Is this interpretation on Hubble's Law/Constant right ?

  • #1
Is this interpretation on "Hubble's Law/Constant" right ?

I referred to this page
http://en.wikipedia.org/wiki/Hubble's_law

The hubbles constant 'H0' is anywhere between 72 to 78 kms per second per megaparsec.
My doubts :-

1) Let us consider a rod (made of streatchable light weight material) of length amounting to one megaparsec be placed in empty space. Neglecting the effects of gravity, If the recessional velocity is contributed by the very expansion of space time, Should the end points recede away by 75 kms per second? (and hence make the rod grow 75 kms every second?)

2) If the rod is elastic, will the space expansion stretch the rod ?. (can that stress be harnessed ?)
 

Answers and Replies

  • #2
Wallace
Science Advisor
1,251
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I didn't read the wiki page, it is possibe that there are poor explanations within it, I'm not sure, but let me address your two points independant of whatever that article states.

1) Let us consider a rod (made of streatchable light weight material) of length amounting to one megaparsec be placed in empty space. Neglecting the effects of gravity, If the recessional velocity is contributed by the very expansion of space time, Should the end points recede away by 75 kms per second? (and hence make the rod grow 75 kms every second?)
It is a verrrrry common misconception that the 'expansion of space' (or spacetime) causes galaxies to receed. In fact 'the expansion of space' does not appear anywhere in the technical description of cosmology, and is nothing more than a metaphor used to simplify the understand of aspects of an expanding Universe.

Galaxies receed because something (inflation) caused everything in the Universe to start moving away from everything else. If you think about any two galaxies, at any point they are moving apart, therefore from either Newtons first law (inertia) or the General Relativistic extension, they will continue to do so unless otherwise acted up. Gravity provides such an action, and acts to slow down the rate of expansion. Due to some unexpected observations, we also suspect that dark energy is also present, and it acts in such as way as to speed up the expansion. At present dark energy appears to be dominate, such that the overall expansion rate is increasing. In any case, 'expanding space' simply describes the expanding universe, it is not something that causes it. If the Universe had a diffferent set of initial conditions then we would possibly not see a Hubble flow and not use the term 'expanding space', it would not be because of different physics operating, just different initial conditions.

This is only true on large scales though. On the scale of say a galaxy, the average density is much higher and hence gravity is much stronger than dark energy. There is not however a balance between gravity and 'the expansion of space' on the scale of a galaxy. If you look at a galaxy, it is not expanding, hence by Newtons first law (inertia) there is not reason for it to start doing so. Their does not exist any special 'new' force called the expansion of space that is continually trying to rip galaxies apart.

So, to more directly answer your questions, the rod will sit there and do nothing. It will not expand and you will not see any recession velocity along the rod. The rod will not stretch.
 
Last edited:
  • #3


I didn't read the wiki page, it is possibe that there are poor explanations within it, I'm not sure, but let me address your two points independant of whatever that article states.

It is a verrrrry common misconception that the 'expansion of space' (or spacetime) causes galaxies to receed. ....................

So, to more directly answer your questions, the rod will sit there and do nothing. It will not expand and you will not see any recession velocity along the rod. The rod will not stretch.
Thanks For The Advise.
 

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