The Universe is expanding a bit slower than we expected. The Universe is expanding, and has been ever since the moment it was born. We can measure the speed of this expansion in various ways; for example, looking at distant exploding stars. We can measure how fast they are moving away from us, swept along with the expansion of space, by seeing how much their light is redshifted (I have details about how this works in an earlier post on redshifts and the expansion of the Universe). We can measure their distance, too, using various methods including how bright they appear to be, and with both their speed and distance we can calculate how fast the Universe is expanding. The farther away you go, the faster the Universe expands, and what Planck found is that the Universe is getting bigger at a rate of 67.3 kilometers per second per megaparsec. A megaparsec is a unit of distance equal to 3.26 million light years (which is convenient to astronomers). That means that if you look at a galaxy one megaparsec away, it appears to be moving away from you at 67.3 km/sec. A galaxy two megaparsecs away would recede at twice that speed, 134.6 km/sec, and so on. This is called the Hubble constant. Various methods have been used to measure it for the past century, and some of the best found it to be about 74.2 km/s/Mpc. Planck’s measurement is smaller, so the Universe appears to be expanding a little more slowly than we thought, which is why the age is a bit higher than measured before, too. Part of the reason the number is smaller from Planck is that it’s looking at light that is very old, and came from very far away, so they extrapolate forward in time to see how fast the Universe is growing. Other measurements use light from objects that are closer, and scientists extrapolated backwards. Since the two numbers are different, this may mean the Hubble constant has changed over time, though that’s way too preliminary to tell. I’ll just note it here as an interesting development. The Hubble constant is notoriously difficult to measure, and I imagine astronomers will be arguing about it for some time yet to come.
Posted on: Sun, 22 Sep 2013 04:52:15 +0000
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