# Acceleration of Universe

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1. Aug 31, 2015

### H-S-D

Is it possible to measure the universe' expansion in m/s^2 instead of metres per second per parsec?

2. Aug 31, 2015

### Bystander

3. Aug 31, 2015

### marcus

Hi HSD, Bystander often has good ideas so that may be a good suggestion that you google "parallax second" the root meaning of the distance measure called "parsec", but I do not understand how that relates to your question

Maybe I don't understand your question. Of course it is possible to measure the Hubble growth rate in different ways. Parsec is a measure of distance, a unit of length. So the conventional version
eg 67 km/s per Mpc
is a L/T per L quantity and the L units cancel so it is basically a 1/T quantity, a reciprocal time

One way to express the present-day Hubble growth rate H0 is as a unitless number per unit time (a fractional part or percentage per unit time)
like 1/144 percent per million years.

Or as the reciprocal of a long interval of time called the "Hubble time" namely 14.4 billion years.

You can express the Hubble growth rate in various equivalent ways. But you would never express it as a
m/s2 quantity because that would be an ACCELERATION. The growth rate is a fractional growth rate or a speed-to-size ratio. It is not an acceleration! So that would not be mathematically equivalent to the growth rate and would not make sense. What particular distance do you imagine would be accelerating?

So the answer is YES there are a lot of different equivalent ways to express this quantity,
and NO you would not want to try expressing it as a m/s^2 acceleration quantity.

Last edited: Aug 31, 2015
4. Aug 31, 2015

### Bystander

Two different "seconds."

5. Aug 31, 2015

### marcus

I see, that makes sense. You suspect HSD of mistakenly imagining that "parsec" is a unit of time, rather than distance.
Could be. There was that famous goof in the Starwars movie.

Anyway, there is a thread (with a poll) about "how do you think of the Hubble growth rate?". Maybe HSD would get something out of it.
I'd be interested in your view, Bystander---what your mental picture of the Hubble rate is, or your concept of it.