1 00:00:01,000 --> 00:00:05,000 The Milky Way galaxy is our cosmic home. 2 00:00:05,000 --> 00:00:09,000 But it is far from being the only galaxy in the Universe. 3 00:00:10,000 --> 00:00:15,000 To get a better understanding of the composition and evolution of our Universe, 4 00:00:15,000 --> 00:00:20,000 astronomers try to answer a deceptively simple question: 5 00:00:20,000 --> 00:00:23,000 How many galaxies are there in the Universe? 6 00:00:44,000 --> 00:00:45,000 On a dark night, 7 00:00:45,000 --> 00:00:50,000 the Milky Way can be seen as a glowing band across the night sky. 8 00:00:50,000 --> 00:00:53,000 And for a long time it was thought 9 00:00:53,000 --> 00:00:57,000 that that the entire Universe consisted of our galaxy alone. 10 00:00:59,000 --> 00:01:02,000 This only changed in 1924. 11 00:01:02,000 --> 00:01:08,000 In that year Edwin Hubble identified variable stars in several spiral nebulae. 12 00:01:09,000 --> 00:01:14,000 He used these stars to calculate the distances to these nebulae. 13 00:01:15,000 --> 00:01:21,000 His observations proved that these stars were far too distant to belong to our Milky Way. 14 00:01:21,000 --> 00:01:25,000 They instead were members of other galaxies, 15 00:01:25,000 --> 00:01:28,000 far outside the Milky Way. 16 00:01:28,000 --> 00:01:31,000 Since then astronomers have tried to find out 17 00:01:31,000 --> 00:01:35,000 just how many galaxies there are in the observable Universe. 18 00:01:43,000 --> 00:01:51,000 A reliable first estimate could only be made after the Hubble Deep Field was observed in 1995. 19 00:01:52,000 --> 00:01:56,000 This was the first really deep look into the Universe 20 00:01:56,000 --> 00:02:01,000 and it revealed hundreds of galaxies that had never been seen before. 21 00:02:03,000 --> 00:02:07,000 Additional deep observations with Hubble and other instruments followed, 22 00:02:07,000 --> 00:02:11,000 detecting even fainter and more distant galaxies. 23 00:02:14,000 --> 00:02:17,000 From these observations astronomers determined 24 00:02:17,000 --> 00:02:23,000 that the Universe contained a total of 120 billion galaxies! 25 00:02:27,000 --> 00:02:32,000 These are only the galaxies within the so called observable Universe: 26 00:02:32,000 --> 00:02:34,000 This consists of all objects, 27 00:02:34,000 --> 00:02:38,000 that can in principle be observed with telescopes like Hubble — 28 00:02:38,000 --> 00:02:43,000 those from which light has had time to reach us since the beginning of the Universe. 29 00:02:43,000 --> 00:02:46,000 Beyond this limit, 30 00:02:46,000 --> 00:02:49,000 known as the cosmological horizon, 31 00:02:49,000 --> 00:02:51,000 the Universe continues. 32 00:02:51,000 --> 00:02:55,000 Just how far it goes is yet an unanswered question. 33 00:03:00,000 --> 00:03:03,000 Now astronomers have carried out a new analysis 34 00:03:03,000 --> 00:03:06,000 of published data from Hubble and other telescopes. 35 00:03:06,000 --> 00:03:09,000 They have concluded that the previous estimate of the number of galaxies 36 00:03:09,000 --> 00:03:15,000 within the observable Universe is at least ten times too low! 37 00:03:15,000 --> 00:03:20,000 They think that some 90% of the galaxies in the Universe 38 00:03:20,000 --> 00:03:26,000 are actually too faint and too far away to be observed by the current generation of telescopes. 39 00:03:27,000 --> 00:03:33,000 Astronomers can only infer their existence, based on models and calculations. 40 00:03:42,000 --> 00:03:45,000 The newly collected data has allowed astronomers 41 00:03:45,000 --> 00:03:49,000 to look back more than 13 billion years into the past, 42 00:03:49,000 --> 00:03:52,000 to the early days of the Universe. 43 00:03:52,000 --> 00:03:55,000 This lookback unveiled an early Universe 44 00:03:55,000 --> 00:04:01,000 in which the density of galaxies was also 10 times higher than today. 45 00:04:03,000 --> 00:04:07,000 Most of these galaxies were relatively small and faint, 46 00:04:07,000 --> 00:04:12,000 with masses similar to those of the satellite galaxies surrounding the Milky Way. 47 00:04:14,000 --> 00:04:20,000 These results are powerful evidence that galaxies evolved and grew via mergers 48 00:04:20,000 --> 00:04:22,000 throughout the Universe’s history, 49 00:04:22,000 --> 00:04:26,000 dramatically reducing their total number over time. 50 00:04:29,000 --> 00:04:34,000 With 90% of the galaxies in our Universe yet to be observed, 51 00:04:34,000 --> 00:04:39,000 there is still much left to explore and to discover for Hubble — 52 00:04:39,000 --> 00:04:42,000 and the upcoming James Webb Space Telescope. 53 00:04:42,000 --> 00:04:48,000 Once again nature has surprised us beyond our wildest imagination. 54 00:04:51,000 --> 00:04:58,000 Transcripted by ESA/Hubble. Translated by ---