Einstein cross spotted: Here's why we see it

Einstein's crosses have been spotted before and may not excite the world as much, but they are still rare and give us another opportunity to peek into Einstein's mind – the mind that also famously gave us the theory of relativity.

Making sense of Einstein's theory

Understanding the theory of general relativity in its entirety is really the job of books, documentaries, and tutors, but we can surely make sense of parts of it to understand new discoveries.

Einstein suggested that massive objects warp the fabric of the universe, which he referred to as space-time. Unlike the school science teacher, who called gravity a force exerted by one body on another, Einstein called it the experience of space-time curving and distorting it in the presence of matter and energy.

If you have already given up, stay with me for another second because it gets interesting from here. The curving of space-time also sets the rules for how matter and energy behave in it.

For instance, light, which is known to travel in straight lines, will curve in the region of space-time around a massive galaxy and even create a halo around it. How the halo looks depends on factors such as the observer's perspective and the strength of gravity. The light coming from behind the galaxy is therefore "lensed" and becomes visible to an observer, say on Earth.

Rare Einstein cross

One such event was spotted by the Dark Energy Spectroscopic Instrument attached to the telescope at Kitt Peak National Observatory in Arizona in 2021. In this case, the lens is made by an elliptical galaxy located six billion light-years away from us and is a warping beam of light coming from a galaxy located 11 billion light-years away from us.

The beam of light is coming from a quasar, where the gas and dust are falling into a supermassive black hole that is emitting electromagnetic radiation, making it appear brighter than the brightest stars in the region.

Due to the galaxy in the fore, this has led to the light appearing split into four smudges of blue light, referred to as Einstein's cross, and also has a ring-like appearance around the lensing galaxy. The rings magnify the light that is bent, and by reconstructing these smears, astronomers can obtain brighter images of distant galaxies, giving them more access to more details in them.

Since the formation of the rings is also dependent on the gravity of the lensing galaxy, the extent of the ring can serve as a gauge for determining the mass of the galaxy and black holes.

The researchers spent two years confirming the occurrence with Multi-Unit Spectroscopic Explorer at the Very Large Telescope in Chile. The results were published on a pre-print server last month.