Gravitational Lensing

Space is so vast and empty that the light from distant galaxies almost never encounters anything else as it travels toward Earth. However, if you look toward any nearby galaxy, or a collection of galaxies like this one, you will almost always see something behind it. In this image, the yellow galaxies are close by, and the blue ones are much farther away. The blue galaxies are stretched into arcs, because the gravity of the galaxies in front have bent the light from the ones in back. The contents of this image are important for many reasons, two of which I will describe below.

First, galaxy clusters are the largest gravitationally-bound structures in the universe. The first attempts to determine how much mass there is in these clusters of galaxies was made by measuring the velocities of the galaxies. Those measurements indicated that the matter that we see in visible light does not contain enough mass to hold the cluster together. Some of the "missing mass" was found to be X-ray emitting matter. The X-ray measurements confirmed our basic understanding of the structure of these galaxy clusters (namely, that many were old, and were close to equilibrium, so that the velocity measurements were a good measure of the mass in the cluster), but the X-ray emitting plasma was still not enough to hold the clusters together. It is generally believed that the clusters are held together by "dark matter", which is composed of particles that only interact gravitationally, and possibly through the weak interaction. These dark matter particles may make up 80% of the matter in the universe.

Now, many have questioned why one would hypothesize that there is some unknown particle that makes up most of the universe, when it might be easier to suppose that we just don't understand gravity well enough to predict how a cluster like this should behave. After all, in some sense we know that the theory that we use for gravity, General Relativity, is wrong, because it can not be reconciled in its current form with observations on the spatial scales of atoms, which are well explained by Quantum Mechanics. (In brief, in General Relativity one must assume space is smooth, whereas in Quantum Mechanics, space must be divided in to discrete quanta). Therefore, many scientists have fairly asked, Could our lack of understanding of General Relativity also applies to the largest scales in the universe? Perhaps the correct theory of gravity would explain how a galaxy cluster is held together without resorting to particles that we have not yet observed directly in an experiment on Earth.

That is where the second importance of this image comes in. The manner in which the light from distant galaxies is bent by a galaxy cluster is predicted as part of General Relativity. Indeed, one of the early triumphs of General Relativity was that it correctly predicted the angle through which light from a star would be bent as it passed close to the sun; the only competing theory, Newtonian gravity, was wrong by a factor of two. This bending of light is referred to as gravitational lensing, and it has now been seen in many situations --- stars lensing the light from more distant stars (microlensing), galaxies lensing the light from more distant ones (strong lensing, producing Einstein rings and crosses), and "weak lensing" caused by groups of galaxies, as in the image above. It turns out that almost all competing theories of gravity have failed to explain gravitational lensing, and therefore either have been dismissed, or need to be modified before they can be considered seriously. Images of gravitational lenses are therefore one of the most important tools for understanding gravity.