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#53928 - 04/28/15 10:40 PM Imagining the expanding Universe
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Thoughts while dog walking. Comments, please, not too rude! smile

What do we non-scientists visualise when we think about the expansion of the Universe? Certainly, a common image is that of an expanding sphere, like an inflating balloon. This, of course, is not to be confused with the “balloon analogy” in which the Universe is represented by the two-dimensional surface, rather than by the whole balloon.

Let’s think briefly about this visualisation. Without in any way suggesting that the Universe actually has a physical boundary, the mental image will almost certainly have one. You, of course, will be at the centre of this sphere, and will see the rest of the Universe moving away from you. The most up to date figure for the expansion rate of the Universe is “67.3 kilometers per second per megaparsec”. A megaparsec is defined as a distance equal to 3.26 million light years.

Just to make the arithmetic easy, let’s say our imaginary sphere has a radius of one thousand megaparsecs. Obviously, this is much too small, but it is just a simple thought experiment. This means that the imaginary boundary will be separating from you at 67,300 kilometres per second. The galaxies are being carried along with the expanding space, so any celestial body that is within the last 67,300 km. on this side of the boundary now, will be outside the present boundary position in one second’s time. It will be in newly created space; space that was not there a second ago.

Think a little more about this, though. An observer, say on a planet that has, from your perspective, just passed into the newly created space, will perceive himself as being at the centre of a spherical universe, and will think that you have just passed into newly created space.

This tells us a couple of things: (1) If the Universe is homogeneous, and in order to do any meaningful cosmology we must assume it is, we cannot define a physical boundary. (2) We cannot define any part of the Universe; rather than any other part; as being the newly created part. This must mean that every part of the Universe is expanding: new space is coming into existence everywhere in the Universe, all the time. No part is newer than any other part.
_________________________
There never was nothing.

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#53932 - 04/29/15 03:04 AM Re: Imagining the expanding Universe [Re: Bill S.]
Bill Offline
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Registered: 12/31/10
Posts: 1858
Loc: Oklahoma, USA
It sounds to me as if you have got it right.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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