WEBVTT 00:00:00.000 --> 00:00:03.240 This is College Physics Answers with Shaun Dychko. 00:00:04.300 --> 00:00:06.420 The further away a galaxy is, 00:00:06.460 --> 00:00:09.160 the faster its receding from us 00:00:09.680 --> 00:00:13.140 and this is because space itself is expanding 00:00:13.140 --> 00:00:17.660 and the more space there is between us and this galaxy 00:00:17.960 --> 00:00:22.200 then the faster it will be expanding 00:00:22.200 --> 00:00:26.920 or going away from us since there's more space expanding in between. 00:00:27.840 --> 00:00:28.800 Okay! 00:00:28.980 --> 00:00:31.200 Now we are going to estimate the age of the universe 00:00:31.320 --> 00:00:35.120 by saying let's suppose we have to go 00:00:35.120 --> 00:00:37.080 1 megalight year 00:00:37.320 --> 00:00:40.640 and at a speed of 20 kilometers per second, 00:00:40.640 --> 00:00:42.580 how long would it take to do that? 00:00:43.520 --> 00:00:47.700 So 1 megalight year is 1 times 10 to the 6 light years 00:00:48.040 --> 00:00:53.300 and we multiply by speed of light for every l 00:00:53.560 --> 00:00:56.860 and l represents the speed of light 00:00:56.900 --> 00:01:00.380 and the y represents a period of a year, 00:01:00.380 --> 00:01:01.860 which we will convert into seconds 00:01:02.120 --> 00:01:05.780 so we multiply that by 365.25 days per year 00:01:05.940 --> 00:01:09.880 and then by 24 hours per day and then 3600 seconds per hour 00:01:10.060 --> 00:01:13.380 and we now have meters on the top 00:01:14.480 --> 00:01:17.400 and then we divide that by the speed of 20 kilometers per second 00:01:17.400 --> 00:01:19.440 converted into meters per second 00:01:19.620 --> 00:01:22.000 by multiplying by 1000 meters per kilometer, 00:01:22.200 --> 00:01:26.140 this works out to 4.7305 times 10 to the 17 seconds 00:01:26.240 --> 00:01:28.000 which I then convert back into years 00:01:28.040 --> 00:01:30.520 and that is 15 billion years— 00:01:30.520 --> 00:01:33.120 that's the estimate for the age of the universe. 00:01:34.580 --> 00:01:38.740 Part (b) says if deceleration is taken into account 00:01:38.800 --> 00:01:42.960 although it is peculiar that they talk about deceleration since 00:01:43.180 --> 00:01:44.800 it turns out that in fact 00:01:44.860 --> 00:01:48.700 the expansion is accelerating— it's getting faster— 00:01:48.700 --> 00:01:52.500 and that's what dark energy has been 00:01:52.540 --> 00:01:55.200 postulated to explain not that it really 00:01:55.220 --> 00:01:57.380 is something that anybody understands but 00:01:57.420 --> 00:01:59.140 it's just an explanation well... 00:01:59.140 --> 00:02:01.920 it's gotta be something that makes it expand faster 00:02:02.780 --> 00:02:04.580 but in any case, the question here 00:02:04.580 --> 00:02:06.580 says suppose there's deceleration 00:02:06.580 --> 00:02:08.460 which I think was the original supposition 00:02:08.460 --> 00:02:13.100 since gravity is expected to, you know, slow down this expansion. 00:02:14.280 --> 00:02:20.340 So if the average 00:02:20.700 --> 00:02:24.160 speed is actually higher than what we have got here 00:02:24.220 --> 00:02:27.720 you know if 20 kilometers per second per megalight year 00:02:27.720 --> 00:02:31.260 or if 20 kilometers per second is 00:02:33.640 --> 00:02:36.380 the result of some deceleration in which case, 00:02:36.380 --> 00:02:38.660 the speed was higher earlier 00:02:38.660 --> 00:02:41.360 that would make the average velocity higher 00:02:41.720 --> 00:02:46.900 and by making the denominator bigger, we make this quotient smaller 00:02:46.960 --> 00:02:50.100 in which case the calculation is off 00:02:50.100 --> 00:02:53.540 and so the universe is actually younger than what we have calculated.