What Is the Universe Expanding Into?

[marcus]

Mark, I thought that it would be more distance between "unbound" objects. In what sense are the objects bound?

Regards,

Noel.

Heh, I think I see what you are driving at, Lino---but what I see is more a verbal ambiguity than a disagreement. A distance between bound objects (in Mark's words) could mean the distance between two objects, each of which is bound, but the two objects are not bound gravitationally to each other.

So the distance could be the distance between two rocks, or two galaxies, or two gravitationally bound clusters of galaxies. Each of them separately is a "bound object" because held together by its own internal forces.

But it's assumed (as I read Mark's post) that the two bound objects do not form a larger bound system. So it is an "unbound" pair of bound objects.

Ordinary language is frustrating, so many chances for ambiguity.

 

[skydivephil]

I thought the best way to define the expansion of the unvierse is the distance between galaxies is growing. It doesn't mean the unvierse is getting bigger (it might have been infinite at the start of the expansion). Also it doesn't mean the distance between objects smaller than galaxies is expanding as these are gravitationally bound. However I have just realized a difficulty here that I am not sure how to square. The propblem is if we define the expansion this way, what do we mean by the expansion before galaxies formed? Woudln't that imply the distance between unbound obejcts is growing and hence invalidate the earlier description or deny that the unvierse was expanding before galaxies formed. Both seem wrong to me although obviously the latter seems a little more absurd. not sure how to make my deifnition correct though.

 

[Lino]

Thanks Marcus (& Mark). Much appreciated.

Regards,

Noel.