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Starts With A Bang

Machine Learning Finds More Gravitational Lenses Than All Astronomers Combined

GAL-CLUS-022058s is one of the largest and most complete Einstein rings ever discovered. This beautiful gravitational lens is created by a bright, distant galaxy which happens to be aligned directly behind a massive galaxy at the center of a massive galaxy cluster. The lensing effect stretches, distorts, and magnifies the background galaxy, as well as creating multiple images of it. (ESA/HUBBLE & NASA, S. JHA; ACKNOWLEDGEMENT: L. SHATZ)

A new survey, the DESI Legacy Imaging Survey, has found more lenses than all others put together.


One of Einstein’s most revolutionary predictions is that mass bends light.

During a total solar eclipse, stars can be visible during the day. Their apparent positions, as you get closer to the Sun’s limb, will be distorted due to the gravitational effect of passing close by the Sun. This image was made from 98 images obtained by means of four different cameras. These 98 images were chosen from totally 275 images in order to minimize the influence of rolling clouds. Images taken during the 2010 solar eclipse. (MILOSLAV DRUCKMULLER, MARTIN DIETZEL, SHADIA HABBAL, VOJTECH RUSIN.)

Starlight bent around the eclipsed Sun in 1919 confirmed this.

In 1919, a total solar eclipse occurred, enabling scientists to test General Relativity. According to Einstein’s predictions, starlight near the limb of the Sun should be gravitationally deflected, and by a different amount than Newton’s theory would predict under any assumptions. The observations agreed with Einstein, confirming General Relativity’s validity. (NEW YORK TIMES, 10 NOVEMBER 1919 (L); ILLUSTRATED LONDON NEWS, 22 NOVEMBER 1919 (R))

The 1930s first developed a prediction for gravitational lenses.

Gravitational lenses, magnifying and distorting a background source, allow us to see fainter, more distant objects than ever before. Similarly, observing the light that experiences a gravitational lensing effect enables us to reconstruct properties of the lens itself, potentially shedding light on the nature of dark matter. (ALMA (ESO/NRAO/NAOJ), L. CALÇADA (ESO), Y. HEZAVEH ET AL.)

Large foreground masses would bend and magnify fortuitously aligned background sources.

This images showcases the effects of both weak and strong gravitational lensing. The strong lensing effect creates multiple images of the same background quasar, while magnifying and distorting background galaxies into rings and arcs. Meanwhile, the shapes of background galaxies are distorted in a circle around the central mass, consistent with weak lensing predictions. (ESA, NASA, K. SHARON (TEL AVIV UNIVERSITY) AND E. OFEK (CALTECH))

Multiple images or even “Einstein rings” could occur.

A horseshoe-shaped Einstein ring, just short of the perfect alignment needed for a 360-degree ring. Systems like this have recently been used to place a strong constraint on the validity of relativity, and can reveal features about ultra-distant galaxies that could never be seen without this serendipitous alignment. (NASA/ESA AND HUBBLE)

For decades, they were solely theoretical.

This illustration shows the physics behind a strong gravitational lensing system. There needs to be a foreground mass that acts as the lens, and the background light source(s) must be properly aligned. If this is the case, it can produce multiple images, distorted light, and highly magnified views of background objects. (NASA/ESA)

Finally, in 1979, the “Twin QSO” was found: two lensed images of the same quasar.

This galaxy cluster appears to be hosting two blue stars, but they are actually the same background object: the distant quasar QSO 0957+561. This was the first gravitationally lensed object ever discovered back in 1979, nearly 50 years after they were predicted within the context of General Relativity. (ESA/HUBBLE & NASA)

Since that time, many more gravitational lenses have been found.

This image showcases six examples of strong gravitational lenses found in the COSMOS survey, which found 67 such lenses total. The lenses were all found in the same 1.6-square-degree field of sky with several space-based and Earth-based observatories. These gravitational lenses often allow astronomers to peer much further back into the early Universe than they would normally be able to. (NASA, ESA, C. FAURE (ZENTRUM FÜR ASTRONOMIE, UNIVERSITY OF HEIDELBERG) AND J.P. KNEIB (LABORATOIRE D’ASTROPHYSIQUE DE MARSEILLE))

Features include:

  • quadruple images,
Two temporally varying images (left) and a 1990 Hubble image (right) of the first quadruple-lens system ever discovered, all resulting from the same distant quasar, known colloquially as an Einstein Cross. We now have scores of quadruple lenses, and that number should only increase as time goes on and we collect more observational data from the deep Universe. (NASA, ESA, AND STSCI)
  • magnified arcs,
A Hubble image showcasing many of the lensed galaxies inside a massive galaxy cluster. The presence of not only these galaxies but the dark matter within them as well as within the larger cluster is responsible for the observed lensing effects: rings, arcs, magnified and distorted light, etc. These observations allow us to compare the actual universe with numerical simulations. (NASA, ESA, G. CAMINHA (UNIVERSITY OF GRONINGEN), M. MENEGHETTI (OBSERVATORY OF ASTROPHYSICS AND SPACE SCIENCE OF BOLOGNA), P. NATARAJAN (YALE UNIVERSITY), AND THE CLASH TEAM)
  • hidden background objects,
The ultra-distant, lensed galaxy candidate, MACS0647-JD, appears magnified and in three disparate locations thanks to the incredible gravity of the gravitational lens of the foreground cluster, MACS J0647. Other weak and strong lensing effects can also be seen elsewhere around this galaxy cluster.(NASA, ESA, M. POSTMAN AND D. COE (STSCI), AND THE CLASH TEAM)
  • and nearly-perfect rings.
Two bright, massive galaxies are relatively close by in space, and their mutual gravity lenses some background galaxies, as shown here. The light from the background galaxies gets stretched and magnified into giant circular arcs, revealing properties of both these background objects as well as the gravitational properties of the lens itself. (NASA & ESA ACKNOWLEDGEMENT: JUDY SCHMIDT)

Hubble’s deep imaging uncovered many more strong lenses.

The streaks and arcs present in Abell 370, a distant galaxy cluster some 5–6 billion light years away, are some of the strongest evidence for gravitational lensing and dark matter that we have. The lensed galaxies are even more distant, with some of them making up the most distant galaxies ever seen. (NASA, ESA/HUBBLE, HST FRONTIER FIELDS)

Lensing affects only 1 in ~10,000 galaxies.

This image highlights more than two dozen galaxy candidates that are red, faint, and extremely distant, as found in the Hubble Ultra Deep Field. Many of these galaxies are found extremely close to massive foreground galaxies, whose mass lenses and magnifies the background sources. This technique has helped identify many of the most distant objects known in the Universe.(NASA, ESA, R. BOUWENS AND G. ILLINGWORTH (UC, SANTA CRUZ))

Hubble, unfortunately, only offers narrow-field capabilities.

This image from the Digitzed Sky Survey shows the area around the Hubble eXtreme Deep Field (XDF), located in the constellation of Fornax (The Furnace). The full Moon is shown to scale for comparison. Over the course of its 30 year lifetime, Hubble has imaged a significant number of square degrees on the sky, but less than 1% of the 40,000 square degrees available. (NASA, ESA, Z. LEVAY (STSCI), T. RECTOR, I. DELL’ANTONIO/NOAO/AURA/NSF, G. ILLINGWORTH, D. MAGEE, AND P. OESCH (UNIVERSITY OF CALIFORNIA, SANTA CRUZ), R. BOUWENS (LEIDEN UNIVERSITY) AND THE HUDF09 TEAM)

After 30 years, it’s imaged less than 1% of the sky.

The Copeland Septet, in the constellation of Leo, was imaged along with about a billion other galaxies as part of the DESI Legacy Imaging Surveys. The survey covers approximately half of the sky, ~20,000 square degrees, to very good depth. With that much data, machine learning was required to extract gravitational lensing signals. (KPNO/CTIO/NOIRLAB/NSF/AURA/LEGACY IMAGING SURVEY)

However, DESI (Dark Energy Spectroscopic Instrument) Legacy Imaging Surveys are both deep and wide.

This image, part of the DESI Legacy Imaging Survey, showcases a gravitational lens at the center that forms a near-perfect ring. Alignments such as this are rare, affecting fewer than 1-in-10,000 galaxies, but with more than a billion galaxies and the advent of machine learning for handling this Big Data, more than 1,000 new lensed galaxies have been found so far. (KPNO/CTIO/NOIRLAB/NSF/AURA/LEGACY IMAGING SURVEY)

Spanning ~20,000 square degrees, its full map requires over 10 trillion pixels.

In this image, a massive set of galaxies at the center causes many strong lensing features to appear. Background galaxies have their light bent, stretched, and otherwise distorted into rings and arcs, where it gets magnified by the lens as well. This gravitational lens system is complex, but informative for learning more about Einstein’s relativity in action. (KPNO/CTIO/NOIRLAB/NSF/AURA/LEGACY IMAGING SURVEY)

Machine Learning is required to handle that much data.

Not every gravitational lens is simple and circular, as this image shows. The irregular arcs and multiple stretched, magnified images of background objects, visible in red and blue, help scientists trace out and reconstruct the locations of matter in the foreground cluster. This was taken as part of the DESI Legacy Imaging Survey. (KPNO/CTIO/NOIRLAB/NSF/AURA/LEGACY IMAGING SURVEY)

That process discovered 1,210 new gravitational lenses.

One of the best examples of a quadruple lens found with the DESI Legacy Imaging Survey. This was just one of 1,210 lensed systems found in the survey that spanned approximately half of the sky. Many of the targets identified will be studied in more detail in the future, and more lenses will likely be revealed before all is said and done. (KPNO/CTIO/NOIRLAB/NSF/AURA/LEGACY IMAGING SURVEY)

That’s more than previously discovered by all astronomers, combined.

This Hubble Space Telescope image shows a gravitational lens (center) that was first identified as a lens candidate with the assistance of a neural network that processed ground-based space images. The lens is artificially colorized and circled in this image. (HUBBLE SPACE TELESCOPE)

Occasionally, Hubble followed up, revealing additional details.

These two two column composites show side-by-side comparisons of gravitational lens candidates imaged by the ground-based Dark Energy Camera Legacy Survey (color) and the Hubble Space Telescope (black and white). Where data from Hubble was available, it not only confirmed these gravitational lenses, but revealed many additional features that the DESI survey could not. (DARK ENERGY CAMERA LEGACY SURVEY, HUBBLE SPACE TELESCOPE)

With Euclid, Vera Rubin, and Nancy Roman telescopes coming soon, we’ll certainly find even more.


Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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