Gravitational lensing
The principle behind gravitational lensing is that whereas the bending of light caused by a single star may be quite small, the light rays emanating from behind a massive object such as a galaxy, would be bent so strongly by gravity that they could converge, producing a magnified image or multiple images of the distant source, in the process revealing the mass and structure of the intervening galaxy. This is similar to the bending of light through a lens, hence the name – gravitational lensing. Apart from using the technique to measure the masses of the largest structures in the universe, the technique can also be employed to test general relativity, to magnify more distant objects that would otherwise be unobservable, and probing the geometry and evolution of the universe on its largest scales[1] Einstein did not recognise either the magnitude or the importance of the lensing effect[2].
The technique has also been described as “using nature's own magnifying glasses in the form of massive galaxy clusters”. These groups of galaxies have so much mass combined that their gravity bends space and time, according to Einstein's general theory of relativity. As light from a more distant object travels through the universe, it follows the bent spacetime around the cluster, becoming magnified along the way. When it reaches Earth, the distant object looks warped and stretched, and sometimes multiple images of it appear. The lensing magnification brings more faint galaxies into view, and it also zooms in on a smaller area containing fewer galaxies. [3]
A recent experiment called the Reionization Lensing Cluster Survey (RELICS) aimed to find some of the first galaxies to form in cosmic history, using gravitational lensing as its basic technique [4]. The technique yielded more than 300 ancient galaxies, including one named SPT0615-JD whose light began its journey toward Earth 13.3 billion years ago, in other words, some 300,000 years after the universe began. The technique thus provides a window into cosmic history.
As Coe explains, the first galaxies were not like those we know today. They were more pristine, composed primarily of hydrogen and helium gas; they had yet to settle into “majestic spiral patterns” or ”puffy elliptical balls” like the galaxies we see around us now. They were far more disordered and much smaller, making them harder to find. Early galaxies were “bathed in cool streams of flowing hydrogen gas, lured inward by gravity”. It is conjectured that galaxies such as SPT0615-JD transformed early space by blasting out ultraviolet light that the gas around them absorbed, turning the universe's first neutral atoms back into the lone protons and electrons that they started out as, in other words - reionisation. However, the details of how and when this process occurred are still unclear. All this occurred in the context that many massive clusters had never been observed by Hubble at near-infrared wavelengths, in which distant galaxies would appear, the light from these objects becoming longer and redshifted as they move further away from us. [5]
A recent experiment called the Reionization Lensing Cluster Survey (RELICS) aimed to find some of the first galaxies to form in cosmic history, using gravitational lensing as its basic technique [4]. The technique yielded more than 300 ancient galaxies, including one named SPT0615-JD whose light began its journey toward Earth 13.3 billion years ago, in other words, some 300,000 years after the universe began. The technique thus provides a window into cosmic history.
As Coe explains, the first galaxies were not like those we know today. They were more pristine, composed primarily of hydrogen and helium gas; they had yet to settle into “majestic spiral patterns” or ”puffy elliptical balls” like the galaxies we see around us now. They were far more disordered and much smaller, making them harder to find. Early galaxies were “bathed in cool streams of flowing hydrogen gas, lured inward by gravity”. It is conjectured that galaxies such as SPT0615-JD transformed early space by blasting out ultraviolet light that the gas around them absorbed, turning the universe's first neutral atoms back into the lone protons and electrons that they started out as, in other words - reionisation. However, the details of how and when this process occurred are still unclear. All this occurred in the context that many massive clusters had never been observed by Hubble at near-infrared wavelengths, in which distant galaxies would appear, the light from these objects becoming longer and redshifted as they move further away from us. [5]
[1] Lawrence M.Krauss, “What Einstein got wrong”, Scientific American, September 2015, Special Issue – 100 years of General Relativity, 40 at 26, 44.
[2] Ibid.
[3] Dan Coe's," Back in time", Scientific American, November 2018, 34 at 37-38.
[4] What follows is an edited summary of portion of Dan Coe's "Back in time" article which in the Scientific American, November 2018, 34 ff.
[5] Ibid, 36, 38.