Modern researchIn 1944, Hendrik van de Hulstpredicted - TopicsExpress

Modern researchIn 1944, Hendrik van de Hulstpredicted microwaveradiation at a wavelength of 21 cmresulting from interstellar atomic hydrogengas; [ 44 ]this radiation was observed in 1951. The radiation allowed for much improved study of the Milky Way Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating bar structurein the center of the Galaxy. [ 45 ]With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s it was discovered in Vera Rubins study of the rotation speedof gas in galaxies that the total visible mass (from the stars and gas) does not properly account for the speed of the rotating gas. This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter. [ 46 ] [ 47 ] Beginning in the 1990s, the Hubble Space Telescopeyielded improved observations. Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. [ 48 ]The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the Universe. [ 49 ]Improved technology in detecting the spectrainvisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the Zone of Avoidance(the region of the sky blocked by the Milky Way) have revealed a number of new galaxies. [ 50 ]