By Jason Wu, Fall 2021.

On October 5th, 2021, the Dark Energy Survey (DES) collaboration—a scientific imaging group on the hunt for the elusive dark energy—officially concluded its six-year mission. Despite sounding like a string of stereotypical astronomical jargon, the DES utilized some genuinely innovative techniques in cosmological research. UChicago involvement by research fellows in the Survey Science Group contributed immensely to the project. Using the Dark Energy Camera in Chile, the team had scanned over one-third of the southern hemisphere sky, producing countless imaging data, and pushing what we know about the universe to its limits.[1]

What scientific interest could have inspired countless astronomers to dedicate this amount of time and effort? O It turns out, one of the main objectives of the DES was detecting a fascinating phenomenon called gravitational lensing—a process in which light from distant stars (source) is was bent by massive galaxies (lens), as shown in Figure 1. To understand why gravitational lensing has generated so much scientific attraction, we should first begin with its history.

Figure 1. The gravitational lensing effect. Light from the distant source is bent by the lens (depicted as a yellow plane), to produce an illusionary image distinct from the source.[5]

Like many discoveries in modern physics, the seeds of gravitational lensing are were rooted in a small room in the University of Zürich, where Einstein first formulated his theory of special relativity in 1907.[2] As his groundbreaking theory became widely understood in the scientific community, physicists began looking for proofs. However, objects massive enough to produce a sizable gravitational pull would be impossible to reproduce here on Earth. As astronomers looked up into the sky, the concept of gravitational lensing emerged—if directly observed, the lensed image would allow us to visualize the gravitational effects of massive celestial bodies.

The initial reception of this proposal was discouraging to say the least. Several physicists conducted geometric calculations on gravitational lenses during the 1920s, but all agreed that the phenomenon would be impossible to observe: it required two objects being lined up exactly in the sky, the chances of which is too trivial to consider.[2] Einstein also published a paper on this topic in 1936, and he too, held a pessimistic view on gravitational lenses.[3]

It is important to note, however, that modern cosmology had barely started taking shape in the early 20th century. The dominant theory saw the universe as nothing more than stars and planets. It was no wonder, then, that astronomers at the time did not believe in this phenomenon. As prophetic as Einstein was, what he did not consider as lenses were quasars and galaxies—both of which are much more massive than individual stars in observable diameter and gravitational effects.

It was not until 1979 that scientific interest in gravitational lensing was renewed by a peculiar discovery of the famous “Twin Quasar” by physicist Dennis Walsh. Initially, the object seemed like a pair of quasars, but the uncanny similarity across the spectrum made Walsh and his team speculate otherwise.[4] They correctly suspected that the “Twin Quasar” was not a pair of twins at all, but instead consisted of two images of the same object. Further investigation revealed the mechanism behind this phenomenon: light from the distant quasar was bent on its way to Earth by a nearby galaxy. This is analogous to optical lenses we normally use, which bend light coming from objects in our line of sight. Unlike glass lenses, however, gravitational lenses are point-like masses that deflect light by their centers. Walsh’s team observed this distortion of light in the “Twin Quasar” because the source and lens were aligned in a straight path. Inspired by their pioneering work, the scientific community then discovered numerous similar systems. For the first time in history, we observed relativistic effects of gravitational pull on an identifiable scale. Further advancements in modern telescopes revealed stunning images of gravitational lensing, including the formation of Einstein rings, arcs, multiple images, and distorted spheres of the source object.

The observation of gravitational lensing shifted the paradigm of cosmological research. After its first discovery, researchers soon realized just how often these events had occurred: in densely populated regions of the sky, it was possible to identify hundreds of micro-lensing events per year. This, combined with the generalist nature of gravitational lenses, meant they could be observed in all wavelengths at all times. With the new-found computational power developed in the late 20th century, astronomers began documenting the brightness of millions of stars to be processed each night. As of today, the Dark Energy Survey revolutionized the field by deploying machine learning on this immense volume of data. Human researchers looked for images with larger, more identifiable lensing radii, while leaving targets with smaller radii to the algorithms. Using this method, the DES was able to identify an extraordinary 247 systems for future research. [1]

Today, gravitational lensing comprises some truly awe-inspiring interplays with cosmology and theoretical physics. It allows us to tangibly probe for matter distributions in the universe. For example, astronomers have observed an unusual abundance of lenses in seemingly less populous regions of the sky. This has led to provocative speculations about the existence of dark matter, a form of mass which cannot be seen. Because gravitational distortions of the lens also concentrate light from source objects, signals from distant galaxies are amplified by this effect, allowing us to see further than otherwise possible.

In a sense, gravitational lensing acts as a telescope for our telescopes. What we would assume as a niche, closed-off astronomical concept is, in reality, one of our most powerful scientific tools. By looking up at the sky, we are grateful to have received a new pair of lenses to explore the deepest secrets of the universe and lend insights into our place in this vast cosmos.

 

[1] O’Donnell, J. H. et al. 2021. “The DES Bright Arcs Survey: Candidate Strongly Lensed Galaxy Systems from the Dark Energy Survey 5,000 Sq. Deg. Footprint.” Phys. Rev. D. FERMILAB-PUB-21-469-E. arXiv:2110.02418. https://arxiv.org/abs/2110.02418.

[2] Tilman Sauer. 2010. “A brief history of gravitational lensing” in: Einstein Online Vol. 04, 1005. https://web.archive.org/web/20160701154224/http://www.einstein-online.info/spotlights/grav_lensing_history.

[3] Einstein, Albert. 1936. “Lens-Like Action of a Star by the Deviation of Light in the Gravitational Field.” Science 84 (2188) 506-507. https://doi.org/10.1126/science.84.2188.506.

[4] Walsh, D., Carswell, R. & Weymann. 1979. “0957 + 561 A, B: twin quasistellar objects or gravitational lens?” Nature 279, 381–384. https://doi.org/10.1038/279381a0.

[5] Sachs, Michael. 2008. “Gravitational-lensing-angles.png.” (image) via Wikimedia Commons. https://upload.wikimedia.org/wikipedia/commons/e/e4/Gravitational-lensing-angles.png.

[6] Yukterez (Simon Tyran, Vienna). 2018. “Naked.Singularity,Overextremal.Kerr.Newman,Raytracing.png.” (image) via Wikimedia Commons. https://upload.wikimedia.org/wikipedia/commons/4/4b/Naked.Singularity%2COverextremal.Kerr.Newman%2CRaytracing.png.