Hubble Constant

[rockystar]

Just been reading about the latest figure for the Hubble Constant (in this month's S@N mag), and had a quick question to confirm my understanding of it. So the figure is stated as 73 km-per-second-per-megaparsec.

So does that mean, galaxies at 1 mp distance are moving away from us at 70 km/sec, galaxies at 2mp are moving at 140 km/sec, 3mp at 210 km/sec, etc, etc...?

 

[acey]

Yes (assuming they are at rest with respect to the Hubble flow; local motions make a slight difference). "Moving away from us" is of course used loosely: when light reaches us from a distant, high-redshift galaxy, the galaxy itself might no longer be there. The Hubble constant is defined by supposing a model universe that is a perfectly smooth (and extremely dilute) fluid. The real universe proves to be a pretty close approximation to that.

[rockystar]

yeah, I know it's not really "moving away from us", and more like everything moving away from everything else.

thanks.

[George Jones]

The Hubble "constant" is poorly named, as it is not constant in time. At any instant in time, the Hubble constant is the same throughout space, but at different times, the Hubble constant is different. For example, in the past, the Hubble constant was larger than it is now.

[dph1nm]

On 23/05/2016 at 16:43, George Jones said:

At any instant in time, the Hubble constant is the same throughout space

Actually not even that is true. The local Hubble constant depends on the local density of matter.

NigelM

[George Jones]

On 2016-05-23 at 08:43, George Jones said:

The Hubble "constant" is poorly named, as it is not constant in time. At any instant in time, the Hubble constant is the same throughout space, but at different times, the Hubble constant is different. For example, in the past, the Hubble constant was larger than it is now.

 

2 hours ago, dph1nm said:

Actually not even that is true. The local Hubble constant depends on the local density of matter.

NigelM

Yes, of course. My statement refers to the standard Friedmann-Lemaitre-Robertson-Walkers models of the universe, which are spatially homogeneous and isotropic. as our universe is on scales larger than 300 million light-years ("Cosmology", by Weinberg, 2008).