2 ms·

I appreciate your elaborate answer to my short post very much!

raattgift 3y ago
You're welcome. To prove the geometry of the cosmos among other reasons, cosmologists actively look for distorted spiral galaxies. We are fortunate that there are several highly common types of spirals that, when not gravitationally lensed, look the same in every direction and at every cosmological redshift ("distance" or alternatively "lookback time"). Distorted spirals, especially if we get a good face-on view, reveal the large scale geometry of the universe. That alone can tell us whether we're in the middle of a sphere-like observable universe or an axisymmetric (or cylindrical) one, like if we were in a black hole or rotating cosmos. We see face-on spirals looking nice and circular in every direction and at pretty much every apparent size and brightness, whereas in something like a truly enormous black hole we'd expect those spirals to be elongated in one direction, towards the centre (of mass, for a black hole; of rotation, for a spinning universe). (We can also use radiotelescopes on nearly edge-on spirals to check whether there's an elongation along our line-of-sight, and whether whole galaxy clusters with multiple spirals at various orientations are elongated in any direction). Spirals are also great for characterizing gravitational lenses! Such lenses are a prediction of general relativity from the 1930s (Einstein 1936, Zwicky 1937), and are a good test for the theory especially since we can now use computers to recover the images of the distant background galaxies (this effectively "weighs" the foreground mass), and the magnified super-distant background spirals look circular face-on like any other spiral, and not like a cigar. (Funnily enough there is the Cigar galaxy, M82, known since the late 1700s and still looking pretty cigar-like through small optical telescopes. Only 20 years thanks to improvements in ground-based radio telescopes and space-based infrared and X-ray telescopes to be a nearly edge-on spiral galaxy behind some dust. It got astronomers' attention in part because it seemed so distorted and elongated, rather than obscured. Thanks to that attention we also know it has a very slight gravitational distortion after all, thanks to its interaction with a larger nearby galaxy, the beautiful spiral M81, "Bode's Galaxy". The latter has also been known since the late 1700s, but its spiral structure wasn't known until the late 1800s, some time after the Whirlpool galaxy's spiral structure was described.) Below is a link to a bunch of spirals and a gravitational lens. They really are everywhere in the sky, in the billions. JWST with thousands of spiral galaxies behind (and distorted by) a foreground elliptical-galaxy-dominated massive galaxy cluster: https://www.nasa.gov/image-article/nasas-webb-delivers-deepest-infrared-image-of-universe-yet/ https://www.nasa.gov/image-article/nasas-webb-delivers-deepe... (You can get a very large PNG of this at <https://www.esa.int/ESA_Multimedia/Images/2022/07/Webb_s_first_deep_field https://www.esa.int/ESA_Multimedia/Images/2022/07/Webb_s_fir...>, under the Download box or directly at <https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2022/07/webb_s_first_deep_field/24354723-1-eng-GB/Webb_s_first_deep_field.png https://www.esa.int/var/esa/storage/images/esa_multimedia/im...> (about 26 MBytes). Be sure to zoom in, especially around the edges and corners of the image! Each fuzzy blur is a galaxy with betwen about a hundred billion and a trillion stars.) Also there is a lot of beauty in recent entries at https://blog.galaxyzoo.org/ https://blog.galaxyzoo.org/

You're welcome. To prove the geometry of the cosmos among other reasons, cosmologists actively look for distorted spiral galaxies. We are fortunate that there are several highly common types of spirals that, when not gravitationally lensed, look the same in every direction and at every cosmological redshift ("distance" or alternatively "lookback time"). Distorted spirals, especially if we get a good face-on view, reveal the large scale geometry of the universe. That alone can tell us whether we're in the middle of a sphere-like observable universe or an axisymmetric (or cylindrical) one, like if we were in a black hole or rotating cosmos. We see face-on spirals looking nice and circular in every direction and at pretty much every apparent size and brightness, whereas in something like a truly enormous black hole we'd expect those spirals to be elongated in one direction, towards the centre (of mass, for a black hole; of rotation, for a spinning universe). (We can also use radiotelescopes on nearly edge-on spirals to check whether there's an elongation along our line-of-sight, and whether whole galaxy clusters with multiple spirals at various orientations are elongated in any direction). Spirals are also great for characterizing gravitational lenses! Such lenses are a prediction of general relativity from the 1930s (Einstein 1936, Zwicky 1937), and are a good test for the theory especially since we can now use computers to recover the images of the distant background galaxies (this effectively "weighs" the foreground mass), and the magnified super-distant background spirals look circular face-on like any other spiral, and not like a cigar. (Funnily enough there is the Cigar galaxy, M82, known since the late 1700s and still looking pretty cigar-like through small optical telescopes. Only 20 years thanks to improvements in ground-based radio telescopes and space-based infrared and X-ray telescopes to be a nearly edge-on spiral galaxy behind some dust. It got astronomers' attention in part because it seemed so distorted and elongated, rather than obscured. Thanks to that attention we also know it has a very slight gravitational distortion after all, thanks to its interaction with a larger nearby galaxy, the beautiful spiral M81, "Bode's Galaxy". The latter has also been known since the late 1700s, but its spiral structure wasn't known until the late 1800s, some time after the Whirlpool galaxy's spiral structure was described.) Below is a link to a bunch of spirals and a gravitational lens. They really are everywhere in the sky, in the billions. JWST with thousands of spiral galaxies behind (and distorted by) a foreground elliptical-galaxy-dominated massive galaxy cluster: https://www.nasa.gov/image-article/nasas-webb-delivers-deepest-infrared-image-of-universe-yet/ https://www.nasa.gov/image-article/nasas-webb-delivers-deepe... (You can get a very large PNG of this at <https://www.esa.int/ESA_Multimedia/Images/2022/07/Webb_s_first_deep_field https://www.esa.int/ESA_Multimedia/Images/2022/07/Webb_s_fir...>, under the Download box or directly at <https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2022/07/webb_s_first_deep_field/24354723-1-eng-GB/Webb_s_first_deep_field.png https://www.esa.int/var/esa/storage/images/esa_multimedia/im...> (about 26 MBytes). Be sure to zoom in, especially around the edges and corners of the image! Each fuzzy blur is a galaxy with betwen about a hundred billion and a trillion stars.) Also there is a lot of beauty in recent entries at https://blog.galaxyzoo.org/ https://blog.galaxyzoo.org/