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May 2002
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## Hubble inconstant

I've read a few places where there's a problem with measuring the Hubble constant. Measurments that go back to the beginning of the universe give an answer around 67 km/sec/Mpc; those that only measure things from more recent eras give an answer around 74 km/sec/Mpc. The most recent reading was in the Bad Astronomer's blog: The Universe is acting funny. Or we’re looking at it wrong.

Based on these various articles, the general thought among cosmologists is that one or the other set of measurements or calculations is wrong. There's some systematic error in one or the other that's throwing them off. I had the thought (and I'm sure I can't be the only one this occured to) that all the measurement and calculations are good and that they represent the actual state of the universe.

My thinking has to do with the expansion of the universe. This expansion is thought to be accelerating, so that in more recent eras, it's expanding faster than in earlier eras. That seems to be what the two sets of measurements are telling us. One set of measurements goes back to the beginning and reflects either only the early values of H0 or an average of the constant thoughout the entire history of the universe. The more recent measurements only reflect the last billion years or even less.

Now as I said, I'm sure this has occured to someone else. But there's no mention of the possibility in any of the articles I've read. So there must be something wrong with it. Most likely is that the difference is too large. The expansion is accelerating, but not by that much.

2. Two things.
One is that all these studies are estimating H0, which is the value of the Hubble parameter right now, indicated by the "0" subscript. Values at other times are denoted by just plain H, and they are, as you say, different at different times.
The second is that accelerating expansion doesn't necessarily imply an increasing value of H. In general, the scale factor increases faster than the rate of change of scale factor, so H actually decreases with time. So while the recession rate of a specific distant galaxy increases with time as it gets farther away, the recession rate of successive galaxies as they pass through a given distance from Earth decreases with time. It's the latter value that defines the Hubble parameter.

Grant Hutchison

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