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 Jun 7th 2017, 08:40 AM #1 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 wad Does anyone know if the Hubble constant for the rate of expansion of the universe (currently estimated at 71,900 meters per second at 1 million parsecs) has been determined from observations in only one direction into the universe (i.e., observing one pair of galaxies moving apart) or is it an average of observations looking in dozens or hundreds of different directions into the universe? Can't find the answer in the "literature." Many thanks for any feedback.
 Jun 7th 2017, 09:22 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 It's an average, looking at a large number of galaxies across all portions of the sky. You need to collect a fair amount of measurements in order to come up with a good estimate for Hubble's Constant, in order to average out the effects of relative motions between the galaxies being observed and us.
 Jun 7th 2017, 10:36 AM #3 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 And is it a measurement of one galaxy moving outward from one another and away from Earth, or two galaxies moving side to side from each other but at the same distance from Earth?
 Jun 7th 2017, 01:18 PM #4 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 It's a measurement of the speed of galaxies receding from Earth. It's calculated by determining the speed of recession (from the red shift of spectral lines) divided by the estimated distance from earth to to the galaxy. Do this for a few hundred galaxies ranging from several million to several billion light years away, and you can calculate an average value in units of kilometers per second per megaparsec. Note this has fundamental units of one over time - if you express Hubble's Constant in terms of Km/s per Km then the inverse is a rough estimate of the time since the Big Bang.
 Jun 7th 2017, 01:42 PM #5 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 Got it. Many thanks!
 Jun 7th 2017, 04:03 PM #6 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 You're welcome. But just to add: although it's a measure of how galaxies are receding from the Earth based on their distance from us, theory says that any observer anywhere in the universe would measure the exact same recession based solely on distance. Stated another way: all points in the universe are the center of the universe!