Gravitational Lensing – Wonders of the Universe III


Weak lensing

In many cases the lens is not strong enough to form multiple images or arcs. However the source still be distorted, both stretched and magnified. The fact is that, there is some dark matter in between us and every distant galaxy we see. It means that all galaxies are lensed, even if it is only slightly. In fact, most galaxies are lensed such that their shapes are altered only by a very small amount. The 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.

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.

Weak lensing is not a noticeable effect by eye; rather, it has to be done statistically. Ellipticities of a field of background galaxies are observed on a grid, and are statistically averaged together to create the weak lensing signal. Shape distortions of these background galaxies due to lensing are on the percent scale. One important assumption is made, however, and that is that galaxies are elliptical, and their orientations are completely random. With that, any net tangential shearing produced is due to gravitational lensing. In the image below (from upper left to bottom right), the upper left frame shows an unlensed field of circular galaxies, and to its immediate right shows the effect of lensing. The image in the bottom right has added shape noise (a ‘realistic’ field of background galaxies), and to its right is how the field would lens.



Microlensing is a regime of lensing which is most common on the scale of the Milky Way galaxy.  Where no distortion in shape can be seen but the amount of light received from a background object changes in time. The lensing object may be stars in the Milky Way in one typical case, with the background source being stars in a remote galaxy, or, in another case, an even more distant quasar. This can occur when background stars pass behind foreground stars. Microlensing is actually strong enough to produce multiple images of the background star, but since the image separations are so small (micro-arc second scale – hence the name), what we observe (since an angular resolution of a micro-arc second is tough to achieve) is a change in the flux as the object moves into and out of alignment with the intermediate massive object. Interestingly, microlensing has actually proven useful in detecting planets around stellar lensing systems.



Importance of gravitational lensing for cosmology

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.

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.

There is a distribution of galaxies far enough away that can be treated as sources, and thus clusters nearby can be “weighed”, or have their mass measured using their lensing. Super clusters have been considered as well. In addition, theories of cosmology predict the distribution of large scale structure, the distribution of matter in the universe. The statistical properties of the large scale structure (e.g. the probability of finding a galaxy at one place when there is another a certain distance away) can also be measured by weak lensing, because the matter will produce shear and convergence in distant sources (which can be galaxies, or the cosmic microwave background, for example). Weak lensing is a useful complement to measures of the distribution of luminous mass such as galaxy surveys. Lensing measures all the mass, in particular the dark matter as well as the luminous matter.


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)



3D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with Hubble Space Telescope.



Gravitational lens discovered at (redshift) z = 1.53. Hubble Space Telescope image of the most distant gravitational lens yet discovered.

Sources : Wikipeadia the free enclyclopedia