Einstein Cross Replays Supernova Over And Over
Astronomers have found for the first time, multiple separate images of one supernova bent around a galaxy. Each image is a bit older or younger than the others plus we may see the whole thing repeat itself in a few years.
This recently spotted phenomenon has so many layers of awesomeness. First of all, these multiple supernova images are a breed of gravitationally lensed images called an Einstein Cross. Gravitational Lensing is an artifact of Einstein’s General Relativity which predicted that light follows the bend in space-time caused by gravity. The intense gravity well around a foreground object in space can therefore act like a huge lens bending and magnifying the light from a more distance background object far behind it. If the alignment is near-perfect, we see a ring (an Einstein ring) of smeared light around the foreground object like a galaxy or galaxy cluster. If the background object is small like a star, we instead see multiple points of light arranged like the outermost extremities of the letter T; this is an Einstein Cross.
In 1964, Norwegian astronomer Sjur Refsdal, predicted that these crosses could be potentially used to see a time-delayed lensed supernova. That means that different streams of light from a distant supernova could bend around a huge mass such that each one takes a different path to the earth. Some would take longer to complete the journey resulting in images of the same supernova from different points in time of the actual explosion process yet all arriving at the same time.
The variation in the time difference among the various images could be a day, or weeks or perhaps far longer. The same is true for any of the other more common lensed objects such as entire galaxies. The key difference here is that galaxies vary little over days, months, even centuries. So the images look essentially the same. A supernova, on the other hand, is transient, it varies greatly over much shorter time spans, i.e. days or weeks. One image for example could show the supernova brightening while the other could show it already well past its peak brightness.
Once astronomers fully appreciated Refsdal’s theory the hunt was on to actually find one of these buggers. They looked for half a century until Dr Patrick Kelly of Berkeley was searching through recent Hubble images for distant galaxies. He spotted something that caught his eye and then became incredibly excited when he realized exactly what he found.
What he found was the light from a type 1a supernova in a galaxy 9.3 billion years away. That light ran through a huge cluster of galaxies 5 billion light years away with the exciting name of MACS J1149.6+2223. The cluster lensed the light making it much brighter than it normally would have. Not only that though, the light was positioned nicely behind a red galaxy within the cluster itself contributing the magnification and giving the Einstein cross a double lensing effect.
At 9.3 billion light-years, this supernova is obviously dim as hell (on its best day). So dim in fact that the most powerful telescope on earth looking at it for an entire day would not know it was there. It took something like the Hubble with its own big lens above the obscuring atmosphere to make it out. Still, without the magnification inherent in the Einstein cross, even Hubble would have probably missed it.
By examining the supernova images that reached us and the time differences, the scientists and engineers were able to make reasonable assumptions about the density of matter in that region of space and further calculate other pathways that the light could take. Interestingly, they realized that we missed an earlier presentation of the supernova 50 years ago and again 20 years ago as the scenario played out for us in another Einstein Cross event. Don’t fret though because they believe it could all happen again in 1 to 10 years as supernova light that took the really long way to us finally reaches our area of space.
As usual, these images aren’t merely time-shifted pretty pictures. This very special Einstein cross gives us a unique opportunity to learn a plethora of new things such as:
- The expansion rate of the universe in that parcel of space. Remember this was a type 1a supernova which was critical for determining that the universe’s expansion is actually accelerating instead of slowing down.
- The four separate images allow us to investigate the shape and distribution of dark matter in the cluster
- More generally, we may be able to refine our knowledge of the Theory of Relativity, gravity’s strength, and dark energy
And finally, I’d like to leave you with this. Remember that time you really screwed up? Perhaps, like that supernova, our earth is gravitationally lensed for some distant aliens who will laugh at you over and over as they watch repeats of that time you…..
Image Credit: NASA, ESA, FrontierSN and GLASS