In this PF Cosmo forum context we have to be sensitive/practical about language. How to state cosmo basics, especially to newcomers who may not have assimilated technical terms like "scalefactor" yet. This does not mean we dumb everything to the max! Introductory terms should lay a basis of understanding that you can build on. So there are issues.(adsbygoogle = window.adsbygoogle || []).push({});

Here's an example and I'd like to hear your opinion.

The Hubble rate is 71 km/s per Mpc----with the new Riess numbers putting it at 74 and a gradual transition seems to be under way. But say, for now, 71.

According to the standard LCDM model, that should decline to about sqrt(0.75) of the current value and level out there. So we are looking at eventually getting down near 61, or with the new Riess numbers 64 km/s per Mpc.

Now a newcomer would not necessarily be familiar with Megaparsecs or be able to picture Hubble law expansion graphically from that number 71. So some of us have gotten into the practice of translating it into concrete terms of distances currently increasing by a certain (non-constant) percentage every million years. The percentage changes very very slowly so it will stay roughly right for a long time and the mental picture is, I think, adequate.

The point of using a percentage is to get across the idea that longer distances increase more.

The current Hubble rate of 71 translates into distances currently increasing 1/140 percent every million years.Do you think this is an OK way to describe things to newcomers? A good thing about it is it conveys the idea of the slowness (1/140 of a percent is a slight increase and a million years is a long time.) And it also implies superluminal rates of distance increase for long distances. If a distance is long enough then 1/140 of a percent of it can be more than a million lightyears, so in a million years it adds more than a million lightyears to its length. So saying things that way can give useful mental pictures.

Now there are some fine points to consider later after one gets across the main idea. The eventual decline to about sqrt(.75) of the current value means that this 1/140 will eventually get down near 1/160.

Or with the new Riess numbers closer to 1/150. So one can say that the asymptotic percentage rate, for Hubble law expansion, is slated to be about 1/150 of a percent per million years.

An important point to make is that currently the increase in distance is not exponential growth, because the percentage rate is declining.

If you would like to check the calculation yourself, type this into the google window and press return:

(71 km/s per megaparsec)*(10^6 year) in percent

The google calculator will give you this response:

(71 ((km / s) per megaParsec)) * ((10^6) year) = 0.00726109501 percent

and you can verify that 0.00726 is approximately 1/140.

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# Expressing the current Hubble rate as a temporary percentage increase in distance

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