Looking at the universe is difficult. Fortunately, the Universe sometimes provides some help, using gravity to give light a quick kick to photons.
I’m talking about the gravitational lens:
Briefly, the gravitational lens is when the gravity of a massive object (a star, a black hole, a galaxy) bends the space around it, causing light to pass into the curve, like a car following the curve of ‘a road. Einstein [wrote about this in relation to his work on Relativity], saying that matter bends space and that we perceive this flexion as gravity. So we call this gravitational lens; the bending space of the object is the target, and the object whose light is distorted is the target object.
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Today we have seen this effect countless times. It’s a little weird; not only can you slightly move the position of an object with lenses in the sky for this purpose, but you can also distort it, stretch it like a sophisticated one, even wrap it around the lens like a ring ( in fact, we call them Einstein’s rings), and it became much brighter through the lens than it would otherwise be.
The Hubble Space Telescope is like a machine designed to observe the target. You can see small details in weak objects, perfect for this kind of thing. And when a gravitational lens is pointed, what you see is spectacular:
This system is called 2M1310–1714 and was discovered by accident in a study of galaxies; the survey was designed to cover distances of more than a million galaxies and a scientist saw this situation slowly when visually inspecting images of a quarter of a million galaxies. That’s … a lot of work. But a dozen lensed systems were found, including 2M1310–1714.
In the center are two galaxies that act as the lens itself. They are about 3.4 billion light years from Earth, which is a very long way. But the object with lenses is even further away. It is a quasar, a galaxy where the central supermassive black hole is swallowing matter and illuminating it like a cosmic beacon. Matter heats up as it falls and can be so bright that we can see it clearly in the observable universe.
And in this case I mean it: the quasar is about an impressive 10.4 billion light-years away.
The pair of slow-moving galaxies distorts so much space around them that quasar light is severely deformed as it passes. The ring is the quasar light erased around the galaxies, and the four dots along it are also images of the quasar, multiplied and amplified by the pair. There is also a much weaker fifth image of the quasar, which also appears in the center between the two galaxies.
Lens quasar images are relatively common, but multiples like this are very rare. They are also extremely useful: they can be used to find out how quickly the Universe expands.
The basic idea here is that quasar light takes different paths as the pair of galaxies passes, and one path may be slightly longer than another. Quasars are variable; they can light up and darken over time, sometimes quite quickly. If it suddenly lights up, for example, due to these different lengths of path we see that images with lenses light up at different times. Images of longer paths will be delayed compared to shorter paths. The duration of this delay depends on many things, including the exact path taken by light, which depends on the distribution of matter in the pair of slow-moving galaxies and, fundamentally, on the distances to the slow-moving galaxies and the quasar with lens.
We can also measure the redshifts of galaxies and the quasar, which tells us how quickly universal expansion takes them away from us. There are many steps in this process, but in the end, if you measure the delay between the different images with lenses, you can measure how quickly the Universe expands. This is the key measure we need to understand, well, all.
The Hubble image was taken to get a good map of the lens galaxies and the structure of the quasar. With this in hand, large terrestrial telescopes can observe it from time to time and look for these changes in brightness. From here, complex mathematical models can be used to measure the deformation of space and, from there, the speed of cosmic expansion. Hubble’s image alone can’t do that, and field-based observations need Hubble’s to tighten restrictions. Together they are much more powerful than they would be alone.
I love the fact that this image alone is a real stunner. But when you look at the science behind it and understand that we can understand the most fundamental aspects of the Universe thanks to that, that also amplifies and multiplies my appreciation.