“Mass tells Spacetime how to curve, and Spacetime tells mass how to move,” the pioneering American physicist John Wheeler (1911-2008) once said. Wheeler was commenting on a result of Albert Einstein’s Theory of General Relativity (1915), describing how Spacetime can bend like the flexible fabric of a trampoline when a heavy object is placed upon it. According to General Relativity, the gravitational force can be explained by the warp a massive object causes on the Space surrounding it, and this Spacetime warpage can result in strange distorted shapes in a way that has been compared to a “funhouse” mirror at a carnival. In October 2019, astronomers released a picture from the Hubble Space Telescope (HST) that reveals a galaxy nicknamed the “Sunburst Arc” that has been divided into a weird and lovely kaleidoscopic illusion of a dozen images created by a massive foreground cluster of galaxies situated 4.6 billion light-years away. This image beautifully demonstrates Einstein’s prediction that gravity from massive objects in Space should bend traveling light, thus causing some very strange distortions.
Imagine a child’s trampoline. A little girl picks up a bowling ball and places it on the flexible fabric of the trampoline. Next, her brother picks up a handful of marbles and tosses them near where the bowling ball rests on the fabric. The marbles travel around the dimple created in the trampoline’s fabric by the heavy bowling ball. If the bowling ball is removed, the marbles travel along straight paths. The mass of the heavy bowling ball created a warpage–a curvature–in the fabric of the trampoline that “told” the marbles how to move. The fabric of the trampoline is Spacetime.
Einstein’s view of Spacetime warpage was proven in 1919 by observations of a solar eclipse where the Sun’s (the bowling ball’s) bending of Space (the fabric of the trampoline) could be measured. An additional prediction was that the warping would create a gravitational lens that, besides distortion, would enlarge the apparent size and brightness of a distant object–behaving much like a magnifying glass, to the delight of astronomers who find such distortions valuable as they observe distant objects in the Universe.
Mother Nature’s Magnifying Glass
The term gravitational lensing itself refers to the path that traveling light has taken when it has been deflected. This happens when the mass of an object situated in the foreground warps the light streaming out from a more remote object located in the background. The light does not have to be visible light. It can be any form of electromagnetic radiation. Because of the effects of gravitational lensing, beams of light that would normally not have been observable are warped in such a way that their paths wander towards the observer. However, light can also be warped so that its beams travel away from the observer.
There are three different forms of gravitational lenses: strong lenses, weak lenses, and microlenses. The differences between the three types depend on the position of the background object that is sending forth its beams of light into space, the foreground object that serves as the lens warping that light, and the position of the observer. Also, the shape and mass of the foreground lens itself plays an important role. This foreground object is what determines how much of the background object’s light will be bent, as well as the path that this light will take through Spacetime.
The Universe that we see today sparkles brilliantly with the furious, fabulous fireworks of billions and billions of stars. The Universe’s sparkling stellar inhabitants populate the billions of galaxies that dwell in the relatively small expanse of Spacetime that we call the visible or observable Universe. Observers are unable to see whatever may exist beyond the cosmological horizon (edge) of the visible Universe. This is because the light emitted from shining objects, inhabiting those unimaginably remote regions, has not had enough time to reach us since the Big Bang birth of the Universe almost 14 billion years ago. The expansion of the Universe and the finite speed of light have made that journey impossible.
The speed of light sets something of a universal speed limit–no known signal can travel faster than light in a vacuum. We cannot see what may exist beyond the cosmological horizon, and the greatest of all mysteries–the unanswered secret of our very existence–may reside in those very remote domains far beyond our visiblility. When we peer deep into Space, we look back in Time. The more distant a luminous object is in Space, the longer it has taken its streaming light to reach us. It is impossible to locate an object in Space, without also locating it in Time (Spacetime). The three spatial dimensions that characterize our familiar world are up-and-down, back-and-forth, and side-to-side. Time is the fourth dimension..
Gravitational lenses can dramatically magnify distant sources in the ancient Universe, if there is a sufficiently massive object lurking in the foreground that is situated between the background source and the prying eyes of curious observers.
It wasn’t until 1979 that the first gravitational lens was confirmed. A galaxy that was otherwise obscure served as a lens and split and magnified the light of a remote quasar situated far behind it into a duo of images. Gravitational lensing observations today are frequently used to discover new exoplanets orbiting stars beyond our Sun. Astronomers zoom in on very remote galaxies, and then map the distribution of the otherwise transparent and invisible dark matter.
Dark matter is thought to be an exotic form of matter that is composed of non-atomic particles, that do not interact with light–which is why it is invisible. It is thought to be the most abundant form of matter in the Universe–far more abundant than the “ordinary” atomic matter that forms our familiar world. Because dark matter is transparent–it does not interact with visible objects except through the force of gravity–its existence has not been directly verified. It is believed to play the important role of gravitational “glue” that holds galaxies together, and its gravitational effects on objects that can be observed indicates that it likely does lurk phantom-like in the Cosmos.
Lenses In The Sky
Gravitational lensing reveals that the foreground galaxy cluster magnifying the Sunburst Arc is so extremely massive that its powerful gravity warps the fabric of Spacetime, both bending and magnifying the light emitted from the Sunburst Arc situated far behind it. This distorting effect also creates multiple images of the same galaxy.
The Sunburst Arc is located almost 11 billion light-years from our planet, and it has been lensed into multple images by the massive foreground cluster of galaxies that are between the Sunburst Arc and Earth. The lensing phenomenon created at least a dozen images of this distant background galaxy, distributed over a quartet of major arcs. Three of these arcs can be seen in the upper right of the HST image, while one counter arc is situated in the lower left. However, the counter arc is partially hidden by a very bright foreground star in our own Milky Way Galaxy.
HST uses these gravitational magnifying glasses in Spacetime to study objects that would otherwise be too dim, too distant and too small for even very sensitive instruments to detect. The Sunburst Arc, even though it is one of the brightest of gravitationally lensed galaxies, is no exception. Without the foreground lens magnifying and distorting its distant light, it would be too faint for astronomers to detect.
The lens created images of the Sunburst Arc are between 10 and 30 times brighter than this background galaxy would be without the effects of gravitational lensing. The magnification enabled HST to peer at structures as small as 520 light-years across that would otherwise be too diminutive to be observed without Mother Nature’s gift of a lens. The structures resemble star-birthing regions in nearby galaxies in the local Universe. This helped astronomers make a detailed study of the remote galaxy and its environment.
HST observations reveal that the Sunburst Arc is very much like galaxies which existed at a much earlier time in the Universe’s history–perhaps as long ago as only 150 million years after the Big Bang.