverge may be focused together by lensing. From the point of view of an observer on the Earth, it looks as if two very similar light rays have travelled along straight lines from different parts of the sky. You can see this in the orange lines in the schematic above - we can see more than one image of the same galaxy in different places. Lensing can also act like a magnifying glass, allowing us to see images of galaxies that would otherwise be too faint to see.

An example of multiple images is shown below in an image from the Hubble Space Telescope. There are 3 images of the same galaxy, and 5 images of a type of galaxy called a quasar. The images are not the same shape or size because each image will have passed through a different region of space on its journey to us, and hence will have been distorted differently. A technique known as spectroscopy is used to determine which images came from the same galaxy.

Image: NASA/ESA, K Sharon (Tel Aviv University), E. Ofek (Caltech)

Weak Lensing

If the lensing effect is strong enough to be seen by the human eye on an astronomical image, like in Abell 2218, we call this strong lensing. Strong lensing only happens when a massive cluster of galaxies lies between us and some other galaxies - it is the further-away galaxies that have their shapes changed by lensing. In this case, it is easy to see and measure the effects of lensing. However, there are not that many clusters in the sky that are so big that they cause such a large lensing effect - most of the time, we don’t see galaxies stretched into arcs or multiply-imaged. So these instances of strong lensing are very useful - and pretty - but rare.

However, the fact that there is some dark matter in between us and every distant galaxy we see means that ALL galaxies are lensed - even if it is only slightly. In fact, most galaxies are lensed such that their shapes are altered by only 1%, an effect we call weak gravitational lensing.

We can never see this shape modification with our own eyes on an image because it is too small - but if we have some way to measure this, it could tell us a lot about how dark matter behaves across the whole sky (and not just in massive clusters) as it is a ubiquitous effect. But if we can’t see the effect, how do we measure it? How do we know how strong the lensing effect is on a particular galaxy?

It turns out that we don't need to know how much an individual galaxy image has been lensed – we can instead work out the average lensing effect on a set of galaxies. To do so, cosmologists have to make a couple of assumptions: firstly, that all galaxies are roughly elliptical in overall shape, and secondly that they are orientated randomly on the sky, as shown in the left hand side of the figure below. In the presence of a lensing effect, we would expect that the galaxies in a patch of sky would appear to align themselves together slightly on the sky, as lensing stretches all their images in the same direction. In this way, any deviation from a random distribution of galaxy shape orientations is a direct measure of the lensing signal in that patch of sky. Weak lensing can thus be used to measure the gravitational lensing signal on any part of the sky.

Image: E Grocutt, IfA, Edinburgh

Why is lensing useful?

Gravitational lensing is useful to cosmologists because it is directly sensitive to the amount and distribution of dark matter. This is because the amount of light bending is sensitive only to the strength of the gravitational field it passes through*, which is mostly generated by the mass of the dark matter in the Universe. This means that to measure the amount of lensing on a patch of sky, we don't need to know anything about what kind of galaxies we are observing, how they form and behave or what colour light they emit. This makes gravitational lensing a very clean and reliable cosmological probe as it relies on few assumptions or approximations.

Lensing can therefore help astronomers work out exactly how much dark matter there is in the Universe as a whole (the fraction of the pie chart at the top of the page that dark matter takes up), and also how it is distributed. An example dark matter map constructed from CFHTLenS data is shown below.

Image: CFHTLenS Collaboration

Lensing has also been used to help verify the existence of dark matter itself. The image below is known as the Bullet Cluster, and it has been observed in both optical (visible) light and in X-ray. The majority of the light coming from the Bullet cluster comes from hot X-ray emitting gas, and has been overlaid onto the visible-light image in pink. Superimposed in blue is the location of the dark matter in the cluster, determined from measuring the lensing signal from the visible-light images of the galaxies. The offset between the pink X-ray gas and the blue dark matter regions tells us that what we are observing is actually the aftermath of a collision between two galaxy clusters. During the collision, the baryonic X-ray gas particles (the 'normal' matter) will interact with each other through both gravity and electrostatic forces, slowing and shocking one another. The dark matter particles, however, only interact through gravity and can pass through each other unimpeded by electrostatic interactions. This means that the X-ray gas lags behind the dark matter as the two clusters escape the collision, causing the observed offset - most of the visible matter is now in the centre of the image, but lensing tells us that most of the mass lies further out.

Image: Composite Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; 
Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. 
Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

Some scientists believe that since the only observed effects of dark matter are gravitational, then perhaps our understanding of gravity is incomplete. It is possible that we are not observing a new type of matter, but that the laws of gravity as we understand them are wrong. As a result, many different modified gravity theories have arisen to explain the dark matter phenomenon. The Bullet cluster provides strong evidence for the existence of dark matter, as this offset between the light and mass is exactly what scientists expect to see if dark matter is real, and it is hard to explain under many theories of modified gravity.

If we know something about the distances to the galaxies we look at with our telescopes, lensing can also tell us about the nature of dark energy because the amount of dark energy affects how galaxies and clusters form and develop. Measuring their distribution with distance through gravitational lensing can help us constrain the amount of dark energy in the Universe to a higher degree of precision. The light from distant galaxies began travelling towards us many millions (or even billions) of years ago, providing a window into the early Universe. This means that it is also possible to work out if the amount of dark energy changes over time by observing galaxy structures at different distances from us. Thus, gravitational lensing is a clean probe of the Universe and has much to tell us about its two most mysterious components - dark matter and dark energy.

*In fact, this is one way in which gravitational lensing differs from optical lensing, as gravitational lensing is independent of the wavelength (colour) of the light. All light rays are bent the same amount by gravity. Optical lenses cause light of different colours to bend by varying amounts in a process called diffraction, resulting in the splitting of light into rainbows. There is no such analogous effect with gravitational lensing.

Author: Emma Grocutt