James Webb Space Telescope's image distortion explained; what is gravitational lensing?

As the James Webb Space Telescope's program scientist Eric Smith rightly said, the world is turning new again after the most powerful observatory ever built released its first image of our universe. The image was unveiled by US President Joe Biden at 2:30 am IST today, July 12, during a media event at the White House, marking the commencement of Webb's operations. Said to be the deepest and sharpest infrared image of the universe to date, the picture is brimming with details of galaxy cluster SMACS 0723.

According to NASA, which has tirelessly worked with its partners - the European Space Agency (ESA) and the Canadian Space Agency (CSA) - in commissioning the Webb telescope, this picture shows the SMACS 0723 as it was 4.6 billion years ago. Moreover, it is made from images at different wavelengths in 12.5 hours and if seen carefully appears to be weirdly distorted, something which cannot be blamed on the $10 billion telescope but on the featured galaxies that warped the space around them owing to the phenomenon of 'gravitational lensing'. 

But before we delve into more details about this historic image, let us first understand what is gravitational lensing and most importantly, infrared imaging, something which Webb has been specially designed for. 

Gravitational lensing and the warping of space-time

Gravitational lensing is the phenomenon that has birthed the term 'cosmic lens' and as the name suggests, it allows a magnified study of celestial objects. This phenomenon occurs when a massive object, say a galaxy cluster, warps space-time around it, which makes the light emerging from objects behind it and travelling toward, say Earth, bend and get magnified. 

Gravitational lensing is just Albert Einstein's theory of general relativity in action. In 1915, Einstein proposed the idea that the gravity of a massive object will warp the space around it causing even light to change its path accordingly. Stronger the gravitational strength of the object, the stronger the effect of gravitational lensing. Interestingly, the magnification due to the lensing distortion has allowed scientists to study objects that are otherwise too far away and too faint to be seen such as the one below.

(Galaxy LRG-3-817 captured by Hubble Space Telescope by gravitational lensing; Image: NASA)

What is infrared imaging?

The Webb space telescope is extremely sensitive to infrared light which is invisible to us but can be felt in the form of heat. A light turns infrared when it has travelled for billions of kilometres and gets stretched. This stretching of light occurs due to a phenomenon called 'redshift' which 'shifts' the light beam further away from the visible light in the spectrum.

Now the reason for making Webb sensitive to infrared is for acquiring images and data which otherwise get obscured due to thick cosmic dust out there. By observing the universe in infrared, Webb will be able to see objects which were shielded by thick dust and make discoveries unimaginable using the Hubble telescope. For instance, the image released by Biden earlier today features many faint galaxies that have emerged for the first time, thanks to Webb's infrared capabilities. 

Apart from taking pictures in infrared, Webb is capable of breaking light beams to study the information stored in them. This information could be about the atmospheric composition of an exoplanet or about the light that has travelled for billions of years after emerging from an early galaxy.

For making such discoveries, the telescope has been equipped with four instruments- the Mid-InfraRed Instrument (MIRI), the Near-Infrared Spectrograph (NIRSpec), the Near-Infrared Camera (NIRCam), and the Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS/NIRISS). Except for NIRSpec, all other three instruments have a camera but every instrument has a spectrograph to scrutinise the captured light. 

Image: NASA, ESA, CSA, and STScI