In my previous blog post, Visual “Proof” of General Relativity, I discussed how gravitational lensing demonstrates the effects of Einstein’s theory of general relativity in a direct, visual manner. Images created by gravitational lenses show features that are not possible in Newton’s version of gravity.

Although seeing general relativity with your own eyes is kinda awesome, there’s one unsatisfying aspect: you only see the result, not the process. Since you don’t know exactly what those galaxies looked like before the gravitational lensing, it is hard to fully appreciate the magnitude of the distortions. We have no on/off switch for the mass of the galaxy cluster to be able to examine the un-lensed image and compare against the lensed one.

A simulation of gravitational lensing by a cluster of galaxies (click on image for larger version). The galaxies of cluster Abell 2744 (left) are inserted into the Hubble Ultra Deep Field (right) to produce the combined image with gravitational lensing (center).

But we can demonstrate the process of gravitational lensing through scientific visualization. The images above show a simulation of gravitational lensing by a galaxy cluster. On the left is an image of only the galaxies that belong to galaxy cluster Abell 2744; all of the foreground and background objects have been removed. On the right is a deep field image of galaxies. In the center is a simulation of how the galaxies of Abell 2744 would distort the galaxy images in the deep field.

By carefully comparing galaxy images between the right and center panels, one can see how the un-lensed galaxies transform to their distorted lensed versions.  The elongated streaks and arcs in the center image generally come from compact, ellipse-shaped galaxies in the right image. But not all galaxies are changed, a fact easily seen by examining the larger, yellow galaxy in the lower right.

The explanation comes from the details of the simulated lensing. The deep field used above is a portion of the Hubble Ultra Deep Field (HUDF), and includes only galaxies for which we have a good measure of their distance. Using those distances and the distance to Abell 2744, we were able to place the galaxies of Abell 2744 at their correct positions within the deep field. HUDF galaxies which are closer than the galaxy cluster would not be lensed, and appear the same in the right and center images. Only those galaxies behind the cluster were transformed by the simulated lensing. Thus, the central image provides a proper simulation of what would be seen if Abell 2744 suddenly wandered across the sky and ended up in the middle of the HUDF.

I note that all of the background galaxies were combined into a single image at a set distance behind the cluster for simplicity. The full, and rather tedious, 3D calculation could have been performed, but was deemed unlikely to provide a significant visual difference for a public-level illustration. I further note that it is an occupational hazard of being a scientist that one feels compelled to provide such full-disclosure details.

The really difficult challenge is to do the reverse of this simulation. Start with an image of gravitational lensing and then work out the mass distribution of the galaxy cluster from the distribution of streaks and arcs. But, hey, no one said being an astrophysicist was easy.

In the final part of this series of blog posts, I’ll provide a more down-to-earth example of gravitational lensing.