Recall a previous article in this series on a very special type of exploding star or supernova called “Supernova Refsdel.” This supernova is so special not because of the great firework display that it made as it blew up, although this part is interesting too. Rather, this particular supernova is drawing attention because it was photocopied on the sky four times.
Yes, one can repeat: we report the detection of four images of the same background supernova. What is the cause for this bizarre trick of nature?
The answer goes back to Einstein, who first predicted the phenomenon that we now call gravitational lensing. Einstein said that mass bends light. He further worked out that the more massive is the object, the stronger is the bending effect. These natural benders, or lenses, act kind of like eyeglasses. When we look through them they magnify the sizes of the images of the objects, and in the case of a poorly-polished eyeglass lens, can even distort the images of objects.
Note of course that while eyeglasses can be made so that we ‘see double,’ they cannot physically change say a dollar bill you are looking at on the table from one bill into two. They change the focus, size, and shape of that dollar bill. Sadly, if you see double it means only that the image of that piece of cash is what has been multiplied. And of course it means that you need to find a new optician.
Einstein predicted that there are natural eyeglass lenses in space, but they are not polished at all. As a result, objects seen through such natural lenses are always distorted in shape and size can even us see double when we look through them (hence the photocopying described in the
first paragraph).
The quadruply-imaged supernova recently discovered is interesting for many reasons. Perhaps the most important one is that we can use it to measure the prescription of that unpolished natural eyeglass lens in space. In turn, knowing the prescription is interesting as natural eyeglasses are made out of a material we call dark matter. What is dark matter? We have no idea, but whatever it is it seems to dominate the total mass in the universe. Stay tuned for the next article on dark matter and another fascinating story about natural eyeglasses in space.
