Q&A results: Gravitational lensing: 57%, Galactic orbits and time dilation: 27%, A photographic glitch: 16%

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Correct answer: Gravitational lensing Cute though it seems, this celestial smiley face spans billions of light years across: the bright yellow lights that form its eyes and nose are in fact entire galaxies found inside an even bigger space object, a galaxy cluster. The bluish smile is a galaxy in the background whose shape is completely deformed by the galaxy cluster’s strong gravitational field. Captured by the Hubble Space Telescope, this Cheshire-cat grin is the result of a strange phenomenon called “gravitational lensing”: gravity that is so strong, it can bend light itself – as seen by an observer. Gravitational lensing goes all the way back to Albert Einstein and general relativity; particularly his idea that space and time exist in a continuum, woven throughout the entire universe. The idea is that gigantic objects like stars or galaxies or galaxy clusters have so much mass that they can warp “spacetime” enough to distort light passing through them. That is what is happening with this smiley face, which is formed by a galaxy cluster affectionately named SDSS J1038+4849. The sheer size of this galaxy cluster warps spacetime so much that light from background distant galaxies is bent into a full circle, which is known as an “Einstein ring”. The resulting circular shape creates the illusion of a smiling face with sinister-looking eyes. Gravitational lensing is ubiquitous in the Universe but only the best telescopes can observe its effects on light. Because of Hubble Telescope’s exceptional sensitivity and resolution, as well as its advantageous position outside of Earth’s atmosphere, it can capture images of faint and distant gravitational lenses that ground-based telescopes cannot. This image was actually discovered by amateur astronomer Judy Schmidt while she was hunting for images to submit for the Hubble's Hidden Treasures competition in 2012. The participants were asked to search through the Hubble Space Telescope archives for overlooked discoveries. However, gravitational lensing does more than create celestial emoticons. Objects or distributions of matter that bend light – called simply “gravitational lenses” – have turned out to be very useful astronomical tools. Because they function as magnifying glasses in space, astronomers can use them to study distant galaxies from the early universe that otherwise would be impossible to see. For example, EPFL’s Laboratory of Astrophysics (LASTRO) is heavily involved in the preparation of an ESA satellite, called EUCLID, which will study dark energy and dark matter by observing billions of gravitational lenses. (Image credit: NASA/ESA/JPL-Caltech)