# Hubble's constant

Why is Hubble's constant called a constant if it isn't constant?

As of 20 December 2012 the Hubble constant, as measured by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and reported in arxiv, is 69.32 ± 0.80 (km/s)/Mpc (or 21.25 ± 0.25 (km/s)/Mega-lightyear). [Reference]

As of 21 March 2013, the Hubble constant, as measured by the Planck Mission, is 67.80 ± 0.77 (km/s)/Mpc. [Reference]

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Simon Bridge
Homework Helper
Why is Hubble's constant called a constant if it isn't constant?
History.

The constant isn't necessarily changing - we're simply getting a more accurate measurement of what's its magnitude is. :)

Chronos
Gold Member
The Hubble constant declines over time [albeit very slowly]. In the distant past it was much larger, in the distant future it will decrease, although not as much as it has in the past 13 odd billion years.

The changes the original post was referring to are not because of change over time, as far as I could tell. :p

marcus
Gold Member
Dearly Missed
Why is Hubble's constant called a constant if it isn't constant?

As of 20 December 2012 the Hubble constant, as measured by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and reported in arxiv, is 69.32 ± 0.80 (km/s)/Mpc (or 21.25 ± 0.25 (km/s)/Mega-lightyear). [Reference]

As of 21 March 2013, the Hubble constant, as measured by the Planck Mission, is 67.80 ± 0.77 (km/s)/Mpc. [Reference]
It's the same number calculated from different sets of data. The actual Hubble expansion rate changes very very slowly. You would not notice a change over 100 years, it would be too small to measure even with much more advanced instruments.

I would encourage people to also know the Hubble rate as a PERCENTAGE rate of distance expansion per million years.

The Planck report figure of 67.8 km/s per Mpc corresponds to about 1/144 % growth per million years.

Everbody should know how to get this using google calculator: just type or paste this into the google window 67.8 km/s per Mpc in percent per million years

the google calculator can convert from one type of unit to another when you say IN. It can tell you x yards in feet or y yards in meters. you just paste that blue thing into the ordinary window that you use for searches, and press return to show you are done.

then it will say "0.00693384038 percent per (million years)"

and if you paste 1/0.00693384038 into the window it will say 144.220222...

So what it is really telling you is that the Planck figure of Hubble growth rate is actually
1/144 percent per million years.

The growth rate is declining very very slowly towards an estimated longterm level of 1/173 percent.

You can see that slow change reflected in the output of Jorrie's calculator, which is the "LightCone" link in my sig. Click on it for a condensed account of the past history and future of cosmic expansion. There are blue popup info buttons that explain stuff.

If you do that you should be able to spot the 1/144 I'm talking about in the row of the present (age 13.8 billion) and the 1/173 I mentioned in the row of a distant future (age 92 billion)
You will see the disguised as distances, e.g. 14.4 and 17.3, in the R column which gives the Hubble radius, a kind of "reciprocal" of the expansion rate. Try the blue dots if you need more explained.

That's how slow the Hubble rate is changing, you can see it in the table. so it's virtually constant in the short run.

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Simon Bridge
Homework Helper
Ah yes - the complete pedantic response should have been:

1. repeated accurate measurements will give different results due to experimental uncertainty - you can try it yourself: make a pendulum (length about 1m is good) and measure the time for one swing using a stopwatch that measures to 100ths of a second. Do this 10 times and you will get 10 different times:does this mean the period different each time?

2. the examples were from different experiments - the subsequent measurements were more accurate.
You can see this effect with the pendulum too: same setup, but make your first measurement with an egg-timer (the sand type) or using your pulse. You get different values don't you? Is the swing different?

3. the Hubble constant does vary - it is still thought of as "constant" because it changes so slowly that it can be safely treated as constant over the time-frame of any experiments we could hope to do. If you compared the period of the pendulum with the usual theory you did much the same thing: you took the (net) acceleration of gravity as a constant even though, technically, it would change a bit according to the relative positions of the Sun and Moon etc. It is safe to do so because the changes are very small compared with the accuracy of your measurements and are unlikely to become larger during the period you will do the experiment.

4. "Hubble Constant" is a name that got stuck at a time when it was thought to be actually constant - much like we still use "Newton's Laws" when we want to model the pendulum motion even though we know they are not quite right.

That should just about cover it.
:D

The growth rate is declining very very slowly towards an estimated longterm level of 1/173 percent.
Does this mean that the universe is expanding but slowing down? I had the impression that it was expanding and accelerating